No abstract

Variation in the intensity of competition has been proposed as a mechanism that may control the species diversity and composition of many kinds of communities, including North American prairie. Competition intensity is predicted to increase with soil fertility and decrease with disturbance, causing variation in the abundances of competitively subordinate species. Variation in competition intensity was examined in an experiment in which fire and fertilizer were applied to native mixed—grass prairie. Treatments consisted of annual burns, a single burn, and untreated control, and fertilization (N ≥ 15 replicates) applied to 7 x 5 m plots for two growing seasons. The experiment was repeated at a second site 15 km away. Fire reduced standing crop and litter, and increased the frequency of bare ground. Nutrient addition increased standing crop and decreased bare ground. Species composition and diversity were largely unaffected by fire. Two common species, Bouteloua gracilis and Carex obusata, were significantly more abundant in fertilized vegetation than in controls at both sites; species diversity decreased in fertilized plots. Vegetation responses to the treatments were similar at the two sites. To tests for variation in competition intensity with fire and nutrient availability, removal experiments were performed in each treatment at one site. Two tussocks each of Festuca ovina and Stipa spartea were chosen in each plot. One plant of each pair had all neighbors within 30 cm clipped and removed during two growing seasons. Each species grew larger and faster in the absence of neighbors, but the magnitude of this effect did not vary with fire, fertilizer, or standing crop. Fire and nutrient addition produced significant changes in the community structure of mixed—grass prairie but did not alter competition intensity in the manner predicted.

Prairie marshes in western Canada occur in the Manitoba, Saskatchėwan, and Alberta plains, from 49 to between 50 and 53° N. They are dynamic habitats characterized by fluctuations in water levels and water chemistry. Marsh habitats form an east–west gradient that involves a decrease in precipitation (525 – 350 mm year−1) and an increase in its variability. Within the region, water salinity ranges from fresh to hypersaline. Water levels and chemistry coupled with life history features influence the distribution and successional role of emergent macrophytes. Five dominant emergents have aboveground standing crops of between 425 (Scirpus lacustris ssp. glaucus) and 1750 g m−2 (Typha latifolia). They vary in reproductive strategy and tolerance to water levels and salinity. Scirpus lacustris ssp. glaucus grows in deep water with conductivities < 15 mS cm−1, and spreads vegetatively. Typha latifolia, tolerant of water fluctuations and salinity < 10 mS cm−1, spreads both vegetatively and by seed, particularly under drawdown. Phragmites australis tolerates a wide range of moisture conditions and salinities (< 20 mS cm−1). It propagates primarily by rhizomes and runners. Scirpus lacustris ssp. validus thrives under unstable water regimes but is saline intolerant (< 2 mS cm−1). It is short lived (±3 years) and reproduces primarily by seed. Scirpus maritimus var. paludosus, which dominates western wetlands, tolerates hypersaline conditions (±45 mS cm−1), prefers changing water levels, and spreads both vegetatively and by seed. These environmental and life history features help elucidate wetland dynamics.

A dense stand of Phragmites australis (Cav.) Trin. ex Steudel in the Delta Marsh was divided into a grid of 20 experimental plots. Three different burn treatments (August 1979, October 1979, and May 1980) were each applied to four plots, with the remaining plots as controls. Shoot biomass was greater after spring and fall burns in comparison with the controls but less on summer-burned plots. Total shoot density was higher after all burning treatments in comparison with the controls. Flowering shoot density was lower after summer and fall burns in comparison with the controls but higher following spring burns. All burn treatments resulted in lower mean shoot weight than on controls primarily as a result of greater densities of shorter, thinner vegetative shoots. Belowground standing crop was higher by mid-September of 1980 on spring- and fall-burned plots but not on those burned in the summer. The seasonal minimum total nonstructural carbohydrate contents of rhizomes were reduced after summer and spring burns in comparison with the controls.

A greenhouse experiment was conducted to examine the effects of water depth on the growth and reproduction of Scirpus maritimus L. Plants grown at or above the water surface had higher shoot survivorship, greater numbers of vegetative tillers, and higher underground biomass, while seed production was small. With increasing water depth plants had taller shoots and greater seed production but total biomass, numbers of vegetative tillers, and underground biomass were reduced. This shift from clonal growth to seed production with increasing water depth is interpreted as a strategy that permits survival of S. maritimus populations through the wet and dry climatic periods of the Canadian prairies.