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

In this study we show that logged tropical forests have reduced leaf nutrient concentrations compared with old-growth forests and this becomes more pronounced as forests recover in stature. Our findings suggest rock-derived nutrients, such as phosphorus, in short supply in tropical forests on old soils, are depleted by as much as 30% by logging. This changes the concentration of these nutrients in leaves and may lead to shifts in species composition, and possibly reduced ecosystem function.

To achieve landscape-scale maps of canopy nutrients, hyperspectral imagery was used to predict ground-based measurements taken directly from trees. Leaves were collected by tree-climbers from the canopies of hundreds of trees in Sabah, Malaysia and their nutrient content was assessed through traditional physical and chemical laboratory analysis. Their findings are published in Both et al. (2018). The area was also mapped by the UK’s Natural Environmental Research Council (NERC) airborne research facility plane. The plane was equipt with an onboard scanning laser (LiDAR) and hyperspectral cameras, which collect precise measurements of forest canopy height and the underlying topography. The hyperspectral imagery was georeferenced to the 3D landscape surface models so that corrections could be made to account for differences in illumination and atmospheric conditions. Finally, we built a statistical model to predict the leaf nutrient concentrations, as well as other traits, from the hyperspectral imagery corresponding with the tree crowns. Our models were able to explain a significant amount of the variation in the observed data, enabling us to make predictions across the entire landscape.

Logged forests were found to contain less nutrients than old-growth forests and these differences were greater in logged forests that had recovered in stature. We modelled the millions of observations of mapped nutrient concentrations as a function of forest canopy height, elevation and disturbance history (i.e. logged vs. primary forest). Our models revealed that elevation is a major driver of nutrient availability in lowland tropical forests but, after holding elevation constant, logged forests contained fewer nutrients than old-growth forests and this difference was greatest in taller logged forests. We conclude that nutrient limitation becomes increasingly pronounced as forests produce more leaves and acquire biomass, with time after logging, that the effects of nutrient limitation become increasingly important. Our findings suggest that logged forests diverge from pristine forests in terms of their canopy chemistry. We caution that this may have resulted from changes in species composition and may make tropical forests less able to recovery, following future rounds of logging.

We acknowledge that our findings were produced through a remote-sensing exercise, expanding a high quality ground-based survey not designed to sample across all the gradients. A valuable next step would be to use predictions of nutrients, such as ours, to guide targeted field surveys so sample over gradients in nutrient availability, forest degradation and recovery.

This work was the culmination of a huge international collaboration between multiple universities, government agencies and funders, without whom it would not have been possible. For that we are extremely grateful.

Swinfield (2019) Global Change Biology