Structural Analysis and Dynamics of the Plant Communities of Wizard Island, Crater Lake National Park

Abstract

Wizard Island, a ½—square—mile volcanic cinder cone located in Crater Lake, Oregon, was sampled by continuous belt transects. Separation of the one herbaceous and four forested community types, which were mapped as major vegetation units, was remarkably distinct, as 60% of the herbaceous species were found in only one or two communities. Physical factors such as the distribution and stability of substrate materials, temperature extremes, and soil—moisture availability exert the greatest control over plant community structure and development. Biotic factors have reduced effects on the generally sparsely vegetated island. The unstable cinder slope community was dominated by Polygonum newberryi Small., Eriogonum pyrolaefolium Hook. var. coryphaeum T. & G., and Arenaria pumicola Cov. & Leib. Pinus albicaulis Engelm. constituted 55% of the importance value of the crater rim community, but appears to be declining in importance. Pinus contorta is invading the crater area and may slowly replace P. albicaulis. Other characteristic crater rim species were Holodiscus microphyllus Rydb. var. glabrescens (Greenm.) Ley. and Castilleja applegatei Fern. The lower cone and north slope forests were dominated by Abies magnifica Murr. var. shastensis Lemm. and Tsuga mertensian (Bong.) Carr., with Pinus monticola Dougl. the only other important tree species. An importance value of 51% for T. mertensiana in the north slope community as compared with 25% in the lower cone reflects the cooler, moist microclimate and greater community maturity on the north slope. The more tolerant T. mertensiana increases in importance as the stands mature. Characteristic shrub and herb species were Vaccinium membranaceum Dougl. and Luzula glabrata (Hoppe) Desv. in the north slope, and Arctostaphylos nevadensis Gray and Pyrola secunda L. in the lower cone communities, respectively. The lava flow community was largely restricted to recesses between lava ridges where moisture and soil conditions are improved. Tsuga mertensiana was the dominant tree species on the lava flow at 51% importance value, with Sambucus microbotrys Rydb. and Cheilanthes gracillima D. C. Eaton as indicators of the shrub community development was and herb strata. Diversity, total plant density, and herbaceous and shrub community development was greatest in the favorable forest microclimates of the north slope. The north slope, crater rim, lower cone, cinder slope, and lava flow communities had 28, 23, 21, 12, and 10 nonarborescent species, respectively. The reported vascular flora of the island comprises 105 species and varieties. The best forest development occurred in an encircling belt at about 6,400—6,500 ft (1,950—1,980 m) elevation, but tree species characteristic of an elevational range of 2,500 ft (820 m) are telescoped onto the 760—ft—(250—m) high island. Substrate and local climatic diversity and climatic moderation due to lake effects and shielding by the caldera wall permit survival of trees above their usual elevation range. Tree invasion on the upper cone (particularly of Tsuga mertensiana) is slowed more by unstable slopes than by climatic severity. Mat—forming plants, such as Arctostaphylos nevadensis and Penstemon davidsonii Greene, anchor the substrate and serve as nurseries for tree seedlings. Ecesis of tree seedlings on the lava flow occurs only in crevices which exhibit some soil development and microclimatic amelioration. Log density—size class plots gave a straight line relationship for north slope tree species, but all other community types had marked density reductions in some size classes. As such, the north slope community is considered to be in equilibrium at the present level of substrate and microclimatic alteration.