Microclimate varies spatially and temporally (Boddy, 1984, 1986) over a range of scales. At the forest scale two important microclimatic gradients are evident: a horizontal gradient from forest interior to forest edge, clearcut or natural canopy gap and a vertical gradient from forest floor to canopy. Forest interiors and lower levels in stands tend to have higher air humidity, lower wind speed, lower maximum and higher minimum temperatures compared to gaps, forest edges and open land, and gradients run from stable to variable microclimatic conditions (e.g. Chen et al, 1993; Morecroft et al, 1998; Ritter et al, 2005). Microclimatic stress is hence low in wood decomposing on the forest floor in closed forests, while fungi in dead wood in the canopy or on the floor of exposed forest edges are subject to stressful conditions. In addition, environmental conditions vary vastly between functional sapwood (with its high water content and low aeration), dysfunctional sapwood and heartwood (both of which are drier and better aerated), and according to wood diameter (Boddy, 1984,1986; Rayner and Boddy, 1988).
The microclimatic regime in dead wood may even be affected biotically by fungi living in the wood. Thus, some fungi are able to regulate the moisture contents of occupied wood. Pyrenomycetes, e.g. Xylaria hypoxylon, seem to maintain wood drier than ambient conditions (Boddy et al., 1989; Heilmann-Clausen, 2001), while Armillaria species occupy wood wetter than ambient, by virtue of the pseudosclerotial plates that they produce (Lopez-Real and Swift, 1975; Chapela, 1987). The strategy seems to be to create an environment stressful for the majority of fungi, thereby avoiding combative exclusion from more competitive species. Microclimatic gradients may also be created and maintained by mosses growing on dead wood (Bader et al., 1995; Chlebicki et al., 1996; Heilmann-Clausen et al., 2005), and ground vegetation around and above it (Heilmann-Clausen, 2001).
Temporal microclimatic fluctuations occur daily and annually, and over the long term—global climate changes. Fluctuations in temperature and water content over the year are probably one factor promoting high fungal diversity in dead wood, by creating a natural variation in the relative fitness of competing species. Indeed, fluctuations in temperature increased the number of inoculated fungal species reisolated after 6 months in a wood-chip microcosm experiment (Toljander et al, 2006).
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