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Sunlight can have a significant impact on freshwater aquatic communities. Using 64‐ bit GRASS, a previously generated 18.2 m resolution forest canopy height grid for the State of North Carolina was used as a base elevation layer for calculation of 18.2 m resolution total irradiance (Watt‐hours/square meter/day) grids for 365 days of the year. Daily calculations were aggregated annually and overlaid on rasterized 1:24,000‐scale USGS hydrology with canopy type and seasonal filters to quantify annual solar irradiation input to streams and rivers in North Carolina. Bare earth calculations of solar irradiation for the aquatic habitats were compared to the canopy filtered calculations. There was substantially less solar irradiation in aquatic habitats under the canopy filtered model. Total solar irradiation for subbasins created from point locations of occurrences of Dwarf wedgemussel (Alasmidonta heterodon) were compared with total solar irradiation for watersheds created from locations with no mussels for the full subbasin, 1 km, and 500 m upstream from the sampling point. There was no significant difference in the amount of total solar irradiation modeled for the subbasins with Dwarf wedgemussel and watersheds with no mussels.  相似文献   
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Soil compaction and substrate modification produced during large-scale armored military maneuvers in the early 1940s were examined in 1981 at seven sites in California’s eastern Mojave Desert Recording penetrometer measurements show that tracks left by a single pass of an M3 “medium” tank have average soil resistance values that are 50% greater than those of the surrounding untracked soil in the upper 20 cm At one site, measurements made along short segments of track that have been visually eliminated by erosion and deposition processes show a 73% increase in penetrometer resistance over adjacent, undisturbed soils Dirt roadways at three former base camp locations could not be penetrated below 5–10 cm because of extreme compaction Soil bulk density was not as sensitive an indicator of soil compaction as was penetrometer resistance Density values in the upper 10 cm of soil are not significantly different between tank tracks and undisturbed soils at most sites, and roadways at two base camps show an average increase in bulk density of only 12% over adjacent soils. Trench excavations across tank tracks show that physical modifications of the substrate can extend vertically beneath a track to a depth of 25 cm and outward from a track’s edge to 50 cm These soil disturbances are probably major factors that encourage accelerated soil erosion throughout the manuever area and also retard or prevent the return of vegetation to pre-disturbance conditions  相似文献   
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