Christopher Doughty, Camille Gaillard, Andrew Abraham, Patrick Burns, Jenna Keany, Jesus Aguirre-Gutierrez, Yadvinder Malhi, Patrick Jantz, George Koch, Alexander Shenkin, Hao Tang

The stratified nature of tropical forest structure had been noted by early explorers, but until recent use of satellite-based LiDAR (GEDI, or Global Ecosystems Dynamics Investigation LiDAR), there has been no way to quantify stratification across all tropical forests. Understanding stratification is important because by some estimates, a majority of the world’s species inhabit tropical forest canopies. Stratification can modify vertical microenvironment, and thus can affect a species’ susceptibility to global warming. A better understanding of structure could also improve predictions of biomass across the tropics. Here we find that, based on analyzing each GEDI 25m diameter footprint in tropical forests (after screening for human impact), most footprints (60-90%) do not have multiple layers of vegetation. We find stratification depends on the spatial resolution of the pixel (e.g. going from a 25m footprint to a 1 ha footprint will impact the results). However, with a 25m footprint, the most common forest structure has a minimum plant area index (PAI) at ~40m followed by an increase in PAI until ~15m followed by a decline in PAI to the ground layer (described hereafter as a one peak footprint). However, there are large geographic patterns to forest structure within the Amazon basin (ranging between 60-90% one peak) and between the Amazon (79±9sd) and SE Asia or Africa (72±14 v 73±11). The number of canopy layers is significantly correlated with tree height (r2=0.12), forest biomass (r2=0.14), maximum temperature (Tmax) (r2=0.05), vapor pressure deficit (VPD) (r2=0.03) and soil fertility proxies (e.g. total cation exchange capacity -r2=0.01). Certain boundaries, like the Pebas Formation and Ecoregions, clearly delineate continental scale structural changes. More broadly, deviation from more ideal conditions (e.g. lower fertility or higher temperatures) leads to shorter, less stratified forests with lower biomass.