摘要:

Fire is an ecosystem property rather than an exogenous force in southern California chaparral, and it interacts with processes of drought—mediated canopy development, production, and mortality to affect stability of community composition. Where species that must reproduce from seed, such as Ceanothus crassifolius or Ceanothus oliganthus, are predominant, composition can be altered by a single fire with little or no recruitment after initial postfire establishment. Water balance apparently regulates subsequent leaf area development; after 15—22 yr of postfire growth, foliage biomass in monospecific C. crassifolius stands in this study had approached a maximum that was unrelated to incident solar radiation and insensitive to initial population density over a 10—fold range. Thus, establishment success, above that required for canopy closure, should have little effect on the foliage biomass that sustains combustion. After canopy closure, total biomass accumulated at an accelerating rate through at least two decades with aboveground net primary production as great as 12—13 Mg · ha—1 · yr—1. C. crassifolius mortality was substantially less than predicted from growth rates and the —3/2 power model of Yoda et al. (1963), and there was no approach to a common asymptotic density by stands of disparate initial density. With low deadwood biomass and absence of ground fuels, c. crassifolius cannot sustain burning in the absence of wind, steep slopes, or exceptionally low live—fuel moisture. Increased Ceanothus abundance in multispecies communities with Adenostoma fasciculatum or Salvia mellifera alters biomass structure and could modify subsequent fire effects even if foliage area fully redevelops in concert with site water balance. Rare, low—intensity fires can devastate Ceanothus chaparral, that reproduces only from seed. Salvia mellifera and Eriogonum fasciculatum can occupy resulting openings in the canopy, and their abundant deadwood and compact biomass can readily spread low—intensity fires, thereby perpetuating the degraded community. Productive stands within a chaparral association are probably subject to especially severe fires that limit nutrient accumulation and may also limit subsequent productivity. Copious nitrogen volatilization during burning is promoted by high nitrogen concentrations in foliage and fine woody biomass of Ceanothus and heavy leaf litter of Quercus dumosa and C. crassifolius. The communities most prone to severe fires also accumulate and cycle nitrogen and phosphorus rapidly.

ISSN:0012-9615    

DOI:10.2307/2937023

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

The concentration, composition, and flux of dissolved organic carbon (DOC) were measured in the Hubbard Brook Valley, New Hampshire. Data on precipitation, throughfall, soil solution, streamside seeps, stream water, and lake water are presented for 1976—1980. Characterization of DOC included analysis of phenolics, monomeric and polymeric carbohydrates, carboxylic acids, primary amines, and aldehydes. DOC concentrations increased with passage of water through the forest canopy and forest floor, decreased due to abiotic sorption in the mineral soil, and remained relatively low in most "downstream" ecosystems (streamside seeps, streams, and Mirror Lake). Average flux of DOC is estimated as 16, 47, 263, 54, 23, and 20 kg · ha—1 · yr—1in precipitation, throughfall, soil solution (E, upper B, and B horizons), and streamflow, respectively. The composition of DOC, in particular the relative proportions of carbohydrates, appears to be related to the degree of biological activity at a given site in the landscape. Carbohydrates are particularly high in both absolute and relative terms in throughfall and lake water, which are the areas of highest photosynthetic activity within the terrestrial and aquatic ecosystems, respectively.

ISSN:0012-9615    

DOI:10.2307/2937024

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

Changes in pH over the past 12 000 yr in Duck Pond in the Cape Cod National Seashore were reconstructed using a diatom—pH transfer function. The reconstructed pH suggested that the pond has been acid for its entire history with a mean reconstructed pH of 5.3 ± 0.3 (standard error of the predictive equation ± 0.5). There was a brief period during the late—glacial (at ≈11 500 BP) of higher pH of ≈6, possibly caused by increased windiness and erosion (inferred from concurrent pollen and sediment changes indicative of an open spruce—Hudsonia parkland), increased leaching of cations from outwash sands of the drainage basin, and/or increased instability of the water levels during lake development. The reconstructed pH history indicates that Duck Pond has become more acid recently (with a mean pH of 5.1 ± 0.1 for the past 150 yr) although the pH has varied both up and down throughout the Holocene with acidity as low as the present at other times in the past 12 000 yr. The significance of these changes in reconstructed pH is problematic because the pH variations lower in the core remain within the standard error of the equation and primarily reflect a long—term naturally acid ecosystem. This finding illustrates the need for more local and regional long—term lake pH histories so that naturally acid lakes can be distinguished from anthropogenically degraded lakes and ecologically sound management decisions can be made. In the pre—European—settlement levels of the core, correlations between charcoal and alkaline Fragilaria diatoms suggest that forest fires in the drainage basin may have affected pond pH. Correlations between charcoal and acid diatoms in post—European—settlement sediments suggest an effect on pond pH from recent increase in windborne industrial acids. Negative correlation between diploxyon Pinus species and the reconstructed pH and positive correlation between haploxylon Pinus and the alkaliphilic diatoms suggest that vegetation changes, which also reflect the climate and fire history of the region, have also affected pH of the pond.

ISSN:0012-9615    

DOI:10.2307/2937025

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

Resource heterogeneity promotes coexistence between two competing species, Sarraceniopus darlingtoniae, a slime mite, and larvae of Metriocnemus edwardsi, a midge, which live together in pitchers of Darlingtonia californica, the California pitcher plant. Both species depend on resources derived from the decomposing insects captured by the pitchers. Experimentation revealed that midge larvae consumed resources at a greater rate than mites and that the densities of the two species are negatively correlated. These results indicate that M. edwardsi is a potential dominant competitor with S. darlingtoniae. Competition theory predicts that the presence of midge larvae should lead to the competitive exclusion of mites, yet these species coexist in field populations. This seeming paradox can be explained by temporal and spatial heterogeneity in resources. Midge larval density is generally associated with expected resource levels of pitchers, whereas mites are found in greatest abundance in pitchers whose resources differ from expected. This pattern suggests that the effects of competition vary and are strongest when actual resource levels equal expected resource levels. When resource levels are above expected levels, coexistence occurs. These results support Levins' (1979) model for coexistence of two species on a single resource. The implications of this study to the general debate over the role of interspecific competition in structuring communities are discussed.

ISSN:0012-9615    

DOI:10.2307/2937026

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

ISSN:0012-9615    

DOI:10.2307/2937027

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY