While the effects of climate change on forests and rangelands are still unclear, they will likely increase in severity and duration over the next century. Warming conditions and changes in precipitation patterns will increase the frequency and severity of droughts. In addition, future droughts are likely to occur with higher temperatures.
The indirect effects of drought on forests and rangelands are widespread and devastating. They include insect and pathogen outbreaks, weakened trees, and increased wildfire risks. Researchers have found a nonlinear relationship between drought intensity and bark beetle outbreaks. Intense droughts are associated with higher outbreaks of bark beetles, which are the most critical biotic agents of tree mortality. Drought causes host trees to become weaker and more vulnerable to diseases, and warming facilitates the spread of the bark beetle range.
Some of these impacts may be positive, including increased productivity in high-elevation forests. However, the effects of higher temperatures are more mixed. Higher temperatures can increase the rate of soil evaporation, reducing plant growth and productivity. A higher CO2 level can boost pollen production in certain trees. However, high temperatures may also decrease tree establishment and increase mortality in seedlings and young trees.
The effects of hotter drought are more diverse in different habitats. For example, tree mortality in semiarid woodlands is associated with shifting to a more alternate state. Conversely, grasses are more likely to recover if they survive the drought, and they may also become more susceptible to post-drought flooding. Increasing temperatures and precipitation levels are expected to worsen these impacts.
Indirect impacts of drought include changes in water quality. Reduced water yields from forests and rangelands can affect domestic water supplies. In some cases, water markets implement quantity controls to limit the amount of water available to farmers and ranchers. Acute impacts on forest water quality include reduced water availability, decreased forage yield, and diminished vegetation cover. Some drought-related disturbance also reduces the economic benefits of skiing.
Climate change is causing a shift in tree distributions. While the climate in these areas is comparatively mild, tree ranges in the Pacific Northwest may be shrinking, resulting in a shortage of climatically suitable sites. Some alpine vegetation may be out of space on mountain ranges. Some plant species’ optimum elevations shift up and down with temperature and water availability changes. These divergent climate and moisture conditions may make trees unstable.
Drought will likely increase in severity over the next century. Adaptation to lack is a crucial process by which forest and rangeland ecosystems respond. However, these adaptations may not be sufficient to prevent a significant change in ecosystems. Drought also intensifies the effects of pathogens, diseases, and pests.
Ecosystem processes affected by drought stress
Human activities affect ecosystems in several ways, including increased water demand and drought stress. Historically, drought conditions were influenced by sea surface temperatures in the major oceanic basins, coupled with internal atmospheric variability and land-atmosphere feedbacks. But now, anthropogenic climate change is increasing the frequency and intensity of droughts and the impacts of these conditions on ecosystems. Droughts also affect how forests and rangelands function, including driving tree mortality globally. Ecosystem transformations, such as shifting from forest to shrubland, also result from drought stress, and these changes can have negative consequences for human communities.
During periods of drought, shrub and woodland ecosystems develop at lower productivity levels than predicted and are more susceptible to fire. In addition, more severe droughts lead to higher levels of woody mortality and a greater abundance of fuels for fires. These changes, in turn, lead to a degrading effect on rangelands. The consequences of drought are often not as straightforward as climate change, but they affect ecosystem processes and functions.
These changes in forest and rangeland ecosystems also affect the biodiversity and productivity of native species. Compared to other regions, the Northeastern U.S. has been experiencing fewer drought-induced mortality events, which may be attributed to the high buffering capacity of carbon stores. During droughts, tree growth and canopy photosynthesis are negatively affected, which may decrease the carbon sink strength of forests.
Tree species in the Northeastern U.S. have adapted to drought through isohydric regulation of leaf water potential, and they also exhibit little change in leaf water potential during drought. Such a decline is expected for species in the Northeastern U.S. and Canada. A similar pattern can be observed for anisohydric species in the Southwest U.S., but the latter has a smaller range of adaptations.
Studies have indicated that drought and other stressors may have an additive effect on tree mortality. In particular, studies of drought and nitrogen have found that the two stressors together have greater effects on tree mortality.
Impacts of drought on tree mortality
Scientists at the U.S. Forest Service authored a report on the effects of drought on tree mortality. The results were particularly severe in California, where 58 million trees died due to a lack of water. In addition, warming has facilitated the growth of bark beetles, one of the essential biotic agents responsible for tree mortality in the Western U.S. Drought has weakened host trees, which provide breeding grounds for beetles. In addition, warming has increased their range. In the Forest Service report, scientists recommend mitigation measures, such as managing reliable water flow and modifying structural components of vegetation.
The authors used information from 131 forest and rangeland sites in North America to assess the effects of drought on tree mortality. Overall, they found that drought-related mortality was much higher than background rates. However, they noted that other factors could contribute to the observed mortality rates. Contributors to the study assessed mortality rates of dominant species and subsequent replacements. Despite the small scale of the research, the authors found that drought-related mortality was associated with significant changes in community structure and composition.
In addition to affecting forest health, drought also affects human well-being. Increased drought severity can lead to water availability and quality changes, decreasing recreational opportunities, decreasing rangeland suitability for livestock grazing, and increasing the risk of wildfire. As climate change worsens, the U.S. Forest Service will continue to assess how drought may affect tree mortality. Tropical rainforests are among the most vulnerable species to drought.