Deep Images require Deep Learning: A Pixel-Based Convolutional Neural
Network Classifier can Accurately Identify Tree Species Using Imaging
Spectroscopy
Abstract
Our study uses field training data, airborne LiDAR (Light Detection and
Ranging), imaging spectroscopy, and a Convolutional Neural Network (CNN)
classifier to identify individual tree species in a mixed conifer forest
in the Southern Sierra Nevada Mountains. The remote sensing data was
collected on the National Ecological Observatory Network (NEON) Airborne
Observation Platform in 2017. We trained the classifier using existing
field plot data, and an independently collected validation dataset which
identifies trees location of the 7 dominant species (Pine, Fir, Cedar
and Oak), including condition and ‘live’ or ‘dead’ status. The LiDAR
canopy height model was used to identify tree crowns and imaging
spectroscopy data around these crowns were created as image ‘labels’.
These species level ‘labels’ were used to train, test and validate a CNN
tree species classifier. Our method achieved greater than 63-90%
accuracy for all field validated stems and worked best for large
diameter trees. On an independent tree stem dataset, we performed a
species-level logistic regression to study which cases the classifier
works most and least effectively. Spatially, in the southern areas
scattered Black Oak were present and tree species often were confused
with shrub species or covered by adjacent conifer species. In the north
where the upper elevation forest is dominated by red and white fir the
classifier achieved greater than 96% accuracy for larger canopy trees,
with accuracy degrading to about 59-70% when smaller trees are included
in the model. There is also genus level mis-classification, particularly
between Red and White Fir species. There was high tree mortality in this
forest and the classifier was effective in detecting large tree
mortality, which also varies as a function of species and size of trees.
This work leverages newly developed ‘deep learning’ tools which have yet
to be extensively applied to the remote detection of large trees or in
plant biogeography generally. This research is a proof-of-concept for
forest community ecologists who want accurate ‘tree species maps’ to
study how plants are distributed across space. Generally, this method
will be of interest to biogeographers or remote sensing scientists
looking to apply novel classification methods to problems beyond remote
tree species identification.