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.