Functional group responses to varying disturbance severities 15 years following biomass harvest in aspen forests

Background/Question/Methods

Evaluating the influence of management actions on resilience in forest ecosystems requires knowledge of the changes in functional groups imparted by such treatments.  We examined the impacts of removing previously non-merchantable material for biofuel production, a practice suggested as a climate change mitigation strategy, on the productivity of different functional groups  in aspen-dominated forests of the Lake States region. We tested whether and how varying levels of organic matter removal (stem only harvest, total tree harvest, total tree and forest floor removal) and compaction (no additional, moderate, and heavy) influence the productivity response of different functional groups.  We identified groups using a hierarchical cluster analysis that classified woody species based on 29 plant traits.  Sampling of species composition and productivity occurred at three Long-Term Soil Productivity Study sites located within the Chippewa National Forest in Minnesota and Ottawa, and Huron National Forests in Michigan.  These sites represent three different soil types (loamy, clay, and sandy, respectively) allowing for assessments of the impacts of soil productivity on disturbance responses 15 years following harvest. 

Results/Conclusions

Of the 29 traits used to define functional groups, 8 proved most useful for distinguishing among species.  These included leaf lifespan, leaf size, leaf type (deciduous vs. evergreen), maximum age, seed mass, wood decay rate, nutrient acquisition strategy, and leaf mass per area.  Responses observed in regeneration varied both among functional groups and among sites.  On clay and sandy soils, where species richness was generally lower, no differences among treatments were observed in above-ground productivity for any functional group.  On loamy soils, however, two functional groups did exhibit significant differences in response to compaction.   Increasing compaction negatively impacted the functional group containing longer living, slower growing shrubs such as Acer spicatum and Dirca palustris (F=5.63, p=0.0126) but appeared to favor the functional group comprising trees with long leaf lifespans and high leaf mass per area like Abies balsamea (F=5.78, p=0.0115).  Interestingly, the group including Populus tremuloides showed no response to treatments despite results indicating that treatments do impact P. tremuloides when considered individually.  Results suggest that practices associated with biomass harvests may influence some functional groups more than others, depending on methods and site conditions, and that responses of some species can be obscured through the functional group approach.