|Abstract or Summary
- A severe wildfire burned 454 hectares of a second-growth Douglas-fir forest
in 1977 on a north-facing slope in Pattee Canyon, near Missoula, Montana. The
slope was aerially seeded with a grass mixture, from which Dactylis glomerata
established best. Community structure, conifer regeneration, and the impact of the
seeded grass on the plant community were evaluated with two data sets. One set
tracked postfire vegetation development from 1979 to 1987 on permanent transects
established on upland sites in areas of varying fire severity. The second data set
was collected in 1989 on upland sites nearest to the bum edge where conifer
regeneration was expected to be greatest.
Most stands converged to a similar ordination space by 1987, showing that
several key species which established in the initial postfire year determined
community structure. These species were largely on-site survivors (including
Calamagrostis rubescens, Physocarpus malvaceus, and Spiraea betulifolia) and the
seeded grass, D. glomerata. The spread of this species by 1987 to stands that
apparently escaped seeding in 1977 suggested that this species may persist at this
Patterns of species abundance and distribution in 1989 were primarily
controlled by factors summarized by a topographic-moisture index and by pre-burn
disturbance history. Three general site types were described by an ordination.
Areas that appeared as open woodlands in 1937 occurred on ridges and had more
xerophytic vegetation in 1989, including native grasses. Conifer regeneration in this
region was limited primarily to Pseudotsuga menziesii (Douglas-fir). Areas that
appeared as an even, young forest in 1937 were on open slopes and were associated
with Pinus contorta (lodgepole pine), Larix occidentalis (western larch), Vaccinum
globulare, and Amelanchier alnifolia. Areas that appeared as an older, uneven-aged
forest in 1937 were on open slopes near the upper burn edge and were characterized
by Douglas-fir and Spiraea betulifolia in 1989.
D. glomerata was more successful on the drier ridges and was negatively
associated with Calamagrostis rubescens. Competition with C. rubescens, rather
than differences in environmental tolerances, most likely restricted D. glomerata to
the more xeric sites. An adverse effect of D. glomerata on conifer regeneration was
most likely for western larch on xeric sites.
Site factors and historical factors were most important in determining patterns
of conifer regeneration. Regeneration was moderate on mesic slopes and sparse on
xeric ridges. The extreme density of lodgepole pine regeneration (13,000
stems/hectare) in one mesic area reflected the importance of serotiny for post-fire
regeneration of this species. Douglas-fir regeneration ranged from 370 stems/ha in
a mesic area where no mature survivors were noted to 4045 stems/ha on a mesic
slope near survivors. On mesic slopes near survivors, western larch regeneration
was 857 stems/ha and was minimal elsewhere.
A regression model confirmed the importance of site factors, site history, and
availability of seed source for Douglas-fir and lodgepole pine. The model for
western larch was only able to explain 14% of regeneration pattern, suggesting that
microsite variation as well as other variables would be needed to predict
regeneration for this species.