|Title||Change in diversity and nitrogen accrual following clearcutting and burning in bigtooth aspen ecosystems in northern lower Michigan|
|Year of Publication||2000|
|Academic Department||School of Natural Resources and Environment|
|Degree||Master of Science|
|Number of Pages||130 pp.|
|University||University of Michigan|
|City||Ann Arbor, MI|
Fire, wind, flooding, and predation maintained natural ecosystem properties and processes in presettlement time. However, today most of these natural disturbance and ecosystem-regulating mechanisms have been eliminated or severely constrained by human activities. The structure and function of forested ecosystems at UMBS, Cheboygan and Emmet Counties, northern lower Michigan, are currently quite different compared to those of presettlement time due to clearcutting, severe or repeated fires, fragmentation, and in the last 80 years, fire exclusion. These disturbances resulted in the establishment and spread of early-successional forests dominated by bigtooth aspen (Populus grandidentata Michx.), trembling aspen (Populus tremuloides Michx.), and other pioneer species where pine-oak-hemlock forests once existed. In addition, populations of white-tailed deer (Odocoileus virginianus Zimmerman), which heavily browse young seedlings of oak, hemlock, and maple, have almost doubled over the last 50 years in northern Lower Michigan (MDNR 1999). Over time the forests at UMBS are slowly reverting back to a condition more similar to the presettlement forest. However, it is unclear to what extent the severe human-caused perturbations have affected the ability of this ecosystem to regain its former composition, structure, and function. The Biological Station has a chronosequence of experimentally clearcut and burned plots located within one high-level outwash plain ecosystem, which provides an opportunity to study changing composition, biomass, and nutrient cycling within these forests at different stages following disturbance. Two adjacent ecosystems were used to statistically distinguish and compare ecosystems. A multivariate discrimiant analysis integrating physiographic, soil, and ground-cover variables verified the ecological distinctness of two closely related outwash plain ecosystems. Total overstory and understory basal area was not statistically different between these ecosystems. A pre-treatment analysis of experimental plots which continued the burn chronosequence at UMBS demonstrated no significant difference for overstory basal area prior to experimental clearcutting and burning of the bigtooth aspen dominated forest and a signficantly greater total understory basal area in the large plot to be treated with clearcutting and burning. Average herbaceous richness and combined richness were signficantly greater in the unburned megaplot and two heterogeneity indices were signficantly greater in the plot to be burned. The burn plot was nearly devoid of all vegetation in June 1998. The number of herbaceous species present in 1999 had increased by 28% from 1997 pre-treatment levels compared to woody plant species, which demonstrated a net decrease of 26% from 1997-1999. Significantly higher maximum daytime temperatures and lower minimum nighttime temperatures were demonstrated in the burn area. Periodic clipping of bigtooth aspen ramets resulted in an increase in the number of woody species, whereas a shaded and relatively cool microclimate provided by aspen ramets allowed for twice as many herbaceous species. Nitrogen mineralization and nitrification showed important accrual patterns for the burn chronosequence, with rates being highest immediately following disturbance, decreasing over the first 20 years of succession, and then increasing asymptotically over the next 70 years. The UMBS permanent sample plots, spanning nearly a century, continue to provide important information on how fire exclusion and competition influence ecosystem dynamics and successional trends over time. In conclusion, it appears that clearcutting, burning, and aspen removal have significantly different effects on the composition and abundance of woody and herbaceous species, and that nutrient availability does limit productivity during succession.