- Understory vegetation in artificially created tree canopy gaps in the Pacific
Northwest was studied to determine 1) variation in understory vegetation cover
between gap edges and gap centers, as well as between control and treatment plots,
2) spatial patterns of biomass and difference in biomass patterns among plots, 3)
individual species responses to gap creation and 4) the relationships between
species dominance and diversity by site and treatment. Data were collected in 1990
and 1997 in 16 plots (two controls and two treatments at each of four study sites:
the H. J. Andrews Experimental Forest and Martha Creek, Panther Creek, and
Trout Creek in the Wind River Experimental Forest). Two of the sites are oldgrowth
stands (250-500 years old), while the other two sites are mature stands (90-
150 years old).
Six growing seasons after gap creation, average percent cover of understory
vegetation was greater within gaps than at gap edges. Vegetation cover had
increased significantly in all artificial gaps, and it had increased more in gaps than
in controls. Vegetation cover increased more at Martha Creek, a mature stand, than at H. J. Andrews and Trout Creek, the two old-growth stands, or at Panther Creek,
a mature stand.
Understory biomass increased more in gaps than in controls and the amount
of increase varied by site. Initial understory biomass was highest in Panther Creek
and Martha Creek, but the magnitude of biomass increase was greatest in Trout
Creek and H. J. Andrews. Biomass patterns in 1990 and 1997 were more patchy in
old-growth stands than in mature stands.
Vegetation cover of most understory species increased from 1990 to 1997 in
artificial gaps, but species' responses were often site specific. In some cases,
species with advantageous dispersal mechanisms (such as rhizomes, stolons, or
clonal growth, e.g. Rubus ursinus, or Acer cinncinatum) increased in cover and
biomass more than species without such advantageous dispersal mechanisms.
Cover of weedy species such as Epilobium angustfolium and Lactuca muralis
increased dramatically (up to 25 times) in artificial gaps, but weedy species
represented less than two percent of the total average cover in 1997.
Species dominance and diversity did not respond consistently following gap
creation. Species dominance was relatively high (species with highest dominance
was 30 g/m2) and diversity relatively low (38 species) at Martha Creek, a mature
stand, whereas species dominance was low and diversity high at H. J. Andrews (9
g/m2, 51 species), an old-growth site. Panther Creek, a mature stand, and Trout
Creek, an old growth stand, had intermediate dominance and high diversity (17
g/m2, 57 species at Panther Creek and 18 g/m2, 41 species at Trout Creek).