Discussion
We found that drought reduced survival and growth for seedlings of 16
tree species from a wet tropical forest region in India (Western Ghats),
but species varied substantially in their responses. Interspecific
variation in survival and growth with drought was primarily explained by
anatomical traits that mediate water use. Generally, species with fewer
stomata and smaller xylem show lower decline in performance, growth and
survival under drought condition. Variation in photosynthetic
performance under drought stress correlated best with anatomical traits
related to water use and gas exchange (Figure 3). Growth constituted a
distinct axis as compared to survival and photosynthesis for variation
in species’ drought response (Appendix Figure A9), but the overall
variation in performance among species was explained by composite
phenotypes determined by soft traits. Species from more seasonal sites
showed larger proportional decline in photosynthetic performance under
drought (Figure 1c), but species affinity to seasonality conditions did
not explain variation in drought response in terms of growth or
survival.
Different aspects of species’ drought performance were regulated by
different traits, but stomata and xylem traits emerged as consistently
important. Seedling survival in well-watered conditions improved with
larger xylem and stomata (Figure 1a, b). Larger xylem also correlated
with faster growth (Figure 2d) in well-watered conditions indicating
that survival under well-watered conditions improves with traits that
allow moving larger volumes of water for carbon assimilation.
Accordingly, species with larger xylem also had higher photosynthetic
rates (Figure 3a). The benefit of larger xylem for survival eroded in
dry conditions (Figure 2a), indicating that species that move more water
through their conduits may be disadvantaged during drought, consistent
with larger xylem being more vulnerable to embolism and cavitation
(Hoffmann et al. 2011, Skelton et al. 2015, Santiagoet al. 2018).
Greater stomatal density increased growth (Figure 2e) and photosynthesis
(Figure 3b) in well-watered conditions, highlighting that tighter
stomatal packing improved physiological function when resources were not
limited (Medeiros et al. 2019). However, in drought conditions, species
with larger, more densely packed stomates experienced substantially
larger declines in photosynthesis (Figure 3 b, f, g) and growth (Figure
2e). Since smaller stomatal size did not correlate with higher stomatal
density, stomatal size and density form different axes to regulate
water-use (Anderegg et al. 2018). Stomatal area fraction (SAF =
SD*SS) showed results consistent with stomatal density (Appendix Figure
A3).
Species with lower specific root length, i.e., ones that proportionally
invested more in root length than area, had higher survival when water
was plenty, but experienced larger decline in survival in drought
conditions (Appendix Table A9, l). Investing in below ground tissues may
help maintain growth during drought by improving access to water (Joslinet al. 2000, Poorter & Markesteijn 2008, Paz et al. 2015,
Chitra-Tarak et al. 2018). The relationship between drought
response and organ-level allocation may be due to higher photosynthesis
in species that invest more above-ground, suggesting coordination
between root traits and stomata in regulating performance. However,
photosynthetic rates in well-watered conditions did not vary with root
mass fraction, or the length and area of roots. Moreover, these
relationships remained similar in drought conditions even as average
photosynthetic rates declined with water scarcity.
In general, trait values that supported better growth in well-watered
conditions resulted in larger declines in growth with drought,
suggesting a safety-efficiency trade-off with respect to water use
(Choat et al. 2012). Species with larger xylem, suggesting
higher hydraulic conductivity, also decreased photosynthesis during
drought, but in this case, reduced photosynthesis translated to greater
mortality suggesting carbon starvation with drought. Variability of
trait effects on different aspects of performance was unsurprising when
considering that survival and photosynthesis comprised distinct axes as
compared to growth for variation among species (see PCA of species
coefficients for performance variables, Appendix Figure A10). Moreover,
different combinations of traits were possible and required to explain
interspecific variation in the overall drought response represented by
changes in survival, growth, and photosynthesis.
A few caveats temper
interpretation of our results. Extrapolating from pot experiments to
performance in field conditions merits caution. Plant size affects soil
water depletion and transpiration rates when growing in a limited volume
of soil, influencing comparisons among species (Comita & Engelbrecht
2014). Initial height, however, did not predict drought responses in
our study. Secondly, in closed canopy forests, shade may buffer
seedlings from drought if shaded conditions decrease evapotranspiration
and/or maintain water in soil (Kupers et al. 2019). In addition,
larger trees often transport water from deeper layers towards the
surface (hydraulic redistribution) and increase water availability for
seedlings to buffer drought stress. Alternatively, drought in natural
settings may increase mortality due to other stressors such as nutrient
limitation or herbivory, which we did not assess. Transplant experiments
in the field will be needed to understand ‘real world’ implications of
drought for seedling performance (Comita & Engelbrecht 2014).