X I

aggravation

S-C

stress

C

alleviation

S and/or R

Figure 2 Schema showing potential fungal community development pathways following colonization of newly available wood (top of diagram), through an open community stage in which there is still unoccupied territory, to a closed community in which all territory is occupied (i.e. colonization must be by secondary resource capture), and finally to communities developing in well-decayed wood, characterized by substrate modification and invasion by soil invertebrates. Major ecological characteristics of dominant organisms are indicated in boxes: S, stress tolerant; C, combative; R, ruderal. Driving forces for change are indicated in italic. Direction in which the driving force pushes the community is indicated by arrows. Aggravation of stresses will push the community to the left, whereas alleviation will push the community to the right. As the community moves from open to closed, combat will be the main driving force for change. In contrast, destructive disturbance will push the community towards species with more ruderal characteristics (Adapted from Boddy, 2001; Rayner and Webber, 1984).

content of functional sapwood. Nevertheless, fungal colonization of wood tissue in living trees is common. Functional sapwood is water saturated, and contains virtually no oxygen and many living cells, while the inner core—heartwood— often contains large quantities of allelopathic compounds, but has an ameliorated gaseous regime. Though heartwood is inimical to fungal growth it is slightly less hostile than functional sapwood; hence, in the standing tree establishment of actively growing mycelia occurs predominantly in the former, and decay is most rapid and extensive there. Paradoxically, in felled and fallen trunks sapwood decays considerably more quickly than does heartwood. This is because sapwood is no longer functional in water conductivity and cells are dead; hence, the gaseous regime improves dramatically, cells are not in possession of host defence mechanisms and nutrients are more easily accessible.

Fungi may gain access to heartwood in living trees by different routes (Rayner and Boddy, 1988), but most commonly via wounds, e.g. after breakage of large branches. Volumes of sapwood can become dysfunctional, e.g. branches may die as a result of drought or light suppression and trunks and branches may experience wounding due to forestry activity, storm damage, mammal activity, invertebrate penetration or fire. Conditions within the sapwood are then ameliorated allowing development of actively growing fungal mycelia. Finally, fungal infection of intact wood may follow direct inoculation during oviposition by an insect vector (Chapter 9), and by pathogenesis, e.g. H. annosum that colonizes via contact of healthy roots with those of trees and stumps that are already colonized, and Armillaria species that arrive as rhizomorphs. In sapwood, primary colonizers are usually followed by more competitive secondary decay fungi prior to fall.

In some cases whole trees may die standing. Standing wood and attached branches are highly subject to microclimatic variation, being particularly prone to desiccation and to high internal temperatures that may occur in exposed standing trunks during sunny days. Community development in these dead wood habitats is hence characterized by many species with S- or R-selective traits.

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