In the boreal forest of eastern North America, the distribution of eastern white cedar (Thuja occidentalis) is characterized by a latitudinal fragmentation gradient from south to north. Marginal populations could be outposts allowing cedar to expand its presence in the north in response to climate change. This study aimed to characterize the spatio-temporal regeneration dynamics of 20 marginal cedar stands in order to evaluate their expansion capacity into adjacent black spruce stands. Cedar recruitment within marginal cedar stands was mainly by layering, which allowed the species to maintain for a long time in the landscape. However, the rate of expansion of marginal stands into adjacent black spruce stands through seed dispersal was very low (0.28 m.year?1) and it was negatively influenced by distance to seed trees. Substrate had no significant effect. Global warming could lead to increased seed production by cedar, which could favour the species’ expansion at its northern distribution limit. However, global warming could also increase the frequency and severity of wildfires, which would have a negative effect on cedar expansion capacity.

Aim

Non?climatic constraints on species northern range boundaries are often overlooked in attempts to predict climate?induced range shifts. Here, we examined the effects of habitat availability and fire disturbance on the distribution of a species that transitions from being common to being found only in marginal populations at the northern boundary of its range.

Location

North?western Quebec, Canada (46–51° N and 74–79° W).

Taxon : Eastern white cedar (Thuja occidentalis L.)

Methods

We used forest inventory data (n = 4,987) to characterize white?cedar habitat based on edaphic and topographic conditions at sampled sites along a 600?km latitudinal gradient. Non?metric multidimensional scaling was used to assess habitat similarity of sites in the south, where white?cedar stands are abundant, and sites in the north, where white?cedar stands are rare. We constructed ensemble white cedar distribution models based on habitat variables in the south and compared ensemble forecast projections of white cedar in the north with observed occurrences to determine if habitat availability was limiting. We independently estimated the age of white?cedar stands and adjacent stands without white cedar along the gradient. ANOVA was performed to test the age difference between white?cedar and adjacent stands to determine whether the location of white?cedar stands was influenced by disturbance, primarily stand?replacing fire.

Results

Habitat availability was not limiting the distribution of eastern white cedar at its northern range boundary. White cedar did not occupy most sites with suitable habitat in the north, suggesting that other factors prevent white cedar from establishing more stands northward. White?cedar stands were older than adjacent stands without white cedar all along the gradient, but the difference was more pronounced in the north. This suggests that white?cedar stands in the north are restricted to undisturbed areas.

Main conclusions

Fire disturbance, more than habitat availability, limits the distribution of white cedar at its northern range boundary. Projections of white cedar distribution under climate change that ignore fire could overestimate the ability of warming temperatures to extend its northern range limit.