Effects of neighborhood trait composition on tree survival differ between drought and postdrought periods

Although direct tree demographic responses to drought are widely recognized, studies of drought-mediated changes in tree interactions are rare. We hypothesize that drought exacerbates soil-water limitation and intensifies competition for water, but reduces light limitation and competition for light. We predict that competition would be stronger for trees (1) consuming more water or more susceptible to water deficits during drought and (2) intercepting more light or more susceptible to shade during postdrought periods. We tested these predictions in a 50-ha tropical forest plot by quantifying the effects of neighborhood mean trait values on tree survival during versus after a severe drought. We used wood density (WD) and leaf mass per area (LMA) as proxies for water and light use strategies, respectively. Tree survival was lower, canopy loss was greater, and sapling recruitment was greater during the drought relative to postdrought census intervals. This suggests that drought pushed water deficits to lethal extremes and increased understory light availability. Relationships between survival and neighborhood WD were independent of drought, which is inconsistent with our first prediction. In contrast, relationships between survival and neighborhood LMA differed strongly with drought. Survival time was unaffected by neighborhood LMA during drought, but was longer for trees of all sizes in low-LMA neighborhoods in the postdrought census interval, consistent with the prediction of reduced competition for light during drought. Our results suggest that severe drought might increase light availability and reduce competition for light in moist tropical forests.