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\begin{document}

\title{Allometric covariation of xylem and stomata across diverse woody
seedlings}



\author[1]{Mengying Zhong}%
\author[2]{Bruno E.L.  Cerabolini}%
\author[3]{Pilar Castro-Diez}%
\author[4]{Jean-Philippe  Puyravaud}%
\author[5]{Johannes Cornelissen}%
\affil[1]{Vrije Universiteit Amsterdam}%
\affil[2]{University of Insubria}%
\affil[3]{Universidad de Alcalá}%
\affil[4]{Sigur Nature Trust}%
\affil[5]{VU University}%


\vspace{-1em}



  
  \date{June 14, 2024}


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\selectlanguage{english}
\begin{abstract}
Leaf stomatal density is known to covary with leaf vein density.
However, the functional underpinning of this relation, and how it scales
to whole-plant water transport anatomy, is still unresolved. We
hypothesized that the balance of water exchange between the vapour phase
(in stomata) and liquid phase (in vessels) depends on the consistent
scaling between the summed stomatal areas and xylem cross-sectional
areas, both at the whole-plant and single-leaf level. This predicted
size-covariation should be driven by the covariation of numbers of
stomata and terminal vessels. We examined the relationships of stomatal
traits and xylem anatomical traits from the entire plant to individual
leaves across seedlings of 53 European woody angiosperm species. There
was strong and convergent scaling between total stomatal area and stem
xylem area per plant and between leaf total stomatal area and midvein
xylem area per leaf across all the species, irrespective of variation in
leaf habit, growth-form or relative growth rate (RGR). Moreover, strong
scaling was found between stomatal number and terminal vessel number
while not in their respective average areas. Our findings have broad
implications for integrating xylem architecture and stomatal
distribution, and deepen our understanding of the design rules of
plants' water transport network.%
\end{abstract}%



\sloppy


\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
Definitions of parameters: \textbf{Average stomatal area:} mean area of
a guard cell pair, i.e. double ellipse 2*a*b*\selectlanguage{greek}π, \selectlanguage{english}where a and b are the
maximum length and maximum width of the guard cells
(\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}). \textbf{Leaf total stomatal area:} the sum of
all stomatal areas per leaf (\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) \textbf{Plant total
stomatal area:} the sum of all stomatal areas per plant
(\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) \textbf{Plant total stomatal number:} the sum
number of all stomata per plant \textbf{Minor vessel area:} mean area of
plant terminal vessels (\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) \textbf{Plant minor
vessel number:} the sum number of all terminal vessels per plant
\textbf{Stem xylem area:} cross-sectional area of the entire xylem
tissue in stems (\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) \textbf{Stem xylem conductance
area:} cross-sectional area of the conducting section in stem xylem
(\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) \textbf{Average leaf area:} mean area of single
leaves (mm\textsuperscript{2}) \textbf{Total leaf area:} the sum of all
leaf areas per plant (mm\textsuperscript{2})\tabularnewline
\bottomrule
\end{longtable}

\textbf{Introduction}

The xylem system of vascular plants generally features a ``tip-to-base''
widening with the maximal number of the narrowest conduits in the
terminal parts; the size of these terminal conduits should not vary with
plant size (or leaf size) (Lechthaler, Colangeli, Gazzabin \& Anfodillo,
2019, Rosell \& Olson, 2019, West, Brown \& Enquist, 1999). With this
hierarchical and basipetally widening xylem architecture, the energy
cost of long-distance water transport is minimized (Anfodillo, Carraro,
Carrer, Fior \& Rossi, 2006, Shinozaki, Yoda, Hozumi \& Kira, 1964,
West\emph{et al.} , 1999). Under the negative pressure created by
stomatal transpiration, water ascends from the soil, progressively
through stem and leaf xylem vessels, all the way up to the terminal
stomata. Covariation of stomatal and xylem traits in leaves is required
to maintain a balance in water exchange between the liquid (water
delivery) and the vapor (water loss) phase (Brodribb, McAdam \& Carins
Murphy, 2017, Carins Murphy, Jordan \& Brodribb, 2014, Zhang, Carins
Murphy, Cardoso, Jordan \& Brodribb, 2018).

There is mounting evidence that vein density is proportional to stomatal
density in leaves, and this pattern is applicable to diverse plants
within and across species (Brodribb \emph{et al.} , 2017, Carins Murphy,
Jordan \& Brodribb, 2016, Fiorin, Brodribb \& Anfodillo, 2016). However,
the causality of this relationship is difficult to interpret for three
reasons. Firstly, leaf vein traits have been proposed to be proxies for
leaf xylem properties (Blonder, Violle, Bentley \& Enquist, 2011, Sack,
Scoffoni, McKown, Frole, Rawls, Havran, Tran \& Tran, 2012). Veins
consist of more than xylem (e.g. they also host phloem), so simply
considering vein density will ignore xylem vessel number and vessel
lumen diameter, which have been deemed the predictors of conductive path
length and leaf area respectively (Echeverr\selectlanguage{ngerman}ía, Anfodillo, Soriano,
Rosell \& Olson, 2019, Rosell \& Olson, 2019). We are not aware of any
studies linking stomatal traits to xylem traits \emph{per se} (i.e. size
covariation of stomata and xylem vessels) (but see Meinzer and Grantz
(1990) about xylem-stomatal conductance relationships) within and across
species. Secondly, stomatal and vein densities reflect leaf water
relations in terms of a leaf plane, while it is a system of conduits
within a three-dimensional system, obviously finely tuned by natural
selection in a way that directs water nearly optimally given carbon
costs, conductance, and embolism resistance (Enquist, 2002, West\emph{et
al.} , 1999). Thirdly, these vein-stomatal density studies (Brodribb
\emph{et al.} , 2017, Carins Murphy \emph{et al.} , 2014, Sack,
Dietrich, Streeter, Sanchez-Gomez \& Holbrook, 2008) use the water
balance of single leaves to implicate the whole-plant water balance.
This approach might be an oversimplification for understanding the
entire liquid phase and the vapour phase relation, even though leaf area
has been proved to predict photosynthetic productivity precisely, from
the single leaf, to the branch, to the whole-tree, to the forest level
(Li, Reich, Schmid, Shrestha, Feng, Lyu, Maitner, Xu, Li \& Zou, 2020).

We address this knowledge gap with a laboratory growth experiment that
enabled us to obtain xylem and stomatal traits both at leaf and
whole-plant level. We grew seedlings of 53 diverse woody species from
cool-temperate and Mediterranean Europe in a standard growing
environment (Cornelissen, Castro-Díez \& Hunt, 1996, Zhong, Castro-Diez,
Puyravaud, Sterck \& Cornelissen, 2019).

