Continental-scale consequences of tree die-offs in North America: identifying where forest loss matters most

Abstract

Regional-scale tree die-off events driven by drought and warming and associated pests and pathogens have occurred recently on all forested continents and are projected to increase in frequency and extent with future warming. Within areas where tree mortality has occurred, ecological, hydrological and meteorological consequences are increasingly being documented. However, the potential for tree die-off to impact vegetation processes and related carbon dynamics in areas remote to where die-off occurs has rarely been systematically evaluated, particularly for multiple distinct regions within a given continent. Such remote impacts can occur when climate effects of local vegetation change are propagated by atmospheric circulation-the phenomena of 'ecoclimate teleconnections'. We simulated tree die-off events in the 13 most densely forested US regions (selected from the 20 US National Ecological Observatory Network [NEON] domains) and found that tree die-off even for smaller regions has potential to affect climate and hence Gross Primary Productivity (GPP) in disparate regions (NEON domains), either positively or negatively. Some regions exhibited strong teleconnections to several others, and some regions were relatively sensitive to tree loss regardless of what other region the tree loss occurred in. For the US as a whole, loss of trees in the Pacific Southwest-an area undergoing rapid tree die-off-had the largest negative impact on remote US GPP whereas loss of trees in the Mid-Atlantic had the largest positive impact. This research lays a foundation for hypotheses that identify how the effects of tree die-off (or other types of tree loss such as deforestation) can ricochet across regions by revealing hot-spots of forcing and response. Such modes of connectivity have direct applicability for improving models of climate change impacts and for developing more informed and coordinated carbon accounting across regions.

Forest Management Under Megadrought: Urgent Needs at Finer Scale and Higher Intensity

“…A first step in anticipatory management for hotter drought is to accurately identify in advance the forested ecosystems most at risk to drought (Millar and Stephenson, 2015). Identification of vulnerable forest stands can be aided by Earth System Models (Swann et al, 2018); forest health monitoring networks (Hartmann et al, 2018); nearterm ecological forecasting of extreme events (Dietze et al, 2018;Redmond et al, 2019); remote sensing (Mu et al, 2013), traits related to drought-induced mortality (Anderegg et al, 2016;Sperry et al, 2016;O'Brien et al, 2017); and ground surveys (Breshears et al, 2005;Redmond et al, 2019).…”

A Dirty Dozen Ways to Die: Metrics and Modifiers of Mortality Driven by Drought and Warming for a Tree Species

“…Tree mortality impacts ecosystem services provided by forests, including carbon storage that helps offset warming impacts of emissions (Bonan, 2008;Kurz et al, 2008;Breshears et al, 2011;Ma et al, 2013). Tree mortality events can cause ecosystem state changes (Cobb et al, 2017) and, if extensive enough, can influence not only local climate but also climate and associated vegetation elsewhere-termed ecoclimate teleconnections (Garcia et al, 2016;Stark et al, 2016;Swann et al, 2018).…”

Macrosystems as metacoupled human and natural systems

“…The choice of this domain was motivated by the recent loss of tree cover in the region (an estimated ~130 million dead trees as of early 2018) in Southern California. Forest removal causes local changes in transpiration rates, influencing atmospheric circulation, which results in distant climate impacts that markedly affect leaf area index (LAI) and gross primary production (GPP) elsewhere (graphical interpretation of the model proposed by Swann et al [2018]).…”