Can native woodlands cope with climate change? Measuring genetic variation & phenotypic plasticity in British populations of ash, rowan and silver birch

Rapid climate change is a significant threat to the long-term persistence of native tree populations. Concern has been expressed that tree populations might fail to adapt due to rate of change, insufficient adaptive variation in tree populations and limits to dispersal. In contrast, others have contended that most tree species have high phenotypic plasticity, maintain high levels of within-population genetic variation and exhibit effective gene dispersal capability, all characteristics which should enable an adaptive response. To assess the potential adaptability of tree populations we need to understand their genetic diversity and phenotypic plasticity to build on the currently limited evidence base and guide decisions about seed sourcing for establishment of new woodlands desired to meet ambitious planting targets. Currently the seed sourcing system divides the island in four regions of similar size although it is not based on any genetic or ecological information. We discuss the suitability of this system with the insight of the data collected from native tree populations growing in experimental trials. In this thesis we study genetic diversity and phenotypic plasticity patterns in over 30 native tree populations across all Great Britain for three broadleaved species: ash (Fraxinus excelsior), rowan (Sorbus aucuparia), and silver birch (Betula pendula). To obtain these data we assessed the variation in multiple traits in several common garden experiments for each species, which were grown in contrasting environments. There is a tendency in provenance experiments to consider height as a proxy for fitness. We demonstrate that tree height is not enough to understand tree fitness and its adaptability capacity. We assessed our tree populations for growth (survival, tree height, DBH), stem form (number of forks), leaf phenology (leaf flushing and senescence) and leaf anatomical traits (leaf area, stomatal density and stomatal size).Great Britain has very distinct and heterogeneous environments likely to have given rise to adaptive differentiation. Knowing the geographical pattern of the genetic differences we can see the direction selective pressures have had on each of the traits studied, and we compare differences in patterns across the traits and species. Comparing populations growing in different environments we assessed the variation in phenotypic plasticity by trait and the direction of these plasticity. We found that tree populations across Great Britain are highly genetically variable and show genetic differences which have a geographical pattern, and that the patterns and size of the differences vary by species. Phenotypic plasticity varies across traits and interactions between genotype and environment make plasticity in some traits more unpredictable than others. We conclude that tree populations of ash, rowan and birch are well adapted to the diverse and oceanic climate of Great Britain, and that levels of genetic diversity and phenotypic plasticity provide a high capacity to respond to environmental change