Changes in vegetation after disturbance are often slow, complex, and involve several different community characteristics. Changes in structure and composition are most often studied, but environmental relationships also define a community, may change during succession, and are seldom studied. Here we describe changes in relationships of understory plant cover with abiotic factors and with importance of other plants following deposition of tephra (aerially-transported volcanic ejecta) in intact old-growth subalpine conifer forests following the 1980 eruptions of Mount St. Helens, Washington. Two sites with 4.5 and two with 15 cm tephra were sampled using permanent 1-m2 plots. At each tephra depth, one site was herb-rich with much snow when tephra was deposited and the other, herb-poor with little snow. We removed tephra from 50 plots per site during 1980 and used 1981 samples from these to represent pre-eruption vegetation. To describe succession, 100 plots per site with undisturbed tephra were sampled repeatedly. We related canopy tree presence, microtopography, large wood, light, snow cover, and cover of understory strata to cover of each growth form and five widespread species for three times: pre-eruption, year 1, and year 30. We considered only relationships significant in both single- and multi-factor analyses.
Results/Conclusions
We identified many significant relationships: 34 pre-eruption, 54 in year 1, and 73 in year 30, but these differed with time. Only 3 pre-eruption relationships persisted in year 1, when most of the significant relationships were with tephra depth and factors modifying its impact. The ratio of relationships with factor-types differed: 7:16:11 pre-eruption; 41:5:8 in year 1; and 46:16:11 in year 30, where the values represent the three factor-types, abiotic factors : canopy trees : understory growth form cover, respectively. The number of significant relationships declined from year 1 until year 20 and then increased rapidly. Relationships with canopy tree presence and tephra depth were primarily negative; those with snow and woody debris included a mixture of signs; and those with canopy gaps, light intensity, and cover of other understory plants were primarily positive. Smaller growth forms had more significant relationships than larger ones. Thus, disturbance and community re-development were reflected clearly in number and type of environmental relationships with plant cover. The disturbance produced an almost complete turnover in relationships, which increased in number over 30 years but included few pre-eruption relationships.