Forest structures are shaped by elevation and precipitation in the Central Himalaya

Forests in the Himalaya occur across a huge elevational range up to the tree-line ecotone. Precipitation also varies strongly; it is usually high at the windward side and low at the leeward side of the central mountain chain. Our objectives were (a) to compare forest structures in the tree-line ecotones at the wind and leeward side, and (b) to test the predictability of forest structural complexity by topographic and climatic variables from lower elevations to the tree-line. The study was conducted in the Annapurna range with 90 plots in the tree-line ecotones and an additional 69 plots at lower elevations. Forest structure was assessed by mobile laser scanning. On the windward side, the tree-line ecotone forest was mainly composed of broad-leaved species such as Rhododendron campanulatum. The stands had a high number of stems, small crowns, low vertical stratification, and dense canopy cover. On the leeward side, the tree-line ecotone forest was predominantly composed of needle-leaved species, including Pinus wallichiana. The stands had a low number of stems, large crowns, greater vertical stratification, and an open canopy. Forest structural complexity, measured by the box dimension (D_b) was similar at the tree-line on both sides. For all available plots (n = 159), generalized additive models explained up to 83% of the variation in D_b with the variable elevation, precipitation, slope, and aspect. Shapley additive explanations (SHAP) analysis underlined the dominant influence of elevation, followed by precipitation on both D_b and forest height. Overall, D_b remained relatively stable up to 3,600 m a.s.l. and then abruptly declined. This contrasts with forest height, which had already declined earlier. Overall, our study highlights the differences between precipitation regimes and underscores the importance of topography and precipitation in shaping forest height and structural complexity differently in the Himalaya.