A comparison of the early growth and survival of lesser-known tree species for climate change adaptation in Britain

Globally, climate change is altering both seasonal climates and the occurrence of extreme climate events, resulting in a drive to ensure our forests are resilient to these changes and the challenges they will bring. In Great Britain (GB), there has been a reliance on a limited number of species grown in monospecific plantations and concerns over the resilience of these forests is leading to a growing recognition of the need to diversify tree species composition. However, evidence of the relative growth rate and survival of alternative tree species and provenances during the critical establishment phase (typically within five or six years of planting) is often limited, hampering consideration of the wider adoption of many potentially suitable species. To address this knowledge gap, we compared tree height and survival data six years after planting from 34 provenances across 18 tree species at five experimental sites established across GB in 2012 in monospecific plots. For coniferous species, we found that Larix decidua, Larix x marschlinsii and Pinus radiata (on drier sites) were consistently amongst the tallest species, but survival could be variable. Pseudotsuga menziesii and Pinus pinaster also showed good early growth, matching the growth rate of Picea sitchensis (the most widely planted conifer in GB) when co-occurring and often exhibited good survival. In contrast Picea orientalis was slow to establish and amongst the smallest species at all five sites after six years, with Cedrus atlantica also performing poorly where planted. Surprisingly, we found very few differences in both mean tree height and survival between most provenances of the same species across all experimental sites, though more obvious differences may emerge as these trees mature. Only a small number of broadleaf species were available for analysis at age six in this study, but as expected Betula pendula generally performed well, while Acer macrophyllum often had very high mortality. While there are concerns around the susceptibility of pine species to Dothistroma septosporum and the future use of larch species in GB forestry is currently limited by the disease Phytophthora ramorum, our results highlight the potential for these species to establish well and exhibit good initial growth and survival on drier sites, with the same true of P. menziesii on moister sites. Future work should aim to understand whether further differences between species and provenances emerge with age and explore the potential of these emerging species as components of mixed-species stands to increase GB forest resilience to climate change.