Climate change impacts on forests

Climate change, in particular increased temperatures and levels of atmospheric carbon dioxide as well as changes in precipitation and in the frequency and severity of extreme climatic events, is having notable impacts on the world's forests and the forest sector. Impacts on forest productivity and health, distribution, and disturbances have been observed with eventual impacts on the forest sector. Some examples include the following.

• Forest productivity and species diversity typically increase with increasing temperature, precipitation and nutrient availability, although species may differ in terms of their tolerance.
• Increased temperatures may relieve plant stress during colder periods but increase it during hotter periods. In some regions, higher temperatures have led to permafrost degradation and early spring melt resulting in diebacks and large ecosystem changes.
• Moisture availability in forests will be strongly influenced by changes in both temperature and precipitation. Warmer temperatures can lead to increased water losses from evaporation and evapotranspiration and reduced water use efficiency of plants resulting in severe moisture stress.
• Higher atmospheric carbon dioxide (CO₂) levels result in increased growth rates and water use efficiency of plants and trees, so long as other factors such as water and nutrients do not become limiting. Elevated CO₂ levels can also result in changes in plant structure such as increased leaf area and thicknessand larger diameter stems and branches.
• Concurrent increases in concentrations of ground-level ozone (O₃) may lower tree productivity and enhance susceptibility to pathogens while nitrous oxide (N₂O) may enhance growth in nitrogen-limited ecosystems such as boreal forests.
• Consistent responses of species and communities to climate change are typically associated with changes in their distribution, particularly at their latitudinal or altitudinal extremes. The distribution of forest plants and trees is expected to shift northwards or to higher altitudes in response to climate warming.
• Climate change is expected to impact the susceptibility of forests to disturbances; affect the frequency, intensity, duration, and timing of disturbances; and alter the disturbance dynamics of native forest insect pests and pathogens, as well as facilitating the establishment and spread of non-indigenous species.

Such impacts of climate change on trees and forests will inevitably have widespread impacts on the forest sector. Changes in the structure and functioning of natural ecosystems and planted forests and extreme events and disasters will have negative impacts on the productive function of forest ecosystems which in turn will affect local economies. Production patterns and trade in forestry commodities will be altered as species are grown more competitively in higher latitudes and altitudes. Markets may become saturated due to increased mortality of trees following pest infestations. Decreased forest ecosystem services, especially water cycle regulation, soil protection and conservation of biological diversity, as a result of climate change may imply increased social and environmental vulnerability.

Climate change impacts on forest pests

Changes in the patterns of disturbance by forest pests (insects, pathogens and other pests) are expected under a changing climate as a result of warmer temperatures, changes in precipitation, increased drought frequency and higher carbon dioxide concentrations. These changes will play a major role in shaping the world's forests and forest sector.

Insects and pathogens have been noted to respond to warming in all the expected ways, from changes in phenology and distribution to influencing community dynamics and composition. While some impacts of climate change may be beneficial in terms of protecting forest health (e.g. increase winter mortality of some insect pests due to thin snow cover; slower larval development and increased mortality during droughts), many impacts will be quite detrimental (e.g. accelerated insect development rate; range expansions of pests).

Climate change can affect forest pests and the damage they cause by:

• directly impacting their development, survival, reproduction, distribution and spread;
• altering host physiology and defences;
• impacting the relationships between pests, their environment and other species such as natural enemies, competitors and mutualists.