Increasing atmospheric humidity and CO ₂ concentration alleviate forest mortality risk

Abstract

Climate-induced forest mortality is being increasingly observed throughout the globe. Alarmingly, it is expected to exacerbate under climate change due to shifting precipitation patterns and rising air temperature. However, the impact of concomitant changes in atmospheric humidity and CO 2 concentration through their influence on stomatal kinetics remains a subject of debate and inquiry. By using a dynamic soil-plant-atmosphere model, mortality risks associated with hydraulic failure and stomatal closure for 13 temperate and tropical forest biomes across the globe are analyzed. The mortality risk is evaluated in response to both individual and combined changes in precipitation amounts and their seasonal distribution, mean air temperature, specific humidity, and atmospheric CO 2 concentration. Model results show that the risk is predicted to significantly increase due to changes in precipitation and air temperature regime for the period 2050-2069. However, this increase may largely get alleviated by concurrent increases in atmospheric specific humidity and CO 2 concentration. The increase in mortality risk is expected to be higher for needleleaf forests than for broadleaf forests, as a result of disparity in hydraulic traits. These findings will facilitate decisions about intervention and management of different forest types under changing climate.forest mortality | drought | climate change | hydraulic failure | stomatal closure F orest mortality can lead to irreversible change in vegetation cover, thereby affecting many processes pertinent to water, carbon, and nutrient budgets (1). Multiple studies (2-10) have noted close association between forest mortality and water and heat stress, owing to shifting precipitation patterns and rising air temperature. However, the influence of concurrent changes in specific humidity (SH) and CO2 concentration, which affect plant response to stress by altering stomatal kinetics (11), have not received similar attention. Although elevated CO2 concentration is expected to promote future forest productivity (12), the extent to which it affects forest mortality in the context of water and heat stress remains a subject of inquiry. Short-term records (3, 4) and long-term manipulative field studies in forests such as the Free Air CO2 Enrichment experiments (13-15) have tried to fill the knowledge gap; however, they do not cover the entire manifold of projected climate conditions. The goals of this study are to evaluate the individual and combined influence of projected changes in precipitation, temperature, SH, and CO2 concentration on forest mortality risk and to investigate whether the response of mortality risk differs among plant functional types (PFTs).Tree mortality may occur through several mechanisms, including hydraulic failure, carbon starvation, phloem transport limitation, and biotic attack (16,17). Hydraulic failure is characterized as the malfunction of xylem water transport associated with cavitation, which is induced by low xylem water potential under limited soil wate...

Tree carbon allocation explains forest drought‐kill and recovery patterns

“…), and other modelling studies that project a decrease in drought‐driven mortality and enhance productivity with climate change (Lloyd & Farquhar ; Liu et al . ). However, caution should be used when interpreting these predictions, because they omit effects of climate variability and ecophysiological trait adaptation (Drake et al .…”

Research frontiers for improving our understanding of drought‐induced tree and forest mortality

“…Dai, ), has been causing elevated levels of both chronic and acute stress often leading to tree mortality across large forested regions of the globe (Breshears et al ., ; Eamus et al ., ; Ruehr et al ., ). By contrast, recent studies suggest that increasing specific humidity and elevated atmospheric CO 2 concentration may partially offset mortality risk from drought and elevated temperature (Liu et al ., ). Nevertheless, periods of drought and heat stress often interact with other forest disturbances like fire and windthrow (Brando et al ., ) and can significantly amplify the incidence and severity of biological disturbances such as outbreaks of damaging insects and diseases (Williams et al ., ).…”

Multi‐model analysis of climate impacts on plant photosynthesis in China during 2000–2015

“…CO 2 fertilization not only increases plant photosynthetic rates but also alleviates the risk of drought by decreasing stomatal conductance and increasing water use efficiency (Jarvis et al, ; Schimel et al, ). Increasing CO 2 concentration and atmospheric humidity also alleviate forest mortality risk (Liu et al, ). Thus, it is our assessment that CO 2 fertilization caused the weak increase in GPP by offsetting the negative impacts of climate in southern China over 2000–2015.…”