Regulation of Climate

Highlights

  • Microclimate including temperature extremes can be strongly buffered by forests. Forests have cooler below canopy maximum temperatures, warmer minimum temperatures and lower daily, seasonal and interannual variability than clearcuts or other open areas. They are also more humid than clearcuts.
  • As temperatures warm under climate change, organisms may be able to buffer themselves from stressful environmental conditions by selecting vegetation conditions such as old growth forests. The temperature difference between forest understories and open areas is of greater magnitude than the warming of land temperatures over the past century.

Microclimate

Microclimatic parameters including temperature, moisture and solar radiation are important because they affect the distribution of organisms by driving biological processes including decomposition and photosynthesis (Chen et al., 1993). Across the world, old forests influence these parameters which have the potential to reduce the severity of climate change impacts on forest biodiversity and functioning (De Frenne et al., 2019).
Forest structure can have substantial influences on microclimate, for example, structure associated with old forests (e.g., high biomass, vertical canopy layers) results in cooler spring and summer temperatures in relation to mature plantations with a simplified structure (Suggitt et al., 2011; Frey et al., 2016). Temperature extremes can be strongly buffered by forests. Forests have cooler below canopy maximum temperatures, warmer minimum temperatures and lower daily, seasonal and interannual variability than clearcuts or other open areas (Chen et al., 1993; De Frenne et al., 2021). During clear and warm days, much of the incoming short-wave solar radiation is absorbed and reflected by forest canopies, which together with increased evapotranspirative cooling, leads to a cooling of the understorey maximum temperature by a global mean of 4.1°C compared to open conditions (De Frenne et al., 2019). Minimum temperatures of forest understories are on average 1°C warmer than in the open, largely due to nighttime shielding of the outgoing long-wave radiation by the canopy. Soil temperatures are also typically cooler in the forest than the open. Additionally, relative humidity tends to be higher and vapour pressure deficit less in old forests than in clearcuts (Chen et al., 1993). Microclimate differences between old forests and clearcuts tend to be greater on clear days than rainy or dry cloudy days (Chen et al., 1993).

Cryptogams on trees, logs and the ground collect water from fog and rain, slowing the rate at which water is lost from the forest (U.S. Geological Survey and National Park Service, u.d.). Water stored in bryophytes and lichens evaporates slowly, helping to maintain a humid environment in forests long after rainfall events, allowing other plants to remain hydrated and keep photosynthesizing during drier weather.

Water dropplet on moss (Phooty by Dru! / Flickr)

Regional Climate

Forests impact not just microclimate but also regional climate. The release of water vapour into the atmosphere by trees through transpiration can affect regional precipitation patterns (Bonan, 2008; Spracklen et al., 2012). For example, in the tropics, air passing over extensive areas of forests produces at least twice as much rain as air that passes over short or no vegetation (Spracklen et al., 2012).

Climate Change

As temperatures warm under climate change, organisms may be able to buffer themselves from stressful environmental conditions by selecting particular habitats. For example, old growth forests can buffer populations of climate-sensitive bird species against the negative effects of warming (Betts et al., 2018). The temperature difference between forest understories and open areas is magnified as temperatures become more extreme and notably is of greater magnitude than the warming of land temperatures over the past century (De Frenne et al., 2019).