Spatial distribution of understory vegetation in tree canopy gaps of the Pacific Northwest

Understory vegetation in artificially created tree canopy gaps in the PacificNorthwest was studied to determine 1) variation in understory vegetation coverbetween 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 betweenspecies dominance and diversity by site and treatment. Data were collected in 1990and 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, andTrout Creek in the Wind River Experimental Forest). Two of the sites are old-growth 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 understoryvegetation was greater within gaps than at gap edges. Vegetation cover hadincreased significantly in all artificial gaps, and it had increased more in gaps thanin 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 amountof increase varied by site. Initial understory biomass was highest in Panther Creekand Martha Creek, but the magnitude of biomass increase was greatest in TroutCreek and H. J. Andrews. Biomass patterns in 1990 and 1997 were more patchy inold-growth stands than in mature stands.
Vegetation cover of most understory species increased from 1990 to 1997 inartificial gaps, but species' responses were often site specific. In some cases,species with advantageous dispersal mechanisms (such as rhizomes, stolons, orclonal growth, e.g. Rubus ursinus, or Acer cinncinatum) increased in cover andbiomass more than species without such advantageous dispersal mechanisms.Cover of weedy species such as Epilobium angustifolium and Lactuca muralisincreased dramatically (up to 25 times) in artificial gaps, but weedy speciesrepresented less than two percent of the total average cover in 1997.
Species dominance and diversity did not respond consistently following gapcreation. Species dominance was relatively high (species with highest dominancewas 30 g/m2) and diversity relatively low (38 species) at Martha Creek, a maturestand, whereas species dominance was low and diversity high at H. J. Andrews (9g/m2, 51 species), an old-growth site. Panther Creek, a mature stand, and TroutCreek, an old growth stand, had intermediate dominance and high diversity (17g/m2, 57 species at Panther Creek and 18 g/m2, 41 species at Trout Creek).