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Abstract Climate change is triggering regional-scale alterations in vegetation including land cover change such as forest die-off. At sufficient magnitudes, land cover change from forest die-off in one region can change not only local climate but also vegetation including agriculture elsewhere via changes in larger scale climate patterns, termed an ‘ecoclimate teleconnection’. Ecoclimate teleconnections can therefore have impacts on vegetative growth in distant regions, but the degrees to which the impact decays with distance or directionally diffuses relative to the initial perturbation are general properties that have not been evaluated. We used the Community Earth system model to study this, examining the implications of tree die-off in 14 major US forested regions. For each case we evaluated the ecological impact across North America as a function of distance and direction from the location of regional tree die-off. We found that the effects on gross primary productivity (GPP) generally decayed linearly with distance, with notable exceptions. Distance from the region of tree die-off alone explained up to ∼30% of the variance in many regions. We also found that the GPP impact was not uniform across directions and that including an additional term to account for direction to regional land cover change from tree die-off was statistically significant for nearly all regions and explained up to ∼40% of the variance in many regions, comparable in magnitude to the influence of El Nino on GPP in the Western US. Our results provide novel insights into the generality of distance decay and directional diffusion of ecoclimate teleconnections, and suggest that it may be hard to identify expected impacts of tree die-off without case-specific simulations. Such patterns of distance decay, directional diffusion, and their exceptions are relevant for cross-regional policy that links forests and other agriculture (e.g. US Department of Agriculture). 

Abstract Earth’s forests face grave challenges in the Anthropocene, including hotter droughts increasingly associated with widespread forest die-off events. But despite the vital importance of forests to global ecosystem services, their fates in a warming world remain highly uncertain. Lacking is quantitative determination of commonality in climate anomalies associated with pulses of tree mortality—from published, field-documented mortality events—required for understanding the role of extreme climate events in overall global tree die-off patterns. Here we established a geo-referenced global database documenting climate-induced mortality events spanning all tree-supporting biomes and continents, from 154 peer-reviewed studies since 1970. Our analysis quantifies a global “hotter-drought fingerprint” from these tree-mortality sites—effectively a hotter and drier climate signal for tree mortality—across 675 locations encompassing 1,303 plots. Frequency of these observed mortality-year climate conditions strongly increases nonlinearly under projected warming. Our database also provides initial footing for further community-developed, quantitative, ground-based monitoring of global tree mortality. 

Abstract Estimates of the percentage of species “committed to extinction” by climate change range from 15% to 37%. The question is whether factors other than climate need to be included in models predicting species’ range change. We created demographic range models that include climate vs. climate‐plus‐competition, evaluating their influence on the geographic distribution ofPinus edulis, a pine endemic to the semiarid southwestern U.S. Analyses of data on 23,426 trees in 1941 forest inventory plots support the inclusion of competition in range models. However, climate and competition together only partially explain this species’ distribution. Instead, the evidence suggests that climate affects other range‐limiting processes, including landscape‐scale, spatial processes such as disturbances and antagonistic biotic interactions. Complex effects of climate on species distributions—through indirect effects, interactions, and feedbacks—are likely to cause sudden changes in abundance and distribution that are not predictable from a climate‐only perspective. 

Summary With climate change, heat waves are becoming increasingly frequent, intense and broader in spatial extent. However, while the lethal effects of heat waves on humans are well documented, the impacts on flora are less well understood, perhaps except for crops. We summarize recent findings related to heat wave impacts including: sublethal and lethal effects at leaf and plant scales, secondary ecosystem effects, and more complex impacts such as increased heat wave frequency across all seasons, and interactions with other disturbances. We propose generalizable practical trials to quantify the critical bounding conditions of vulnerability to heat waves. Collectively, plant vulnerabilities to heat waves appear to be underappreciated and understudied, particularly with respect to understanding heat wave driven plant die‐off and ecosystem tipping points. 

