Climate change & trees


Beech Fagus wood © Ita McCobb

Effects of climate change

Climate influences the structure and function of forest and woodland ecosystems and plays an essential role in a forest’s health.

Climate change, particularly increased temperatures and levels of atmospheric CO2 as well as changes in precipitation and in the frequency and severity of climatic events, is having notable impacts on the world’s woodland, forests and forest sector.

Climate change may worsen many of the threats to forests, woodland and trees, such as disease outbreaks, pest infestation, fires, drought and human development. This could result in widespread tree die-off.

Trees are one of our best defences against climate change as they use energy from the Sun to convert CO2, a greenhouse gas, into sugars to provide food for growth, releasing oxygen and storing carbon in their trunks, root structures and soil.

Trees sequester (lock away) carbon in a more permanent way than other plant species due to their size and relatively long lifespan; however, when they die or are cleared or degraded, most of the stored carbon is released into the atmosphere thereby contributing to global carbon emissions. The more trees that die, the less carbon is stored from the atmosphere, increasing temperatures further.

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Threats to trees – Drought

Mimosa Acacias dealbata © Jennifer Hope-Morley

Although many trees are resilient to a certain degree of drought any increases in temperature could make droughts more damaging to them.

Causes of more frequent droughts include higher temperatures, earlier snow melt affecting the seasonal availability of water and increased variability in precipitation leading to greater fluctuations in the water table, limiting root depth and reducing tree stability on exposed sites.

During a strong drought many trees shed leaves, reducing transpiration and photosynthesis, which affects growth. If this continues the tree will eventually die.

As drought weakens trees they become more susceptible to insect attack and pathogens, which increases the risk of wildfires.

Changing temperatures could make it more difficult for some trees, particularly seedlings, young trees and drought-sensitive species on shallow free-draining soil to survive, as they would be left with insufficient water to endure a period of drought.

The rise in CO2 levels would be expected to increase the growth rate of trees if sufficient water and nutrients are available, but this might be contradicted by increased levels of ozone pollution damaging foliage and reducing trees’ ability to withstand a drought.

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Threats to trees – Insect infestations, pathogens & competing non-native plants

Infested tree trunk © Ita McCobb

Insect infestations often defoliate, weaken and even kill trees. Climate change increases the frequency and severity of insect infestations and pathogens and the introduction of non-native plants that compete.

Warmer temperatures overall contribute to the survival of diseases and pests during warmer winters, leading in some cases to multi-voltinism (frequency and number of broods/generations produced in a season), meaning some species will develop faster and expand their range.

The increased number of generations also enables pests to evolve and adapt more effectively to climate change than their tree hosts. While native pests may become more damaging and problematic, the impact and profile of native species, not currently seen as pests, could also increase.

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Threats to trees – Wildfires

Climate change is projected to increase the extent, intensity and frequency of wildfires. Warmer Spring and Summer temperatures, plus lack of water, dry out woody materials in woodland increasing wildfire risk.

The fires also play a part in climate change since the loss of trees can cause a large rapid release of CO2 into the atmosphere.

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Waldo canyon fire © NASA

Threats to trees – Storms

Global warming is accelerating the hydrologic cycle by evaporating more water, transporting that water to higher latitudes and producing more intense and more frequent storms.

Severe storms along with an increase in precipitation increases instability, causing more storm falls among trees.

Windstorms and lightning strikes damage trees allowing the entry of pathogens and pests as well as causing chemical breakdown in normal physical tree function.

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Threats to trees – Change in phenology

Land surface temperature, Dec 2016 © NASA

Since phenology (cyclic, seasonal and natural phenomena) is in many cases temperature dependent it is influenced by climate change.

Warmer Spring temperatures bring forward the leafing of tree species. Although this has the positive effect of extending the growing season for certain tree species, it also increases the risk of a negative impact on the wider woodland ecosystem.

Earlier leafing does not seem to correspond to the date of the last Spring frosts, thus leading to the risk of frost damage to young leaves.

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References: University of Michigan; UN-REDD programme; United Nations Environment Programme (UNEP); National Geographic; World Wildlife Fund;; Rainforest Connection (RFCx); The Forestry Commission UK; Forestry, An International Journal of Forest Research; US Department of Agriculture (USDA) Forest Service; The Woodland Trust;; US Environmental Protection Agency; Cornell University; The Forestry Commission UK; Cornell University; Food & Agriculture Organization of the UN; Institute of Chartered Foresters; Botanical Gardens Conservation International; International Plant Sentinel Network (IPSN); Zac Goldsmith in collaboration with The Countryside Restoration Trust, The Threat to England’s Trees; United States Department of Agriculture (USDA) Forest Service.