We examined relations between xylem dimensions (Zhong \emph{et al.} ,
2019) and stomatal dimensions of these seedlings both at whole-plant
level and at leaf level. Specifically, this study presents, for the
first time, the allometric scaling relationships at two scales: (i)
between stem xylem cross-sectional area (as well as stem xylem
conductance area) and total stomatal area at the whole-plant level, and
(ii) between leaf midvein xylem area and leaf total stomatal area at
single leaf level. We hereby introduce uniformity in the analyzed
pairwise traits as they are expressed in the same physical units, which
helps to represent more directly the selection effect on the water flux
and enlightens our understanding of the whole-plant water balance.

Furthermore, as was proposed by the West-Brown-Enquist (WBE) model, the
terminal vessels should be `invariant' with plant size (or leaf size)
along with plant growth for a given individual (Roddy, Théroux-Rancourt,
Abbo, Benedetti, Brodersen, Castro, Castro, Gilbride, Jensen \& Jiang,
2020, Simonin \& Roddy, 2018, West \emph{et al.} , 1999). We tested the
relations between terminal vessel traits (i.e. minor vessel number and
individual minor vessel area) and plant size (represented by stem xylem
area and/or total leaf area per plant) across these 53 species, which
were grown in a similar environment, in order to understand whether the
terminal vessels `depended' on plant size across diverse species.
Additionally, the relations between stomatal traits (i.e. stomatal
number and individual stomatal area) and leaf size (represented by
midvein xylem area and/or average leaf area) were tested empirically, in
order to link with the large body of studies on the allocation mechanism
of leaf surface to stomata (Boer, Price, Wagner-Cremer, Dekker, Franks
\& Veneklaas, 2016, Franks \& Farquhar, 2007, Parlange \& Waggoner,
1970). Additionally, as stomatal allocation at the leaf surface tends to
simultaneously minimize water loss (e.g. water exchange from the minor
vessels to stomata) while maximizing gas exchange to maintain a constant
photosynthetic productivity per unit leaf area (Boer \emph{et al.} ,
2016), we expect that the number of stomata should scale linearly with
the number of minor vessels (Fig. 1).

Fig.1 Conceptual framework of this research concerning allometric
relations of plant hydraulics across woody species. Natural selection
acts on heritable variation between individuals within the same species.
Individuals with vessels that do not widen with height growth, or widen
little, will experience continual declines in leaf-specific conductance
with height growth and therefore declining growth and reproductive
output per unit leaf area. Individuals with vessels that widen very
markedly would have conduits of low resistance, in contrast to the
high-resistance variants with conduits that are `too narrow', but they
would have their own set of disadvantages. For example, for a given leaf
area and transpirational demand, the wider conduits cost more for the
same service provided. Each unit of carbon invested in excessively wide
vessels is a unit that is not invested in further growth or
reproduction, and so these variants should be at a selective
disadvantage (Banavar, Cooke, Rinaldo \& Maritan, 2014). Moreover, wider
vessels are more vulnerable to gas embolisms that obstruct conductance,
both from freezing ((Sevanto, Holbrook \& Ball, 2012, Zanne, Tank,
Cornwell, Eastman, Smith, FitzJohn, McGlinn, O'Meara, Moles, Reich,
Royer, Soltis, Stevens, Westoby, Wright, Aarssen, Bertin, Calaminus,
Govaerts, Hemmings, Leishman, Oleksyn, Soltis, Swenson, Warman \&
Beaulieu, 2014) and likely drought as well (Cai \& Tyree, 2010,
Jacobsen, Pratt, Venturas \& Hacke, 2019, Liu, Gleason, Hao, Hua, He,
Goldstein \& Ye, 2019). As a result, plants with vessels that are `too
wide' would also be at a selective disadvantage (Zhong \emph{et
al.}(2019)). The variants that should have the largest amounts of
surplus carbon to devote to growth and reproduction are those in the
intermediate zone, in which conduits widen just enough that conductance
remains constant per unit leaf area, but not so much as to incur
excessive carbon costs and embolism vulnerability.

In our previous study, based on the same woody seedling populations, we
found that, at the whole-plant level, the stem xylem cross-sectional
area (\emph{X\textsubscript{stem}} ) of stem medium (a) closely scales
with stem height (\emph{H} ) and total leaf area per plant (\emph{LA} )
as\emph{X\textsubscript{stem}} \selectlanguage{english}[?] \emph{H} \textsuperscript{1.52}
and\emph{X\textsubscript{stem}} \selectlanguage{english}[?] \emph{LA} \textsuperscript{0.75}
across all the studied species. For individual leaves, vessel diameter
(\emph{D} \textsubscript{leaf}) in the medium of leaf midvein (b)
closely scales with average leaf area (\emph{MLA} ) as\emph{D}
\textsubscript{leaf} \selectlanguage{english}[?] \emph{MLA} \textsuperscript{0.21} (Zhong\emph{et
al.} (2019). In this study, we test the poorly understood xylem-stomata
covariation from the size perspective We ask: are there scaling
relationships between total stomatal area and xylem cross-sectional area
across species, at the entire plant and at individual leaf level?
Specifically, we zoom in on the terminal part of water exchange (from
minor vessels to stomata), and ask: does the minor vessel number (which
scales with leaf area; see (Lechthaler \emph{et al.} , 2019, Rosell \&
Olson, 2019)) scale with stomatal number per leaf and per plant across
these woody seedlings? The conceptual picture should deepen our
understanding of plants' water transport system and have broad
implications for integrating xylem architecture and stomatal
distribution.

Based on the expectations above, we test the hypothesis that, despite
large interspecific differences in leaf-habit, growth-form and relative
growth rate (RGR), similar scaling should exist between total stomatal
area and xylem area across woody seedlings, both at the entire plant and
at individual leaf level, to ensure the balance between liquid- and
vapour-phase water conductance. We also expect that the hypothesized
scaling of total stomatal area to xylem area should be driven by the
number covariation of stomatal and minor vessel elements; we also expect
a scaling relation between mean stomatal area and mean minor vessel area
as we presumed the distal element size for a given plant to be limited
under long-term nature selection.

\textbf{Materials and Methods}

Seedling growth protocol

Seeds of 53 diverse woody species, belonging to different growth-forms
(19 trees, 22 shrubs, 6 subshrubs and 6 climbers or scramblers) and leaf
habits (34 deciduous and 19 evergreens), were collected from
cool-temperate and Mediterranean Europe (Table S1). These species are a
subset of those used by Cornelissen \emph{et al.} (1996), and the
seedlings sampled for this anatomical study were subpopulations of those
grown in that growth rate focused study, which was conducted in standard
environmental conditions at the Unit of Comparative Plant Ecology,
Sheffield University. In brief, throughout 1994 and 1995, all seeds were
first germinated and then transplanted into experimental pots that were
filled with quarried, prewashed silica sand. An environmental condition
of 14 h 20-22 \selectlanguage{english} : 10 h 15-17 \selectlanguage{english} light : dark was provided, with 135 +- 10
\selectlanguage{greek}µ\selectlanguage{english}mol m\textsuperscript{-2}s\textsuperscript{-1} of photosynthetically
active radiation (classified as partial shade, see Hendry and Grime
(1993)).

