Logging residue harvest effects on plant community dynamics in commercial Populus stands of the Great Lakes region

Background/Question/Methods

Contemporary energy policies have created an increased demand for biomass feedstock.  In forested regions, these are expected to take the form of short rotation woody crops and logging harvest residues (topwood and non-merchantable bolewood).  Commercial aspen (Populus spp.) forests of the Great Lakes region provide roundwood for traditional timber products such as pulp and panel board, while harvest residues offer potential utilization to the emerging bioenergy market.  In some regions, pulp and paper mills already utilize these byproducts as fuel in combustion for heat and electricity.  The effects of removing high nutrient content topwood are poorly understood when assessing vegetative biodiversity and long-term site productivity.  We hypothesize that a decrease in the structural and nutrient gradients across the site associated with residue removal will result in: (1) an initial decrease in species richness and diversity; (2) an increase in vegetative evenness and; (3) an increase in plant community diversity, richness, and evenness through stand development.  We established a chronosequence of 27 stands along a commercial aspen stand rotation (40 years), both with and without removal and on a single soil taxon.  Plant species between treatments and across the chronosequence were sampled to identify changes in plant community structure.   

Results/Conclusions

Results show that vascular plant species richness and Shannon’s diversity index exhibited a significant positive relationship with stand development, from harvest year across the 40 year chronosequence.  However, there was no significant difference detected between residue retention and removal treatments.  In addition, there was no observable pattern in plant community evenness between treatment types or stand age. We suspect that the primary effect of residue removal may be related to: (1) additional site disturbance, if any, from either (a) multiple passes from forwarding equipment for topwood removal, or (b) soil scarification from whole tree skidding and; (2) within-stand spatial heterogeneity in the plant community induced by retention of logging residues in discrete piles.  Further, any effect of residue removal could be masked by inefficient collection methods or incidental breakage during a whole tree harvest skid operation.   We have compiled an inventory of forest floor and mineral soil carbon, nitrogen and macronutrient content to further investigate residue harvest effects to site productivity and plant community response. We will extend this inventory to examine the effect of residue distribution in space.