ISSN:0012-9658    

DOI:10.2307/1940294

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1940295

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1940296

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1940297

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

In the state of Para, Brazil, in the eastern Amazon, we studied the potential for sustained fire events within four dominant vegetation cover types (undisturbed rain forest, selectivity logged forest, second—growth forest, and open pasture), by measuring fuel availability, microclimate, and rates of fuel moisture loss. We also estimated the potential tree mortality that might result from a wide—scale Amazon forest fire by measuring the thermal properties of bark for all trees in a 5—ha stand of mature forest, followed by measurements of heat flux through bark during simulated fires. Partial logging resulted in dramatic increases in downed woody debris. Total fuel mass was significantly greater in the logged forest (180 Mg/ha) compared to the other cover types (30—60 Mg/ha). However, the readily combustible fine—fuels (e.g., grasses, litter, herbs) were significantly greater in pastures (° 11 Mg/ha) than in all other cover types (° 6 Mg/ha). Anthropogenic disturbance altered microclimate, which in turn affected rates of fuel moisture loss and the dynamic equilibrium of fuel moisture contents. In pastures of average midday temperature was almost 10oC greater, and the average midday relative humidity was 30% lower, than in primary forest. There was a sixfold difference in average vapor pressure deficit between the primary—forest and the open—pasture cover types. Given the relatively steep gradient between the vapor pressure of the fuel particles and the surrounding atmosphere in the disturbed communities, fuel moisture loss was more rapid and equilibrium moisture contents were lower than in primary forest. Based on the changes in fuels and microclimate, we determined that cattle pastures were the most fire—prone ecosystem. During much of the 6—mo "dry" season (total rainfall: 200—400 mm), sustained combustion was possible in this community within 24 h following rainfall events. Openings in the selectively logged forest would burn after 5—6 rainless days and in the second—growth forest after 8—10 d. In contrast, sustained combustion was not possible in the primary forest even after prolonged rainless periods (e.g.,>30 d). Through an examination of bark tissues and simulated fires in primary forest we found that only a small percentage of the standing vegetation would likely survive even a low—intensity, surface fire. Mean (@+ SE) bark thickness for trees>20 cm in diameter was 7.3 @+ 0.14 mm (n = 699) with values ranging from 1.5 to 28.9 mm. We found a significant relationship (r2= 0.77) between bark thickness and maximum cambium temperatures during fire simulations, and thereby estimated that in the event of a surface fire in the primary forest, 98% of all stems ° 1 cm diameter at breast height would be killed. Even though the autogenic factors in primary forest of the eastern Amazon create a microclimate that virtually eliminates the probability of fire, they are currently a common event in disturbed areas of Amazonia. As many as 8 x 106ha burned in the Amazon Basin of Brazil in 1987 alone. In terms of current land—use patterns, altered microclimates, and fuel mass, there are also striking similarities between the eastern Amazon and East Kalimantan, Indonesia (the site of recent rain forest wildfires that burned 3.5 x 106ha).

ISSN:0012-9658    

DOI:10.2307/1940299

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Bog development, in terms of the rates of horizontal and vertical accumulation of peat and the timing of landform development of open—water pools, was examined on two concentric raised bogs in central Sweden. The results are compared with three models (allogenic, autogenic, and neutral models) of bog development in order to evaluate the relative contribution of environmental factors vs. ecosystem control of developmental processes. Both mires began to grow on discrete upland sites °6000 BP and then spread concentrically and accumulated peat vertically at approximately continuous rates to the present. Radiocarbon dates from pool sediments are progressively younger from the center to the margin of the mire, suggesting that pool formation is triggered autogenically by changes in hydrology. The results conform to hydrological models of mire formation based on groundwater mound equations and suggest that autogenic processes exert a major control over bog expansion, landform development, and the formation of conspicuous features in the stratigraphic record.

