A spectral three-dimensional color space model of tree crown health.

Protecting the future of forests in the United States and other countries depends in part on our ability to monitor and map forest health conditions in a timely fashion to facilitate management of emerging threats and disturbances over a multitude of spatial scales. Remote sensing data and technologies have contributed to our ability to meet these needs, but existing methods relying on supervised classification are often limited to specific areas by the availability of imagery or training data, as well as model transferability. Scaling up and operationalizing these methods for general broadscale monitoring and mapping may be promoted by using simple models that are easily trained and projected across space and time with widely available imagery. Here, we describe a new model that classifies high resolution (~1 m2) 3-band red, green, blue (RGB) imagery from a single point in time into one of four color classes corresponding to tree crown condition or health: green healthy crowns, red damaged or dying crowns, gray damaged or dead crowns, and shadowed crowns where the condition status is unknown. These Tree Crown Health (TCH) models trained on data from the United States (US) Department of Agriculture, National Agriculture Imagery Program (NAIP), for all 48 States in the contiguous US and spanning years 2012 to 2019, exhibited high measures of model performance and transferability when evaluated using randomly withheld testing data (n = 122 NAIP state x year combinations; median overall accuracy 0.89-0.90; median Kappa 0.85-0.86). We present examples of how TCH models can detect and map individual tree mortality resulting from a variety of nationally significant native and invasive forest insects and diseases in the US. We conclude with discussion of opportunities and challenges for extending and implementing TCH models in support of broadscale monitoring and mapping of forest health.

Drought-herbivory interaction disrupts competitive displacement of native plants by Microstegium vimineum, 10-year results.

Biological invasions are often exacerbated by disturbance or deviations from historic disturbance regimes. Dense understory layers of invasive exotic plants can alter successional trajectories, resulting in consequences that cascade through the biota. However, it is unclear if such layers are self-sustaining or maintained by chronic disturbances that asymmetrically depress native competitors. We examined the role of white-tailed deer (Odocoileus virginianus Zimm.) herbivory and drought on the permeability of recalcitrant understory layers dominated by the invasive exotic Microstegium vimineum (Trin.) A. Camus in 15 exclosures and 15 control plots from 1997 to 2006. This study was conducted in Cades Cove, Great Smoky Mountains National Park, Tennessee, USA. M. vimineum cover exhibited high inter- and intra-annual variation in both exclosures and controls, but displayed a significant correspondence to drought severity. Native species richness and the abundance of woody plants increased within exclosures, but not controls, following a drought-induced nadir in M. vimineum cover that occurred in 2000. By 2003, all height classes of native tree seedlings were present in exclosures and seedlings were advancing into the sapling layer (>or=50 cm tall). After 10 years, no tree seedling on a control plot had been able to attain and maintain a height >or=20 cm. Our results suggest that chronic herbivory inhibits state transitions that could occur in response to intermittent disturbances, which reduce the abundance of the invader. Consequently, recalcitrance is likely reinforced by chronic herbivory.