Thinning of a Ponderosa Pine/Douglas-Fir Forest in South-Central BC: Impacts on Understorey Vegetation

Abstract

The ponderosa pine and Douglas-fir forests in the Interior of British Columbia, Canada, are facing problems such as forest in-growth mainly due to fire suppression, reducing grazing land area. This study focused on the use of thinning to reduce forest stand density and restore understorey species diversity and increase aboveground biomass productivity. Data were collected over a 4-year period. Species richness and diversity were generally lower under the canopy of ponderosa pine and Douglas-fir trees than outside the canopy. Species diversity was reduced by thinning at one site but unaffected by thinning at the other. Total understorey biomass increased up to 80% within 3 to 4 years after thinning. Depending on site and year, biomass production of one or more plant groups, such as forb, shrub, or graminoid, increased. Reduction in litter depth was observed at both sites due to the removal and/or reduction of needle sources. Variations in species composition existed among blocks and between sites, suggesting greater sampling size may be needed in future research to better capture the spatial variability. Thinning reduces stand density and thereby reduces fuel load and enhances understorey species productivity.