The interactions between the environment and plant functional type in controlling global maximum forest height
J. DAI; Q. YU; G. BAO; M. RYAN; N. P. HANAN
Department of Plant & Environmental Sciences, New Mexico State University, Las Cruces, New Mexico 88001, USA

Forest maximum height is a critical determinant of ecosystem structure and function, but the relative importance of genetic and environmental controls on maximum individual tree heights at macro-ecological scales have not been fully elucidated. Specifically, it remains unknown how the mechanical and physiological limits to tree height associated with plant genetics and plant functional type (PFT) interact with critical resources impacting tree physiology and growth (e.g., water, light, and soil nutrient availability) at regional scales (determined by large scale climate patterns and geomorphology) and local-scales terrain complexity (impacting landscape resource distributions). Here we disentangled the contribution and interaction of environmental determinants and broad-scale PFT in controlling global patterns in maximum tree height (Hm). We use data from the Global Ecosystem Dynamics Investigation (GEDI) lidar instrument on the International Space Station and a combination of ensemble random forest and structural equation algorithms. While at global scale water availability positively contributes ~75% to Hm variations, terrain complexity is surprisingly the most important non-water determinant on Hm. Complex terrain increases Hm in arid regions, perhaps because of resource redistribution and accumulation in depressions, while Hm is suppressed in flat landscapes in wetter environments, perhaps due to poor drainage. PFTs play a relatively small role in regulating Hm globally. Instead, they follow the global water-Hm relationship, with different PFTs occupying different segments of climate space.