Forests across the country have been impacted by increasingly prevalent drought conditions in the last two decades, according to a new study by scientists from 14 research institutions.

Drought has the potential to impact the distribution of species, biodiversity, and the goods and services that forests provide. These and other responses have already been observed in the West, a region which has experienced its fair share of drought and resulting forest diebacks (when a community of trees has above average mortality rates), changes in the composition and structure of forests, and wildfires. Droughts are also now affecting forests in the eastern U.S., in particular in the South and Upper Midwest—regions where there is comparatively less information on how drought affects forests, particularly at the larger ‘stand’ (forest community) scale.

Understanding how drought has already affected forests will help managers anticipate what the broader scale impacts of drought will look like in the future as the climate continues to change. To address this need, researchers - including Southwest Climate Science Center director Stephen Jackson - produced a synthesis of the current state of knowledge of the effects of drought on forests across the continental U.S. The results were recently published in the journal Global Change Biology.

Researchers synthesized studies on the effects of drought on tree growth and mortality, tree recruitment, range shifts, and community composition. They also examined the role of forest management practices in mitigating and exacerbating the effects of drought, and summarized various modeling approaches used to estimate forest change.

Key findings include:

1. In the West, stand- and regional-level consequences of drought are well-documented. Extensive drought has coincided with reduced tree growth and an increased risk of mortality. In the East, there is very little information available at the stand-scale, though numerous studies have looked at individual tree response.
2. Human fire suppression in the West has resulted in higher density stands comprised of small-diameter trees. High density forests face increased moisture stress during drought, and fires that occur in these forests have more fuel and are more severe than those that occur under low-density forest conditions.
3. In the eastern U.S., where there are large areas of low relief, even a modest change in climate can result in large shifts in the location of suitable habitat. Meanwhile, in the more mountainous West, moisture and temperature gradients are more compact and migration has the potential to be more effective.
4. Models suggest that suitable habitats of many tree species are shifting faster than populations are adjusting by adaptation or migration.

Interestingly, the researchers note that forest managers already engage in several practices that can be harnessed to mitigate the effects of drought. For example, thinning can be used to selectively remove moisture-demanding species, allowing drought tolerant species to take their place and improving moisture access to remaining trees. Artificial regeneration (i.e. plantings) can also be implemented during drought to help forests recover.

Anticipating the future of forests is a challenging task that relies heavily on observational data, which is difficult to extrapolate to larger scales and is unavailable in some parts of the country. Furthermore, other variables that affect forests are changing simultaneously, such as rising CO₂ levels, nitrogen deposition, and invasive species, making it difficult to attribute observed changes in forests to drought.

The results of this study show that we have some understanding of how drought affects the individual trees that make up a forest. However, we do not yet have sufficient information to predict the future of forest ecosystems in the face of a changing climate. The authors emphasize that future research should prioritize identifying how drought effects forests at the larger community scale, so that managers can take appropriate action to protect forests.