Comparative physiological ecology of lupines colonizing early successionalhabitats on Mount St. Helens

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 Mo unt St. Helens. In this study, interspecific and age-specific variation in the water relations and photosynthetic properties of these lupines were ass essed 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. Stoma tal conductance and xylem pressure potential of seedlings and adults were c losely coupled with soil water potential and leaf-to-air vapor-pressure gra dients. Relatively high osmotic potentials were linked with high leaf water contents and cell-wall elasticity. Collectively, these water-relations pro perties 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 highe r light saturation point and was able to acclimate photosynthetically to hi gher temperatures, relative to L. latifolius. The photosynthetic activity o f L. lepidus seedlings was significantly greater at higher light and temper ature regimes compared to seedlings of L. latifolius. Differing photosynthetic responses to temperature and light help explain th e contrasting demographic patterns of these lupines and the relative domina nce of L. lepidus in early successional habitats on Mount St. Helens, The p hysiological capacity to respond to extreme temperatures and periodic water deficits also appears to be an important determinant of the natural distri bution patterns of these two species in the Pacific Northwest: L, lepidus o ccupies windy, exposed alpine habitats, whereas L. latifolius is common wit hin subalpine meadows and coniferous treeline communities.