Biotic interactions shape plant community dynamics, wherein demographic response to increasing density of conspecifics relative to heterospecifics–termed conspecific density dependence (CDD)–plays a critical role. CDD may vary with abiotic conditions, which remains understudied despite its relevance to plant community dynamics in spatiotemporally variable habitats.We examined how gradients of light, soil moisture and nutrient availability altered conspecific effects on seedling survival in a tropical humid forest of the Western Ghats (India), from 9148 seedlings of 39 tree species over three years. Then, we examined whether changes in CDD with abiotic conditions were explained by traits representing tradeoffs along fast-slow strategies: specific leaf area (SLA), leaf dry matter content (LDMC) and specific root length (SRL).Community-average CDD was negative at wetter and nutrient-rich sites and became positive at dry, nutrient-poor sites, and these shifts were driven by a few species. In bright, dry and nutrient rich conditions, resource-acquisitive species (high SLA, low LDMC and high SRL) had positive CDD while conservative species (low SLA, high LDMC and low SRL) had negative CDD.Synthesis: Drier and low nutrient conditions weakened CDD on average for the community, largely driven by changes to dominant species. Resource-acquisitive species escaped self-limitation and experienced positive conspecific interactions in brighter, drier, and nutrient-rich conditions. With global environmental change, drier conditions and nutrient deposition may diminish population constraints on resource-acquisitive species to modify community structure in the seedling bank of humid forests.

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.