David A. Perry

Net transfer of carbon between ectomycorrhizal tree species in the field

Different plant species can be compatible with the same species of mycorrhizal fungi, and be connected to one another by a common mycelium,. Transfer of carbon, nitrogen, and phosphorus, through interconnecting mycelia has been measured frequently in laboratory experiments, but it is not known whether transfer is bidirectional, whether there is a net gain by one plant over its connected partner, or whether transfer affects plant performance in the field,. Laboratory studies using isotope tracers show that the magnitude of one-way transfer can be influenced by shading of ‘receiver’ plants,, fertilization of ‘donor’ plants with phosphorus, or use of nitrogen-fixing donor plants and non-nitrogen-fixing receiver plants,, indicating that movement may be governed by source–sink relationships. Here we use reciprocal isotope labelling in the field to demonstrate bidirectional carbon transfer between the ectomycorrhizal tree species Betula papyrifera and Pseudotsuga menziesii, resulting in net carbon gain by P. menziesii. Thuja plicata seedlings lacking ectomycorrhizae absorb small amounts of isotope, suggesting that carbon transfer between B. papyrifera and P. menziesii is primarily through the direct hyphal pathway. Net gain by P. menziesii seedlings represents on average 6% of carbon isotope uptake through photosynthesis. The magnitude of net transfer is influenced by shading of P. menziesii, indicating that source–sink relationships regulate such carbon transfer under field conditions.

Influence of red alder on soil nitrogen transformations in two conifer forests of contrasting productivity

Abstract We conducted laboratory studies to determine the effects of red alder ( Alnus rubra Bong.) on soil N transformations and N availability indices at two conifer forest sites of contrasting productivity. The inclusion of red alder in conifer forests significantly increased gross rates of N mineralization, N immobilization, nitrification and NO 3 − immobilization, and the effects of alder were generally similar for soils from low- and high-productivity sites. However, the addition of alder to the conifer stand at the high productivity site increased gross N mineralization and immobilization processes more than at the low productivity site. At both sites, gross N and NO 3 − production were enhanced by alder more than gross N immobilization processes, leading to higher rates of net N mineralization and nitrification. At the fertile site, most microbial N assimilation occurred from the NO 3 − pool, compared with less than half at the infertile site (none as NO 3 − in the less productive pure conifer stand). Heterotrophic nitrification (as indicated by a lack of C 2 H 2 inhibition) accounted for 65–72% of the gross nitrification in all stands that exhibited nitrification (no nitrification was detected in the pure conifer stand at the infertile site). The inclusion of red alder had no effect on the proportion of total nitrification that was heterotrophic, despite the lower soil pH in mixed alder-conifer stands compared to conifer stands. Gross rates of N mineralization correlated well with both autotrophic and heterotrophic nitrification across all soils. Gross N mineralization may be a good index of NH 4 + availability to autotrophic nitrifiers, as well as the quality of organic N as a substrate for heterotrophic nitrification. Most estimates of microbial biomass and activity, N availability and N transformation rates were significantly correlated with each other. In general, gross N transformations were better correlated with other indices of N availability and microbial activity than estimates of net N transformations. Similar N cycling rates and microbial biomass N pool sizes in pure alder and adjacent alder-conifer stands at the fertile site suggest that continued inputs of N via symbiotic N-fixation by red alder in coniferous forest stands can lead to the elimination of N-limitation to forest ecosystem production.

Ectomycorrhizal mediation of competition between coniferous tree species

To test the effect of ectomycorrhizal fungi (EMF) on interactions between host plants, Pseudotsuga menziesii (Mirb.) Franco and Pinus ponderosa Dougl. ex. Laws., seedlings were grown in replacement series in pasteurized soil with (a) no EMF added, (A) two EMF species added - Rhizopogon vinicolor Smith (specific to Douglas-fir) and R. ochraceorubens Smith (specific to pine), and (c) tour EMF species added - the two Rhizopogon species plus two host generalists, Laccaria laccata (Scop, ex Fr.) Bk. & Br, and Hebeloma crustuliniforme (Bull.) Quel. A replacement series in unpasteurized forest soil also was included. Seedlings without added EMF were colonized by the greenhouse contaminant, Thelephora terrestris. Without added EMF (but with T. terrestris), the tree species mutually inhibited one another, producing Relative Yield Totals significantly < 1; with EMF added, mutual inhibition disappeared. With four EMF species added, Pseudotsuga menziesii seedlings were significantly larger in mixture than in monoculture, with no corresponding decrease in the size of Pinus ponderoso seedlings; this was due solely to seedlings with L. laccata, which apparently enhanced nitrogen (N) and phosphorus (P) uptake by Pseudotsuga menziesii at the expense of luxury consumption by Pinus ponderosa. Graphical analysis suggested that better growth of Pseudotosuga menziesii in mixture with EMF added was related to improved P nitrogen. Both N and P nutrition of Pinus ponderosa was better in mixture with two than no EMF species added; there was no clear nutrient effect with four EMF species added. Results indicate that EMF can reduce competition between plant species and perhaps increase overall community P uptake. However, patterns were specific to both EMF and tree species and were quite different in unpasteurized soils. Hence generalizations about the effects of EMF on plant-plant interactions must be made cautiously.