The population of each species was evenly divided into two halves for
initial and final harvest. After the seedlings opened the first true
leaf or leaf pair (i.e. at standardized ontogenetic stage), we harvested
the first half population and determined the total plant dry weight. The
second half of the population was cultivated for another 21 days within
the same standard environment with 0.25 ml per sand volume full-strength
Rorison nutrient solution (N, P and K at 56, 31 and 78 mg
l\textsuperscript{-1}, respectively, plus Ca, Mg, Fe and trace elements)
and sufficient deionized water on alternate days. The seedlings were
then harvested, dry-weighed and further treated for anatomy analysis
(details see Cornelissen \emph{et al.} (1996)).

Relative growth rate (RGR) and leaf area

Mean relative growth rate (RGR) was derived as: RGR =
(log\textsubscript{e}\emph{W\textsubscript{2}} -
log\textsubscript{e}\emph{W\textsubscript{1}}
)/(\emph{t\textsubscript{2}} -\emph{t\textsubscript{1}} ), where
\emph{W\textsubscript{1}} and\emph{W\textsubscript{2}} was the plant dry
weight at the first (\emph{t\textsubscript{1}} ) and second
(\emph{t\textsubscript{2}} ) harvest, respectively. At the final
harvest, plant total leaf area was measured after saturation in wet
tissue paper at 5 overnight, with a Delta-T Area Meter (Burwell,
Cambridge, UK) for most species, while a 1 mm paper grid was used to
calculate leaf area visually for some species with tiny leaves. Average
leaf area was calculated as the ratio of total leaf area to leaf number
per individual. For each species, 8-30 individuals were used for
quantifying the above parameters (details see Cornelissen \emph{et al.}
(1996), Cornelissen, Cerabolini, Castro-Diez, Villar-Salvador,
Montserrat-Marti, Puyravaud, Maestro, Werger and Aerts (2003a)).

Xylem traits

At the final harvest, three to four seedlings per species were chosen
randomly for xylem traits measurements. For each individual, one fully
expanded leaf as well as the stem was pickled, and the middle part of
each leaf (including the middle of the midvein) as well as the middle of
each stem were cut transversely. The materials were embedded in 5\% agar
and progressively dehydrated in 50, 70 and 95\% ethanol (2h per
solution), after which the small blocks of agar were infiltrated for 15
days with resin JB 4 Polysciences (Polysciences Inc., Warington, Pa.,
USA). After polymerisation of the resin, 2 \selectlanguage{greek}µ\selectlanguage{english}m thick cross-sections were
obtained with a glass ultra-microtome, then sections were stained with
5\% toluidine blue and permanently mounted onto slides with DPX (dibutyl
phthalate in xylene). The cross-sections of leaves and stems were
studied with a light microscope (Zeiss Axioskop; Carl Zeiss, Jena,
Germany) on a computer screen with image analysis software (Aequitas IA
v. 1.25) (Castro-D\selectlanguage{ngerman}íez, Puyravaud \& Cornelissen, 2000, Castro-Díez,
Puyravaud, Cornelissen \& Villar-Salvador, 1998).

For stems, stem xylem area and stem xylem conductance area (stem xylem
area minus cell wall area) were circled and measured. The proportion of
cell wall area relative to xylem area in transverse section was measured
in three to four microscopic fields per slide using Aequita tools
(Castro-Díez \emph{et al.} , 1998). For leaves, leaf midvein xylem area
and minor vessel area were circled in light microscope images and
measured, and the minor vessel area was calculated as the average area
of the ten smallest vessels of the cross-section of leaves, which were
defined as the distal conduits. The plant minor vessel number (\emph{N}
\textsubscript{vessel}) was theoretically approximated as:\emph{N}
\textsubscript{vessel} = A/( \selectlanguage{greek}π\selectlanguage{english}*\emph{R} \textsubscript{s}*\emph{R}
\textsubscript{m}), where A is stem xylem conductance area, \emph{R}
\textsubscript{s} is the radius of the biggest vessel in stem medium,
and \emph{R} \textsubscript{m} is the radius of the minor vessel in
leaves. This calculation was based on the pipe model, which states that
the sum of all vessel inner diameters at each vein order is equal
(Shinozaki \emph{et al.} , 1964). We used the stem (rather than leaf)
xylem conductance area to calculate\emph{N} \textsubscript{vessel}
because it is difficult to gain the leaf xylem conductance area in a
representative way from entire leaf cross-sections, especially for
species that have big leaves.

Stomatal traits

At the final harvest, one leaf from each of three different seedlings
was randomly selected for epidermal prints; the representative leaf
section at about one third from the apex and one third from the midvein
was examined. We first brushed acetone onto surfaces of these leaves,
and then pressed an acetate layer onto them firmly for 30 s and waited
for them to dry. Subsequently, we peeled off the acetate layer and
mounted it onto a slide for stomatal analysis. Stomatal number of each
of ten randomly selected views (0.12 mm\textsuperscript{2} at 100 \selectlanguage{ngerman}×
magnification) was counted and averaged. When prints did not have
sufficient large undamaged and clear areas, smaller areas (0.01- 0.05
mm\textsuperscript{2} at 400 × magnification) were examined. Stomatal
density was determined as the summed number of stomata on both upper and
lower surface per one-sided leaf area. The stomatal area was defined as
the area of a guard cell pair, \emph{i.e.} double ellipse 2*a*b*\selectlanguage{greek}π, \selectlanguage{english}where
a and b are the maximum length and maximum width of the guard cells of
ten randomly selected closed stomata, respectively (Cornelissen \emph{et
al.} , 2003a). Leaf total stomatal area was defined as stomatal density
multiplied by average leaf area and average stomatal area.
Correspondingly, plant total stomatal area was defined as stomatal
density multiplied by plant total leaf area times average stomatal area.
Leaf total stomatal number was defined as stomatal density multiplied by
average leaf area. Similarly, plant total stomatal number was defined as
stomatal density multiplied by plant total leaf area.

Statistics

Bivariate line-fitting of pair-wise traits across species with
contrasting life strategy (\emph{i.e.} different leaf habits,
growth-forms and RGRs) was conducted with the standardized major axis
(SMA) model using the `smatr' package in R (R Development Core Team,
2014). All data were first ln-transformed before line fitting.
Homogeneity among slopes and Y elevations of fitted lines were
determined referring to different groups (leaf habits and growth-forms).
Elevation homogeneity, as well as the overall slope homogeneity with 1,
were analyzed when these individual slopes of ecological groups were
homogeneous. As the absolute values of stomatal area or xylem area
should vary due to different measuring methods, we did not compare the
elevations of these regression against the one-to-one line. The impact
of RGR on these scaling relationships was defined by fitting lines of
Y/X to RGR.

