. Les tendances 2003-2004 Le Programme d'évaluation de rapport. Projet d'évaluation des tendances critiques ; la gestion des écosystèmes ; la surveillance de l'environnement ; les ressources naturelles des sondages. Programme d'évaluation des tendances critiques Rapport de 2003-2004. -1.5 -1 0 12 3 4 5 6 7 Coefficient de conservatisme Figure 4. Dire ( +/- 1 S.E.) les espèces individuelles nestedness espèces contre score coefficient de conservatisme (Q pour les plantes de sous-bois dans les forêts des plaines inondables [A] et [B] Les forêts des hautes terres. Les valeurs positives de l'axe des y indiquer qu'une espèce est plus susceptible de se produire dans des sites de haute qualité floristique, tandis que les valeurs négatives indiquent t

• Contributeur: The Book Worm / Alamy Banque D'Images
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. Critical trends assessment Program 2003-04 report. Critical Trends Assessment Project; Ecosystem management; Environmental monitoring; Natural resources surveys. Critical Trends Assessment Program 2003-2004 Report. -1.5 -1 0 12 3 4 5 6 7 Coefficient of Conservatism Figure 4. Mean (+/- one S.E.) individual species nestedness score versus species coefficient of conservatism (Q for understory plants in floodplain forests [A] and upland forests [B]. Positive values on the y-axis indicate that a species is more likely to occur in sites of high floristic quality, whereas negative values indicate that a species is more likely to occur in sites of low floristic quality. Data points are means for all species within a coefficient of conservatism category. Discussion Forest maturity and ground-layer plants Following European settlement, most forests in eastern and central North America incurred heavy human use and disturbance, and in many cases they were completely cut over for timber and farmed. In both North America and Europe the amount of forested land has begun to increase over the past few decades (Bretthauer and Edgington 2002) (Figure 2). However, observations suggest that the species occupying these re-grown forests are different from those present in old-growth forests that were never cleared and were minimally disturbed. It has been shown that species diversity is greater in old-growth forests compared to mature second- growth forests that have a history of being clear-cut (Duffy and Meier 1992), and sensitive forest interior plants that are specialized to grow in mature forest (late-successional) account for these differences in diversity. These so called "ancient forest specialists" tend to be shade tolerant, slow growing, and have low seed production; they are often eliminated by massive disturbances and re-colonize very slowly (Meier et al. 1995). Effects on diversity are most pronounced in forests that have re-grown from cropland, as compared to for

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