There is growing scientific evidence that climate change is profoundly altering the distribution, composition and dynamic of all ecosystems. Several North American temperate tree species are expected to expand their distributions northward where boreal forest is already established and dominated by conifers. The northward migration of temperate trees may be difficult primarily because of their slow demography and limited dispersal and competition by resident species which could hinder their establishment.
Our objective is to describe the transition dynamics of forest communities of Quebec in the last decades and identify the factors and processes that drove it. Specifically, the following questions will be addressed: How have temperate and boreal forests changed in recent decades? What are the likely causes of community transitions? What environmental factors influence the speed of these transitions? To answer these questions, we used Quebec forest inventory plots, which have been sampled approximately every ten years south of the 52nd parallel since 1970. More than 5000 plots were selected to analyze temporal change in community composition. A cluster analysis of tree abundance data was used to assign each plot, from the first to the last inventory, to one of the main community states: temperate, boreal, mixed and pioneer.
Results/Conclusions
Although forests showed a slow response to recent climate change, we found that most of the observed transitions were conversion from mixed to temperate stands. We then evaluated the strength of the relationship between transition probabilities among states and climate (change in temperature and precipitation between each inventory) and disturbance variables (fire, insect outbreak, timber harvesting, planting) using multinomial regressions. The transition probabilities between the different states were mainly related to logging and secondarily to temperature change. Following this result, one question emerges: does logging accelerate the effect of climate change? By analyzing the rate of change in forest communities, we showed that harvested forests change more and faster than undisturbed forests. We conclude that disturbances could accelerate forest community transition, or even trigger a regime shift, by providing opportunities of establishment for migrating species in otherwise competitive environments.