Detecting early warning signals of tree mortality in boreal North America using multiscale satellite data

Abstract

Increasing tree mortality from global change drivers such as drought and biotic infestations is a widespread phenomenon, including in the boreal zone where climate changes and feedbacks to the Earth system are relatively large. Despite the importance for science and management communities, our ability to forecast tree mortality at landscape to continental scales is limited. However, two independent information streams have the potential to inform and improve mortality forecasts: repeat forest inventories and satellite remote sensing. Time series of tree-level growth patterns indicate that productivity declines and related temporal dynamics often precede mortality years to decades before death. Plot-level productivity, in turn, has been related to satellite-based indices such as the Normalized difference vegetation index (NDVI). Here we link these two data sources to show that early warning signals of mortality are evident in several NDVI-based metrics up to 24 years before death. We focus on two repeat forest inventories and three NDVI products across western boreal North America where productivity and mortality dynamics are influenced by periodic drought. These data sources capture a range of forest conditions and spatial resolution to highlight the sensitivity and limitations of our approach. Overall, results indicate potential to use satellite NDVI for early warning signals of mortality. Relationships are broadly consistent across inventories, species, and spatial resolutions, although the utility of coarse-scale imagery in the heterogeneous aspen parkland was limited. Longer-term NDVI data and annually remeasured sites with high mortality levels generate the strongest signals, although we still found robust relationships at sites remeasured at a typical 5 year frequency. The approach and relationships developed here can be used as a basis for improving forest mortality models and monitoring systems.

Spatiotemporal remote sensing of ecosystem change and causation across Alaska

“…Our results (Figure a) confirm the idea that continued warming will increase aboveground biomass throughout portions of Alaska (Ackerman, Griffin, Hobbie, & Finlay, ), which may partly help curb carbon emissions from thawing permafrost (Abbott et al., ). In contrast, our results (Figure c) also suggest that plant productivity may decline in warming regions due to drought stress which would predispose woody vegetation to the risk of disease, fire, and mortality (Rogers et al., ; Trugman, Medvigy, Anderegg, & Pacala, ). Further research is needed to better understand climates impact on short and long term shifts in vegetation phenology, as well as to identify and quantify higher order interactions among environmental factors (e.g., climate, soil moisture) that influence vegetation productivity at a multitude of scales.…”

Testing early warning metrics for drought‐induced tree physiological stress and mortality

“…In particular, the combination of the NPV during the drought and the change in NPV between pre‐drought and drought were valuable early warning metrics of subsequent dieback and mortality. NDVI‐based early warning metrics have been tested rigorously in North American boreal forests and found to capture both episodic pulses of mortality and more gradual “spiral of decline” in these forests (Rogers et al, ). This underscores that there may be multiple satellite‐based optical remote sensing metrics that capture incipient mortality with different advantages and disadvantages.…”

“…NDVI-based early warning metrics have been tested rigorously in North American boreal forests and found to capture both episodic pulses of mortality and more gradual "spiral of decline" in these forests (Rogers et al, 2018). This underscores that there may be multiple satellite-based optical remote sensing metrics that capture incipient mortality with different advantages and disadvantages.…”

Contrasting resistance and resilience to extreme drought and late spring frost in five major European tree species

“…These two metrics can be used as early warning signals of an impending critical transition (Dakos, Carpenter, Nes, & Scheffer, 2015). For instance, a decrease in the ability of a tree to recover after a severe drought may be indicative of declining growth and vigor and thus a higher mortality risk (Rogers et al, 2018).…”