PlHELLINUS WEIRII: CONIFER INTERACTIONS IN WESTERN NORTH AMERICA
EM HANSEN, J HOLAH and P. LATTIN
Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon, USA
Background and objectives
Results and conclusions
Local distribution is determined by stand history and topographic barriers to root-to-root spread. If stand successional history includes a prolonged period of occupancy by resistant angiosperm species, such as alder or maple, then the fungus may exhaust its resources in old conifer roots and die. If on the other hand, periods without susceptible conifers are relatively short, as when Douglas-fir is replanted after harvest or seeds naturally from scattered trees that survive wildfire, the fungus will persist and increase on the site. In many forests, it is the most important disturbance agent, affecting stand structure and composition in the long intervals between catastrophic stand-replacing fires or timber harvest. The consequences to the forest are determined by the abundance of the fungus and the existing and potential vegetation of the site .
In the vast Douglas fir dominated forests west of the Cascade Mountains in Oregon, P. weirii is common. Stand-replacing fires occur at 300- to 400-year intervals. The 'climax' species is shade-tolerant western hemlock, but without intermediate disturbance events to kill the Douglas fir, hemlock is confined to the understory. By killing Douglas fir, P. weirii allows the disease-tolerant hemlock to achieve dominance. Thus, root rot advances the succession in a patchy but slowly increasing manner in the forest. Because hemlock forms a very dense stand with little light penetration, the diversity of understory vegetation in infection centre occupied by this tree is often lower than in the surrounding Douglas-fir stand. In stands that lack hemlock, the slowly expanding mortality pockets may allow persistence of the diverse early successional vegetation.
At high elevations in the Cascade Mountains, the conifer forest grows under harsh conditions. In many places, on pumice soils, mountain hemlock is the dominant and climax species. It is very susceptible to laminated root rot, and the disease reaches its most dramatic expression in these forests. Individual genets of the fungus may span many hectares and be thousands of years old. The huge infection centre contain a younger, more diverse forest, with increased decomposition and nutrient cycling rates and hence greater productivity than the surrounding old growth. Although the specific consequences differ in different forest ecosystems, wherever it is found laminated root rot is a major factor in shaping forest structure, composition, and process.