OOS 19-6 - Living on the (northern) edge: Holocene dynamics of marginal, northernmost maple stands in eastern North America

Background/Question/Methods
The use of historical climate and model-tree-species interactions helps in our understanding of present and future forest dynamics. Changes in climatic conditions during the Holocene and consequent shifts in the composition of the north-eastern North American temperate-boreal forest ecotone might provide key insights to the importance of marginal populations for forest resilience in the context of ongoing climate change. Sugar and red maples are examples of temperate species whose northernmost stands are isolated within the boreal forest. Responses of these stands to past climate change could be indicative of greater changes in the ecotone. This study establishes a comprehensive historical reconstruction of Holocene dynamics of these two maple species at their current northern limit to decipher ecological factors driving past occurrence and abundance of temperate species within the boreal biome. Macrofossil wood charcoal particles recovered from inorganic forest soil were taxonomically identified and radiocarbon dated to assess historical species presence at the stand-scale at 16 sampled sites across the southern boreal landscape. Local comparisons of sites with and without maple helped disentangle the relative contributions from past climate and fire regimes in structuring the northern limit of temperate species.

Results/Conclusions
Preliminary results from 6700 charcoal identifications and 183 radiocarbon dates indicate that maple colonized its current northern limit ca. 9600 years ago. For at least 6000 years, maple was found only sporadically within these marginal populations while white pine, which is currently absent in the area, was found regularly in most sites. Hence, maple remained scattered and did not take advantage of the warmer conditions prevailing throughout the Holocene Thermal Optimum. Instead, maple became more abundant throughout its northern limit ca. 3500 years ago, during the Neoglacial cooling. This rapid shift might be attributed to a change in the fire regime, opening the canopy for opportunistic species such as red maple. Such an alteration of the fire behaviour could trigger a shift from dominant conifers (white pine, balsam fir) to deciduous species, potentially removing priority effects inhibiting sugar maple regeneration. Our results suggest that a further increase in the frequency of natural or anthropogenic disturbances coupled with temperature warming, could foster expansion of northernmost maple stands. This study emphasizes the importance of marginal populations for the rapid colonisation of landscapes during sudden shifts in environmental conditions such as those prompted by ongoing global changes.