Anderson, Paul D. 2007.Understory vegetation responses to initial thinning of douglas-fir plantations undergoing conversion to uneven-age management. In: Deal, R.L., ed. Integrated Restoration of Forested Ecosystems to Achieve Multi-Resource Benefits: Proceedings of the 2007 National Silviculture Workshop; 2007; Portland, OR: U.S. Department of Agriculture, Pacific Northwest Research Station.18p.
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.