Population Age and Size

In young populations, the size of ramets and genets is small, and many spore infections start new individual genets. With time populations tend to consist of a few large individuals, both for mycorrhizal and decay fungi (Dahlberg and Stenlid, 1990; Swedjemark and Stenlid, 1993). Many small individuals die out for stochastic reasons. As a genet grows older and proliferates in the environment it also grows larger. The fact that populations have a discrete distribution of genet sizes indicates that recruitment to a typical population is not continuous, but rather occurs at certain windows of opportunity following disturbance events. For unit restricted species (see Chapter 1) the size of ramets will not have a linear relationship with age, since they will grow in size until the outer border of the resource unit(s) is reached.

Expansion rates for saprotrophic basidiomycetes with non-unit-restricted growth is in the order of 0.3-1.5 m-1 year-1 (Hansen and Hamelin, 1999). To get a reliable measure of the expansion rate an independent measure of age is needed. This is hard to achieve from markers within the mycelium and has to be sought from known historical events. Normally the maximum age can be calculated but there are also a number of cases where minimum age has been inferred from forestry operations or construction of roads etc. For example, genets of Armillaria were estimated to be older than a road that crossed through its distribution (Kile, 1983; Lygis et al., 2005), and the age of H. annosum was dated from the onset of thinning operations in previously untouched stands (Bendz-Hellgren et al., 1999). Another possibility may be to use the spike in 14C from nuclear weapon trials date biological material to certain years (Levin and Kromer, 2004). This requires that mycelial structures are built during that period but maybe of use for dating fruit bodies of some perennial species. An indication of fruit-body age can also be obtained from the number of 'growth rings', though these may not always equate to years, as in trees (Niemela, 1986).

The size distribution of genets within a population represents a footprint of how and when new genets were recruited and how they have expanded over time (Hansen and Hamelin, 1999). Obviously, non-unit-restricted fungi have the potential to grow much larger than those that are restricted to a single resource. Non-unit-restricted mycelia often alter between a foraging phase in their vegetative growth and an exploitative phase in resources encountered (Boddy,

1999; Chapter 1) and can form large territorial clones. Large areas occupied by single genets have been reported for species of Armillaria (Korhonen, 1978a; Kile, 1983; Rishbeth, 1991; Smith et al., 1992; Guillaumin et al., 1996; Legrand et al., 1996; Ferguson et al., 2003; Lygis et al., 2005), the largest being 965 ha (Ferguson et al., 2003), with biomasses in the order of 80 tons (Smith et al., 1992). The implication is that spore infection is uncommon and that vegetative growth, mediated by cords, rhizomorphs and root-to-root contact infections of living root systems, is a much more common mode of spread in this genus. The competitive capacity as a saprotroph, as well as the inoculum potential as a pathogen, are both strengthened by the energy provided through the mycelial network as compared to a situation where basidiospores would be the initiating agent. Other species with a similar ability to spread between resource units are the pathogenic root-rot fungi Phellinus weirii (Dickman and Cook, 1989), H. annosum s.l. (Stenlid, 1985, 1987; Piri, 1996; Piri and Korhonen, 2007), Inonotus tomentosus (Lewis and Hansen, 1991), Phellinus noxius (Hattori et al., 1996), and the saprotrophic Mega collybia platyphylla (Thompson and Rayner, 1982), Resinicum bicolor (Kirby et al., 1990), Phallus impudicus (Boddy, 1999). Hymenochaete corrugata and H. tabacina can even move from tree-to-tree via branches glued together by pseudosclerotia (Ainsworth and Rayner, 1990; Stenlid and Holmer, 1991).

