PHYSIOLOGICAL AND MORPHOLOGICAL ATTRIBUTES OF 2 COLONIZING PLANT-SPECIES ON MOUNT-ST-HELENS

Substrates deposited during the 1980 eruption of Mount St. Helens are being colonized by numerous plant species. Anaphalis margaritacea and Epilobium angustifolium are herbaceous species that are common colonists on lower elevation pyoclasic flows (Pumice Plains). Precious studies have shown that exogenous seed rain of E. angustifolium is higher than that of A. margaritacea on to the Pumice Plains but the abundance of individual colonists is reversed. This study compared water relations, nutrient relations, mycorrhizal associations and root morphology of these species to determine factors controlling seedling establishment on the Pumice Plains. Diurnal patterns of water potential were similar between the two species, and turgor loss points of -1.3 to -1.4 MPa indicate that neither species is particularly drought-tolerant. Anaphalis margaritacea had higher leaf conductance and closed stomata at lower water potentials than E. angustifolium. There was a strong growth response to nitrogen-amended, but not to phosphorus-amended, pyroclastic material by both species, with a significantly greater response in E. angustifolium. On the Pumice Plains, mycorrhizal associations were not found on plants of either species, but in a nearby clear-cut forest, mycorrhizal infection rates were high for both species. Four-week-old seedlings of A. margaritacea developed a deeper, more branched root system and had a higher relative growth rate in low nitrogen pyroclastic soils than E. angustifolium. This morphological attribute appears to be an important reason why A. margaritaceae is the more successful colonizer at Mount St. Helens.