Seasonality affiliation and drought response
Species’ seasonality affiliation did not affect seedling survival (β =
-0.89, CI = -2.38 to 0.61, z = -1.16, P =0.25, Figure 1a) but overall
seedling had lower survival in drought vs well-watered conditions (β =
-2.17, CI = -2.95 to -1.39, z = -5.47, P < 0.05, Figure 1a).
However, photosynthesis rates for species from less seasonal forests,
i.e., larger values of seasonality index experienced larger decline
under drought condition (β = -0.45, CI = -0.82 to -0.08, n=51, t =
-2.46, P = 0.02, Figure 1c). Seasonality index did not explain
interspecific variation in growth for plants in drought vs well-watered
conditions (β = -0.001, CI = -0.003 to -0.002, n=339, t = -0.64, P =
0.52, Figure 1b).
Of the 12 traits we measured, traits related to leaf structural
investment, anatomy of stomata and xylem best-explained variation in
drought response of seedlings. Species with larger xylem had lower
survival in drought conditions as compared to well-watered conditions (β
= -1.3, CI = -2.14 to -0.45, n = 412, z = -3.0, P = 0.002, Figure 2a).
Smaller stomata correlated with poorer survival in well-watered
conditions (β = 1.94, CI = 0.11 to 3.76, n = 412, z = 2.08, P = 0.04,
Figure 2b), and survival also declined more steeply for species with
smaller stomata in drought (β = -3.29, CI = -5.08 to -1.5, n = 412, z =
-3.61, P < 0.001, Figure 2b). Although survival in
well-watered conditions decreased with higher LDMC, this difference
disappeared under drought conditions (β = 1.2, CI = 0.41 to 2.0, n =
412, z = 2.97, P = 0.003, Figure 2c).
In well-watered conditions, growth rates increased with increasing xylem
diameter (β = 0.01, CI = 0.003 to 0.022, df = 13, t = 2.97, P = 0.01,
Figure 2d) and higher stomatal density (β = 0.01, CI = 0.005 to 0.025,
df = 14, t = 3.29, P < 0.05, Figure 2e). However, in drought
condition, species with larger xylem and higher SD showed larger
declines in growth but the effect sizes were weak (XD : β =
-0.01, CI = -0.008 to -0.004, df = 321, t = -5.09, P < 0.01;SD : β = -0.004, CI = -0.009 to -0.0002, df = 339, t = -2.09, P
= 0.04, Figure 2d, 2e). Species with lower LDMC and VD grew less under
drought condition, however with increasing values for both the traits,
growth rates became more similar in well-watered and drought conditions
(LDMC : β = 0.003, CI = 0.001 to 0.004, df = 321, t = 3.37, P
< 0.05, Figure 2f; VD : β = 0.01, CI = 0.003 to 0.009,
df = 339, t = 4.31, P < 0.05, Figure 2g). Species with larger
RD showed greater decline in growth under drought (β = -0.003, CI =
-0.006 to -0.0001, df = 296, t = -2.04, P = 0.04, Figure 2h).
Physiological function (photosynthesis) increased with larger xylem (β =
1.14, CI = 0.08 to 2.19, df = 13, t = 2.3, P = 0.04, Figure 3a) and
greater stomatal density (β = 1.6, CI = 0.65 to 2.49, df = 14, t = 3.65,
P < 0.05, Figure 3b) as expected, and both slopes decreased in
drought conditions (XD : β = -0.8, CI = -1.01 to -0.60, df = 49,
t = -7.73, P < 0.05; SD : β = -0.83, CI = -1.22 to
-0.45, df = 51, t = -4.37, P < 0.05, Figure 3a, 3b). SLA did
not explain overall differences in photosynthetic rates of species in
well-watered conditions, but in drought, photosynthesis rates increased
with SLA (β = 0.53, CI = 0.17 to 0.89, df = 49, t = 2.98, P <
0.05, Figure 3c), as well aswith increasing LDMC (β = 0.4, CI = 0.15 to
0.64, df = 49, t = 3.2, P < 0.05, Figure 3d). Drought led to
larger proportional decline in photosynthesis for species with lower
vein density (β = 0.79, CI = 0.40 to 1.18, df = 51, t = 4.1, P
< 0.05, Figure 3e). Additionally, photosynthesis decreased
with larger and longer stomata in dry conditions (SS : β =
-0.55, CI = -0.86 to -0.24, df = 49, t = -3.6, P < 0.001,
Figure 3f; SL : β = -0.37, CI = -0.72 to -0.01, df = 49, t =
-2.08, P = 0.04, Figure 3g).
Photosynthetic performance under drought was not explained by root area,
length and SRL. Yet, under well-watered conditions, photosynthesis
increased with greater root area, longer roots and higher SRL
(RA : β = 1.36, CI = 0.47 to 2.26, df = 12, t = 3.32, P
< 0.05; RL : β = 1.12, CI = 0.13 to 2.11, df = 12, t =
2.48, P = 0.03; SRL : β = -0.97, CI = -1.10 to 0.07, df = 11, t
= -2.06, P = 0.06; Appendix Table A8 h, A8 i, A8 k). Species with
thicker roots had higher photosynthetic rates in well-watered
conditions, but experienced larger proportional decline in
photosynthesis than species with thinner roots under drought stress (β =
-0.89, CI = -1.28 to -0.50, df = 47, t = -4.6, P < 0.05,
Figure 3h).