Assessing the resilience of UK forests to drought

Widespread impacts on forest productivity from extreme drought events have now been documented on every forested continent on earth, with the frequency and severity of these events expected to increase across much of the world. To meet the challenges of a changing climate, an understanding of how forest systems have responded to extreme droughts in the past and how we can increase the resilience of these systems to future events is needed. This thesis investigates how resilient Scots pine (Pinus sylvestris) and Sitka spruce (Picea sitchensis) (the two most economically important and abundant UK conifer species) are to historic extreme drought events in the UK. In doing so we aimed to understand how differences in short vs longer term responses might influence our understanding of how these forests recover, how both forest composition and the nature of the drought itself might modify forest resistance and resilience to drought, and whether there are any thresholds of drought tolerance or evidence of drought induced shifts in competitive dominance. Following the implementation of a new methodological approach to quantifying drought resilience over different timescales, we documented evidence of post-drought compensatory growth in both Scots pine and Sitka spruce, which for some trees resulted in the complete recovery of tree size to what might have been expected in a 'no-drought' scenario. We also found evidence that small increases in drought severity were associated with large reductions in the radial growth of Scots pine and a shift in tree growth dominance. Surprisingly, monospecific stands of both species were also more drought resilient than intimate mixtures of the same two species. This research highlights the complexity of operationalising resilience concepts but contributes a strong and comprehensive foundation of evidence which can be used with future modelling work to identify ways to build resilience to future extreme drought events.