Self-organizing systems across scales

Over the past few years, ecologists have increasingly recognized the existence of strong self-reinforcing (or self-organizing) interactions within systems at a variety of scales. Positive feedback within food chains has been reported from terrestrial and aquatic ecosystems. Accumulating evidence supports the existence within communities of cooperative guilds - tit-for-tat relationships based on diffuse mutualisms and favored by environmental unpredictability. At the landscape level, both real world experience and models indicate that processes such as hydrology and the propagation of disturbance can be strongly self-reinforcing (i.e. the landscape structure supports the process, and vice versa). Hence the picture emerges of a hierarchy of self-organizing systems that span food chains, communities and landscapes/regions.

Allelopathic effects of litter on the growth and colonization of mycorrhizal fungi

In laboratory studies, water-soluble extracts of the litter of four shrub and three conifer species had variable effects on the growth of four species of ectomycorrhizal fungi. In general, low concentrations (parts per thousand) stimulated fungal growth; while, high concentrations (parts per hundred and parts per ten) either stimulated growth, inhibited growth, or had no effect, depending on both fungal and litter species. In greenhouse studies, litter applied to the surface of a sand-soil mixture reduced the formation ofRhizopogon sp. on Douglas-fir seedlings. Allelochemicals in the litter may inhibit seedling growth and suppress fungai growth and root colonization in the field, thus explaining the failures of reforestation by conifer species in disturbed sites.

Mycorrhiza inoculum potentials in tropical secondary succession

Vesicular-arbuscular mycorrhizae can be indispensable for establishment and growth of tree seedlings in infertile lowland wet tropical soils. Persistence of mycorrhizal fungi after disturbance, however, is problematic to assess. We used a greenhouse bioassay employing Psidium guajava L. and Allium cepa L. to estimate the most probable number of mycorrhizal fungus propagules in two Costa Rican soils (Oxic Dystropepts) with different vegetation histories. We collected soils at the La Selva Biological Station from sites in secondary forest, abandoned pasture, and plots kept bare of vegetation for four and six years. Both bioassay hosts yielded positively correlated estimates of mycorrhiza propagule numbers. Median propagule estimates per 100 gm dry soil for the pasture site are 57 and 63; these estimates significantly exceed those for the other sites which range from 0.2 to 10 for bare plots and were estimated to be 0.6 and 10 for secondary forest. These bioassay estimates are positively correlated (Pearson r = 0.66 and 0.72) with counts of whole spores in these soils, but not with counts of sporocarps, spore clusters, or the most numerous, spores apparently empty of cytoplasmic contents or parasitized. Growth of both bioassay hosts in pasture soil significantly exceeded growth in soils from the other three sites in accord with the bioassays and whole spore counts.

The Effects of Postfire Salvage Logging on Aquatic Ecosystems in the American West

Abstract Recent changes in the forest policies, regulations, and laws affecting public lands encourage postfire salvage logging, an activity that all too often delays or prevents recovery. In contrast, the 10 recommendations proposed here can improve the condition of watersheds and aquatic ecosystems.

The functioning of ectomycorrhizal fungi in the field: linkages in space and time

Individual trees, either of the same or different species, can be linked spatially and temporally by the hyphae of ectomycorrhizal (ECM) fungi that allow carbon and nutrients to pass among them and promote forest establishment following disturbance. Spatial and temporal linkages between plants influence the function of ECM fungi in the field. Studies indicate that ECM linkages can reduce plant competition for resources, promote forest recovery, and influence the pattern of plant succession. The degree of influence depends on many factors, including the composition and arrangement of the vegetative community and soil and climatic conditions. Management practices that create intense disturbance and loss of organic matter or promote the introduction of non-ectomycorrhizal host species can decrease the ability of plants to form linkages with ECM fungi. Management practices that retain living trees and shrubs and input of organic matter provide the energy source and substrate necessary for ECM linkages. More research is needed to determine the degree to which ECM fungal linkages occur in the field and their role in ecosystem function and long-term health.

Ecosystem concepts and current trends in forest management: Time for reappraisal

Abstract Large areas of natural forest ecosystems are being converted into industrial plantations in tropical, subtropical and temperate regions. In the short term, this seems to maximize returns from investment and to homogenize the raw material base for forest industries. It is argued here that society will continue to use more wood and foresters must produce it; in doing so, however, it is imperative that the immense importance of biotic diversity in containing pests and pathogens, in maintaining current levels of production in perpetuity, and in moderating the global climate should be addressed. Current criteria for economic evaluation do not adequately address these and other important considerations accruing from the management of whole forest ecosystems.

Seedling growth and mycorrhizal formation in clearcut and adjacent, undisturbed soils in montana: A green-house bioassay

Abstract In a series of greenhouse bioassays, tree seedling growth and root-tip development were compared among soils from two areas that had been clearcut and site prepared 16 years earlier and adjacent, undisturbed forest. Seedlings grown in soil from logged areas were shorter and had fewer root tips than those grown in undisturbed forest soil; however, effects on seedling weights were not consistent among species or experimental series. Fertilizing with nitrogen eliminated size differences due to soil source. In Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], the only species tested in sterilized soils, sterilization of logged soils resulted in increased seedling size and root-tip formation, whereas sterilization of unlogged soils had either no or a negative effect.