Understory Vegetation Responses to Initial Thinning of Douglas-fir Plantations Undergoing Conversion to Uneven-Age Management

Since the mid-1990’s, several studies have been undertaken to evaluate thinning treatments designed to promote development of complex forest structure in order to enhance ecological functioning and biological diversity. Most of these studies have focused on forest responses to one or possibly two thinning entries. In contrast, this study, the Unevenaged Management Project (UAMP), is evaluating alternative silvicultural regimes for converting young Douglas-fir stands to mixed-species, uneven-aged condition. Conversion is to be achieved through regulation of stocking with repeated thinnings over an indefinite period. The relatively low intensity thinning treatments applied as a first entry in the conversion process had little impact on the abundance, size, or diversity of understory vegetation. Disturbance resulted in short-term decreases in understory vegetation cover, particularly tall shrubs. However, within five years of treatment, understory vegetation abundance returned to approximate pretreatment condition. Regardless of treatment, shrubs and ferns dominated the understory with coverage that was two-three times that of forbs and grasses. Species richness averaged near 12 species per 0.1-ha plot before treatment and five years post treatment. Community composition was dominated by a few very abundant species regardless of treatment. Post-thinning increases in conifer regeneration were consistent with the density of underplanting. Substantial increases in tree regeneration occurred in the light thinning treatment due to an increased seedling density of the shade-tolerant bigleaf maple. The persistence of planted and natural seedlings of less shade-tolerant species remains to be determined. The general lack of understory vegetation response to the thinning treatments was likely due to the inherent resistance and resilience of the plant communities to disturbance, as well as the low intensity of disturbance attributable to the treatments. Over time, repeated frequent thinning entries may challenge this apparent community stability.