Although the maintenance of biodiversity has become one of the goals in ecosystem management, the relationships of diversity to ecosystem characteristics such as level of herbivory, productivity, and vegetation structure are still poorly understood. We examined these relationships in 8 native grassland sites differing in grazing histories and range condition in the Mixed Grassland (6), Moist Mixed Grassland (1) and Aspen Parkland (1) ecoregions of southern Saskatchewan. Range condition, assessed using standard methods, ranged from fair to excellent. The Shannon's diversity index followed a curvi-linear relationship with range condition, increasing from fair to good, but decreasing from good to excellent condition, within a range between 0.66 and 2.58. Species evenness was affected by range condition in a similar manner ranging from 0.44 to 0.86. Species richness varied among sites and plots between 4 and 28 plants 0.25 m -2 , but changed little with range condition. Most structural parameters, such as the cover, height, or thickness of standing plants (live or dead) and litter, increased with range condition especially from good to excellent. The Shannon's diversity index was positively correlated with forb biomass, but not with biomass of any other group or their combination. Grazing regimes that maintain good range condition also maintain species and structural diversity of grasslands.
Disappearance of dead plant material was studied in ungrazed mixed grass prairie in Western North Dakota. Disappearance rates varied both within and between years as well as with method used. The highest rates (2.07 to 3.15 g/m/day) were recorded early in the season and disappearance continued during the winter but at a slower rate (0.99 g/m/day). Some differences were found in rates of disappearance of dead material of different species during the first two months. Annual decomposition rates ranged from 400 g/m to 526 g/m. The ash content of dead material showed generally increasing values with advancing decomposition. The turnover time of above-ground biomass appeared to be approximately three years.
Ready-to-plant nursery stock of jack pine (Pinus banksiana Lamb.) and white spruce (Picea glauca (Moench) Voss) were subjected to simulated acid rains ofpH 5.6 (control), 4.6, 3.6, and 2.6 under controlled growth room conditions to determine the threshold for damage. The two species exhibited contrasting response patterns to simulated acid rain treatments. Growth and chlorophyll content of jack pine were not adversely affected at any of the levels applied. Furthermore, no macroscopic foliar injury symptoms were evident and foliar nutrient levels remained unaffected. White spruce showed no statistically significant reduction in growth rate at any of the treatments. Needle chlorophyll a content of white spruce was not affected by any of the treatments, while chlorophyll b level was significantly reduced at pH 2.6. Symptoms of visible injury were observed to increase with rain acidity. No statistically significant change in the levels of foliar nutrients of white spruce were found, with the exception of the elevated S concentration at pH 2.6.
Carbon dioxide effluxes from plants, litter and soil were measured in two mixed-grassland sites in Saskatchewan, Canada. Ecosystems at both locations were dominated by Agropyron dasystachyum (Hook.) Scribn. Respiration rates of intact and experimentally-modified systems were measured in field chambers using alkali-absorption. Removal of green leaves, dead leaves, and litter from a wet sward reduced respiration to as low as 58% of the rate in an intact system. In a dry sward green shoots were the only significant above-ground source of CO.Carbon dioxide effluxes from different parts of A. dasystachyum plants, and from soil samples were measured in laboratory vessels at 20° using alkali-absorption. Respiration of green leaves (1.46 mg CO g h) was significantly higher than microbial respiration in moist, dead leaf samples (0.79 mg CO g h) or litter (0.75 mg CO g h). Microbial respiration in air-dried, dead plant material was very low. Average repiration rates of roots separated from soil cores (0.24 mg CO g h) were lower than many values reported in the literature, probably because the root population sampled included inactive, suberized and senescent roots. Root respiration was estimated to be 17-26% of total CO efflux from intact cores.Laboratory data and field measurements of environmental conditions and plant biomass were combined in order to reconstruct the CO efflux from the shoot-root-soil system. Reconstructed rates were 1.3 to 2.3 times as large as field measured rates, apparently because of stimulation to respiration caused by the experimental manipulations. The standing dead and litter fractions contributed 26% and 23% of the total CO efflux in a wet sward. Both field-measured and reconstructed repiration values suggest that in situ decomposition of standing dead material under moist conditions can be a significant part of carbon balance in mixed grassland.
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