\textbf{Results}

All relations reported below are based on linear SMA regressions on
ln-transformed values. We found strong similarity in scaling
relationships across seedlings of woody species between total stomatal
area and xylem tissue area from the whole-plant level to single-leaf
level. Across the 53 species, plant total stomatal area scaled to stem
xylem area (slope = 1.29, \emph{r} \textsuperscript{2} = 0.81, \emph{P
\textless{}} 0.001; Fig. 2a), and to stem xylem conductance area (slope
= 1.22, \emph{r} \textsuperscript{2} = 0.77, \emph{P \textless{}} 0.001;
Fig. 2b); leaf total stomatal area scaled to midvein xylem area (slope =
1.30, \emph{r} \textsuperscript{2} = 0.79, \emph{P \textless{}} 0.001)
(Fig. 3). The slopes of the regression lines were substantially and
significantly larger than 1 (the 95\% confidence intervals did not
bracket zero, Table 1).

Fig. 2 Size-covariation of stomata and xylem at the whole plant level,
across seedlings of 53 European woody species varying in leaf-habit,
growth-form and relative growth rate (RGR). (a) Convergent scaling of
plant total stomatal area and stem xylem transect area. (b) Convergent
scaling of plant total stomatal area and stem xylem conductance area.
Growth forms: \emph{T} tree, \emph{S} shrub, \emph{SS}
subshrub,\emph{C+Sc} scrambler or climber. Regression coefficients of
standardized major axis (SMA) are documented in Table 1.

Fig. 3 Convergent scaling of leaf total stomatal area and midvein xylem
transect area, across seedlings of 53 European woody species varying in
leaf-habit, growth-form and relative growth rate (RGR). Lines indicate
significant scaling relationships. Growth forms: \emph{T} tree,
\emph{S}shrub, \emph{SS} subshrub, \emph{C+Sc} scrambler or climber.
Regression coefficients of standardized major axis (SMA) are documented
in Table 1.

Table 1. Ln -- Ln scaling relationships were analyzed with standardized
major axis regression (SMA) analyses, with additional reference to the
contributions of different growth forms, leaf habits and relative growth
rates (RGRs) to these relationships. Y-intercept and slopes as well as
slope homogeneity with 1 are reported for pairwise relationships with
significant results. 95\% confidence intervals (CI) are in parentheses.
Growth-form: \emph{T} tree, \emph{S} shrub, \emph{SS}
subshrub,\emph{C+Sc} scrambler or climber; Leaf-habit: \emph{D}
deciduous,\emph{E} evergreen. Y/X \emph{-- RGR} SMA regression of Y/X
(ratio of Y values to X values) and RGR. ***, \emph{P \textless{}}
0.001; **\emph{P \textless{}} 0.05; ns, not significant.

Moreover, leaf total stomatal number had a strong scaling relation with
leaf midvein xylem area (slope = 1.32, \emph{r} \textsuperscript{2} =
0.79, \emph{P \textless{}} 0.001) as well as average leaf area (slope =
0.97, \emph{r} \textsuperscript{2} = 0.97, \emph{P \textless{}} 0.001)
(Fig. 4a). For the former, the slope of the regression line was
significantly larger than 1 (the 95\% confidence intervals did not
bracket zero), while slope of the latter was convergent to 1 (Table 1).
In contrast, average stomatal area was independent of leaf midvein xylem
area (\emph{r} \textsuperscript{2} = 0.01, \emph{P} \textgreater{} 0.05)
or average leaf area (\emph{r} \textsuperscript{2}
\emph{\textless{}}0.01, \emph{P} \textgreater{} 0.05) (Fig. 4b; Table
1). Similarly, plant minor vessel number scaled with stem xylem area
(slope = 0.88,\emph{r} \textsuperscript{2} = 0.88, \emph{P \textless{}}
0.001) and total leaf area (slope = 0.69, \emph{r} \textsuperscript{2} =
0.62,\emph{P \textless{}} 0.001; Fig. 4c) (Table 1). Average minor
vessel area was independent of stem xylem area (\emph{r}
\textsuperscript{2}\emph{\textless{}} 0.01, \emph{P} \textgreater{}
0.05) or total leaf area (\emph{r} \textsuperscript{2}
\emph{\textless{}} 0.01, \emph{P}\textgreater{} 0.05) (Fig. 4d; Table
1).

Fig. 4 (a) Covariation of leaf total stomatal number and midvein xylem
area (or average leaf area, insert). (b) Relationship between average
stomatal area and midvein xylem area (or average leaf area, insert). (c)
Covariation of plant total stomatal number and stem xylem area (or total
leaf area, insert). (d) Relationship between minor vessel area and stem
xylem area (or total leaf area, insert). Lines indicate significant
scaling relationships. Growth forms: \emph{T} tree, \emph{S}
shrub,\emph{SS} subshrub, \emph{C+Sc} scrambler or climber. Regression
coefficients of standardized major axis (SMA) are documented in Table 1.

Furthermore, plant total stomatal number strongly scaled with plant
minor vessel number (slope = 1.51, \emph{r} \textsuperscript{2} =
0.62,\emph{P \textless{}} 0.001; Fig. 5a); while the average area of
stomata and minor vessels did not show any relation with each other
(\emph{r} \textsuperscript{2} \emph{=} 0.05, \emph{P} \textgreater{}
0.05; Fig. 5b) (Table 1).

Fig. 5 (a) Covariation of plant total stomatal number and plant minor
vessel number. (b) Relation between average stomatal area and average
minor vessel area. Lines indicate significant scaling relationships.
Growth forms: \emph{T} tree, \emph{S} shrub, \emph{SS}
subshrub,\emph{C+Sc} scrambler or climber. Regression coefficients of
standardized major axis (SMA) are documented in Table 1.

\textbf{Discussion}

Woody seedlings are a convenient life stage to acquire water conductance
parameters at the whole-plant level because of their size advantage,
even though patterns of seedlings may could not completely reflect the
water relation in adult woody plants. Nevertheless, how woody seedlings
regulate water relations, in terms of the xylem-stomatal covariation, is
important for their survival and growth into adulthood. Based on an
anatomical analysis across ontogenetically comparable seedlings of 53
diverse woody species (Table S1), we have presented here key new
findings on xylem-stomata coordination from a previously neglected
aspect: we started from the water balance between the liquid (water
delivery) and vapor (water loss) phase at the whole-plant level by
scaling plant total stomatal area to stem xylem (conductance)
cross-sectional area (Fig. 2; Table 1). This area-scaling pattern was
driven by the covariation of stomata numbers and minor vessel numbers
per plant (Fig. 5a; Table 1). We then zoomed in on the water exchange in
individual leaves by showing the coordination of leaf total stomatal
area and midvein xylem area (Fig. 3; Table 1). We also found that plant
size (or leaf size) scales with stomatal (or minor vessel) number, while
it does not scale with individual stomatal (or minor vessel) area (Fig.
4; Table 1), which has an important implication for our understanding of
the design of xylem structure and stomatal distribution.

