Résumé - CAFD

Modern pollen-representation of some boreal species on islands in a large lake in Canada.

Isabelle Larocque, I.D. Campbell, R.H.W. Bradshaw, Yves Bergeron.

Studies of pollen source areas of closed-canopy sites are contradictory. Some authors found that closed-canopy sites mainly collect local pollen while others found more distant sources. This dichotomy might stem from the use of canopies of varying degrees of closure, and from variations in the pollen productivity of the local vegetation and the background pollen rain. Here, 30 islands were used to evaluate the pollen sources of closed-canopy sites. We compared pollen with the forest inventory in three quadrat sizes: 100, 400 m2 and on the whole island. Regression analyses showed that most pollen of Picea spp., Pinus spp., and Betula spp. comes from within the 400 m2 quadrat. Abies balsamea and Thuja occidentalis showed no relationship with vegetation in any of the quadrats considered, suggesting a more regional source. Insularity and island size are important factors influencing the pollen source area; correlations were stronger on islands located 1200 m from the nearest shore and on islands > 50 ha. These results suggest that closed-canopy sites on islands may be useful in stand-level vegetation history reconstruction through pollen analysis, but that caution must be exercised in separating the local and regional signals. (C) 2000 Elsevier Science B.V. All rights reserved.