Journal Article

Mapping canopy nitrogen‐scapes to assess foraging habitat for a vulnerable arboreal folivore in mixed‐species Eucalyptus forests

Baker, Patrick J., Nitschke, Craig R., Moore, Ben D., Wagner, Benjamin

2021

Overview

Herbivore foraging decisions are closely related to plant nutritional quality. For arboreal folivores with specialized diets, such as the vulnerable greater glider (Petauroides volans), the abundance of suitable forage trees can influence habitat suitability and species occurrence. The ability to model and map foliar nitrogen would therefore enhance our understanding of folivore habitat use at finer scales. We tested whether high‐resolution multispectral imagery, collected by a lightweight and low‐cost commercial unoccupied aerial vehicle (UAV), could be used to predict total and digestible foliar nitrogen (N and digN) at the tree canopy level and forest stand‐scale from leaf‐scale chemistry measurements across a gradient of mixed‐species Eucalyptus forests in southeastern Australia. We surveyed temperate Eucalyptus forests across an elevational and topographic gradient from sea level to high elevation (50–1200 m a.s.l.) for forest structure, leaf chemistry, and greater glider occurrence. Using measures of multispectral leaf reflectance and spectral indices, we estimated N and digN and mapped N and favorable feeding habitat using machine learning algorithms. Our surveys covered 17 Eucalyptus species ranging in foliar N from 0.63% to 1.92% dry matter (DM) and digN from 0.45% to 1.73% DM. Both multispectral leaf reflectance and spectral indices were strong predictors for N and digN in model cross‐validation. At the tree level, 79% of variability between observed and predicted measures of nitrogen was explained. A spatial supervised classification model correctly identified 80% of canopy pixels associated with high N concentrations (≥1% DM). We developed a successful method for estimating foliar nitrogen of a range of temperate Eucalyptus species using UAV multispectral imagery at the tree canopy level and stand scale. The ability to spatially quantify feeding habitat using UAV imagery allows remote assessments of greater glider habitat at a scale relevant to support ground surveys, management, and conservation for the vulnerable greater glider across southeastern Australia. For arboreal folivores such as the greater glider, which is threatened by habitat loss and climate change, the abundance of suitable forage trees can influence habitat suitability and species occurrence. We developed a method to quantify spatial variability in foliar nitrogen, a key metric in forage suitability using UAV multispectral imagery. Our approach allowed us to scale leaf‐level measures of foliar nitrogen to the stand level. This method can provide critical habitat assessments to support management and conservation of this vulnerable species across southeastern Australia.

• Publisher: John Wiley & Sons, Inc,John Wiley and Sons Inc,Wiley
• Subject: Algorithms / Canopies / Commercial vehicles / Dry matter / Elevation / Eucalyptus / folivores / Forage / Foraging habitats / Forests / Gliders / greater glider / habitat mapping / Habitat utilization / Habitats / Herbivores / herbivory / Imagery / Learning algorithms / Leaves / Machine learning / Nitrogen / Nutritive value / Petauroides volans / plant–animal interactions / Polls & surveys / Reflectance / remote‐sensing spectroscopy / Sea level / Species / Trees / UAV / Unmanned aerial vehicles