<p>Climate and elevational gradients influence the spatial variation of tree diversity and community composition across tropical forest ecosystems. The present study examines how water–energy dynamics regulate tree species diversity and community turnover along the elevational gradient of Sri Lanka, a tropical island biodiversity hotspot, using data from 100 National Forest Inventory plots. We tested two hypotheses: that tree diversity declines with elevation in response to interacting reductions in temperature and precipitation, and that community compositional turnover is continuous and gradual rather than punctuated by abrupt transitions along the elevation gradient. Generalized additive models revealed a significant hump-shaped relationship between Shannon diversity and elevation, peaking at approximately 800&#xa0;m a.s.l., with climate variables jointly explaining 39.7% of diversity variation, which is more than twice the explanatory power of elevation alone (16.3%; ΔAIC = 34.9). Variance partitioning confirmed that precipitation was the dominant driver (explained 34.6% of the variation), while temperature contributed only 3.2% independently. Both predictors operated as largely non-redundant axes of environmental constraint, with a shared collinearity fraction of only 0.6%. Beta diversity analysis revealed high overall compositional dissimilarity across the gradient. Mantel tests confirmed significant distance decay in Jaccard dissimilarity with elevational distance (<i>r</i> = 0.182, <i>P</i> = 0.001), consistent with continuous climate-mediated compositional turnover with no evidence of abrupt vegetation zone boundaries. These findings demonstrate that moisture availability is the primary determinant of tree diversity and community composition in Sri Lankan tropical forests, with thermal energy availability imposing a significant but secondary constraint that intensifies with elevation.</p>