Spatial and temporal niche characterization
The forest ground is characterized by exfoliated volcanic rocks (0 to 45
cm high) and rock outcrops. With geophysical imaging (GPR), we
visualized differential species-specific vertical root placement in the
soil surface, regolith, and rock fractures (Fig. 2). For oak, the finest
detectable roots (diameter < 3 cm) occurred preferentially in
soil pockets and rock fractures at depths > 25 cm, whereas
the thicker roots (5 to 7 cm) were located at the soil surface
(differences in vertical root distribution, P < 0.001 ,
Figs. 2b; 3d, f). Pine in contrast, concentrated its finest roots
(< 3cm) near the soil surface (top 10 cm, P
<0.001; Figs. 2a; 3a), however the thicker roots were
observed in the regolith between 20 – 30 cm depth (P
<0.001; Figs. 2a; 3c).
We confirmed that oak and pine trees used different water sources
throughout the year by comparing monthly stable isotope ratios of xylem
water with soil and rainwater; these data provide direct evidence for
spatial and temporal niche occupation by the two species. When combining
geophysical methods with stable isotope ratio information of water from
different soil/rock substrates and xylem of trees, we discovered that
trees of both species occupied between one to four spatial niches (Table
1, Fig. 4) distributed in three geologically distinct substrates (Fig.
S2). In the regolith layer, in several periods two separate pools of
water were apparent, one in the upper regolith at the interface with the
shallow soil and the other in the lower regolith within rock pockets at
its deeper distribution (Figs. 2, 4).
When considering the soil/rock strata and the wetting/drying cycles in
the course of three years, as well as specific tree functional traits
employed, we identified nine niche configurations together forming a
complex nichescape. With nichescape we refer to the set of traits
employed by plants to garner water, that is spatially and temporally
distributed in the soil-rock continuum (Table 1, Fig. 4).Single-niche-sharing only occurred during the rainy season of
September 2012, when oak and pine satisfied all their water needs from
surface soil (Table 1(A), Figs. 4a and S3.1 i).Double niche sharing occurred in the wet season 2013, when oak
and pine roots acquired water from both the surface soil and the upper
regolith (Table 1(D), Figs. 4d, S3.2 f-i), however the
two tree species exploited differential proportions from each niche.
While pine got most of the water from the surface soil (90%), oak
acquired most from the upper regolith (85%). Another case, yet ofindirect double niche sharing occurred in the dry season
(depletion period 2012-2013, Table 1(C), Figs. 4c, S3.1
d, S3.2 c), when oak in both stands acquired most water (92%) from rock
fractures and a small portion (8%) from the surface soil/upper
regolith. In contrast, pine (in mixed stands) took up water from surface
soil/upper regolith plus water originating from rock fractures, which
had previously been hydraulically lifted by oak roots from the rock
fracture to the surface soil, where pine could access it (xylem water δD
= -38.6‰).
Oak and pine exhibit remarkable differences in wood and root anatomy.
While in pine roots, a parenchyma is surrounding the tracheids, in oak
roots this tissue is lacking (Figs. S6g, S4). Oak trunks show a
particular structural anatomy of specialized tissue formed by fiber
tracheids connecting vessel tissue (Cai,
Li, Zhang, Zhang, & Tyree, 2014) (conductive structures, Fig. S6b, d),
which serves for water storage (Fig. S6c). Besides, oak has a
considerable number of vessels adapted to xeric conditions, i.e., it has
mixture of vessels with different diameters. The placements of oak’s
finest roots in rock fractures suggests specific adaptations in
functional anatomy (Rodríguez-Robles et
al., 2017) (Fig. 3f). These roots exhibit a triple layer of epidermal
tissue and contain calcium oxalate crystals (druzes), and under
extremely dry conditions, oak vessel diameter of roots decreases through
the formation of tyloses (Gottwald, 1972)
(Fig. S5d). Deep oak roots exhibited 83% more vessels with tyloses and
60% more druzes than surface roots (Fig. S5). Pine roots did not
exhibit these kinds of anatomical adaptations.