Other species have a limited ability to spread vegetatively between resource units and individual mycelia do not frequently extend outside, for example, a single tree. This was the case for Phaeolous schweinitzii (Childs, 1937; Barrett and Uscuplic, 1971), and Collybia fusipes (Marcais et al., 1998). Inside the bole of a single tree, several genets of Phellinus tremulae were detected, indicating multiple establishments by basidiospores (Holmer et al., 1994). Another interesting case of interspecific competition was observed in Douglas fir (Pseudotsuga mensiezii). Adams and Roth (1969) reported on a population structure of Fomitopsis cajanderi where several genets were present at the site of entrance into the broken tree stems while further down the stem only a few genotypes had expanded at the expense of the others. Typically, multiple spore infections will give rise to small populations of up to tens of decay mycelia in dead stems of conifers (Norden, 1997; Hogberg et al., 1999; Kauserud and Schumacher, 2002, 2003a).

Pathogenic root-rot fungi normally require a tight connection between root systems of host plants in order to spread vegetatively. Among the pathogenic species, it is interesting to compare H. annosum and I. tomentosus both of which have intermediate genet sizes in unmanaged forests, indicating that occasionally they establish via basidiospores in wounds etc. in natural conditions, and thereafter spread by root-to-root contacts (Stenlid, 1985; Lewis and Hansen, 1991). However, H. annosum, in contrast to I. tomentosus, is able to respond to disturbance in an ecosystem, caused by forestry, by increasing its spore establishment and become a true pest to conifer forestry in the northern hemisphere (Woodward et al., 1998). This is mirrored by a decrease in average genet and ramet sizes (Swedjemark and Stenlid, 1993). Clear responses to forestry activities have also been described for the saprotrophic R. bicolor (Kirby et al., 1990). Different size classes in the population indicated pulses of genet growth and resource colonization in response to thinning operations and availability of dead root systems.

Species with unit-restricted mycelia can form populations of genets inside resources. One early study of this was of Fomitopsis cajanderi infecting ice broken tops of Douglas fir. Close to the entry point several genets were present in the same cross-section of the tree while further away from the broken top one of the competing genotypes dominated (Adams and Roth, 1969). In a field experimental study on felled beech (Fagus sylvatica) logs, Coates and Rayner (1985) showed that in dense competing populations none of the genets may obtain large enough resource pools to support fruiting (see also Chapter 5). This illustrates that interspecific competition can reduce the fitness of all genotypes in an overcrowded population.

The distribution of genotypes in the boles of living trees has also been taken as an indication of the infection biology. Latent infection in beech has, for example been implied by the large-sized genets developing rapidly in trunks and branches after drought (Chapela and Boddy, 1988). Such large mycelia are thought to have been established during earlier phases of tree growth, and dormant propagules distributed extensively in the xylem were triggered to grow as mycelia by the onset of wood drying and increased aeration. Two other examples are provided by P. tremulae and P. pini. They initially establish in branches, the stubs of which ultimately become incorporated into trunk wood. In due course the sapwood becomes heartwood, again with decreased water content and improved aeration, triggering the development of active decay in the branch stubs buried in the heartwood (Haddow, 1938). The population structure of P. tremulae was consistent with this type of establishment (Holmer et al., 1994).

In a set of studies of decay fungi invading bark stripping wounds of Norway spruce, Vasiliauskas et al. (1996) illustrated how mating system and infection biology affects the population biology of wood-inhabiting basidiomycetes. Populations of the outcrossing C. evolvens (Vasiliauskas et al., 1998a) consisted of distinct genotypes in each tree. The insect vectored basidiomycetes Amylostereum areolatum and A. chailletii had populations with dispersive clones with ramets of the same genet detected in Sweden and Lithuania (Vasiliauskas et al., 1998; Vasiliauskas and Stenlid, 1999). The pseudohomothallic Stereum sang-uinolentum also had groups of vegetatively compatible (vc) isolates obtained from widely separated places — Lithuania, Finland, Sweden and Iceland (Vasiliauskas and Stenlid, 1998b; Stenlid and Vasiliauskas, 1998). However, these did not represent true clones as variation occurred inside the vc groups. Although genetic variation between vc groups was high and distributed in concordance with random mating, inside the vc groups the variation was much lower within populations from a single forest stand than between the stands. The pseudoho-mothallism apparently had lead to highly inbred lines within vc groups.

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