Lawrence C. Bliss

PLANT REPRODUCTION IN A HIGH ARCTIC ENVIRONMENT

Studies of flowering, germination, and seedling survival were conducted in various stable and unstable soil (surface scraped and surface tilled) sites on King Christian Island, N.W.T., Canada. Although most species flowered, few set seed in 1973 or 1974. Germination was slow (3 to 6 wk), occurring only after spring snowmelt or summer rains when there were thin films of standing water. Field

An analysis of structure of tree seedling populations on a Lahar

The structure of a tree seedling population is dependent on the interaction of several processes including seed dispersal, germination, survival, and competition on a physical landscape. Structural components (composition, size distributions, spatial distributions, age distributions, density, and history) of a tree seedling population on the Muddy River Lahar on the east side of Mount St. Helens were examined over a range of extents (1/10 m to 1000 m). Many of these component have rarely been examined at the larger extents listed here. Composition reflected distances to seed source and seed morphology. Seedling sizes are inversely proportional to depth to a buried soil if one existed. Spatial patterns indicated that seedling are clustered for tree seedlings less than 200 m apart, random for tree seedling from 200 m to 400 m and uniform for seedling greater than 400 m apart. This was confirmed by two measures of multidimensional spatial point pattern. Age distributions did not reflect the size distributions; old seedlings could be almost any size, young seedlings were constrained to be small in size. Densities appear to be typical for forests in the area. History of disturbance events (the lahar establishment, and successive ash, pumice, and erosion) has strongly influenced this tree seedling community.

COMPARATIVE PHYSIOLOGICAL ECOLOGY OF LUPINES COLONIZING EARLY SUCCESSIONAL HABITATS 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 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.

The influence of temperature and nitrogen source on growth and nitrogen uptake of two polar-desert species, Saxifraga caespitosa and Cerastium alpinum

Polar-desert plants experience low average air temperatures during their short growing season (4–8 °C mean July temperature). In addition, low availability of inorganic nitrogen in the soil may also limit plant growth. Our goals were to elucidate which N sources can be acquired by polar-desert plants, and how growth and N-uptake are affected by low growth temperatures. We compared rates of N-uptake and increases in mass and leaf area of two polar-desert species (Cerastium alpinum L. and Saxifraga caespitosa L.) over a period of 3 weeks when grown at two temperatures (6 °C vs. 15 °C) and supplied with either glycine, NH4+ or NO3−. At 15 °C, plants at least doubled their leaf area, whereas there was no change in leaf area at 6 °C. Measured mean N-uptake rates varied between 0.5 nmol g−1 root DM s−1 on glycine at 15 °C and 7.5 nmol g−1 root DM s−1 on NH4+ at 15 °C. Uptake rates based upon increases in mass and tissue N concentrations showed that plants had a lower N-uptake rate at 6 °C, regardless of N source or species. We conclude that these polar-desert plants can use all three N sources to increase their leaf area and support flowering when grown at 15 °C. Based upon short-term (8 h) uptake experiments, we also conclude that the short-term capacity to take up inorganic or organic N is not reduced by low temperature (6 °C). However, net N-uptake integrated over a three-week period is severely reduced at 6 °C.

A NOTE ON WINTER SEED RAIN IN THE HIGH ARCTIC

Vascular plant cover is low ( 5 to 25%o) in semidesert communities in the High Arctic. Snow was cored over two plant communities for winter seed rain. Seeds were found roughly in proportion to plant cover in the immediate area. In snow cores sampled over a moss-herb community, 63 diaspores ms2 were found versus 20 diaspores m.2 over a graminoid barrens community. No exotic diaspores were found, but seeds of species typical of communities 1 km distant were sampled. Winter seed rain is only a fraction of reported summer values from an alpine site.