FACTORS AFFECTING ECTOMYCORRHIZAE AND FOREST REGENERATION FOLLOWING DISTURBANCE IN THE PACIFIC NORTHWEST

Michael P. Amaranthus

ABSTRACT

Studies from the Pacific Northwest indicate that disturbance can reduce ectomycorrhizal formation and forest regeneration. However, the degrees of reduction and impact on forest regeneration vary widely and depend on many factors. Among these are the type and severity of disturbance, ectomycorrhizal diversity, climatic conditions, biotic conditions, and the effects of nonhosts over time. Mycorrhizal formation and regeneration are most greatly impacted on severely disturbed and environmentally limited sites. The rapid occupation of such sites by ectomycorrhizal host plants following disturbance is critical to stabilizing native mycorrhizal populations that may aid forest regeneration.

INTRODUCTION

Much of the research on effects of forest disturbance on soil biology has focused on ectomycorrhizae. Because most forest-tree species in the Pacific Northwest require ectomycorrhizae for nutrient and water uptake, the importance of understanding the relationship between disturbance, site conditions, and mycorrhizae cannot be overstated. Numerous authors have reported reductions in mycorrhiza populations due to forest disturbance (Amaranthus and others 1987; Harvey and others 1976, 1980; Parke 1984; Perry and others 1982). However, the degree of reduction and its impact on forest regeneration varies widely and depends on many factors.

TYPE AND SEVERITY OF DISTURBANCE

Timber harvest and site preparation are the most widespread forest activities in the Pacific Northwest that alter both the aboveground and belowground environments and therefore potentially impact ectomycorrhizae. In the Pacific Northwest, clearcutting and prescribed burning are the common harvesting and site preparation practices. Soil nutrient status, moisture, temperature, pH, and organic matter contents, litter inputs, and species composition affect the community structure of soil organisms (Harvey and others 1980) and all of these are influenced by harvesting and site preparation. Clearing vegetation and disturbing the forest floor remove nutrients and reallocate them within the ecosystem. Harvesting host trees eliminates the photosynthate source for dependent ectomycorrhizal fungi and associated microbes. Converting a mature forest to a clearcut increases soil temperatures by removing the protective canopy. Prescribed broadcast burning increases soil pH, creates a nutrient flush, and can reduce litter and duff levels (Amaranthus and McNabb 1984). Soil organic matter, humified material, and decaying wood are centers of microbial activity and are substantially diminished as a result of intense fire. Changes in aboveground community composition alter the quality and quantity of root exudates and litter leachates.

Wright and Tarrant (1958) found fewer ectomycorrhizae on Douglas-fir seedlings growing in burned, compared to unburned, clearcuts. The greatest reductions were associated with the hottest burns. Thus, not only the type of activity, but its severity, is critical. Parke and others (1984) compared mycorrhiza formation in soils from burned and unburned clearcuts of 36 "difficult to regenerate" sites in northwestern California and southwestern Oregon. Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and ponderosa pine (Pinus ponderosa Laws.) seedlings grown in soils from the burned clearcuts formed 40 percent fewer ectomycorrhizae and seedlings grown in soils from the unburned clearcuts 20 percent fewer ectomycorrhizae than seedlings grown in undisturbed forest soil. Amaranthus and others (1987) found 90 percent less native mycorrhizae and 44 percent less basal area growth on Douglas-fir seedlings grown in clearcut and severely burned soils compared to undisturbed forest soil. However, all studies do not report mycorrhizal reductions following clearcutting and prescribed fire (Pilz and Perry 1984; Schoenberger and Perry 1982). It is difficult to generalize about effects of burning on ectomycorrhizal populations because they are highly dependent on duration and intensity of fire as well as soil and site conditions (Perry and Rose 1983).

