EFFECTS OF DISTRUBANCE ON MICROBIAL COMMUNITIES

Ken Cullings

This research addresses Astrobiology Objective 14 which deals with ecosystem responses to disturbance. Functional biodiversity results from both absolute species numbers in any ecosystem and interactions among members of the community, and plays a pivotal role in the resilience of ecosystems to disturbance and environmental change. Ecosystem reliability can increase when the number of species per functional group increases, thus illustrating the value of functional redundancy or similarity in an ecosystem. On the other hand, gains or losses of species that perform different functions can cause ecosystem processes to be significantly altered. Thus, a controversial aspect of functional diversity is whether species exhibiting similar function are actually redundant, and thus expendable.

This research targets microbes, and a common view is that the potential for redundancy or similarity among microbial species is high. Specifically, we focus on one pivotal mutualistic interaction, the ectomycorrhizal (EM) symbiosis, a plant/fungal interaction that controls both carbon and nitrogen cycles in forest ecosystems. This research has two steps. First, to assess changes in the EM community in response to disturbance. Second to determine if there are functional changes in the ecosystem that results from any changes.

1) Artificial Defoliation (manuscript in press at Oecologia). No previous study has been conducted of effects of altered carbon available to roots in systems comprised of more than a single tree species. Results indicated no significant effect on either EM colonization or on species richness. However, the relative abundance of EM of the two tree species shifted from a ratio of approximately 6:1 without treatment (lodgepole EM:spruce EM), to a near 1:1 ratio post-treatment. In addition, EM species composition changed significantly post-defoliation. Species of EM fungi associating with both lodgepole pine and Engelmann spruce were affected, indicating that alteration of photosynthetic capacity of one species can affect mycorrhizal associations of neighboring non-defoliated trees. Finally, while some fungal species may exhibit consistent specificity patterns (for example Suillus tomentosus to P. contorta) other fungal species shifted host preference in response to the change in source of fixed carbon induced by defoliation.

2) Effects of litter addition on a stand of pure lodgepole pine, P. contorta, (data complete, manuscript to be submitted to Oecologia). Molecular analyses indicate that 1) litter addition significantly increases EM infection levels in the top soil layer, directly adjacent to the added litter. No change is seen with perlite addition. Thus, this response is due solely to nutrient changes imposed by litter; 2) the EM community is altered significantly by litter addition. Species dominant in controls may be lost in response following treatment, and some species increase only in response to litter but not to perlite, further illustrating the role of changes in nutrient status;

3) Effects of litter removal on a mixed lodgepoile pine/Engelmann spruce (P. engelmannii) stand (data are complete, manuscript is in preparation): Results of molecular analyses indicate that, 1) litter removal significantly decreased EM fungal species richness, from 3.0 to 1.5 species/core; 2) as expected from previous studies that indicate that increased nitrogen in litter can inhibit EM infection, litter removal induced a significant increase in EM infection, from a mean of 228 EM/core in controls to 326 in treatments; 3) furthermore, molecular analyses indicate that while many basidiomycete fungal species are common to both treatments and controls, the ratio of basidiomycetes to ascomycetes changed significantly in response to litter removal, from 12:1 ratio of basidiomycete to ascomycete EM, to a 3:1 ratio.

Together, these results indicate that these disturbances can causes changes in the EM fungal community. Because different species may perform different functions, these results indicate that it is now necessary to assess changes to pivotal ecosystem functions. Thus, our next step will be to perform assessments of changes in enzyme systems that are responsible for controlling both nitrogen and carbon cycles in forest ecosystems.