Abstract
Drought, ozone (O3), and nitrogen deposition (N) alter foliar pigments and tree crown structure that may be remotely detectable. Remote sensing tools are needed that pre-emptively identify trees susceptible to environmental stresses could inform forest managers in advance of tree mortality risk. Jeffrey pine, a component of the economically important and widespread western yellow pine in North America was investigated in the southern Sierra Nevada. Transpiration of mature trees differed by 20% between microsites with adequate (mesic (M)) vs. limited (xeric (X)) water availability as described in a previous study. In this study, in-the-crown morphological traits (needle chlorosis, branchlet diameter, and frequency of needle defoliators and dwarf mistletoe) were significantly correlated with aerially detected, sub-crown spectral traits (upper crown NDVI, high resolution (R), near-infrared (NIR) Scalar (inverse of NDVI) and THERM Δ, and the difference between upper and mid crown temperature). A classification tree model sorted trees into X and M microsites with THERM Δ alone (20% error), which was partially validated at a second site with only mesic trees (2% error). Random forest separated M and X site trees with additional spectra (17% error). Imagery taken once, from an aerial platform with sub-crown resolution, under the challenge of drought stress, was effective in identifying droughted trees within the context of other environmental stresses.
Highlights
Recent, massive tree die-offs have drawn world-wide attention
This study was focused on utilizing the relatively few bands commonly available to land managers in order to identify trees “at risk” of drought stress and more likely to be susceptible to extreme drought events or multiple years of chronic stress, and successful bark beetle attack
N amendment decreased the frequency of needle defoliators of trees in both mesic and xeric microsites
Summary
Unanticipated, large-scale, climate- and insect outbreak-induced forest mortality has been reported on most continents since the mid-20th century, increasing in frequency after the late 20th century to present [1]. A plethora of studies link increased insect and pathogen outbreaks and tree mortality with drought [3]. Remote sensing tools that are low in data latency and of sufficient resolution to discern individual tree vigor and/or stress could support forest management, especially as unanticipated environmental events and biotic pressures evolve. Assessment of tree status at the seasonal maximum of ground water extraction late in the growing season, in a “drought” year could inform managers of location and level of risk
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