Assessing Alaskan Boreal Forest Landcover Affected by Climate-wildfire
Interactions from Ground Truth Surveys and NASA Airborne Remote Sensing
Abstract
Alaska’s boreal forest is facing unprecedented challenges under rapid
climate warming (increasingly severe fires, droughts, pest/disease
outbreaks) that may destabilize its function as a global carbon sink.
Forests near Fairbanks may be especially vulnerable, impacting air
quality and ecosystem services. We combined GT (ground truthing) with
Airborne Visible InfraRed Imaging Spectrometer (AVIRIS-NG) images
collected by the NASA Arctic-Boreal Vulnerability Experiment (ABoVE)
program (2017-2019) to assess landcover change at five recently burned
sites (2001-2019) of different fire severities and moisture regimes
within 30 miles of Fairbanks. GT included tree seedling counts,
understory % cover and >50% leaf canopy color assessment.
36 circular plots (1/30 ha radius) including 6 moderate to severely
burned plots were selected across sites. 31 additional sites including
12 burned sites were geotagged in photos. AVIRIS images were processed
from 29 spectral bands selected to identify changes in chlorophyll and
water content. Images were segmented into natural boundaries (polygons)
using ENVI 5.5 software. A spectral library of 8 AVIRIS bands with high
between-class/low within-class variation was used in two random forest
models to predict vegetation classes (model 1: 12 classes, model 2: 14
classes) in each AVIRIS scene, using 20% of the data as training data.
Model 2 classified 20% more polygons overall, but only 42% of
GT/geotagged polygons were correctly classified by both models. More
forest sites were correctly classified (63%) than open vegetation
(32%) or post-fire sites (46%). 50% of aspen forest and post-fire
polygons were misclassified as shrubland. GT revealed that post-fire
plots supported 134,000 (± 48,000) tree seedlings and saplings
ha-1 (0.2 - 4 m height, 64% deciduous) versus 2500 (±
2100) shrubs ha-1 (1-6 m height). > 50%
canopy browning was observed in conifer forest (8 plots) with no signs
of insect infestation. Canopy herbivory > 50% (leaf miner,
leaf beetle) and moose herbivory of tree bark was seen across aspen
sites. Our study suggests: 1) low canopy vegetation presents challenges
for improved landcover classification, and 2) aspen forest should be
differentiated in vegetation maps which would aid in tracking herbivory.