Combined Effects of Changes in Habitat Composition and Fragmentation

Several studies from boreal Fennoscandia (Siitonen et al., 2001; Sverdrup-Thygeson and Lindenmayer, 2003; Stokland and Kauserud, 2004; Pentilla et al., 2006) have documented differences in species diversity (based on sporocarp surveys) and composition of communities of wood-decay fungi depending on the fragmentation and management history at the landscape scale. In all cases certain rare or endangered species were found to be significantly less frequent in long fragmented landscapes compared to more recently, or less fragmented landscapes, even in the presence of suitable habitats. For instance, the red-listed polypore Phellinus nigrolimitatus was five times less frequent on suitable conifer logs in managed forests compared to similar logs in natural forests (Stokland and Kauserud, 2004).

In the European deciduous forest zone actual studies on fragmentation effects are few, but there is some evidence from beech forests, at various geographical scales, that changes in species composition similar to those in boreal forest has occurred (Holec, 2003; (Odor et al., 2006). In a study of old-growth forest remnants in Denmark, the incidence of red-listed species (as evidenced by sporocarps) declined along a geographical gradient, related to differences in gross forest history and climate (Heilmann-Clausen and Christensen, 2005). However, in contrast, overall species density (i.e. the average number of species per fallen tree) did not differ or was even higher in less natural sites. Similarly, along a gradient from SE to NW Europe the relative fraction of threatened species similarly declined from largely coherent forest landscapes in Hungary and Slovenia to highly fragmented landscapes in Belgium and The Netherlands ((Odor et al., 2006).

The selective decline of specialized fungi in relation to fragmentation of old-growth forests is likely to be strengthened synergistically by the relative increase in other dead-wood resources, and their associated fungi, in managed forests, which make up the matrix in most forested landscapes. Thus, the overall and asymmetric scarcity of large logs at the landscape scale, compared with, for example stumps and branches, implies that the relative fraction of spores from species depending on large logs will decrease relative to spores of generalists or stump or branch specialists (cf. Edman et al., 2004). This implies that, even if a

Figure 3 Optima (dead wood continuity) for heart-rot fungi, early combative invaders, ruderal primary colonizers and latent invaders (see Table 1 of Chapter 12) on beech logs along a dead-wood continuity gradient in Denmark. Species optima are based on a DCA-ordination based on frequency data for each species in 14 localities across Denmark. The preference for sites with long dead wood continuity is distinct for heart-rot agents, while many early combative invaders were most frequent in sites with lower continuity in the presence of dead wood. (Source: Modified from Heilmann-Clausen, 2004).

Figure 3 Optima (dead wood continuity) for heart-rot fungi, early combative invaders, ruderal primary colonizers and latent invaders (see Table 1 of Chapter 12) on beech logs along a dead-wood continuity gradient in Denmark. Species optima are based on a DCA-ordination based on frequency data for each species in 14 localities across Denmark. The preference for sites with long dead wood continuity is distinct for heart-rot agents, while many early combative invaders were most frequent in sites with lower continuity in the presence of dead wood. (Source: Modified from Heilmann-Clausen, 2004).

suitable resource for species with special requirements is created, the chance of establishment is decreased because non-selective species are likely to arrive first or with such a majority of propagules, that the selective species is out-competed (Gourbiere and Gourbiere, 2002). Along a gradient of old to recent forest reserves in Denmark the species composition of early decay agents did differ according to this hypothesis (Heilmann-Clausen, 2004). Thus, heart-rot agents were much more frequent on beech logs in old forest reserves, compared to recent reserves while early secondary invaders showed the opposite pattern (Figure 3). This has important implications for conservation of rare wood-inhabiting species, because restoration of dead-wood habitats in highly fragmented (in time and space) landscapes may have much less impact on rare specialists than enlargement of reserves in less fragmented areas (see Chapter 18).

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