Impact of fungi in natural forest ecosystems: A focus on Eucalyptus.

Introduction Fungal pathogens are integral components of healthy natural forest ecosystems. where they play a major role in eliminating weak and unfit trees (Manion 1981; Burdon 1991; Castello et al. 1995). They also affect species occurrence and distribution, especially in the regeneration layer (Castello et al. 1995). Soil-borne pathogens, in particular, are thought to be important in maintaining plant species diversity and distribution (Augspurger and Kelly 1984; Bever et al. 1997; Mills and Bever 1998; Packer and Clay 2000). It is hypothesised that seedlings close to their parents or other conspecific trees are more likely to he killed by host specific soil-borne pathogens than seedlings further away. Over time, this results in a shift in the juvenile population away from the adults. This relationship has been demonstrated between the temperate tree Prunus serotina and the pathogen Pythium (Packer and Clay 2000) and also for seedling damping off of the tropical tree Platypodium eIegans caused by a variety of soil-borne pathogens (Augspurger and Kelly 1984). Interestingly, canker fungi also have been shown to impart a similar effect on the distribution of the tropical tree Ocotea whitei (Gilbert et al. 1994). Higher levels of light intensity, such as those experienced in light gaps caused by fallen trees, reduce both pathogen activity and the net impact of pathogens (Augspurger and Kelly 1984; Castello et al. 1995). The epidemiology and visual impact of indigenous pathogens in natural forest ecosystems is greatly minimised by genetic diversity. Here, trees and their pathogens have co-evolved and the population structure of the hosts is characterised by genetic and age diversity (Manion 1981; Hansen 1999). Consequently, individuals in a mature tree population will vary in their susceptibility to a particular pathogen. Usually, the tree is not susceptible throughout its life cycle, consequently, some individuals in some age classes may be susceptible, but not the whole population. Large populations of susceptible trees do not develop and therefore, widespread disease epidemics cannot occur. This immunity of natural ecosystems to disease epidemics can be overcome in two ways. Firstly, either a natural or human disturbance could result in (lie growth of an even aged stand of a single species that may then be susceptible to an indigenous pathogen. Secondly, virulent pathogens, to which the trees have no resistance, could be introduced to a natural ecosystem. Introduced pathogens often lead to the elimination of entire species and result in permanent changes in the species composition of an ecosystem. Ineffective quarantine has often led to epidemics in indigenous and exotic tree populations caused by introduced fungal pathogens (Old and Dudzinski 1998; Palm 1999). Examples of mass destruction of indigenous forests due to fungal pathogens include Chestnut Blight caused by Cryphonnectria parasitica (Anagnostakis 1987), Dutch Elm Disease caused by Ophiostoma ulmi and 0. novo-ulmi in Europe and North America (Brasier 1991; Hubbes 1999) and Cypress Canker in Europe caused by Seiridium cardinale (Graniti 1998). Destructive epidemics caused by fungi in natural woody ecosystems, particularly in the Northern Hemisphere, have been covered extensively elsewhere (Anagnostakis 1987; Brasier 1991; Granti 1998; Hubbes 1999). In this chapter, we will rather focus on pathogens of eucalypts in their native range in Australia and in exotic plantations elsewhere in the world. Emphasis is placed in the importance of the origin of a pathogen and the structure of the host plantation.