forests and drought

Forest and Drought: Hanging by a Thread

Forests hold the climate together. They are also at extreme risk due to global warming, drought, and other ecological stresses created by industrial civilization. New research shows that forests may be “hanging by a thread.” This excerpt from a recent peer-reviewed article in Science magazine details some of the threats to forests. Despite the academic language, it paints a frightening picture of the near future.

The article can be read in its entirety here.

By Timothy J. Brodribb, Jennifer Powers, Hervé Cochard and Brendan Choat.

The Living Foundations

Trees are the living foundations on which most terrestrial biodiversity is built. Central to the success of trees are their woody bodies, which connect their elevated photosynthetic canopies with the essential belowground activities of water and nutrient acquisition. The slow construction of these carbon-dense, woody skeletons leads to a slow generation time, leaving trees and forests highly susceptible to rapid changes in climate.

Other long-lived, sessile organisms such as corals appear to be poorly equipped to survive rapid changes, which raises questions about the vulnerability of contemporary forests to future climate change. The emerging view that, similar to corals, tree species have rather inflexible damage thresholds, particularly in terms of water stress, is especially concerning. This Review examines recent progress in our understanding of how the future looks for forests growing in a hotter and drier atmosphere.

Temperature and Atmospheric CO2

No tree species can survive acute desiccation. Despite this unambiguous constraint, predicting the death of trees during drought is complicated by the process of evolution, whereby the fitness of tree species may benefit equally from traits that either increase growth or enhance drought resilience. Complexity arises because improving either of these two beneficial states often requires the same key traits to move in opposite directions, which leads to important trade-offs in adaptation to water availability. This conflict promotes strategic diversity in different species’ adaptations to water availability, even within ecosystems.

Understanding how the diversity of tree species will be affected by future droughts requires a detailed knowledge of how the functions of different species interact with their environment. Temperature and atmospheric CO2 concentration are fundamental elements that affect the water relations of all tree species, and the rapid rise in both of these potent environmental drivers has the potential to markedly change the way trees behave during drought. The future of many forest systems will be dictated by how these atmospheric changes interact with tree function.

Rising temperature and drought

Ultimately, the impact of elevated CO2 on forest trees is likely to come down to the intensity of the CO2-associated temperature rise and its effect on trees’ water use. This is because the distributions of tree species, in terms of water availability, broadly reflect their intrinsic tolerance of water stress. In other words, species from rainforests to arid woodlands face similar exposure to stress or damage during periods of drought.

Hence, any increase in the rate of soil drying caused by elevated temperatures is likely to lead to increasing damage to standing forests during drought. Improved tree WUE could ameliorate the temperature effect, but this argument remains highly debatable because most reports of improvements in tree WUE with rising atmospheric CO2 refer to intrinsic WUE, a value that converts to real plant water use only with a knowledge of leaf temperature and atmospheric humidity.

Thus, rising atmospheric temperature and the associated increase in evaporative demand is likely to reverse the improvements in tree WUE that are proposed to result from higher CO2. Recent evidence suggests that this is the case, with observations of reduced global tree growth and vegetation health associated with enhanced evaporative gradients and warming temperatures.

Predicting Tree Mortality

Tree mortality is most commonly observed when drought and high temperature are combined, likely owing to the compounding effects of the increased evaporative gradient and the increased porosity of leaves at high temperature. The inevitable rise in the intensity and/or frequency of such events as global temperatures climb has already been associated with an increase in tree mortality globally , especially in larger trees which raises a grave concern about the capacity of existing forests to persist into the future. Establishing the magnitude of this threat is an important challenge that requires a fundamental understanding of how water deficit leads to tree mortality.

Much research has focused on the possible mechanisms behind tree death during drought. Possible mechanisms primarily include vascular damage, carbon starvation, and enhanced herbivory . These studies reveal the complex nature of tree death, where the moment of death is difficult to pinpoint or even define. Although it remains difficult to connect cause and effect at the point where drought injury becomes lethal, strong and consistent correlational data from trees suffering mortality or growth inhibition across the globe point unequivocally to the plant water transport system as a fundamental axis dictating the long-term survival of trees .

Forests on a Thread

The massive woody structure of trees provides mechanical support for their photosynthetic crowns; however, the matrix of microscopic threads of water that is housed within the porous woody cells of the xylem is even more fundamental to tree survival. These liquid threads provide a highly efficient mechanism to transport large quantities of water over long distances under tension, from the roots to the leaves. Relying on this passive pathway to replace the water transpired by leaves has the major drawback that the internal water column in trees becomes increasingly unstable during times of water stress, as the tension required to draw water from the soil increases.

The water transport system in plants lies at center of interactions between rainfall, soil water, carbon uptake, and canopy dehydration, which makes xylem hydraulics an obvious focus for understanding and predicting the thresholds between tree death or survival during exposure to drought and heat stress. Xylem vulnerability to cavitation varies markedly among species, not only indicating sensitivity to water deficit but also enabling the quantification of functional impairment if trees are not immediately killed by drought.

The characteristics of tree species that are classically associated with adaptation to water availability—such as rooting depth, water storage, stomatal behavior, root and canopy area, and leaf phenology—can be predictably integrated to determine how plant water content will respond to environmental conditions. The combination of environmental conditions with biological attributes results in a highly tractable framework for understanding the dynamics of mortality or survival during slow dehydration.

Modeling forest mortality in the future

Modeling provides the most credible view of how forests may cope with different intensities of future global warming, with most models suggesting large-scale mortality, range contraction, and productivity loss through this century under the current warming trajectories. Greater precision as to the nature and pace of forest change is urgently needed, requiring dedicated work on key knowledge gaps that limit model precision accuracy. These gaps are apparent in even the basic physiological processes of trees, such as stomatal behavior, tree water acquisition, and interactions between water and carbon stores in trees.

Critical components such as the dynamic connection between trees and the soil are highly simplified inmodels owing to a lack of knowledge about water transfer and storage in the roots under conditions of water stress. The triggering of mortality is also highly oversimplified because the negative feedbacks likely to operate during acute tree stress are difficult to capture in a model. Avoiding this complexity, a commonly used proxy for lethal water stress is the point of 50% xylem cavitation in stems.

Although this threshold is not strictly correct (because trees can survive with a 50% impairment of water transport capacity), it does provide a readily measurable indication of rapid vascular decline incipient to complete failure of the vascular connection between roots and leaves. More-precise understanding of the post-drought transition to recovery or tree death is needed to accurately represent the legacy effects of drought in large-scale models.

Additional Disturbances

Predicting or modeling the impacts of drought on forest communities is also complicated by interactions between changes in climate and interactions with other disturbance agents, such as fire, insects and pathogens, or logging . The catastrophic wildfires that have affected Australia in 2019 and 2020, after years of extreme drought, is just one such example of drought-fire interactions. Such interactions are also affecting forests in North America, Amazonia , and elsewhere .

Increases in vapor-pressure deficit and temperature during drought dry out fuel, thereby increasing fire activity and the area that is burned. Drought-fire interactions may also cause tipping points and shifts among vegetation types in areas such as the southwestern Amazon. There, tree mortality is elevated during intense fires experienced in drought years , resulting in altered microclimatic conditions and grass invasion into the understories, which further increases flammability and fire risk.

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