Multi-trait phenotype and drought response
The first two PCA axes explained 77.2% of the variation in performance
among species-level coefficients for survival, growth and photosynthesis
(Appendix Figure A10 and Table A11). The first performance axis
separated species according to their survival under drought vs. control
conditions, with positive values indicating poor drought survival
(Appendix Table A11). As a whole, the PCA showed that species that grew
well also had higher photosynthetic rates and had higher survival in
both drought and well-watered conditions (Appendix Figure A9 and table
A10).
After varimax rotation, PCA for traits required four dimensions to
explain ~80% of the variation among species (Figure 4a,
Appendix Table A13). Most axes had at least two traits with loadings
> 0.5. The first PCA axis separated species based on their
stomatal, leaf structural and xylem traits and explained 29% variation.
Positive values of axis 1 corresponded to lower SLA, larger stomates,
higher leaf structural tissue, smaller xylem, and larger root mass
fraction (Appendix Table S14). Axis 2 separated species based on water
acquisition traits, explaining ~21% variation. Species
with positive values on axis 2 had larger, longer and denser roots
(Appendix Table S14). However, the third and fourth PCA axes reveal
alternative combinations of measured traits. For instance, compared to
axis 1, axis 3 comprised species with higher LDMC that also had higher
SRL and smaller stomates. Axis 4 showed that, in contrast to axis 1, low
SLA species could have smaller xylem and higher vein density.
Procrustes analysis between the ordinations of species performance (two
axes) and traits (four axes) showed substantial concordance (sum of
squares = 38.24, Figure 4b, r=0.66, P = 0.03; permutation test for
significance at alpha 0.05). This indicates that species’ performance in
drought conditions could be explained by phenotypes discerned from soft
traits.