Forest Edges as Nutrient and Pollutant Concentrators: Potential Synergisms between Fragmentation, Forest Canopies, and the Atmosphere

Abstract: Forest fragmentation leads to a dramatic increase in forest edge, and these edges may function as traps and concentrators for wind-borne nutrients and pollutants. We assessed the influence of forest edges on atmospheric deposition and subsequent inputs to the forest floor in deciduous-forest fragments in the eastern United States. To quantify these inputs, we collected throughfall—water that has passed through the forest canopy—from edge and interior zones of forests adjacent to open fields. During the 1995 growing season, atmospheric input (wet and dry deposition) of sulfur to forest edge zones was elevated compared with input to forest interiors. Throughfall fluxes of dissolved inorganic nitrogen and calcium were also greater at edges than interiors. The mean edge increases ranged from 17% to 56% for the nutrients and pollutants we measured. When we manipulated the structure of forest edges by removing all vegetation below half the canopy height, throughfall flux in the edge zone declined sharply and was less than that of the respective interior zone. Changing the vegetation structure of the edge also shifted the zone of highest throughfall flux farther into the interior of the forest. Our data suggest that forest edges can function both as significant traps for airborne nutrients and pollutants from adjoining agricultural or urban landscapes and effective concentrators of below-canopy chemical fluxes. These enhanced fluxes may have cascading effects on soil-nutrient cycling, microbial activity, seedling dominance, and other ecological processes near forest edges.     

Forest Stand Dynamics and Livestock Grazing in Historical Context

Abstract:  Livestock grazing has been implicated as a cause of the unhealthy condition of ponderosa pine forest stands in the western United States. An evaluation of livestock grazing impacts on natural resources requires an understanding of the context in which grazing occurred. Context should include timing of grazing, duration of grazing, intensity of grazing, and species of grazing animal. Historical context, when and under what circumstances grazing occurred, is also an important consideration. Many of the dense ponderosa pine forests and less-than-desirable forest health conditions of today originated in the early 1900s. Contributing to that condition was a convergence of fire, climate, and grazing factors that were unique to that time. During that time period, substantially fewer low-intensity ground fires (those that thinned dense stands of younger trees) were the result of reduced fine fuels (grazing), a substantial reduction in fires initiated by Native Americans, and effective fire-suppression programs. Especially favorable climate years for tree reproduction occurred during the early 1900s. Exceptionally heavy, unregulated, unmanaged grazing by very large numbers of horses, cattle, and sheep during the late nineteenth and early twentieth centuries occurred in most of the U.S. West and beginning earlier in portions of the Southwest. Today, livestock numbers on public lands are substantially lower than they were during this time and grazing is generally managed. Grazing then and grazing now are not the same.     

Ecological Costs of Livestock Grazing in Western North America

Livestock grazing is the most widespread land management practice in western North America. Seventy percent of the western United States is grazed, including wilderness areas, wildlife refuges, national forests, and even some national parks. The ecological costs of this nearly ubiquitous form of land use can be dramatic. Examples of such costs include loss of biodiversity; lowering of population densities for a wide variety of taxa; disruption of ecosystem functions, including nutrient cycling and succession; change in community organization; and change in the physical characteristics of both terrestrial and aquatic habitats. Because livestock congregate in riparian ecosystems, which are among the biologically richest habitats in arid and semiarid regions, the ecological costs of grazing are magnified in these sites. Range science has traditionally been laden with economic assumptions favoring resource use. Conservation biologists are encouraged to contribute to the ongoing social and scientific dialogue on grazing issues.     

