Friday, August 8, 2008

PS 74-42: Community reassembly potential following proposed thinning in a mixed conifer forest: Lessons from the seedbank

Gabrielle Snider, Oregon State University, Becky Kerns, Western Wildlands Environmental Threat Assessment Center, USFS, and Michelle Buonopane, Pacific Northwest Research Station, USFS.


Millions of acres of forest land in the West are slated for fuels reduction under the Healthy Forest Restoration Act of 2003. High priority for thinning is placed at the Wildland Urban Interface where the consequences of a wildfire are great. These areas tend to have a high occurrence of noxious weeds due to the presence of roads, multiple dispersal agents, and repeated disturbance from human activities. Land managers face a paradox: restoration efforts to reduce wildfire threats may simultaneously increase threats of noxious weed spread into interior forestland from thinning operations. Understanding the reassembly potential of the plant community from the seedbank will help managers determine which mitigations may be necessary to reduce noxious weed spread following thinning operations. Our primary question asks how the seedbank in a closed canopy, mixed conifer forest located at the Wildland Urban Interface may contribute to plant community reassembly following forest restoration efforts like thinning.  We evaluated seedling emergence in a greenhouse from litter/duff and mineral soil samples from seven harvest units. Samples were grown in the greenhouse for five months following three months in cold-moist stratification. We evaluated differences in species composition and density between the litter/duff and mineral soil layers and compared them to the existing above-ground vegetation. We also evaluated differences in species composition between herbicided and un-herbicided units and in seedbank samples from herbicided and un-herbicided units.  


Significantly more seedlings emerged from the litter/duff layer than from the mineral soil layer. Seedlings of forest herbs were more common in both layers than were trees, shrubs or grasses.  Noxious weeds made up less than 1% of the above ground vegetation and were confined to sites nearest to roads. Noxious weed seedling emergence was generally low in the greenhouse study.  Total seedling emergence was low and indicated the seedbank may not play a large role in community reassembly following thinning in these systems.