Seedling Growth, Physiology, and Survivorship in a Subalpine, Volcanic Environment

The relationship of seedling survivorship to specific physiological or mor- phological traits is usually only inferred in demographic studies. We compared seedling survivorship, growth, morphology, and water relations of two subalpine herbs (Eriogonum pyrolifolium and Polygonum newberryi) on Mount St. Helens in order to examine the relationship between population and functional biology more directly. Survivorship of two cohorts of seedlings (1982 and 1983) was followed through 4 and 3 yr, respectively. Seedling growth was measured in two substrate types (pre-eruption soil and 1980 deposited tephra) and under different levels of nutrients and water under controlled environmental conditions. Water relations parameters of the two species were determined in conjunction with the laboratory growth experiment. The 1982 cohort of Eriogonum had a significantly higher rate of survival than did that of Polygonum over four growing seasons, while the 1983 cohort did not differ in survival over three growing seasons. Survival rates were closely related to the amount of rainfall, with Eriogonum having significantly higher survival during dry periods than Polygonum. Growth of both species was greater in pre-eruption soil than in 1980 deposited tephra; the difference was much more pronounced in Polygonum. Growth of Eriogonum seedlings was also less sensitive to lower nutrient and water availability in the laboratory experiment than growth of Polygonum. Eriogonum had lower osmotic potentials, higher stomatal conductance, greater fine root biomass, and lower intrinsic growth rate than did Polygonum. These appear to be important characteristics contributing to greater stress tolerance and higher seedling survivorship in Eriogonum.