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 eastern white cedar (Thuja occidentalis L.) at the northern boundary of its range.

Location

North-western Quebec, Canada (46-51° N and 74-79° W)

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 if 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.

We created a combined dataset from three types of forest inventory databases in Quebec, Canada: permanent (MFFP 2016a), temporary (MFFP 2016b), and ecological (Gosselin & Major, 2012; Saucier, Berger, d’Avignon & Racine, 1994) sample plots. We randomly sampled the combined dataset to select approximately equal numbers of stands with and without eastern white cedar (n= 4,987). Sites occurred across four bioclimatic domains: ioclimatic domain-3 sugar maple-yellow birch (Acer saccharum Marsh. - Betula alleghaniensis Britt.); bioclimatic domain-4 balsam fir-yellow birch (Abies balsamea (L.) Mill. - Betula alleghaniensis); bioclimatic domain-5 balsam fir-white birch Abies balsamea - Betula papyrifera Marsh.); and bioclimatic domain-6 black spruce-feathermoss (Picea mariana (Mill.) Britt., Sterns & Poggenb.). Climate data were obtained from the USDA Forest Service Rocky Mountain Research station.

References:

Gosselin, J., & Major, M. (2012). Vegetation database of Québec (MRNF). In J. O. J. Dengler, F. Jansen, M. Chytrý, J. Ewald, J. Ewald, M. Finckh, F. Glöckler, G. Lopez-Gonzalez, R. Peet, & J. Schaminée (Eds.), Vegetation databases for the 21st century. Biodiversity and Ecology. (4, 432). University of Hamburg, Hamburg, Germany

Ministère des Forêts, de la Faune et des Parcs (MFFP). (2016). Placettes-échantillons temporaires - Normes techniques. Direction des inventaires forestiers, Government of Québec, Québec City, Québec, Canada. (pp. 172). Retrieved from https://mffp.gouv.qc.ca/publications/forets/connaissances/Norme-PET.pdf. Accessed January 5, 2020.

Ministère des Forêts, de la Faune et des Parcs (MFFP). (2016). Placettes-échantillons temporaires - Normes techniques. Direction des inventaires forestiers, Government of Québec, Québec City, Québec, Canada. (pp. 238). Retrieved from https://mffp.gouv.qc.ca/publications/forets/connaissances/Norme-PEP.pdf. Accessed January 5, 2020.

Saucier, J.-P., Berger, J.-P., d’Avignon, H., & Racine, P. (1994). Le point d’observation écologique. (pp. 116). Government of Québec, Ministère des ressources naturelles, Québec City, Québec, Canada.