ISSN:0012-9658    

DOI:10.2307/1940300

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

To investigate desert vegetation dynamics, I undertook an open—ended study of a site that offers a combination of multiple observations through time with continuous protection from domestic livestock and other human impacts. The site is MacDougal Crater in the Sierra del Pinacate Reserve, Sonora, Mexico. Three sources of data have been used: a series of exactly matched photographs, begun in 1907; detailed permanent—plot maps, dating from 1959—1960; and an age—distribution analysis of a 170—yr—old population of Carnegiea gigantea. The crater vegetation is dominated by the woody perennials Cercidium microphyllum, Encelia farinosa, Prosopis sp., and Larrea tridentata, and the columnar cactus Carnegiea gigantea. Various populations of Larrea tridentata declined 50—90%, and Cercidium declined 60%, during the first half of this century with little or no recruitment since. Carnegiea numbers increased fourfold over the same period. A 200—fold increase in Prosopis in the playa—like crater center occurred in the early 1970s. Elsewhere on the crater floor, Encelia density increased markedly during the same period from insignificant levels in the early 1960s. Age distribution analysis for the Carnegiea population reveals three major establishment peaks during the 1790 period. Recruitment and morality records from the three sources of data are compared with regional climate records. The high mortality for some of the species was probably the result of the prolonged drought during 1936—1964. Establishment surges for some appear related to periods of unusually heavy precipitation during certain seasons. Clearly, desert communities are highly responsive to changes in the climate regime under which they grow.

ISSN:0012-9658    

DOI:10.2307/1940301

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Non—native annual grasses dominate most Californian mediterranean—climate grasslands today. However, native Californian grassland flora persists in grasslands on serpentine—derived soils. We manipulated soil nutrient resources to explore the links between nutrient availability, plant productivity, and the relative abundances of native and non—native species in serpentine grassland. Factorial combinations of nitrogen, phosphorus, and other components of a nutritionally complete formula were added to field plots over two growing seasons. Fertilization with nitrogen and phosphorus increased biomass of the resident vegetation substantially in the first season, and within two years allowed the invasion and dominance of non—native annual grasses in patches originally dominated by native annual forbs. Species richness declined with fertilization, as the increased biomass production by invaders suppressed some native forbs. Increased macronutrient availability can increase production on serpentine—derived soil, even when other serpentine characteristics (such as low Ca/Mg ratios and high heavy—metal concentrations) have not been mitigated. Observed changes in community structure and composition demonstrate that the invasibility of plant communities may be directly influenced by nutrient availability, independent of physical disturbance.

ISSN:0012-9658    

DOI:10.2307/1940302

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Long—term, natural emissions of sulfur dioxide and acidic aerosols have had an impact on remote tundra at the Smoking Hills. The emissions have caused plant damage by SO2toxicity, and have severely acidified soil and freshwater. At the most intensively fumigated locations closest to the sources of emission, pollution stresses have devegetated the terrestrial ecosystem. The first plants that are encountered along a spatial gradient of decreasing pollution stress are Artemisia tilesii and Arctagrostis latifolia, which dominate a characteristic, pollution—tolerant community. Farther away at moderately polluted sites there are mixed communities with floristic elements of both fumigated and reference, unfumigated tundra. This pattern of ecosystem response to a concatenation of stresses caused by natural air and soil pollution is qualitatively similar to the damage that occurs in the vicinity of anthropogenic point sources of air pollution, such as smelters.

ISSN:0012-9658    

DOI:10.2307/1940303

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Knowledge of the dynamics of seed production and seedling recruitment of individual tree species is crucial for a complete understanding of tropical forest dynamics, yet multiyear studies on the seed and young seedling stages of tropical trees are virtually nonexistent. In a 4—yr study of the understory tree Faramea occidentalis on Barro Colorado Island, Panama, I quantified natural levels of viable seedfall, seedling emergence, and seedling establishment, and experimentally estimated postdispersal seed predation. The levels of viable seedfall, seed predation, seedling emergence, early seedling survival, and seedling recruitment all differed significantly among years. The proportion of fallen seeds destroyed by predators before germination was not related to the quantity of F. occidentalis seedfall. Within a year, however, F. occidentalis seed predation appeared to be influenced by community—wide seedfall, with high predation rates during times of low seed abundance and very low predation during the late dry season peak in seedfall by the community. Most of the annual variation in recruitment can be explained by the combination of seedfall and seed predation; in 3 of the 4 yr seedling emergence could be predicted from a knowledge of viable seedfall an the probability of a seed surviving until the peak of germination. The 4th yr, however, demonstrated that environmental conditions provide a sporadic, though important, limitation to recruitment. In comparison to many tree species, early seedling survival was relatively high, as was the ratio of seedlings recruited per seed falling. The highly successful recruitment of F. occidentalis is associated with a high population density of both saplings and adults in the study area.

ISSN:0012-9658    

DOI:10.2307/1940304

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY