Todor Minchev, Université du Québec à Rimouski, discusses his article: Early departures and delayed arrivals: Holocene dynamics of temperate tree species in the boreal temperate ecotone.
Forests are the backbone of most terrestrial ecosystems and form some of the largest biomes on the planet, excluding the oceans. Such is the case of the circumboreal forest that encircles the Northern Hemisphere, spanning across North America and Eurasia below the Arctic circle. From raw materials and economic value to wellbeing and social benefits, our connection to trees is intrinsic and essential. However, the increase in anthropogenic land use means that most forests are now affected by, and subject to, pressures from humans. To predict how these pressures will impact forests, scientists developed computer models.
These models use ecosystem data along with historical and projected climate data to simulate how these complex webs of interacting organisms might respond to climate change. However, these models are only as reliable as the data they are built on. As the number of assumptions needed to fill knowledge gaps increases, the accuracy of the models tends to decrease. For species with rapid turnover rates and short lifespans that we can observe over several generations, identifying long-term patterns can be relatively straightforward. Trees, however, are long-lived, with generation times spanning tens, hundreds, or even thousands of years. As such, present-day studies cannot track an entire tree lifespan or monitor millennial-scale patterns. Thankfully, various palaeoecological methods have been developed to reconstruct how trees responded to past climate change, by detecting the different types of fossils they left behind. This means that to build accurate forecasting models, we need a thorough understanding of past tree species dynamics.
Using some of these residual fossilized particles, namely, charcoal fragments deposited in forest soils, we reconstructed the dynamics of three temperate tree species at their northern limit of distribution, within the boreal forest. Such small and marginal populations, located at the leading edge of a species’ range, are particularly susceptible to environmental change, thus they serve as excellent model populations. During the Holocene, i.e. the last 11 500 years, there was a period when temperatures were significantly higher than today. This provides the opportunity to extrapolate how temperate species might respond to future climate change by examining how they reacted to past warming events. It is generally expected that such warm-adapted species will migrate northward in response to human-induced climate change. But was this their response in the past as well? If not, how did they react to analogous climate warming of the mid-Holocene.
Our data showed that temperate and warm-adapted species did not respond uniformly to past climate change. Instead, each of the three studied species exhibited a distinct, individual response. White pine (Pinus strobus), a temperate conifer, was the first to occupy the study area, nearly 7000 years before present (BP), at a time when climate was still relatively cool but already warming. However, as temperatures rose between 7000 and 4000 years BP, reaching levels higher than today, competing maple species did not increase in abundance. Instead, white pine maintained its dominance. Surprisingly, it was only once the climate began to cool, between 4000 and 3000 years BP, that red maple (Acer rubrum), a hardy temperate species, became more abundant. Even more surprising, it took nearly two additional millennia of continued cooling for the warm-dependent sugar maple (A. saccharum) to colonize 2000 years ago. While populations of the two deciduous maple species were expanding, that of white pine followed general expectations for temperate species at their northern limit and retreated southward during Neoglacial cooling. It is likely that the deciduous species were not limited by the low temperatures, but rather by biotic interactions. As the climate cooled, fire regimes intensified, likely becoming detrimental for white pine while creating openings in the forest canopy that fostered red maple establishment. In turn, red maple may have gradually facilitated sugar maple establishment and growth by improving soil conditions, thereby enabling its northward expansion.
Our results demonstrate that there are no shortcuts in science. Although temperate tree species may share commonalities in terms of their ecological requirements, coexisting species cannot be treated as a single forest assemblage. Species responses are idiosyncratic: each must be analysed individually to be able to accurately predict how forest assemblages will be altered under the influence of anthropogenic climate change.
