Environmental controls on canopy foliar nitrogen distributions in a Neotropical lowland forest

Abstract

Distributions of foliar nutrients across forest canopies can give insight into their plant functional diversity and improve our understanding of biogeochemical cycling. We used airborne remote sensing and partial least squares regression to quantify canopy foliar nitrogen (foliar N) across ~164 km of wet lowland tropical forest in the Osa Peninsula, Costa Rica. We determined the relative influence of climate and topography on the observed patterns of foliar N using a gradient boosting model technique. At a local scale, where climate and substrate were constant, we explored the influence of slope position on foliar N by quantifying foliar N on remnant terraces, their adjacent slopes, and knife-edged ridges. In addition, we climbed and sampled 540 trees and analyzed foliar N in order to quantify the role of species identity (phylogeny) and environmental factors in predicting foliar N. Observed foliar N heterogeneity reflected environmental factors working at multiple spatial scales. Across the larger landscape, elevation and precipitation had the highest relative influence on predicting foliar N (30% and 24%), followed by soils (15%), site exposure (9%), compound topographic index (8%), substrate (6%), and landscape dissection (6%). Phylogeny explained ~75% of the variation in the field collected foliar N data, suggesting that phylogeny largely underpins the response to the environmental factors. Taken together, these data suggest that a large fraction of the variance in foliar N across the landscape is proximately driven by species composition, though ultimately this is likely a response to abiotic factors such as climate and topography. Future work should focus on the mechanisms and feedbacks involved, and how shifts in climate may translate to changes in forest function.

A Spectral Mapping Signature for the Rapid Ohia Death (ROD) Pathogen in Hawaiian Forests

“…Plotting this metric for all wavelengths in the measured spectrum can indicate which leaf or structural traits most strongly differ between the brown and green classes. We repeated all canopy-level spectral reflectance analyses using brightness-normalized reflectance, which has been found to improve chemical retrievals using airborne imaging spectroscopy (e.g., [26][27][28]). Brightness normalization minimizes differences in observed brightness in reflectance data due to canopy leaf orientation and depth.…”

Towards comparable assessment of the soil nutrient status across scales—Review and development of nutrient metrics

“…As the concentration of a certain nutrient in a plant or canopy typically increases with increasing availability in soil (when the nutrient is limiting—but see Ostertag, ; Peñuelas et al, ; Zechmeister‐Bolternstern et al, ), assessing tissue concentrations and stoichiometry is a common practice to evaluate the plant nutrient status in ecological and agronomical research (Sullivan et al, ). However, multiple studies have shown that factors like phylogeny, phenology and climate are proximal determinants of plant nutrient concentrations and stoichiometry, rather than the soil nutrient status (Balzotti et al, ; Di Palo & Fornara, ; Kokaly, Asner, Ollinger, Martin, & Wessman, ; Sardans et al, ). Indeed, in large‐scale studies including several species and strong climate gradients, plant stoichiometry is explained in great part by long‐term evolutionary processes in which species adapted to soil nutritional conditions along the gradient (Asner et al, ; Sardans et al, , ).…”

Remotely sensed canopy nitrogen correlates with nitrous oxide emissions in a lowland tropical rainforest

“…In our study area in southern Costa Rica, forests are characterized by hundreds of species per hectare (compared with just a few dominants in Hawai'i) and imaging spectroscopy has identified wide variation in canopy N (Balzotti et al. ) that is linked to differences in soil N availability (Osborne et al. ).…”

“…Combining these maps with spatial field sampling, Hall and Asner (2007) generated predictive relationships to describe how high density invasive stands increase soil N cycling rates and N oxide (N 2 O and NO) emissions. In our study area in southern Costa Rica, forests are characterized by hundreds of species per hectare (compared with just a few dominants in Hawai'i) and imaging spectroscopy has identified wide variation in canopy N (Balzotti et al 2016) that is linked to differences in soil N availability (Osborne et al 2017). Thus, this site offers potential to test links between canopy N and soil N 2 O fluxes in a diverse tropical forest.…”