Convergent size coordination of stomata and xylem from whole-plant level
to single-leaf level

The convergent size coordination of stomata and xylem in the case of
entire plants and individual leaves implies that individual leaves have
a tight control over the whole-plant water conductance (Fig. 2; Fig. 3;
Table 1). Previously we showed, based on the same seedling populations,
that the xylem vessels widen basipetally from the tip to the base for
both single leaf (midvein) and the whole plant (stem) and that the
remarkably tight covariation in vessel diameter between different organs
(especially between leaf and stem) (Zhong \emph{et al.} , 2019). When we
now combine all three findings, we can conclude that natural selection
has led to rather tight regulation of water-related architecture
featuring similar size-driven variation across seedlings of diverse
woody species, both for single leaves and entire plant individuals.
Their xylem vessels widen basipetally from the tip to the base, from
leaves to the entire individuals, in a way that maintains a constant
leaf-specific conductance (Sterck \& Zweifel, 2016, Zhong \emph{et al.}
, 2019) and a constant xylem-stomatal size scaling. Using hydraulic
properties of single leaves to predict the entire plant water transport
is an alternative choice, as numerous studies have done
(Brodribb\emph{et al.} , 2017, Carins Murphy \emph{et al.} , 2014,
Meinzer, 2002), especially when the conductance-related parameters of
entire plants are difficult to acquire, for example in adult trees.
Specifically, knowing leaf size (i.e. leaf area) is of the utmost
importance, not only to predict photosynthetic productivity precisely
(Li \emph{et al.} , 2020), but also to understand plant water transport
(Echeverr\selectlanguage{ngerman}ía \emph{et al.} , 2019), from the single leaf, to the branch,
to the whole-tree, and even to the forest level.

The slopes of the ln-scaling regression lines between stomatal and xylem
traits are notably larger than the slope of 1 (Fig. 2; Fig. 3; Table 1),
which means that stomata do not scale linearly to xylem but
exponentially. In actual fact, it should be the stem xylem conductance
area, rather than stem xylem area, that scales with the total stomatal
area, while we gave the pattern for both in order to enable comparison
with a previous study which used the same seedling population
(Zhong\emph{et al.} , 2019). In that study, the total leaf area
(\emph{LA} ) scaled with stem xylem area (\emph{X\textsubscript{stem}} )
at mid stem height as \emph{LA} \selectlanguage{english}[?]
\emph{X\textsubscript{stem}}\textsuperscript{1.25} (Zhong \emph{et al.}
, 2019). Together with isometric scaling of leaf area and total stomatal
area, we could elicit that total stomatal area should scale with
\emph{X\textsubscript{stem}} with an exponent approximating 1.25. Our
finding in the current paper (exponents 1.29 and 1.30 for entire plants
and individual leaves respectively) is in line with this theoretical
prediction. When considering the total water path length (e.g. by
sampling the anatomical cross-section at the stem base), our results are
in line with our prediction that there should be isometric scaling both
between total leaf area and xylem conductance area and between total
stomatal conductance area and xylem conductance area (see also
Echeverria\emph{et al.} , 2019, Fiorin \emph{et al.} , 2016, Lechthaler
\emph{et al.} , 2019, Meinzer \& Grantz, 1990). Further studies are
needed to integrate the relations between leaf area, stomatal area and
xylem conductance area from the perspective of the (3-dimensional) water
transport system from single-leaf level to whole-plant level. Ideally,
such studies should be carried out also on adult woody plants and across
different biomes.

Number coordination of stomata and minor vessels and its implication

We also found strong covariation between terminal xylem vessel number
and stomatal number per plant (Fig. 5a; Table 1). That is: in order to
ensure the balance between liquid- and vapour-phase water conductance,
convergent scaling exists between total stomatal area and xylem area,
both at the entire plant and at individual leaf level; and this
area-scaling pattern was driven by the covariation of stomata numbers
and minor vessel numbers per plant.

These findings provide empirical support for, as well as a better
functional understanding of the xylem structure models and have broad
implications for integrating xylem widening (Anfodillo \emph{et al.} ,
2006, Olson, Anfodillo, Rosell, Petit, Crivellaro, Isnard, Leon\selectlanguage{english}-Gomez,
Alvarado\selectlanguage{english}-Cardenas \& Castorena, 2014, Zhong \emph{et al.} , 2019) and
stomatal distribution; these linkages are illustrated in Fig. 1. Further
studies on plant water relations should incorporate the transport
mechanism of water from the minor vein xylem vessels to stomata with
xylem architecture.

\textbf{Conclusion}

Woody seedlings across ecologically and morphologically wide-ranging
species modulate the balance between the vapor (water loss) and liquid
(water delivery) phase, via a convergent allometric covariation of xylem
area and total stomatal area from entire individuals to individual
leaves. Having a sufficient number of stomata relative to the minor vein
xylem number is imperative for ensuring the force (generated by
evaporation through stomata) of water delivery (through xylem vessels).
The whole-leaf and whole-plant allometric relationships related to water
transport and export in this study deepen our understanding of the
vascular structure models and has broad implications for integrating
xylem architecture and stomatal distribution across species.

\textbf{Acknowledgements}

We thank the China Scholarship Council (CSC, grant no. 201606350163) for
financial support to MYZ. We thank Dr. Mark E. Olson and an anonymous
reviewer for very constructive comments on the submitted manuscript. We
thank Dr. Frank J. Sterck for useful comments on this manuscript.

\textbf{Author contributions}

JHCC, BELC, PC-D and J-PP carried out the experimental work. JHCC, BELC,
PC-D, J-PP and MYZ gathered the data. MYZ and JHCC conceived the present
study and designed the analyses. MYZ analyzed the data and wrote the
first draft of the manuscript, and all authors made important
suggestions on at least two drafts of the manuscript.

\textbf{ORCID}

Mengying Zhong https://orcid.org/0000-0002-1032-0775

Johannes H. C. Cornelissen https://orcid.org/0000-0002-2346-1585

Jean-Philippe Puyravaud https://orcid.org/0000-0001-7866-389X

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Fig.1 Conceptual framework of this research concerning allometric
relations of plant hydraulics across woody species. Natural selection
acts on heritable variation between individuals within the same species.
Individuals with vessels that do not widen with height growth, or widen
little, will experience continual declines in leaf-specific conductance
with height growth and therefore declining growth and reproductive
output per unit leaf area. Individuals with vessels that widen very
markedly would have conduits of low resistance, in contrast to the
high-resistance variants with conduits that are `too narrow', but they
would have their own set of disadvantages. For example, for a given leaf
area and transpirational demand, the wider conduits cost more for the
same service provided. Each unit of carbon invested in excessively wide
vessels is a unit that is not invested in further growth or
reproduction, and so these variants should be at a selective
disadvantage (Banavar, Cooke, Rinaldo \& Maritan, 2014). Moreover, wider
vessels are more vulnerable to gas embolisms that obstruct conductance,
both from freezing ((Sevanto, Holbrook \& Ball, 2012, Zanne, Tank,
Cornwell, Eastman, Smith, FitzJohn, McGlinn, O'Meara, Moles, Reich,
Royer, Soltis, Stevens, Westoby, Wright, Aarssen, Bertin, Calaminus,
Govaerts, Hemmings, Leishman, Oleksyn, Soltis, Swenson, Warman \&
Beaulieu, 2014) and likely drought as well (Cai \& Tyree, 2010,
Jacobsen, Pratt, Venturas \& Hacke, 2019, Liu, Gleason, Hao, Hua, He,
Goldstein \& Ye, 2019). As a result, plants with vessels that are `too
wide' would also be at a selective disadvantage (Zhong \emph{et al.}
(2019)). The variants that should have the largest amounts of surplus
carbon to devote to growth and reproduction are those in the
intermediate zone, in which conduits widen just enough that conductance
remains constant per unit leaf area, but not so much as to incur
excessive carbon costs and embolism vulnerability.

