Drought makes life harder for trees. For example, an estimated 129 million trees have died in California’s national forests since 2010 due to unprecedented drought and bark beetle infestation. Remote sensing studies indicate that slow and subtle degradation of forest canopies has increased in extent during recent decades, possibly due to hotter and drier droughts.
Tree mortality is notoriously complex and governed by many contributing factors. To unravel the effect of drought, Forest Serivce scientist David Bell and colleagues used a tool called TimeSync, developed by Forest Service scientists and collaborators, to interpret 3 decades of Landsat (satellite) data on vegetation status and change through time. They integrated annual TimeSync observations with a time-series modeling approach to characterize the timing of forest canopy decline events in the western United States as a function of drought.
Their work provided a new perspective, showing broad trends across five broadly-distributed coniferous forest types they studied while also revealing the importance of site-level conditions. They found that the timing and duration of forest canopy declines depended on conditions associated with the length and severity of summer drought during the 1 to 3 years preceding the decline. They also identified substantial geographic variation in forest sensitivity to drought, indicating that local factors, such as forest structure and composition, insect and disease outbreaks, topography, and soils, can amplify or dampen drought impacts.
This research quantifies the rising importance of forest decline events as a major disturbance agent in the western United States and the likely contributions of drought to those disturbances. Information and approaches like this make a big difference in our ability to understand and plan for threats to forests across the West.