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The relationship of seedling survivorship to specific physiological or morphological 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.

Lupinus lepidus, a prostrate, evergreen perennial and Lupinus latifolius, a robust, deciduous perennial were prominent survivors and among the first colonizers of early successional habitats following the 1980 eruption of Mount St. Helens. In this study, interspecific and age-specific variation in the water relations and photosynthetic properties of these lupines were assessed to determine their physiological capacity to colonize these volcanic habitats. Comparative water-relations studies showed that seasonal and diurnal trends in plant water balance were similar between species and age classes. Stomatal conductance and xylem pressure potential of seedlings and adults were closely coupled with soil water potential and leaf-to-air vapor-pressure gradients. Relatively high osmotic potentials were linked with high leaf water contents and cell-wall elasticity. Collectively, these water-relations properties are indicative of mesophytic, herbaceous species with leaf tissues sensitive to soil and atmospheric water deficits. The photosynthetic capacity of L. lepidus was higher over a wider range of environmental conditions than that of L. latifolius. L. lepidus had a higher light saturation point and was able to acclimate photosynthetically to higher temperatures, relative to L. latifolius. The photosynthetic activity of L. lepidus seedlings was significantly greater at higher light and temperature regimes compared to seedlings of L. latifolius. Differing photosynthetic responses to temperature and light help explain the contrasting demographic patterns of these lupines and the relative dominance of L. lepidus in early successional habitats on Mount St. Helens. The physiological capacity to respond to extreme temperatures and periodic water deficits also appears to be an important determinant of the natural distribution patterns of these two species in the Pacific Northwest: L. lepidus occupies windy, exposed alpine habitats, whereas L. latifolius is common within subalpine meadows and coniferous treeline communities. Key words: bulk-tissue water relations; drought stress; Lupinus latifolius; Lupinus lepidus; Mount St. Helens, Washington, USA; photosynthesis; plant colonization; plant demography; seedling ecophysiology; seedling establishment, constraints; volcanic environments, colonization., INTRODUCTION Volcanic eruptions in 1980 radically modified the physical and biological structure of alpine and subalpine plant communities on Mount St. Helens (Washington, USA). Over 20% of the alpine-subalpine vascular [...]