In our previous study, based on the same woody seedling populations, we
found that, at the whole-plant level, the stem xylem cross-sectional
area (\emph{X\textsubscript{stem}} ) of stem medium (a) closely scales
with stem height (\emph{H} ) and total leaf area per plant (\emph{LA} )
as\emph{X\textsubscript{stem}} \selectlanguage{english}[?] \emph{H} \textsuperscript{1.52}
and\emph{X\textsubscript{stem}} \selectlanguage{english}[?] \emph{LA} \textsuperscript{0.75}
across all the studied species. For individual leaves, vessel diameter
(\emph{D} \textsubscript{leaf}) in the medium of leaf midvein (b)
closely scales with average leaf area (\emph{MLA} ) as\emph{D}
\textsubscript{leaf} \selectlanguage{english}[?] \emph{MLA} \textsuperscript{0.21} (Zhong\emph{et
al.} (2019). In this study, we test the poorly understood xylem-stomata
covariation from the size perspective We ask: are there scaling
relationships between total stomatal area and xylem cross-sectional area
across species, at the entire plant and at individual leaf level?
Specifically, we zoom in on the terminal part of water exchange (from
minor vessels to stomata), and ask: does the minor vessel number (which
scales with leaf area; see (Lechthaler \emph{et al.} , 2019, Rosell \&
Olson, 2019)) scale with stomatal number per leaf and per plant across
these woody seedlings? The conceptual picture should deepen our
understanding of plants' water transport system and have broad
implications for integrating xylem architecture and stomatal
distribution.Fig. 2 Size-covariation of stomata and xylem at the whole
plant level, across seedlings of 53 European woody species varying in
leaf-habit, growth-form and relative growth rate (RGR). (a) Convergent
scaling of plant total stomatal area and stem xylem transect area. (b)
Convergent scaling of plant total stomatal area and stem xylem
conductance area. Lines indicate significant scaling relationships.
Growth forms: \emph{T} tree, \emph{S} shrub, \emph{SS}
subshrub,\emph{C+Sc} scrambler or climber. Regression coefficients of
standardized major axis (SMA) are documented in Table 1.

Fig. 2 Size-covariation of stomata and xylem at the whole plant level,
across seedlings of 53 European woody species varying in leaf-habit,
growth-form and relative growth rate (RGR). (a) Convergent scaling of
plant total stomatal area and stem xylem transect area. (b) Convergent
scaling of plant total stomatal area and stem xylem conductance area.
Growth forms: \emph{T} tree, \emph{S} shrub, \emph{SS}
subshrub,\emph{C+Sc} scrambler or climber. Regression coefficients of
standardized major axis (SMA) are documented in Table 1.

Fig. 3 Convergent scaling of leaf total stomatal area and midvein xylem
transect area, across seedlings of 53 European woody species varying in
leaf-habit, growth-form and relative growth rate (RGR). Lines indicate
significant scaling relationships. Growth forms: \emph{T} tree,
\emph{S}shrub, \emph{SS} subshrub, \emph{C+Sc} scrambler or climber.
Regression coefficients of standardized major axis (SMA) are documented
in Table 1.

Fig. 4 (a) Covariation of leaf total stomatal number and midvein xylem
area (or average leaf area, insert). (b) Relationship between average
stomatal area and midvein xylem area (or average leaf area, insert). (c)
Covariation of plant total stomatal number and stem xylem area (or total
leaf area, insert). (d) Relationship between minor vessel area and stem
xylem area (or total leaf area, insert). Lines indicate significant
scaling relationships. Growth forms: \emph{T} tree, \emph{S}
shrub,\emph{SS} subshrub, \emph{C+Sc} scrambler or climber. Regression
coefficients of standardized major axis (SMA) are documented in Table 1.

Fig. 5 (a) Covariation of plant total stomatal number and plant minor
vessel number. (b) Relation between average stomatal area and average
minor vessel area. Lines indicate significant scaling relationships.
Growth forms: \emph{T} tree, \emph{S} shrub, \emph{SS}
subshrub,\emph{C+Sc} scrambler or climber. Regression coefficients of
standardized major axis (SMA) are documented in Table 1.

