from which the first colonists are pe- 
riodically removed differ from control 
plots. Another question of interest is 
whether succession tends to converge 
toward a single community type or 
whether initial site differences persist 
indefinitely. This question may be an- 
swered by comparing the initial degree 
of vegetation heterogeneity with that 
found five or ten years later. Signifi- 
cantly reduced variety would imply 
that convergence is indeed occurring. 
Several slower and less publicized 
mudflows were driven by glaciers 
melting on the east, south, and west 
slopes of Mount St. Helens. Last Sep- 
tember, I visited the upper portion 
of the six-mile-long Pine Creek mud- 
flow, which was propelled by the now 
nearly defunct Shoestring Glacier on 
the southeast flank of the volcano. 
Although the preeruption vegetation 
in this location is not known, the area 
still reveals interesting aspects of vege- 
tation recovery. The heat and toxic 
fumes of the main eruption killed the 
sparse lodgepole pine ( Pinus contorta) 
and subalpine fir ( Abies lasiocarpa) 
scattered on the high ridges above 
Pine Creek. The subsequent mudflow 
destroyed creek vegetation but it 
merely buried the snow-covered dor- 
mant ground cover of the ridge. Ini- 
tially there was no vegetation on the 
mudflow, which varied in thickness 
from a few inches near the treeline 
to more than five feet at its upper 
end. The Cascade aster (Aster led- 
ophyllus) was the first plant to emerge 
through this mud. Other herbs, in- 
cluding the broad lupine (Lupinus lat- 
ifolius) and Newberry’s knotweed 
(Polygonum newberryi), did not 
emerge until light rains created small 
erosion channels along which these 
plants were confined. Where mud was 
more than about a food deep, no vege- 
tation emerged in 1980. 
Since mud seal's the soil and can 
limit oxygen exchange, buried vege- 
tation may be suppressed through the 
inhibition of root respiration. An al- 
ternative mechanism, however, may 
operate on mudflows and in areas of 
heavy ash deposition. Both mud and 
ash insulate the soil and create a light 
barrier, two characteristics of a snow- 
pack. Many plant species here and 
elsewhere may be fooled into “think- 
ing” that winter continues. If this is 
so, as the 1980/81 w inter snow pack 
melts and further erosion cleanses the 
mountain, surviving vegetation will 
emerge, having missed a growing sea- 
son but otherwise unscathed. 
North of Pine Creek lies Abraham 
Plains, a site that supported only 
limited vegetation prior to the May 
1980 eruption. After the initial erup- 
tion, pyroclastic and hot-ash flows 
melted snowfields on the mountain’s 
northeastern flank. The resultant mud- 
flows covered or removed all vege- 
tation on these plains. Revegetation 
in this area, unlike Pine Creek, will 
thus depend entirely on seed immi- 
gration. 
The differences between recovery 
due completely to immigration and 
that abetted by residual vegetation 
will be documented in permanently 
marked plots at Abraham Plains and 
Pine Creek. Questions about succes- 
sion under stressful conditions will be 
addressed. Do survivors and immi- 
grants belong to the same species? Or, 
as in the case of Abraham Plains, is 
the habitat so severe that pioneers are 
the only species capable of survival? 
Are differences in species composition 
due to dispersal failures or simply to 
the absent species’ inability to grow 
in unaltered mudflow material? An- 
swers to such questions can be applied 
to the future reclamation of derelict 
Soon after the eruption, herbaceous 
vegetation began returning to recent 
clear-cuts within the blast zone. 
The pink flowers of fireweed, a 
colonizing species of disturbed 
areas, were a common sight. 
43 
