| dc.description.abstract | 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. | en |
