Does climate control the northern range limit of eastern white cedar (Thuja occidentalis L.)?
Véronique Paul, Yves Bergeron, Francine Tremblay.
Our aim was to test whether or not climate influence the northern distributional limit of eastern white cedar (Thuja occidentalis L.) by affecting its radial growth and regeneration. Twenty-two sites were selected along the bioclimatic gradient in Northwestern Québec, Canada. The gradient was divided into three zones based on the abundance of white cedar stands: continuous, discontinuous, and marginal zones. Radial growth and regeneration (numbers of sedds, saplings) were determined for each zone. Results showed that basal area increment (b.a.i.) was the same along the gradient. Seed production and germination did not differ between zones. One-year-old seedlings and small individuals (<30 cm) were less abundant in the northern sites than in stands from the continuous and discontinuous zones. More saplings were found in the northernmost sites (389/400 m2 in the marginal and 354.7/400 m2 in the discontinuous zones) than in the south (130.6/400 m2). Layering seemed to compensate for the low recruitment observed in the marginal sites. Recruitment of seedlings originating from sexual reproduction in the discontinuous zone was not different from the stands in the southern areas. Thus, a climatically driven decrease in recruitment cannot explain the observed decrease in white cedar abundance, which occurred at this latitude. Although seedlings were less abundant in isolated northern marginal stands, sexually based regeneration was still possible. Therefore, the direct effect of climate seems to only have a minor influence on white cedar northern distributional limit and other factors, such as natural disturbances, might better explain its actual boundary.