. Il campo Canadese-naturalista. 284 Il canadese Field-Naturalist Vol. 114 100 90 80 70 60 50 40 30- 20- 10 178 I /V=451. 40 1 ^ I I I ^ I I I ^ I I I I I I R 80 120 160 200 240 280 320 360 400 440 480 ALTEZZA (cm) di figura 4. Struttura di altezza (20-cm classi) della betulla {Betula spp.) popolazione del vuoto-minata unità del Cacouna bog nel 1998. campionamento (Lavoie e Rochefort, 1996). La pres- ence/assenza di tutte le specie di vascolare e non-vas- cular piante a copertura di un punto di piccole dimensioni (diametro: 1 cm) ogni 4 m lungo otto seca, 4 m di distanza e 264 m di lunghezza, è stato notato. Inoltre, poiché il

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. The Canadian field-naturalist. 284 The Canadian Field-Naturalist Vol. 114 100 90 80 70 60 50 40 30- 20- 10 178 I /V=451. 40 1 ^ I I I ^ I I I ^ I I I I I I r 80 120 160 200 240 280 320 360 400 440 480 Height (cm) Figure 4. Height structure (20-cm classes) of the birch {Betula spp.) population of the vacuum-mined unit of the Cacouna bog in 1998. sampling (Lavoie and Rochefort 1996). The pres- ence/absence of all species of vascular and non-vas- cular plants covering a small point (diameter: 1 cm) every 4 m along eight transects, 4 m apart and 264 m long, was noted. Furthermore, since the vacuum- mined section has been invaded by birches {Betula spp.), 128 birch individuals were randomly selected and a stem section of each individual was taken at the collar. In the laboratory, tree rings were counted on finely sanded cross-sections. Birch species {Betula cordifolia, B. papyrifera, B. populifolia, B. X caerulea) were not differentiated because most individuals were hybrids which cannot be easily identified in the field during the summer season (DeHond and Campbell 1989). No exotic birch species (e.g., Betula pendula) has been found. In August 1998, the vegetation sampling was repeated at the same place using exactly the same method (Figure 2). The population structure of the birches was constructed by measuring the total height of all stems included in a 25 X 28 m quadrat located at the center of the vacuum-mined unit. Cross-sections of the stems were not taken so as to minimize distur- bance to the tree population. The 1994 and 1998 surveys were compared using a Wilcoxon's signed-ranks test to verify whether they were significantly different (Scherrer 1984). Furthermore, the surveys were compared for each plant species using a McNemar's test to detect whether a species was significantly more or less abundant in 1998 than in 1994. Each sampling point was then considered to detect if the species was pre- sent/absent in 1994 and in 1998 (Scherrer 1984). The Bonferr

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• bookauthorottawafi
• bookcentury1900
• bookdecade1910
• bookyear1919