Session: Paleopalynology

Title: Warm climate could reduce trees diversity in mixed-wood boreal forest

Cécile C. Remy, Dominic Senici, Han Y. H. Chen, Yves Bergeron, Adam A. Ali & Martin Lavoie

In the global change context, understanding the role of climate and wildfires on the boreal forest composition is crucial to predict vegetation trajectories. We investigated the incidence of fire regime and climate conditions on multi-millennial vegetation diversity in two mixed-wood boreal forest sites in northeastern Canada (Ontario). We hypothesized that potential positive feedbacks between fires, climate and early successional taxa may increase the vegetation diversity during warm periods.

We performed charcoal (>160 µm)  analyses using CharAnalysis to reconstruct fire regime from 6 lacustrine cores and pollen analyses (identifications and counting) to reconstruct vegetation and vascular species diversity histories from 2 of these lacustrine cores, since the last deglaciation (ca. 9,000 years cal. BP). Lakes are situated in two near zones. Due to their spatial proximity, we assumed that these zones were under the same climate conditions and with the same potential vegetation during Holocene period. However, these two zones differ by their degree of landscape connectivity and drainage.

Preliminary results show that vascular trees diversity was higher during the colder periods (Neoglacial period). This period was marked by an increase of coniferous trees abundance in detriment of deciduous trees until then dominant in poorly connected and drained zone. In contrary, in the highly connected and drained zone, this cold period coincided with a decrease of coniferous trees abundance in favor of deciduous trees until then under-represented in landscape.

The best conditions to optimized the mixed-wood forest diversity seems thus to be a cold and/or moist climate rather than warm and/or dry. While mesic zones embody what might be called ‘diversity’s refuges’, xeric zones could be the most fragile areas in these forests and deserve a high watchfulness in the context of the global warming. Future research should focus on understanding the complex dynamics behind vegetation trajectories in order to preserve high diversity level through a maximum equity between conifers and broadleaf trees proportion in mixed-wood boreal forests.