Timing – dry 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.