Hemlock Leaf Radiometry

Our lab has partnered with Lula Lake Land Trust to address questions regarding the physiological status and recovery of Eastern Hemlocks (Tsuga canadensis) amidst an ongoing infestation by Hemlock Woolly Adelgid (Adelges tsugae). Hemlocks are an important part of forest ecosystems in the south, providing shade for headwater streams, and cool/moist microclimates for numerous native species. The Adelgid is particularly adept at killing mature Hemlock trees, and this infestation has the potential to fundamentally change forest ecosystems throughout the Eastern US. Fortunately, a variety of control methods have been introduced, the most reliable of which is treatment with systemic insecticides. These insecticides accumulate in plant tissues and provide protection for multiple years. Some insecticides actually affect plant physiology in addition to killing damaging insects, stimulating immune responses that help speed recovery.

Leaves infested with Hemlock Woolly Adelgid (Notice the white cotton-like secretions left by reproducing females).

We used reflectance spectroscopy to document the structure and chemistry of leaves from treated and untreated Hemlocks. We found evidence that the systemic insecticide Imidacloprid® alters Hemlock leaf physiology in a manner that is consistent with a beneficial stress response. However, as the pest and the insecticide both appear to stimulate the same stress-response pathway, questions remain as to how the pest is able to overcome plant defenses in the absence of the insecticide.

2019. Garris HW, Settle TH, Crossman JE, Grider SJ, Michaels SL. Combined effects of hemlock woolly adelgid (Adelges tsugae) infestation and treatment with imidacloprid on eastern hemlock (Tsuga canadensis) leaf radiometry. Journal of Forestry 1-11 doi:10.1093/jofore/fvz021.

Our initial study was limited by the logistics of accessing samples near the ground. We collected leaves from the apex of basal branches, which may exhibit different degrees of infestation than those in the canopy. McClure et al. (2001) suggested that infestation progresses from basal branches to those in the canopy, but this leaves questions as to whether stress responses are detectable solely by those leaves/branches exhibiting heavy infestation. The canopy (accessible to remote-sensing technology) may provide spectral warning signals that could aid in HWA response efforts before trees are irreparably damaged (Hemlocks in the southern Appalachians can survive 2-5 years after initial infestation). We acquired a drone-mounted multispectral imaging system in the spring of 2021 and training for commercial drone certification to use on this project. We plan to track spectral shifts in canopy reflectance for sample trees in the Lula Lake Land Trust and compare these results with infestation levels recorded for basal branches. We plan to develop a spectral profile for early HWA infestation that can be used to aid in large-scale tracking efforts.

Upper : Sample imagery from Parrott Sequoia+ reflectance spectrometer.
Middle : Parrot Sequoia+ canopy reflectance output (4 bands) relative to leaf –scale reflectance within the visible/NIR range.  Data outputs are uncorrected for ambient light (this requires processing in photogrammetry software).
Lower : Drone setup with integrated Parrot Sequoia+
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