Because ectomycorrhizae predominate in the organic layers of the soil (Harvey and others 1976, 1979; Trappe and Fogel 1977), the degree of organic matter lost from a site can influence mycorrhiza populations. Harvey and others (1976) found up to 95 percent of the active ectomycorrhizal fungus types in humus and decaying wood in a mature Douglas-fir/western larch (Larix occidentalis Nutt.) forest. The importance of decaying wood to support ectomycorrhizal populations may be most critical following disturbance such as wildfire. Immediately following the 1987 wildfires in southern Oregon and northern California, decaying wood contained 25 times more moisture compared to mineral soil and was a center of ectomycorrhizal activity for recovering vegetation (Amaranthus and others 1989). Decaying wood also acts as habitat for small mammals that are important in distributing fungal spores of several belowground mycorrhizal fungi (Maser and others 1978).

ECTOMYCORRHIZAL DIVERSITY

Ectomycorrhizal diversity within the soil may buffer the impact of disturbance on forest sites. Disturbance did not reduce ectomycorrhiza formation on Douglas-fir seedlings grown in soil from extremely productive sites in the Oregon Cascades, where diversity of ectomycorrhizal fungi is high (Pilz and Perry 1984; Schoenberger and Perry 1982). However, the proportion of each mycorrhizal fungus type shifted significantly with both soil disturbance and plant community; different environments reduced ectomycorrhiza formation by some fungus types and apparently stimulated formation for others. In contrast, clearcutting significantly lowered ectomycorrhiza formation on Douglas-fir seedlings grown in soil from a harsh, less productive site in southwestern Oregon, where soil contained relatively few ectomycorrhiza types (Amaranthus and others 1987). Reduced ectomycorrhiza formation correlated positively with decreased basal area Douglas-fir growth after outplanting. The southwestern Oregon soil, with low fungus diversity, was poorly buffered against disturbance, compared to the soil from the Oregon Cascades.

CLIMATIC CONDITIONS

Climate influences seedling growth and ectomycorrhiza formation (Harvey and others 1980; Pilz and Perry 1984). The importance of early mycorrhiza formation in dry areas has been emphasized (Mikola 1970; Parry 1953). Dry climates may limit the activity of mycorrhizal fungi by decreasing the length of time for spore production, germination, and optimal mycelial growth, which in turn can decrease the chances for planted seedlings to become colonized (Amaranthus and Perry 1987). Seedlings in moist climates may be able to survive longer without mycorrhizae than those in dry climates, increasing their chances of becoming colonized. Moisture content also affects uptake of certain nutrients by mycorrhizae (Gadgil 1972).

Seedlings growing in cold climates may also require rapid, early mycorrhizal colonization to take advantage of the short growing season and obtain the necessary nutrients and water to survive the long cold season and early frosts. In studies in the Klamath Mountains of northwestern California and southwestern Oregon, Amaranthus and Perry (1987, 1989a, 1989b) found that mycorrhiza formation most strongly influences seedling survival and growth on sites limited by both moisture and temperature.

BIOTIC CONDITIONS

The importance of aboveground community structure to belowground biological functioning is unclear. It is increasingly apparent, however, that an ectomycorrhizal fungus can link some plant species together through shared fungal mycelia (Bjorkman 1970; Finlay and Read 1986; Read and others 1985). In the natural forest environment, ectomycorrhizal fungi, supported by non coniferous hosts, can actively colonize conifer seedlings. Root-chamber analysis of the development of ecto mycorrhizal mycelium has shown that expanding hyphal fans not only act as nutrient-absorbing structures but also can colonize nonmycorrhizal feeder roots in host-plant combinations within and among host species. Using radioactive labeling, Finlay and Read (1986) have demonstrated the movement of carbon among plants connected by mycorrhizal mycelia. Clearly, the existence of "pipelines" for distributing materials among plant species has important implications for forest regeneration following disturbance.

Little is known of the persistence and distribution of ectomycorrhizae in the absence of living hosts. It has been suggested (Hacskaylo 1973) that ectomycorrhizal fungi do not persist long in the absence of host-supplied substrate. In the Klamath Mountains of northwestern California and southwestern Oregon, sites that have been logged and burned are often rapidly invaded by woody shrubs (Gratkowski 1961). These shrubs, members of the Ericacae and Fagaceae, form mycorrhizae with many of the same fungi as do members of the Pinaceae (Molina and Trappe 1982). These shrubs may preserve mycorrhiza diversity during periods of rapidly changing aboveground community structure.

