Climate change effects on UK woodlands: can species' interactions mitigate the impacts of increased drought?

Abstract

Anthropogenic climate change threatens the structure and function of forest ecosystems which will in turn affect the provision of goods and services. It is crucial that we are able to predict the effects that climate change will have on species so that management strategies can be put in place to alleviate these impacts. As well as the direct effects on plants of climate variables, such as increased temperatures and changes to the precipitation regime, it is thought that biotic interactions between species can modify the direct impacts. For my PhD I used a spatially-explicit individual based forest stand model, SORTIE, to consider both the direct effect of climate change, and the indirect effects of competition for light between species. I predicted that the lengthening of growing seasons caused by temperature-mediated phenological changes will: (i) give early leafing species a competitive advantage by increasing its own growth whilst reducing resources for neighbouring individuals and (ii) be a means to mediate the negative effects of drought on drought-intolerant species. My results show that plant-plant competition can be a stronger driver of species composition, with the only species to benefit from prolonged growth seasons in woodlands both in the northeastern US (Great Mountain Forest) and Southern England (Wytham Woods) being canopy species. These outcompete sub-canopy species for light, inhibiting their expansion. I provide evidence that current codominant drought-intolerant sycamore is significantly impacted even under the current precipitation regime, with ash becoming the dominant species at Wytham after 1000 years. Lengthened growing seasons did not mitigate the effect of drought for drought-intolerant species. Future predictions for the population at Wytham will however need to consider the impact of dieback events such as ash dieback or oak sudden death.