Canopy structure and topography jointly constrain the microclimate of human-modified tropical landscapes.

Tommaso Jucker,Robert M Ewers,Dafydd M.O Elias,Sabine Both,Stephen R Hardwick,Tom Swinfield,David T Milodowski,David A Coomes

doi:10.1111/gcb.14415

Abstract

Local-scale microclimatic conditions in forest understoreys play a key role in shaping the composition, diversity and function of these ecosystems. Consequently, understanding what drives variation in forest microclimate is critical to forecasting ecosystem responses to global change, particularly in the tropics where many species already operate close to their thermal limits and rapid land-use transformation is profoundly altering local environments. Yet our ability to characterize forest microclimate at ecologically meaningful scales remains limited, as understorey conditions cannot be directly measured from outside the canopy. To address this challenge, we established a network of microclimate sensors across a land-use intensity gradient spanning from old-growth forests to oil-palm plantations in Borneo. We then combined these observations with high-resolution airborne laser scanning data to characterize how topography and canopy structure shape variation in microclimate both locally and across the landscape. In the processes, we generated high-resolution microclimate surfaces spanning over 350 km2 , which we used to explore the potential impacts of habitat degradation on forest regeneration under both current and future climate scenarios. We found that topography and vegetation structure were strong predictors of local microclimate, with elevation and terrain curvature primarily constraining daily mean temperatures and vapour pressure deficit (VPD), whereas canopy height had a clear dampening effect on microclimate extremes. This buffering effect was particularly pronounced on wind-exposed slopes but tended to saturate once canopy height exceeded 20m-suggesting that despite intensive logging, secondary forests remain largely thermally buffered. Nonetheless, at a landscape-scale microclimate was highly heterogeneous, with maximum daily temperatures ranging between 24.2 and 37.2°C and VPD spanning two orders of magnitude. Based on this, we estimate that by the end of the century forest regeneration could be hampered in degraded secondary forests that characterize much of Borneo's lowlands if temperatures continue to rise following projected trends.

Highlights

Local‐scale microclimatic conditions in forest understoreys shape ecological processes at all levels of organization, from the metabolic and demographic rates of individual organisms to whole‐ecosystem nutrient cycling (Chen et al, 1999; Clarke, 2017)
Canopy structure and topography emerged as strong, interactive drivers of fine‐scale variation in understorey microclimatic conditions across a land‐use intensity gradient that increasingly typifies much of Borneo's lowland tropical landscapes (Bryan et al, 2013; Gaveau et al, 2014, 2016)
Given the importance of local‐scale microclimatic conditions in shaping ecosystem responses to global environmental change (De Frenne et al, 2013)—and the fact that currently available climate data tend to be unrepresentative of understorey conditions (De Frenne & Verheyen, 2016; Faye et al, 2014)—our results highlight the potential of remote sensing technologies such as ALS for characterizing microclimate at ecologically relevant scales (Bramer et al, 2018; Frey et al, 2016; Lenoir et al, 2017)

Summary

| INTRODUCTION

Local‐scale microclimatic conditions in forest understoreys shape ecological processes at all levels of organization, from the metabolic and demographic rates of individual organisms to whole‐ecosystem nutrient cycling (Chen et al, 1999; Clarke, 2017). By coupling ALS data with on‐ground networks of microclimate sensors, we are in a position to robustly assess the relative importance of different topographic and canopy structural features in determining microclimate (Frey et al, 2016; Lenoir et al, 2017) These same data can be used to generate high‐resolution microclimatic surfaces for entire landscapes using either empirical of physical‐based modelling approaches (Hardwick, 2015; Tymen et al, 2017). We combine ALS data with almost one million hourly readings of near‐surface air temperature and vapour pressure deficit (VPD) taken across a land‐use intensity gradient that spans from old‐ growth tropical forests to oil‐palm plantations in Malaysian Borneo These data were used to fit empirical models relating variation in microclimate among sites to topographic and canopy structural attributes derived from ALS. We aimed to determine whether the two differ systematically, and if so whether accounting for the buffering effect of forest canopies on near‐surface air temperature can explain these differences

| MATERIALS AND METHODS

| RESULTS

Findings

| DISCUSSION