Mortality of tree species around the globe is increasingly driven by hotter drought and heat waves. Tree juveniles are at risk, as well as adults, and this will have a negative effect on forest dynamics and structure under climate change. Novel management options are urgently needed to reduce this mortality and positively affect forest dynamics and structure. Potential drought-ameliorating soil amendments such as nanochitosan – a biopolymer upcycled from byproducts of the seafood industry – may provide an additional set of useful tools for reducing juvenile mortality during hotter droughts. Nanochitosan promotes water and nutrient absorption in plants but has not been tested in the context of drought and heat stress. We evaluated factors affecting mortality risk and rate for drylandPinus edulisjuveniles (2–3 years old) in a growth chamber using a factorial experiment that included ambient and +4°C warmer base temperatures, with and without a 10 day +8°C heat wave, and with and without a nanochitosan soil amendment. The nanochitosan treatment reduced the relative risk of mortality, emphasizing a protective function of this soil amendment, reducing the relative risk of mortality by 37%. Importantly, the protective effects of nanochitosan soil amendment in delaying tree mortality under hotter drought and heat waves provides a new, potentially positive management treatment for tree juveniles trying to survive in the climate of the Anthropocene. 

Tree loss is increasing rapidly due to drought- and heat-related mortality and intensifying fire activity. Consequently, the fate of many forests depends on the ability of juvenile trees to withstand heightened climate and disturbance anomalies. Extreme climatic events, such as droughts and heatwaves, are increasing in frequency and severity, and trees in mountainous regions must contend with these landscape-level climate episodes. Recent research focuses on how mortality of individual tree species may be driven by drought and heatwaves, but how juvenile mortality under these conditions would vary among species spanning an elevational gradient—given concurrent variation in climate, ecohydrology, and physiology–remains unclear. We address this knowledge gap by implementing a growth chamber study, imposing extreme drought with and without a compounding heatwave, for juveniles of five species that span a forested life zones in the Southwestern United States. Overall, the length of a progressive drought required to trigger mortality differed by up to 20 weeks among species. Inclusion of a heatwave hastened mean time to mortality for all species by about 1 week. Lower-elevation species that grow in warmer ambient conditions died earlier (Pinus ponderosain 10 weeks,Pinus edulisin 14 weeks) than did higher-elevation species from cooler ambient conditions (Picea engelmanniiandPseudotsuga menziesiiin 19 weeks, andPinus flexilisin 30 weeks). When exposed to a heatwave in conjunction with drought, mortality advanced significantly only for species from cooler ambient conditions (Pinus flexilis: 2.7 weeks earlier;Pseudotsuga menziesii: 2.0 weeks earlier). Cooler ambient temperatures may have buffered against moisture loss during drought, resulting in longer survival of higher-elevation species despite expected drought tolerance of lower-elevation species due to tree physiology. Our study suggests that droughts will play a leading role in juvenile tree mortality and will most directly impact species at warmer climate thresholds, with heatwaves in tandem with drought potentially exacerbating mortality especially of high elevation species. These responses are relevant for assessing the potential success of both natural and managed reforestation, as differential juvenile survival following episodic extreme events will determine future landscape-scale vegetation trajectories under changing climate. 

Since the 1980s, the U.S. Forest Service (USFS) has transformed from an agency predominantly focused on timber production to one focused on recreation and ecosystem management. This shift is particularly remarkable because it occurred without major substantive national forest policy changes. During this period, many national forests changed their forest planning processes in ways that provided greater opportunity for public input into forest plans, and in 2012 the USFS issued new planning rules that institutionalized these practices. In this study, we ask: how has the planning process changed over time, and how have these changes shaped forest plan outcomes? To answer these questions, we conduct a comparative case study of two national forests—the Lake Tahoe Basin Management Unit and the Inyo National Forest—that produced forest plans in the 1980s and again in the 2010s. We use the Network of Action Situations (NAS) approach to compare planning processes over time and across forests. We find that in addition to the changes mandated by the 2012 rules, both forests developed a series of forums to engage the public in plan development and review, and that increased stakeholder engagement has helped shape forest priorities. These findings suggest that greater involvement by regional stakeholders could pressure the USFS to adopt more regional approaches for addressing challenges like climate change and wildfire risk.