Amaranthus and Perry (1989b) planted Douglas-fir seedlings at two locales: (1) a site cleared of whiteleaf manzanita (Arctostaphylos viscidi Parry); and (2) a meadow cleared of annual grasses. In the first year, there was more dramatically improved seedling survival and growth on the manzanita site than the adjacent meadow with similar moisture and temperature conditions. By the second year, if basal area growth of surviving individuals is taken into account, Douglas-fir at the manzanita site, on an area basis, was nearly 10 times that of seedlings in the cleared meadow. Douglas-fir seedlings, outplanted at the manzanita site, formed mycorrhizae more rapidly than those seedlings outplanted at the meadow. There were also dramatic shifts in the types of mycorrhizae found on seedlings grown at the two sites. Douglas-fir seedlings, at the manzanita plots, contained significantly higher proportions of Rhizopogon sp. mycorrhizae. Certain Rhizopogon species have been demonstrated to decrease seedling moisture stress and improve seedling outplanted performance. Rapid mycorrhiza formation, with fungi well adapted to site conditions, is key to seedling establishment on sites difficult to regenerate.

Some woody shrub species may act as biological reservoirs, not only of mycorrhizal fungi but of other microflora as well. Significantly higher rates of nitrogen fixationand increased seedling survival and growth were found in association with the mycorrhizae of Douglas-fir seedlings in a stand cleared of whiteleaf manzanita than in a meadow cleared of annual grass (Amaranthus and others 1990). Azospirillium, a nitrogen-fixing bacterium, was isolated with Douglas-fir mycorrhizae at the manzanita site. Whiteleaf manzanita occupies particularly hot, dry sites where fire is frequent. Because high nitrogen losses can accompany intense fire, natural mechanisms by which nitrogen is returned to the soil can be important to forest regeneration.

EFFECT OF NONHOSTS OVER TIME

Many grass and shrub species, such as Ceanothus and Rubus (for example, blackberry, salmonberry), form vesicular-arbuscular mycorrhizae (VAM) with fungi incompatible with members of the Pinaceae (Rose and Youngberg 1981; J. M. Trappe, unpublished data, USDA Forest Service, Pacific Northwest Research Station). On sites long dominated by VAM species, the ectomycorrhizal fungi needed by members of the Pinaceae will gradually diminish, and the soil microbial complex associated with ectomycorrhizae can be reduced. Invasion of sites by nonectomycorrhizal plants over years can seriously affect reforestation (Amaranthus and Perry 1987), particularly in the case of ectomycorrhizal tree species growing on difficult sites where seedlings must establish ectomycorrhizae early to survive.

How long soils retain their mycorrhizal colonization potential in the absence of living hosts is unknown. Ectomycorrhizal spores and hyphal fragments have remained metabolically active after 2 years in Scandinavian forests, though the number of active fragments dropped dramatically over that period (Ferrier and Alexander 1985; Persson 1982). In the Pacific Northwest, mycorrhiza formation generally decreases as the length of time between disturbance and reforestation increases (Perry and others 1989; Pilz and Perry 1984).

CONCLUSIONS

Studies from the Pacific Northwest indicate that reductions in mycorrhizal formation may affect the performance of outplanted seedlings, particularly on severely disturbed or environmentally limited sites where rapid early growth is important. On these sites, planting seedlings with well-developed mycorrhizae and protecting native populations of mycorrhizal fungi aid regeneration. Why mycorrhizal formation is reduced following disturbance in some areas and not others is not understood. Great variability in the physical and biotic environment and disturbance history and intensity are likely contributing factors. One factor that appears to be important is the presence of ectomycorrhizal host plants. On many sites, noncommercial plants may serve as reservoirs of ecto mycorrhizal fungal inocula while conifers are becoming established.

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Paper presented at the Symposium on Management and Productivity of Western-Montane Forest Soils, Boise, ID, April 10-12, 1990.

Michael P. Amaranthus is Soil Scientist, Pacific Northwest Research Station, Forest Service, U.S. Department of Agriculture, 3200 Jefferson Way, Corvallis, OR 97331.