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

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.

Recovery of logged forest fragments in a human-modified tropical landscape during the 2015-16 El Niño

“…Our results suggest that topography contributes to landscape-scale variation in soil water availability, as well as water deficit and temperature of air, which in turn affect the degree of dynamics rates associated with leaf shedding, decreased productivity and higher mortality within ENSO years. The topography is known to modulate species composition, functional diversity and vegetation dynamics 19 , 25 , 59 ; forests growing on ridges may have strong competition for nutrients and water compared to those growing on deep alluvial soils or valley bottoms 60 . A previous study in Borneo reported that lowlying forests (i.e.…”

“…Large uncertainty remains regarding the responses of regenerating logged forests to climate change 15 , particularly because rising CO 2 concentrations are expected to increase biomass growth of degraded forests 16 and drought resistance differs among tree species 15 . Early successional species associated with recovering logged forests have characteristics that allow them to grow fast when water is plentiful [17][18][19] . These characteristics include large vessel diameters and high maximum stomatal conductances, which are associated with high transpiration and assimilation rates when the water supply is sufficient 20 but can make them susceptible to drought 21 .…”

Carbon flux and forest dynamics: Increased deadwood decomposition in tropical rainforest tree‐fall canopy gaps

“…Light detection and ranging data were collected using a Leica ALS50‐II LiDAR sensor, which emits 120 kHz frequency pulses, has a 12° field of view and a footprint of approximately 40 cm. See Swinfield et al (2019) for details of LiDAR data processing to generate canopy height and digital terrain models at a 0.5 m resolution. Using these data, we analysed canopy height models to identify gaps, defined as areas with a canopy height of less than 5 m. Gaps larger than 1 ha were filtered out to remove LiDAR artefacts, manmade clearances and the river running through Maliau Basin.…”

A Research Agenda for Microclimate Ecology in Human-Modified Tropical Forests

“…These human-modified tropical forests differ from their old-growth counterparts in a number of important ways Pfeifer et al, 2017;Riutta et al, 2018), as the impacts of logging and land clearing go well beyond the simple removal of biomass. For instance, logging often results in soil compaction and erosion (Putz et al, 2008), while the selective removal of tree species can directly alter the stoichiometry and function of the entire ecosystem (Riutta et al, 2018;Both et al, 2019;Swinfield et al, 2019). But perhaps the most pervasive impact of logging is that by opening and thinning the canopy, it increases solar radiation and air flow in the understorey and decreases evapotranspiration-thereby altering the forest's microclimate (Breshears, 2006;Hardwick et al, 2015;Senior et al, 2017).…”