Trees and Climate Change
Predicting the effects of climate change on trees is difficult. The following is a best guess of possible effects based on current research and recent evidence. The consensus of opinion is that we will experience hotter, drier summers and milder, wetter winters.
Trees grew exceptionally well during the drought in 1976, where they were not limited by water and nutrient availability.
The double drought of 1975/76 did have serious impacts and clear patterns of drought-induced dieback are evident in the results of the Forest Condition Surveys undertaken since 1987, with peaks of severe defoliation following the droughts of 1989/90, 1995 and 2005.
There will also be less dramatic effects of climate change; we have seen an advance in the date of budburst of up to 2-3 weeks over the past 30-40 years. As a rule of thumb for broadleaf species, for each week's lengthening of the growing season, carbon uptake will increase by 10%.
Early flushing makes trees more susceptible to late spring frost damage, which affects both growth and timber quality. Although the climate is warming, the potential for late spring frosts has not diminished and counter-intuitively, frost damage could increase.
The rising concentration of carbon dioxide in the atmosphere, the principal driver of human-induced climate change, also has a direct effect on tree growth; rates of photosynthesis, and thus growth, will increase.
Milder winters may reduce the potential for the chilling requirements of seed germination, leading to less successful natural regeneration while increased soil mineralization may enhance nutrient supply.
While natural pests play a valuable role in the forest ecosystem, invasive or exotic pest species, without any naturally occurring predators or pathogens of their own, can cause widespread damage.
Warmer and wetter winters may lead to more active root pathogens and hence to increased root disease. Consequently, when the root systems come under stress in the hotter, drier summers, more trees may die.
In the southern half of the UK, changing rainfall patterns are likely to have a major impact on tree growth with the combination of more frequent winter flooding and summer droughts leading to widespread losses, particularly of hedgerow trees, street trees, young trees and newly planted trees.
Some species will be affected to greater extents than others which, in natural woodland ecosystems, will change the competitive advantage of one species over another and hence change the appearance of our woodlands. Small-leaved lime, once a major component in our woodland, may make a comeback. Predictions are that although current 'native' species will not disappear, their distribution will change significantly.
These changes will be almost imperceptible at first but, if fossil fuel emissions on a global level are not curbed, the impacts will become increasingly far-reaching.
It’s likely that many species of trees native to Southern France and Northern Spain will become more common - such as Walnut, Poplar and Sweet Chestnut.
Drought crack is likely to increase in England and to cause structural defects in timber. There is likely to be a greater risk of trees spontaneously dropping limbs, which could be bad news for tree owners, who are liable in law for damage resulting from trees or parts of trees failing.
Subsidence could also increase as wet winters swell the soil and dry summers shrink them.
Whatever we learn about how trees may respond to climate change, the effect of climatic warming on the prevalence of pest and disease outbreaks could have a greater impact on Britain’s woodlands than any of the direct effects of climate change.
Impacts on tree growth and function by Mark Broadmeadow
Trees in a Changing Climate from the RHS