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

Swinfield, T. and Both, S. and Riutta, T. and Bongalov, B. and Elias, D. and Majalap-Lee, N. and Ostle, N. and Svátek, M. and Kvasnica, J. and Milodowski, D. and Jucker, T. and Ewers, R.M. and Zhang, Y. and Johnson, D. and Teh, Y.A. and Burslem, D.F.R.P. and Malhi, Y. and Coomes, D. (2019) Imaging spectroscopy reveals the effects of topography and logging on the leaf chemistry of tropical forest canopy trees. Global Change Biology. ISSN 1354-1013

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 km(2) 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.