Aspen harvest residue removals over 40 years increase understory plant diversity

Forest harvesting that removes residues traditionally left on site can supply traditionally unused biomass for products and energy. In response to questions and concerns over the ecological implications of harvest residue removal, biomass harvesting guidelines that include provisions to retain specific quantities or proportions of residues or restrict harvesting altogether on some sites have been developed by states and other entities. Many such provisions are based on “best professional judgment” with supporting technical evidence lacking. One such question receiving relatively little attention is the effect of harvest residue management on understory plant species. 

The long-term effects of harvest residue management on plant species diversity were studied as part of a larger investigation initiated in 2011 across a 40-year chronosequence of aspen sites in Michigan’s Upper Peninsula. The sites are owned by Weyerhaeuser Company (formerly owned by Plum Creek Timber Company) and have a documented history of residue removal or retention, providing a unique opportunity to examine impacts on forest productivity, soils, carbon storage, and ecosystem characteristics.

A recently published paper from the work reported residue removals across the chronosequence had no effect on overstory communities but increased understory plant species diversity and other abundance metrics, presumably a result of increased site disturbance and light penetration to the forest floor.

The paper, which appears in Forest Ecology and Management, was written by M.I. Premer, R.E. Froese, and C.R. Webster of the School of Forest Resources and Environmental Science at Michigan Technological University, and L.M. Nagel of the Department of Forest and Rangeland Stewardship at Colorado State University. The study was supported by Weyerhaeuser Company (and formerly Plum Creek Timber Company), the Ecosystem Science Center at Michigan Technological University, and NCASI. Dr. Eric D. Vance managed NCASI involvement in the study. Following is the abstract.

“Conservation of vegetation diversity has been integrated into forest management regimes as stands with high levels of diversity have been shown to possess enhanced ecological services and resilience to disturbance. Effects of intensified forest management and harvesting practices such as logging residue removals for bioenergy on vegetation diversity and community dynamics has been largely overlooked, however. We examined forest vegetation communities under residue removal treatments along a unique 40-yr chronosequence of commercial aspen (Populus spp.) in Upper Michigan to examine potential impacts on species abundance metrics, community structure, and heterogeneity. We hypothesized that residue removals would negatively impact species diversity of both the over- and understory communities, and that these patterns would occur in parallel with shifts in community structure of both strata. Our results show no effect of residue treatment on overstory tree communities; however we did observe an increase in understory species richness, diversity, and evenness in stands where residues were harvested. The understory community structure was correlated with soil total nitrogen levels, yet this pattern was independent of harvest residue treatment. In addition, we observed higher levels of variance in species representation of the understory community when residues were removed, but did not detect any difference in community organization. Overall, our results suggest that the removal of harvest residues can increase variability in vegetation community relationships. We believe that these patterns can be attributed to increased site disturbance and soil scarification through additional equipment trafficking and subsequent changes in micro environmental conditions when residues are harvested.”

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Reference 

Premer, M.I., R.E. Froese, and C.R. Webster. 2016. Vegetation response to logging residue removals in Great Lakes aspen forests: Long-term trends under operational management. Forest Ecology and Management 382:257-268. http://dx.doi.org/10.1016/j.foreco.2016.09.048