Dead vegetation in drought-stricken area, Senegal © FAO / Ch. Errath

Droughts are caused by a deficiency of precipitation over time and as such can develop slowly, sometimes over years. Often associated with the arid regions of Africa, particularly the Sahel, in recent years, droughts have also struck India and parts of China, the Near East, the Mediterranean, Australia, parts of North America, South America and Europe. Increases in the frequency, duration, and/or severity of drought and heat stress associated with climate change could fundamentally alter the composition, structure and biogeography of forests in many regions. Of particular concern are increases in tree mortality associated with climate-induced physiological stress and interactions with other disturbances such as pest outbreaks and fire.

Dozens of episodes of increased mortality due to drought and heat have been identified throughout a variety of forest types, from monsoonal savannas with mean precipitation <400 mm/year, to subalpine conifer forests with a Mediterranean climate, to tropical rainforests with mean precipitation >3000 mm/year, illustrating that drought-induced mortality is not restricted to forests normally considered water-limited.

A primary response of forests to future drought will be a reduction in net primary production (NPP) and stand water use. Forest drought usually results in reduced shoot growth, reduced nitrogen and water foliar concentrations, and increased allocation to secondary defensive compounds, such as tannins. Drought-induced reductions in decomposition rates may cause a buildup of organic material on the forest floor which could influence nutrient cycling and increase susceptibility to fire.

Young plants such as seedlings and saplings are particularly susceptible to drought whereas large trees with a more developed rooting system and greater stores of nutrients and carbohydrates tend to be less sensitive, though they are affected by more severe conditions. Shallow-rooted trees and plants as well as species growing in shallow soils are more susceptible to water deficits.

Strategies for forest managers to adapt to future drought events might include thinning stands to reduce competition or selecting appropriate genotypes, such as those with improved drought resistance.