SEEDLING ESTABLISHMENT IN FORESTS AFFECTED BY TEPHRA FROM MOUNT ST. HELENS

Seedling density on permanent plots at five sites was monitored for the first four summers following the deposition of 4.5 to 15 cm of tephra from the 1980 eruption of Mount St. Helens. Because the old-growth forests at the sites were not destroyed by the volcanic eruption, the plots were under a normal tree canopy. Almost no seedlings established in 1980. By 1983 tree seedling density exceeded 35 m-2 at all sites. Tsuga heterophylla seedlings were most common, with Abies amabilis seedlings next in abundance. A dense layer of small trees may develop on the tephra and long-term forest dynamics could be affected. There was no successful invasion by species not already present in the stands. Seedlings of some forest herbs, especially Tiarella trifoliata, were common, but many other common forest species produced none. It is still problematical if or when these species will return to areas of the forest from which they were extirpated. FOLLOWING DISTURBANCE, vegetation cover can be restored either by survival and regrowth of plants initially present or by establishment of new genets. The relative importance of these two sources for revegetation depends on the kind of vegetation affected and on the intensity and type of disturbance. Only a few studies have examined vegetation changes following deposition of tephra (volcanic aerial ejecta) in adequate detail to separate these two processes (Griggs, 1918, 1919a, b; Eggler, 1959, 1963). The 18 May 1980 eruption of Mount St. Helens produced a wide array of disturbances (Lipman and Mullineaux, 1981; Means et al., 1982). Beyond the devastated zone, an extensive area of forest was affected by tephra of insufficient depth to kill canopy trees. Damage to the herb and shrub layers varied from little to almost complete destruction, depending upon tephra depth and the condition of the site at the time of the eruption (Antos and Zobel, 1982, 1985). In a previous paper (Antos and Zobel, 1985) we reported patterns of recovery of plants present before the disturbance. Here we present data on seedling populations on the tephra during the first four summers following the volcanic eruption. MATERIALS AND METHODS-Study sites were at 880 to 1,290 m elevation in old-growth, subalpine forests northeast of Mount St. Hel' Received for publication 29 May 1985; revision accepted 30 October 1985. We thank Tom Hill for field assistance. This work was supported by the National Science Foundation, USDA Science and Education Administration, and the U.S. Forest Service Pacific Northwest Forest and Range Experiment Station. ens (Antos and Zobel, 1985). Abies amabilis, Tsuga heterophylla, T. mertensiana, and Pseudotsuga menziesii were the most common tree species. Prior to the eruption, a shrub layer dominated by Vaccinium spp. occurred at all sites, although the herb layers varied from rich to depauperate. The climate is wet maritime with a pronounced winter precipitation maximum, producing snowpacks in excess of 2 m deep. Tephra was 4.5 cm deep at sites B and C, 7.5 cm deep at site D and 15 cm deep at sites E and F. The bulk of the deposit was coarse-textured, bounded both above and below by layers of silt-sized material. Particle size increased with tephra depth. At each site we permanently marked 150 plots (1 m2) along transects. During summer 1980, we cleared all tephra from 50 of these (cleared plots) and left the remaining 100 plots unmanipulated (natural tephra plots). We recorded cover and density of plants established prior to the volcanic eruption and seedlings established subsequent to it during the summers of 1980 through 1983. Observations were made once each summer. We tallied seedlings by species and separated current year from previous year seedlings where possible. To monitor survival, we recorded the location of some seedlings in the plots. In addition, we mapped all seedlings in 10 plots at site B and 9 plots at site D two or three times during 1981 and 1982, and once during 1983. Characteristics of the tephra surface are critical to seedling establishment; we recorded these in the plots. Because the data were not normally distributed, we used nonparametric statistical tests (Sokal and Rohlf, 1981), Wilcoxon's signed ranks test for paired comparisons of plots be-