Water availability regulates plant community dynamics but the drought response of seedlings remains poorly known despite their vulnerability, especially for the Asian tropics. In particular, discerning how functional traits of seedlings mediate drought response can aid generalizable predictions of tree responses to global environmental change. We assessed interspecific variation in drought response explained by above- and below-ground seedling traits. We conducted a dry-down experiment in the greenhouse using 16 tree species from the humid forests of Western Ghats in southern India, chosen to represent differences in seasonality affiliations–sites of high and low seasonal drought. We compared seedling survival, growth, and photosynthetic performance under drought and well-watered conditions, and assessed the extent to which species responses were explained by seasonality affiliation and 12 traits of root, stem and leaf. We found that the species from seasonally dry forest reduced photosynthetic rate in drought compared to well-watered conditions, but seasonality affiliation did not explain differences in growth and survival. Performance changes in drought vs well-watered conditions were best explained by anatomical traits of xylem, veins and stomata. Species with larger xylem reduced their growth and photosynthesis to tolerate desiccation. In drought, species with smaller stomata showed reduced survival even though photosynthetic activity decreased by a larger extent with larger stomata. Our study shows that anatomical traits related to xylem and stomata, directly related to water transport and gas-exchange, played a more prominent role than commonly used soft traits (e.g., specific leaf area, wood density) in explaining species response to drought, and may offer a better proxy for physiological traits related to drought tolerance of seedlings.

Species traits influence their response to environmental conditions and the match between phenotypes and environment mediates spatial changes in species composition. These trait-environment linkages can be disrupted in human-modified landscapes. Human land-use creates habitat fragments where dispersal limitation or edge effects can exclude species that may otherwise suit a given macro-scale environment. Furthermore, stressful micro-environments in fragments may limit viable trait combinations resulting in stronger trait covariance compared to contiguous forest, especially in harsher macroenvironments. In a wet tropical forest landscape in the Western Ghats Biodiversity Hotspot of peninsular India, I compared fragments with adjacent contiguous forest for signatures of trait-mediated assembly of tree communities along macroenvironmental gradients. Using four key plant traits—seed size, specific leaf area (SLA), wood density, and maximum height—I evaluated changes in trait-mediated abundances and trait covariance across environmental gradients. Trait-mediated abundances primarily changed along the elevation gradient in contiguous forest, smaller-seeded, shorter, thinner-leaved species increased at higher elevations. In fragments, higher SLA species increased in more seasonal climate and decreased with higher precipitation, and larger seeds decreased at warmer sites. However, traits only weakly predicted abundances and only contiguous forests experienced significant compositional change via traits, driven by trait syndromes varying along a composite environmental gradient defined by elevation, water deficit, and soil C:N ratio. Covariance of seed size and maximum height along gradients of precipitation and temperature revealed divergent constraints on viable phenotypes in fragments and contiguous forest. Notably, local biotic conditions (functional diversity) had stronger effects than environment in explaining trait covariance. Overall, the results imply that trait syndromes and trait covariance, rather than single traits, determine the phenotypes best suited to different macroenvironmental conditions and should inform management or restoration goals in fragments.