Table 1. Ln -- Ln scaling relationships were analyzed with standardized
major axis regression (SMA) analyses, with additional reference to the
contributions of different growth forms, leaf habits and relative growth
rates (RGRs) to these relationships. Y-intercept and slopes as well as
slope homogeneity with 1 are reported for pairwise relationships with
significant results. 95\% confidence intervals (CI) are in parentheses.
Growth-form: \emph{T} tree, \emph{S} shrub, \emph{SS}
subshrub,\emph{C+Sc} scrambler or climber; Leaf-habit: \emph{D}
deciduous,\emph{E} evergreen. Y/X \emph{-- RGR} SMA regression of Y/X
(ratio of Y values to X values) and RGR. ***, \emph{P \textless{}}
0.001; **\emph{P \textless{}} 0.05; ns, not significant.\selectlanguage{english}
\begin{longtable}[]{@{}lllllllllll@{}}
\toprule
\begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Model\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Intercept (95\% CI)\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Slope (95\% CI)\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\emph{r}\textsuperscript{2} (\emph{P})\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Slope.test= 1 r (P)\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Y\textasciitilde{}X+ Leaf habit (\emph{P})\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Y\textasciitilde{}X+ Leaf habit (\emph{P})\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Y\textasciitilde{}X + Growth form (\emph{P})\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Y\textasciitilde{}X + Growth form (\emph{P})\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Y/X -- RGR (\emph{P})\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
Figures\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Slope homogeneity\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Elevation homogeneity\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Slope homogeneity\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Elevation homogeneity\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Plant total stomatal area \textasciitilde{} Stem xylem area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
2.52 (0.50, 4.55)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
1.29 (1.14, 1.47)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.81***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.51***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Fig. 2a\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Plant total stomatal area \textasciitilde{} Stem xylem conductance
area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
3.76 (1.70,5.82)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
1.22 (1.06, 1.41)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.77***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.39**\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Fig. 2b\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Leaf total stomatal area \textasciitilde{} Midvein xylem area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
4.78 (3.16, 6.39)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
1.30 (1.13, 1.49)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.79***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.50***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Fig. 3\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Leaf total stomatal number \textasciitilde{} Midvein xylem area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
-1.48 (-3.09, 0.13)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
1.32 (1.16, 1.51)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.79***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.53***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Fig. 4a\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Leaf total stomatal number \textasciitilde{} Average leaf area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
5.38 (5.02, 5.74)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.97 (0.91, 1.04)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.94***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
-0.12ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
**\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Stomatal cell area \textasciitilde{} Midvein xylem area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
8.08 (7.47, 8.68)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
-0.22 (-0.30, 0.17)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.01 ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Fig. 4b\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Stomatal cell area \textasciitilde{} Average leaf area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
6.93 (6.66, 7.20)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
-0.16 (-0.22,-0.13)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.006ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Plant minor vessel number \textasciitilde{} Stem xylem area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
-2.73 (-3.86, -1.61)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.88 (0.79, 0.98)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.87***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
**\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
**\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
**\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Fig. 4c\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Plant minor vessel number \textasciitilde{} Total leaf area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
3.17 (2.24, 4.10)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.69 (0.57, 0.83)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.62***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
**\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
**\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Minor vessel area \textasciitilde{} Stem xylem area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
4.91 (4.08, 5.74)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
-0.23 (-0.31, -0.17)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.003ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Fig. 4d\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Minor vessel area \textasciitilde{} Total leaf area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
3.46 (3.04, 3.88)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
-0.19 (-0.25, -0.14)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.01ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Plant total stomatal number \textasciitilde{} Plant minor vessel
number\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.19 (-2.10, 2.47)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
1.51 (1.25, 1.82)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.62***\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Fig. 5a\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Stomatal cell area \textasciitilde{} Minor vessel area\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
3.48 (2.71, 4.24)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
1.22 (0.91, 1.62)\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
0.05ns\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
--\strut
\end{minipage} & \begin{minipage}[t]{0.09\columnwidth}\raggedright\strut
Fig. 5b\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table S1} Growth-form, Leaf-habit and seedling traits of the 53
studied woody species. Growth-form: \emph{T} tree, \emph{S}
shrub,\emph{SS} subshrub, \emph{C+Sc} scrambler or climber;
Leaf-habit:\emph{D} deciduous, \emph{E} evergreen. \emph{RGR} relative
growth rate (dataset could be found in Cornelissen \emph{et al.}
(1996)). Majority part of these datasets -\emph{Stem xylem area} ,
\emph{Leaf (midvein) xylem area} , \emph{Total leaf area} and
\emph{Average leaf area} - could be found in (Zhong \emph{et al.} ,
2019). The dataset of \emph{average stomatal area} could be found in
(Cornelissen, Cerabolini, Castro-D\selectlanguage{ngerman}íez, Villar-Salvador,
Montserrat-Martí, Puyravaud, Maestro, Werger \& Aerts, 2003b). The
