Remote sensing of forest die-off in the Anthropocene: From plant ecophysiology to canopy structure

Human activities have fundamentally changed Earth's climate and land surface in the latter half of the 20th century, leading to the proposal of a new geologic epoch known as the Anthropocene. One major ecological perturbation underway over the past several decades of the Anthropocene is extensive tree and shrub mortality, or forest die-off, triggered by elevated temperatures and prolonged drought, and/or insect and pathogen outbreaks. This increasingly common disturbance has affected forests and woodlands on all vegetated continents and may alter terrestrial carbon fluxes in the biosphere. Remote sensing has enabled mapping of the extent and ecological patterns of tree loss and damage, assessing potential carbon emissions and monitoring ecosystem trajectories after tree mortality. In this review article, we cover drought-induced changes in plant physiology, chemistry, and structure that occur as an individual tree progresses from healthy to stressed to standing dead or coarse woody debris, and corresponding responses in remotely sensed data that provide the opportunity and potential for observation and analysis at large spatial scales and early detection. The linkages between changes associated with tree mortality and remote sensing show exceptional promise for strategic and adaptive natural resource management as climate models project warmer and drier climates in the coming decades.