Open AccessVisual interpretation and time series modeling of Landsat imagery highlight drought's role in forest canopy declines
Author(s) -
Bell David M.,
Cohen Warren B.,
Reilly Matthew,
Yang Zhiqiang
Publication year - 2018
Publication title -
ecosphere
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.255
H-Index - 57
ISSN - 2150-8925
DOI - 10.1002/ecs2.2195
Subject(s) - canopy , forest dynamics , environmental science , tree canopy , vapour pressure deficit , precipitation , geography , ecology , transpiration , biology , meteorology , photosynthesis , botany
Remote sensing studies indicate that slow and subtle degradation of forest canopies, or forest canopy declines, has increased in extent during recent decades, possibly due to hotter and drier droughts. However, it is difficult to collect consistent, high‐quality time series of forest canopy decline occurrence observations needed for examining multi‐year drought contributions to forest canopy decline at an annual scale. In this research, we leveraged a tool for visual interpretation of annual Landsat satellite imagery (TimeSync) and a hierarchical Bayesian time series modeling approach (stochastic antecedent modeling, SAM) in five forest type groups located in the western United States to assess (1) what seasonal and interannual patterns in vapor pressure deficit (VPD) and precipitation preceded forest canopy decline events, (2) how drought effects on forest canopy decline events differed by forest type group, and (3) whether or not drought effects on forest canopy decline events were uniform within forest type groups. We examined observations of forest canopy decline over three decades (1985–2013) at 126 plots where we collected annual TimeSync observations. Stochastic antecedent modeling indicated that January–March VPD and July–September precipitation anomalies for the current year and 1–3 yr in the past contributed to defining drought conditions in relation to forest canopy decline dynamics. The probability of forest canopy decline decreased with summer precipitation for all forest type groups and increased with winter VPD for the warmest and the coldest forest type groups. However, the magnitude and direction of forest canopy decline sensitivity to drought varied substantially within forest type groups. The ubiquitous, but not uniform, effects of drought on forest canopy decline dynamics implied that local biotic (e.g., forest structure and composition, tree genetics) and abiotic (e.g., topography and soils) factors act to mediate effects of drought on forest change. The integration of the TimeSync satellite image interpretation tool with SAM provides a promising approach to link ecological understanding of tree drought responses to forest and landscape responses at regional and continental scales.