Relationships between individual‐tree mortality and water‐balance variables indicate positive trends in water stress‐induced tree mortality across North America

Abstract

Accounting for water stress-induced tree mortality in forest productivity models remains a challenge due to uncertainty in stress tolerance of tree populations. In this study, logistic regression models were developed to assess species-specific relationships between probability of mortality (P ) and drought, drawing on 8.1 million observations of change in vital status (m) of individual trees across North America. Drought was defined by standardized (relative) values of soil water content (W ) and reference evapotranspiration (ET ) at each field plot. The models additionally tested for interactions between the water-balance variables, aridity class of the site (AC), and estimated tree height (h). Considering drought improved model performance in 95 (80) per cent of the 64 tested species during calibration (cross-validation). On average, sensitivity to relative drought increased with site AC (i.e. aridity). Interaction between water-balance variables and estimated tree height indicated that drought sensitivity commonly decreased during early height development and increased during late height development, which may reflect expansion of the root system and decreasing whole-plant, leaf-specific hydraulic conductance, respectively. Across North America, predictions suggested that changes in the water balance caused mortality to increase from 1.1% yr in 1951 to 2.0% yr in 2014 (a net change of 0.9 ± 0.3% yr ). Interannual variation in mortality also increased, driven by increasingly severe droughts in 1988, 1998, 2006, 2007 and 2012. With strong confidence, this study indicates that water stress is a common cause of tree mortality. With weak-to-moderate confidence, this study strengthens previous claims attributing positive trends in mortality to increasing levels of water stress. This 'learn-as-we-go' approach - defined by sampling rare drought events as they continue to intensify - will help to constrain the hydraulic limits of dominant tree species and the viability of boreal and temperate forest biomes under continued climate change.

Low Tree-Growth Elasticity of Forest Biomass Indicated by an Individual-Based Model

“…Although periodic droughts, warming, and the increasing evaporative demand of the atmosphere are clearly implicated in many reports of increasing tree mortality [26,[66][67][68], considerable uncertainty exists in the magnitude of interactions between external hydrological conditions and competition and between external hydrological conditions, tree size, and age [26,32,[69][70][71][72][73]. If one assumes that global change factors have increased tree growth, then a proportion of recent evidence indicating increasing time trends in tree mortality may reflect negative feedback responses to growth enhancement.…”

“…For this study, we focused on calibrating and applying relatively simple equations for each species, herein called "Default 1" equations, with the intention of representing basic patterns of ontogeny, aging, and competition among trees. The Default 1 equations intentionally avoided additional complexity and, thus, provide a high-bias/low variance alternative to equations that attempt to represent additional system complexity [26,30].…”

Climate change-associated trends in biomass dynamics are consistent across soil drainage classes in western boreal forests of Canada

“…The increased tree mortality in tropical forests has been attributed to shortened tree longevity associated with greater climate variability and feedbacks of faster growth on mortality (Brienen et al 2015). In boreal and temperate forests, increases in tree mortality (van Mantgem et al 2009;Michaelian et al 2011;Luo and Chen 2013;Hember et al 2017) and decreases in aboveground biomass accumulation (Ma et al 2012;Chen and Luo 2015;Hogg et al 2017) have been hypothesized to be the result of warming-induced increased demand for evapotranspiration exceeding water availability (McDowell et al 2008).…”

“…Recent studies have begun to support this theory. For example, across North America, more arid sites have higher tree mortality during drought conditions than more mesic and wet sites (Hember et al 2017). Alternatively, tree species growing in drier sites have adapted to water deficit, and may be more resistant to droughts.…”

“…Not only is it topographically diverse, but it has also been shown to be strongly affected by climate change. Studies generally demonstrate a loss in net above ground biomass change (Ma et al 2012;Chen and Luo 2015;Chen et al 2016) and increases in mortality either due to declining water availability (Peng et al 2011;Hember et al 2017). The effect of climate change on tree growth in the region is more complex, with studies demonstrating: increases in young stands but no change, or even decreases, in old stands (Chen et al 2016); a slight increase early successional conifer forest types but a decrease in latesuccessional and mixed forest types (Chen and Luo 2015); and, strongly spatially variable trends dominated by heat and drought stress (Girardin et al 2016).…”