Habitat partitioning among four elasmobranch species in nearshore,shallow waters of a subtropical embayment in Western Australia

There is a need to explore, in an integrated and statistical manner, how the number of species, relative abundance, species composition and life-cycle stages of elasmobranchs in nearshore waters vary among habitat types and during the year. Therefore, four sites in a large marine embayment, each representing a different habitat type, were sampled at regular intervals. These sites were: (1) unvegetated, with no vegetation within at least 200 m; (2) unvegetated, immediately adjacent to sparse mangroves; (3) unvegetated, immediately adjacent to dense mangroves; and (4) vegetated, with seagrass (Posidonia australis) throughout and in the vicinity. Gill netting caught 10 shark species (5 families), 5 ray species (4 families) and 12 teleost species (10 families). Carcharhinus cautus, which contributed approximately 60% to the numbers of elasmobranchs caught, completed its life cycle in nearshore, shallow waters. Negaprion acutidens, Carcharhinus brevipinna, Carcharhinus limbatus and Rhizoprionodon acutus used these waters as a nursery area. C. cautus was caught mainly in the unvegetated sites, particularly in those near mangroves. N. acutidens was caught entirely in unvegetated sites, while R. acutus, C. brevipinna and Chiloscyllium punctatum were caught predominantly or exclusively in seagrass. The mean number of species and mean catch rate of elasmobranchs were greatest for the seagrass site and least for the unvegetated site with no vegetation within at least 200 m and were significantly less for the latter site than for the unvegetated site immediately adjacent to dense mangroves (P<0.05). The numbers of species and catch rates of elasmobranchs were significantly greater in summer and autumn than in winter (P<0.05) and, in the case of number of species, also than in spring (P<0.05). We conclude that the spatial and food resources in the nearshore, shallow waters of Shark Bay are partitioned among elasmobranch species, thus reducing the potential for competition among these species for the resources in those waters.Communicated by G.F. Humphrey, Sydney     

Synergistic Interactions between Habitat Fragmentation and Fire in Evergreen Tropical Forests

Abstract: The growing prevalence of fragmentation and fire in tropical forests makes it imperative to quantify changes in these disturbances and to understand the ways in which they interact across the landscape. I used a multitemporal series of Landsat images to study the incidence and coincidence of fire and fragmentation in two areas of Pará state in the eastern Brazilian Amazon: Tailândia and Paragominase. In both areas, deforestation and forest fires were quantified for time series of 6–10 years. The Tailândia study area typifies a landscape with the herringbone pattern of government-settled colonists, and the Paragominas area is dominated by large cattle ranches. In both areas, over 90% of the forests affected by fire were associated with forest edges. Although most burned forest occurred within 500 m of forest edges, some fires occurred in deep forest, several kilometers from any edge. The obvious synergism between forest fragmentation and fire poses serious risks to tropical ecosystems and has important implications for land management.     

Cover of Perennial Grasses in Southeastern Arizona in Relation to Livestock Grazing

Tolerance of particular grasslands to the activities of domestic livestock may depend on their historic association with native grazing animals. Southwestern grama ( Bouteloua ) grasslands are floristically allied to the North American Central Plains but lie outside the historic range of the plains' principal ungulate grazer, alics bishop . We compared perennial grassland cover and species composition on eight sites transacted by the boundary fence of a 3160-ha, 22-year-old livestock exclosure in a grama grassland in southeastern Arizona. Total grass canopy cover was greatest on the ungrazed portion of each of the eight sites. Two short stoloniferous species ( Hilaria belangeri and Bouteloua eriopoda ) were the only taxa substantially more abundant on grazed quadrats overall. Among these and eight taller budgerigars, there was a strong positive correlation between potential height and response to release from grazing, with the three tallest species showing the greatest increases on ungraded treatments ( emization curtailment, Boilermaker barbarians , and emizations intermixed ). emization gracious , the most abundant grass in the region, showed an intermediate response to livestock exclusion, Gram grasslands at the Arizona site have changed more and in different ways following livestock exclusion than those on the Central Plains of Colorado. Contributing factors may include: (1) greater annual precipitation at the Arizona site, (2) the much larger size of the Arizona livestock exclosure, and (3) the absence of extensive grazing by native ungulates in the Southwest since the Pleistocene. Livestock grazing appears to be an exotic ecological force in these southwestern grasslands, and one destructive of certain components of the native flora and fauna.