entire dataset of this research was provided here for easy access.\selectlanguage{english}
\begin{longtable}[]{@{}llllllllllllll@{}}
\toprule
Species & Growth form & Leaf habit & RGR & Plant total stomatal area
(\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) & Leaf total stomatal area
(\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) & Leaf total stomatal number & Average stomatal
area (\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) & Stem xylem area (\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) &
Xylem conductance area (\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) & Leaf midvein xylem area
(\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) & Minor vessel area (\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}) &
Total leaf area (mm\textsuperscript{2}) & Average leaf area
(mm\textsuperscript{2})\tabularnewline
\midrule
\endhead
\emph{Acer platanoides} & T & D & 0.0755 & 223804929 & 106603260 &
468138 & 228 & 343755 & 162768 & 34838 & 11.26 & 4449 &
2119\tabularnewline
\emph{Acer pseudoplatanus} & T & D & 0.0808 & 186377155 & 89310035 &
232903 & 383 & 1075797 & 680388 & & 11.50 & 3093 & 1482\tabularnewline
\emph{Aesculus hippocastanum} & T & D & 0.0935 & 2537142026 & 253714203
& 929899 & 273 & 6874015 & 4529976 & 293560 & 18.97 & 43988 &
4399\tabularnewline
\emph{Alnus glutinosa} & T & D & 0.1106 & 23416451 & 8519951 & 13635 &
625 & 188371 & 130840 & 7134 & 10.91 & 407 & 148\tabularnewline
\emph{Arbutus unedo} & T & E & 0.0919 & 14208189 & 3830456 & 7225 & 530
& 31345 & 16743 & 3157 & 17.95 & 204 & 55\tabularnewline
\emph{Berberis vulgaris} & S & D & 0.0844 & 21345164 & 5157241 & 9532 &
541 & 96960 & 39624 & 5684 & 14.38 & 354 & 85\tabularnewline
\emph{Betula pendula} & T & D & 0.1280 & 8928559 & 3989847 & 4665 & 855
& 53181 & 37014 & 2738 & 12.80 & 113 & 50\tabularnewline
\emph{Buddleja davidii} & S & D & 0.2000 & 66779051 & 11589874 & 20622 &
562 & 55563 & 38530 & 3137 & 9.06 & 861 & 149\tabularnewline
\emph{Buxus sempervirens} & S & E & 0.0579 & 14440719 & 3759371 & 5187 &
725 & 27728 & 12212 & 3684 & 14.88 & 169 & 44\tabularnewline
\emph{Calluna vulgaris} & SS & E & 0.0541 & & 82431 & 192 & 428 & 5206 &
3202 & 301 & & & 1\tabularnewline
\emph{Castanea sativa} & T & D & 0.0746 & 659912449 & 173246507 & 366410
& 473 & 951942 & 518570 & 114990 & 6.70 & 8352 & 2193\tabularnewline
\emph{Cornus sanguinea} & S & D & 0.1028 & 56448859 & 14903255 & 40747 &
366 & 159184 & 100657 & 4650 & 7.21 & 1177 & 311\tabularnewline
\emph{Corylus avellana} & S & D & 0.1309 & 597659121 & 99609853 & 196083
& 508 & 626847 & 388287 & 58223 & 7.68 & 11584 & 1931\tabularnewline
\emph{Crataegus monogyna} & S & D & 0.1059 & 59703754 & 11651733 & 20795
& 560 & 226117 & 114415 & & 7.49 & 1306 & 255\tabularnewline
\emph{Cytisus scoparius} & S & E & 0.0918 & 41138286 & 5072802 & 25123 &
202 & 174629 & 92112 & 2666 & 7.09 & 1014 & 125\tabularnewline
\emph{Empetrum nigrum} & SS & E & 0.0587 & 2220093 & 261786 & 595 & 440
& 9699 & 5593 & 378 & NA & 17 & 2\tabularnewline
\emph{Fagus sylvatica} & T & D & 0.0412 & 304853187 & 152426594 & 375785
& 406 & 280394 & 154667 & 39645 & 5.87 & 3389 & 1694\tabularnewline
\emph{Frangula alnus} & S & D & 0.1164 & 38324802 & 9223801 & 26465 &
349 & 117499 & 60241 & 3283 & 8.26 & 501 & 121\tabularnewline
\emph{Fraxinus excelsior} & S & E & 0.1017 & 147861778 & 36789017 &
90052 & 409 & 536288 & 297372 & & 7.92 & 3971 & 988\tabularnewline
\emph{Hebe x franciscana} & C+Sc & E & 0.1025 & 12333030 & 3866248 &
10173 & 380 & 27081 & 16023 & 2547 & 9.71 & 69 & 22\tabularnewline
\emph{Hedera helix} & C+Sc & E & 0.0257 & 68535534 & 29306295 & 56902 &
515 & 86503 & 54324 & 18421 & 11.33 & 442 & 189\tabularnewline
\emph{Helianthemum nummularium} & SS & E & 0.1129 & & 2941751 & 5383 &
546 & 39081 & 15315 & 3940 & 8.39 & & 17\tabularnewline
\emph{Hippophae rhamnoides} & S & D & 0.0744 & 37317548 & 5519993 &
14444 & 382 & 64016 & 39092 & 5895 & 8.43 & 336 & 50\tabularnewline
\emph{Ilex aquifolium} & T & E & 0.0137 & 17026164 & 7218047 & 11564 &
624 & 92510 & 92510 & 6906 & 7.17 & 96 & 41\tabularnewline
\emph{Juglans regia} & T & D & 0.0643 & 2186734626 & 838486789 & 1765820
& 475 & 2564331 & 2564331 & 43642 & 6.73 & 21178 & 8121\tabularnewline
\emph{Laburnum anagyroides} & T & D & 0.0824 & 62620395 & 18136573 &
62059 & 292 & 176117 & 176117 & 7302 & 11.31 & 2016 & 584\tabularnewline
\emph{Ligustrum vulgare} & S & E & 0.072 & 92734988 & 16910829 & 30565 &
553 & 237776 & 237776 & 5586 & 8.18 & 608 & 111\tabularnewline
\emph{Lonicera periclymenum} & C+Sc & D & 0.0773 & 27182339 & 6170674 &
10937 & 564 & 73158 & 46415 & 3551 & 10.59 & 295 & 67\tabularnewline
\emph{Malus sylvestris} & T & D & 0.1087 & 181004476 & 35085012 & 98027
& 358 & 516712 & 516712 & 23994 & 7.81 & 2685 & 520\tabularnewline
\emph{Prunus laurocerasus} & S & E & 0.0794 & 156932580 & 74277221 &
97945 & 758 & 302786 & 302786 & 13868 & 7.81 & 1806 & 855\tabularnewline
\emph{Prunus lusitanica} & S & E & 0.1001 & 113721032 & 36432187 & 64014
& 569 & 406418 & 406418 & 12132 & 7.93 & 1272 & 407\tabularnewline
\emph{Prunus spinosa} & S & D & 0.1416 & 345051252 & 40525534 & 110922 &
365 & 407299 & 251252 & 8692 & 8.43 & 4620 & 543\tabularnewline
\emph{Quercus cerris} & T & D & 0.0645 & 794869756 & 215056379 & 592037
& 363 & 212333 & 212333 & 88171 & 6.55 & 4258 & 1152\tabularnewline
\emph{Quercus ilex ilex} & T & E & 0.0630 & 419506213 & 83111179 &
184718 & 450 & 316025 & 316025 & 63637 & 8.33 & 2197 &
435\tabularnewline
\emph{Quercus petraea} & T & D & 0.0615 & 320599019 & 67059123 & 198294
& 338 & 430118 & 430118 & 59064 & 7.57 & 3610 & 755\tabularnewline
\emph{Quercus robur} & T & D & 0.0472 & 446422644 & 87661045 & 244593 &
358 & 939504 & 939504 & 106646 & 5.96 & 3530 & 693\tabularnewline
\emph{Rhamnus alaternus} & S & E & 0.054 & 23379549 & 4553052 & 15014 &
303 & 63217 & 27894 & 5654 & 7.44 & 243 & 47\tabularnewline
\emph{Rhamnus cathartica} & S & D & 0.0720 & 56354993 & 15417573 & 50842
& 303 & 172466 & 172466 & 4471 & 6.27 & 586 & 160\tabularnewline
\emph{Rhododendron ponticum} & S & E & 0.0570 & & 1111068 & 2457 & 452 &
16204 & 16204 & 284 & 7.78 & & 10\tabularnewline
\emph{Ribes nigrum} & S & D & 0.1761 & 223802657 & 24133735 & 48511 &
497 & 168779 & 168779 & 4537 & 8.48 & 2422 & 261\tabularnewline
\emph{Ribes uva-crispa} & S & D & 0.1207 & 48489246 & 10525089 & 17938 &
587 & 207779 & 207779 & & 7.99 & 992 & 215\tabularnewline
\emph{Rosa arvensis} & C+Sc & D & 0.1439 & 65827185 & 13143722 & 40272 &
326 & 142694 & 86488 & 4328 & 6.27 & 1672 & 334\tabularnewline
\emph{Rubus fruticosus} & C+Sc & D & 0.1778 & 127053969 & 18354321 &
47090 & 390 & 153453 & 110145 & 13137 & 9.18 & 2335 & 337\tabularnewline
\emph{Salix caprea} & T & D & 0.1913 & 24352804 & 4847936 & 20860 & 232
& 114703 & 114703 & 5003 & 5.87 & 560 & 112\tabularnewline
\emph{Sambucus nigra} & S & D & 0.1393 & 181928719 & 43208882 & 39266 &
1100 & 408946 & 408946 & 11100 & 10.05 & 2209 & 525\tabularnewline
\emph{Solanum dulcamara} & C+Sc & D & 0.2271 & & 47749004 & 153250 & 312
& 256711 & 172567 & 16659 & 8.16 & & 599\tabularnewline
\emph{Sorbus aucuparia} & T & D & 0.1167 & 36145255 & 10221650 & 19561 &
523 & 80505 & 80505 & & & 696 & 197\tabularnewline
\emph{Thymus polytrichus} & SS & E & 0.1308 & & 369414 & 922 & 401 &
40231 & 40231 & 1567 & & & 3\tabularnewline
\emph{Ulex europaeus} & S & E & 0.0781 & 32604769 & 3818019 & 7178 & 532
& 192394 & 192394 & 5807 & 7.36 & 293 & 34\tabularnewline
\emph{Ulmus glabra} & T & D & 0.1200 & 240512411 & 60001258 & 118453 &
507 & 131718 & 131718 & 6578 & 5.08 & 1975 & 493\tabularnewline
\emph{Vaccinium myrtillus} & SS & D & 0.0524 & 1102862 & 277584 & 805 &
345 & 11885 & 11885 & 1021 & & 11 & 3\tabularnewline
\emph{Vaccinium vitis-idaea} & SS & E & 0.0492 & 801863 & 201557 & 558 &
361 & 9745 & 9745 & 1054 & 6.65 & 11 & 3\tabularnewline
\emph{Viburnum opulus} & S & D & 0.0767 & 66860566 & 38275083 & 62941 &
608 & 93487 & 50483 & 3639 & 9.62 & 721 & 412\tabularnewline
\bottomrule
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