Comparisons of Tree Damage Indicators in Five NASA ABoVE Forest Sites Near Fairbanks, Alaska

Abstract

As global warming affects sensitive northern regions, forests near Fairbanks, Alaska may be undergoing attack from pests and pathogens that could impact their ability to store carbon. Visual tree surveys are quick and useful for assessing forest health in remote sensing studies using GT (ground-truthing). Initial spectral analysis of leaf pigments, canopy water content, and non-photosynthetic carbon of one site near Fairbanks, Alaska imaged with AVIRIS-NG by NASA for the Arctic and Boreal Vulnerability Experiment (ABoVE) showed high fire fuel loads in 2017 that burned in 2019. In 2021-2022 we visually assayed damage of 359 deciduous and 309 coniferous trees at five ABoVE sites of different moisture regimes and burn severities. Using indices of 0 - 5 (0 = healthy, 5 = severe damage) we calculated average damage per tree from: 1) leaf damage (holes or defoliation); 2) stem damage (changes in stem color, texture, growth, heartwood, sap ooze, or stem loss); 3) non-photosynthetic tissue, aka "browning"; and 4) wilting. We also characterized crown color tree-1. Least squares models found low overall average tree damage, but damage types were varied and complex. Deciduous trees suffered greater herbivore damage than conifers. A third of trees showed broadleaf insect damage, a tenth of trees across species showed stem damage associated with pathogens. Aspen and conifers showed heartwood rot, but we found no visual signs of spruce beetle at our sites. Structural equation models found greater stem damage and wilting in warmer soils and post-burned sites supporting seedlings. Browning was associated with understory branches of conifers in late-successional sites with colder, shallower soils. Our study suggests that deciduous trees and seedlings near Fairbanks, Alaska are experiencing herbivory and midsummer wilting, and conifer understory browning is common.