Environmental change and species traits determine postglacial forest dynamics: An example from the North Pacific coast of North America

Background/Question/Methods

A central focus of community ecology is identifying relationships between species traits and environmental conditions, but paleoecological and paleoenvironmental data have rarely been used in this regard. I investigated the interaction between species traits and paleoenvironmental change in determining forest composition on northern Vancouver Island over the last 15,000 cal yr. I used RLQ analysis, a three-table ordination technique, to directly link life history and ecological performance traits of the region’s dominant woody species to paleoenvironmental variables, through a fossil pollen record derived from lake sediments. Then, I used fourth-corner analysis to measure and test the significance of relationships between species traits and paleoenvironmental variables.

Results/Conclusions

RLQ axis 1 explained 92% of the total covariance between species traits and paleoenvironmental variables and was correlated most strongly with temperature and traits such as relative growth rate and waterlogging tolerance. The RLQ ordination separated samples into three main groups: mid- to late Holocene (0-8000 cal yr BP), early Holocene (8000-11,000 cal yr BP), and late-glacial (11,000-15,000 cal yr BP). In general, rapid changes in climate during the relatively cold late-glacial period favoured species such as Alnus sinuata and Pinus contorta that exhibit a relatively ‘fast’ life history strategy (e.g., high relative growth rate, low shade tolerance), while the relative stability of warm, wet climate over the last 8000 cal yr has favoured species such as Thuja plicata and Tsuga heterophylla that exhibit a ‘slow’ life history strategy (e.g., low relative growth rate, high shade tolerance). Fourth-corner analysis revealed weak but significant correlations between all paleoenvironmental variables i.e., temperature, precipitation, summer insolation, and total pollen accumulation rates (used here as an analogue for vegetation density) and most species traits (relative growth rate, minimum seed-bearing age, canopy height, longevity, and shade, drought and waterlogging tolerances). The strongest correlation was between paleotemperature and height, reflecting the overarching competitive advantage that height confers. The importance of interactions between environmental conditions and species traits in directing vegetation dynamics has been demonstrated in modern ecological studies, but this is the first instance where this has been confirmed over long timescales with paleo-data. Paleoecological studies routinely ignore the role of species traits in postglacial vegetation change, but this research illustrates that a comprehensive understanding of postglacial vegetation dynamics requires consideration of differences in species traits as well as potential links between species traits and the environment.