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Drought

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The projected warmer temperatures, drier summers, and increase in frequency of extreme drought and heatwave events will increase water stress in trees, with the largest risk of drought in southern England.

Drought can lead to reduced tree growth, crown dieback and tree mortality. Secondary impacts include increase risk of windthrow, wildfire, pest and disease outbreaks.

The risk of drought needs to be assessed and adaptation measures put in place to help manage these risks. Research and ongoing monitoring can help better understand the risk and impacts of drought on different tree species, provenances and sites.

Risk factors

Risk of drought is influenced by the following

  • Timingdry springs may affect the current year’s growth, late summer droughts may affect the next season’s growth more. Repeated droughts may have a cumulative effect, leading to growth reductions several years later.

  • Location – summers are projected to become drier across most of the UK, with the largest reductions in southern England, where temperatures will also be warmest.

  • Site soil – shallow, light-textured and freely draining soils hold less water than deep, heavier soils, and therefore trees growing on shallow, lighter soils are more prone to water stress and drought impacts. 

  • Tree species – drought sensitivity varies between tree species. For example, species such as beech, birch and sycamore are more sensitive than hornbeam or native oak species. Species such as Douglas fir and western hemlock are more tolerant of drought and show lower susceptibility to stem cracking than species such as Sitka spruce. Differences in drought tolerance can also exist between different local populations (provenances). 

  • Root depth – ground vegetation and shallower rooting shrubs and trees may be more affected than deeper rooted tree species. Newly planted trees will be particularly vulnerable to droughts, especially those on exposed open sites. Smaller, more fragmented woodlands are more likely to dry out than extensive areas. In addition, trees growing on soils that are deep, but poorly drained so that they are waterlogged in winter may have shallow rooting zones, and therefore may be at risk if upper soil layers dry out in summer, even if there is water in deeper layers. 

  • Age – New planting, older, and veteran trees are more likely to be affected by drought. Because the effects of drought differ with species and tree age, this may result in changes to stand species composition and age structure. 

  • Stand health – Forest recovery after a drought disturbance is linked to the condition before the disturbance. Forests are adaptable ecosystems, however constraints on natural processes can impact recovery. 

Drought impact

  • Reduced tree growth and productivity.
  • Increased vulnerability to pest and disease damage and mortality.
  • Increased risk of mortality if drought is prolonged, severe or repeated.
  • Reduced carbon sequestration due to reduced growth and increased mortality.
  • Stem crack can appear in the years following a drought, due to impacts on the structural properties of the tree, lowering the commercial value.
  • Increased risk of wildfire.
  • Increased risk of windthrow.
  • Where trees establish under natural conditions, their distribution is expected to shift north and to higher elevations as the climate warms.

Research

Drought risk research is underway at Forest Research, including through the PRAFOR project. 

Case study

Find out how drought risk influenced which adaptation measures to adopt and changes to operations, silvicultural systems and species choice.

Assessing tree species suitability with ESC

Forest Research’s ESC decision support tool can be used to indicate the suitability of a site for a range of tree species, now and into the future. ESC considers the tree species’ response to average climate conditions of temperature, moisture deficit (the accumulative difference between rainfall and evapotranspiration) and the site’s soil nutrient status and water-retention properties (soil moisture regime). ESC does not yet include an assessment of the impact of possible extreme weather events, such as periods of drought, on different species.