Imaging spectroscopy reveals the effects of topography and logging on the leaf chemistry of tropical forest canopy trees.

Tom Swinfield,Yadvinder Malhi,Jakub Kvasnica,Terhi Riutta,Tommaso Jucker,Dafydd Elias,Robert M Ewers,Noreen Majalap‐Lee,David Milodowski,Boris Bongalov,David Coomes,David Johnson,Sabine Both,David F R P Burslem,Yit Arn Teh,Martin Svátek,Nicholas Ostle,Yi Zhang

doi:10.1111/gcb.14903

Abstract

Logging, pervasive across the lowland tropics, affects millions of hectares of forest, yet its influence on nutrient cycling remains poorly understood. One hypothesis is that logging influences phosphorus (P) cycling, because this scarce nutrient is removed in extracted timber and eroded soil, leading to shifts in ecosystem functioning and community composition. However, testing this is challenging because P varies within landscapes as a function of geology, topography and climate. Superimposed upon these trends are compositional changes in logged forests, with species with more acquisitive traits, characterized by higher foliar P concentrations, more dominant. It is difficult to resolve these patterns using traditional field approaches alone. Here, we use airborne light detection and ranging‐guided hyperspectral imagery to map foliar nutrient (i.e. P, nitrogen [N]) concentrations, calibrated using field measured traits, over 400 km2 of northeastern Borneo, including a landscape‐level disturbance gradient spanning old‐growth to repeatedly logged forests. The maps reveal that canopy foliar P and N concentrations decrease with elevation. These relationships were not identified using traditional field measurements of leaf and soil nutrients. After controlling for topography, canopy foliar nutrient concentrations were lower in logged forest than in old‐growth areas, reflecting decreased nutrient availability. However, foliar nutrient concentrations and specific leaf area were greatest in relatively short patches in logged areas, reflecting a shift in composition to pioneer species with acquisitive traits. N:P ratio increased in logged forest, suggesting reduced soil P availability through disturbance. Through the first landscape scale assessment of how functional leaf traits change in response to logging, we find that differences from old‐growth forest become more pronounced as logged forests increase in stature over time, suggesting exacerbated phosphorus limitation as forests recover.

Highlights

Lowland tropical forests have been logged extensively, yet the influence of timber extraction on nutrient cycling remains poorly understood
Through the first landscape scale assessment of how functional leaf traits change in response to logging, we find that differences from old-growth forest become more pronounced as logged forests increase in stature over time, suggesting exacerbated phosphorus limitation as forests recover
These differences in soil nutrient availability are reflected in the positioning of foliar traits along the leaf economics spectrum (Díaz et al, 2016; Wright et al, 2004) with nutrient rich soils filtering for traits associated with fast growth and rapid acquisition of resources while nutrient-poor soils filter for conservative traits that confer survival

Summary

| INTRODUCTION

Lowland tropical forests have been logged extensively, yet the influence of timber extraction on nutrient cycling remains poorly understood. Airborne imaging spectrometry ( known as ‘spectranomics’; Asner & Martin, 2009) makes it possible to study the spatial distribution of foliar plant traits at broad spatial scales, enabling the effects of logging to be unpicked from the natural background variability of leaf properties. This approach provides an unrivalled tool for scaling-up point-based measurements of ecosystem processes that would otherwise be impossible with traditional field measurements alone P becomes increasingly limiting with elevation and logging, exhibited as increased foliar N:P ratios

| MATERIALS AND METHODS

Findings

| DISCUSSION