摘要:

The dwarfed conifer forests occurring on the coastal terraces of Mendocino County, California, are oligotrophic communities containing edaphic endemics as dominants. Species distributions and community characteristics are traced along a gradient in podzolization of the soils from these pygmy conifer stands on highly weathered terrace spodosols, to redwood stands on weakly developed slope spodsols. Vegetation stature, canopy closure, species richness, litter biomass and pH generally decrease along the gradient as podzolization and nutrient impoverishment increase, whereas soil organic matter and available water capacity tend to be highest at the gradient extremes. Analyses of pygmy forest soils show low levels of macro— and micro—nutrients, and high levels of exchangeable aluminum. When ordinations of stands based on vegetational data are compared with rankings based on environmental data, changes in pH of the soil A horizon are found to correlate most highly with vegetation changes. Nutrient loss and acidification, initiated by varying rates of soil weathering in the pygmy forest region, may be aggravated, in the case of pygmy forest soils, by a series of feedback effects, including solubilization of possibly toxic amounts of aluminum by low pH, which may contribute to the observed plant stunting and shrinking pools of nutrients in the biomass. A ranking of stands along a soil nutrient gradient, based on field observations and presence data for species, correlated successively less strongly with ordinations computed by similarity—projection, Bray—Curtis method, importance—projection, principal components analysis, and factor analysis. Although these ordinations were expected to produce distortions in the interstand relations due to the modal and even polymodal nature of species response curves and to the presence of species not spanning the range of the data set, additional distortions due to variability in sample data were also highlighted.

ISSN:0012-9615    

DOI:10.2307/1942403

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

The mechanisms by which various species exert influence disproportionate to their abundance or mass on the structure of a lower intertidal algal community were evaluated experimentally. These functional roles were evaluated experimentally. These functional roles were evaluated by various controlled manipulations at seven stations along the Washington coastline ranked according to an exposure/desiccation gradient. The algae were divided into three categories: canopy species, which grow above the other species and apparently succeed in competitively dominating the light resources as demonstrated by algal blooms following their removal; obligate understory species, which die after the canopy species are removed; and fugitive species, which are quick to colonize new space. Ecological dominance was exerted in areas of moderate wave exposure by Hedophyllum sessile, which competitively displaces a large number of fugitive algal species and which furnishes a protected habitat for many obligate understory algae that die or defoliate after the removal of Hedophyllum. Hedophyllum loses this dominance in the most exposed areas, although such sites apparently represent its physiologically optimal habitat, because in these areas it is out—competed by Laminaria setchellii and Lessoniopsis littoralis. In these wave exposed habitats Lessoniopsis was demonstrated to exert a strong competitive dominance over all the other species in the association. The molluscan herbivores were not observed to express any measurable effects on the recruitment or survival of the algae. However, the echinoid Strongylocentrotus purpuratus often overexploits its prey and has a pronounced influence on most of the algal species. In this respect S. purpuratus enjoys an important community role singular among the many herbivores. Similarly, Pycnopodia helianthoides and Anthopleura xanthogrammica are disproportionately important carnivores, because their predation on Strongylocentrotus, clearing large areas of urchins, results in patches in which algal succession follows. The rate of algal succession following removal of the dominant algal species or of Strongylocentrotus is proportional to the degree of wave exposure. The Hedophyllum canopy recovery at the Eagle Point area of San Juan Island, a site exposed to relatively little wave action and thus high levels of desiccation, was relatively slow, with only 10%—26% cover reestablished after 3 yr. In contrast, Hedophyllum canopy developed up to 66% cover in only 1 yr in the exposed area of Waadah Island; it then quickly lost its dominance to Laminaria and Lessoniopsis. Algal succession in deeper Portage Head tidepools was found to be relatively slow with no clear dominance expressed after 5 yr.

ISSN:0012-9615    

DOI:10.2307/1942404

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Succession of vascular plant communities was studied on fields in central New York abandoned for the previous 4 to 36 yr. Changes within fields as they aged were consistent with the overall pattern of change between fields. Over the period covered, Phleum pratense was replaced successively as the major species by Solidago altissima, Picris hieracioides, and Cornus racemosa. Succession was accompanied by increased biological diversity, reduced dominance, decreased energy flow per unit biomass, and increased stability to a pulse perturbation of inorganic nutrients. Although soil phosphorus is an important factor influencing community function on the fields studied, the fertilization experiment indicated a fundamental reorganization of community relationships during succession which reduces community responsiveness to fluctuations in soil nutrients. Increased diversity resulted from the partitioning of constant density and net productivity among increasing numbers of species. Accompanying the increase in diversity was decreasing seasonal synchrony of resource demands among the constituent species pool. Although successional stages could be arbitrarily recognized by the taxonomic identification and growth forms of dominant species, there was a gradual and continuous change in structural and functional properties of the plant community, and no objectively definable stages could be delimited. These studies indicate that a failure to determine productivities of individual species may seriously distort the evaluation of community properties.

ISSN:0012-9615    

DOI:10.2307/1942405

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY

摘要:

Lichen species richness and abundance are reduced by approximately 90% in lichen communities near a zinc smelter at Lehigh Water Gap in comparison with the lichen communities of Delaware Water Gap. The principal cause of the impoverished lichen flora in the Lehigh Water Gap area is probably high concentrations of zinc. Of the non—pollution and pollution factors considered, only abnormal soil concentrations of zinc and cadmium extend beyond the limits of the lichen impoverishment zone. Because Zn is present in concentrations generally 100 times higher than that of Cd and because Cd is experimentally shown to be no more toxic to lichens than is Zn, it is probable that Zn is the more important, detrimental factor to lichens in the Palmerton area. Near the smelters, Zn, Cd and sulfur dioxide are all present in sufficiently high concentrations to be detrimental to lichen growth and survival. However, at the perimeter of the lichen—impoverished zone, only Zn is present in high enough concentrations to be phytotoxic. Although microclimatic alteration in forested areas near Palmerton may be sufficiently great to be a stress for a few lichen species that are adapted to shaded conditions, no microclimatic differences were demonstrable between Delaware and Lehigh Water Gaps for open habitats where most lichen species normally occur in this area. Variations in such factors as lichen geographical range, climate, substrate composition and abundance, and fire history are probably of negligible importance in explaining the reduction in lichen numbers or abundance in Lehigh Water Gap.

ISSN:0012-9615    

DOI:10.2307/1942406

出版商:Ecological Society of America    

年代:1975

数据来源: WILEY