. La Canadian field-naturaliste. 370 Le Canadian Field-Naturalist Vol. 92 p. 445). Un transect de 10 m de large a été examinés à travers la marge de la grove à travers la dépression en son centre et une série de stations de sondage a été mis en place à partir de laquelle pour tracer la position de chaque arbre par relèvement et distance. Des mesures ont été effectuées de tronc d'un diamètre de 1,2 m du niveau du sol (dhp) pour chaque arbre dans le transect. Trente arbres, sélectionné comme un représentant si pas strictement échantillon aléatoire de diamètre du tronc, ont été évidées à la même hauteur afin de produire un graphique de l'anneau contre dbh-count age (Fi

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. The Canadian field-naturalist. 370 The Canadian Field-Naturalist Vol. 92 p. 445). A transect 10 m wide was surveyed across the margin of the grove through the depression at its center and a series of survey stations was set up from which to plot the position of each tree by bearing and distance. Measurements were made of trunk diameters at 1.2 m aboveground level (dbh) for each tree in the mapped transect. Thirty trees, selected as a representative though not strictly random sample of trunk diameters, were cored at the same height in order to produce a graph of dbh against ring-count age (Figure 1). The annual rings of aspen, like those of many other hardwoods, are not easily distinguished and many methods have been proposed to solve this problem (Maini and Coupland 1964). A very simple method and one which served well in this research was to dip the air-dry cores into hot black coffee for about 1 min. Air was expelled from the porous wood and the rings were clearly visible while they remained wet. Dead trees were included in the mapping and their dbh meas- ured, a distinction being made between dead trees still standing and stumps with rotting trunks beside them. To evaluate the spatial pattern of trees, we divided the transect into six quadrats of 10 X 10 m and applied nearest- neighbor analysis (Clark and Evans 1954) to the mapped distributions." The significance of the nearest-neighbor index (R) was tested using the z-score technique of King (1969). The value of R is found by defining a sample area, measuring the mean distance from each point (in this case, tree trunk) to its nearest neighbor, then dividing by the mean distance to be expected from a corresponding number of points randomly distributed over the same area. Mean random distance is calculated as the reciprocal of twice the square root of points per unit area, or ^ /., , .. R approaches 0 forcom- 2>yN/A ^^ pletely clustered points, is 1.0 for a random dis- tribution, and reaches 2.15 for a per

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