摘要:

Studies were conducted in four distinct geographic areas (biomes/sites) in northern United States to examine changes in key ecosystem parameter: benthic organic matter (BOM), transported organic matter (TOM), community production and respiration, leaf pack decomposition, and functional feeding—group composition along gradients of increasing stream size. Four stations ranging from headwaters (1st or 2nd order) to midsized rivers (5th to 7th order) were examined at each site using comparable methods. The results for each parameter are presented and discussed in light of the River Continuum Concept of Vannote et al. (1980). The postulated gradual change in a stream ecosystem's structure and function is supported by this study. However, regional and local deviations occur as a result of variations in the influence of: (1) watershed climate and geology, (2) riparian conditions, (3) tributaries, and (4) location—specific lithology and geomorphology. In particular, the continuum framework must be visualized as a sliding scale which is shifted upstream or downstream depending on macroenvironmental forces (1 and 2) or reset following the application of more localized "micro"—environmental influences (3 and 4). Analysis of interactions between BOM and TOM permitted evaluation of stream retentiveness for organic matter. Headwaters generally were most retentive and downstream reaches the least. Estimates of organic matter turnover times ranged between 0.2 and 14 yr, and commonly were 1—4 yr. Both turnover times and distances were determined primarily by the interaction between current velocity and stream retention. Biological processes played a secondary role. However, the streams varied considerably in their spiraling of organic matter due to differences in the interplay between retentiveness and biological activity. Differences in the relative importance of retention mechanisms along the continuum suggest that headwater stream ecosystems may be functionally more stable, at least to physical disturbances, than are the r intermediate river counterparts.

ISSN:0012-9615    

DOI:10.2307/1942585

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Unlike most phytophagous insects, the reproduction of primary bark beetles (Coleoptera: Scolytidae) is contingent on host mortality. Consequently, there have been intense selective pressures on trees for properties which confer resistance to attack, and likewise, on the insect for increased behavioral complexity by which to overcome these defenses. In this study, we examined the relationship between the physiology of Pinus contorta var. latifolia and the behavior of the bark beetle, Dendroctonus ponderosae. Host mortality is a discrete outcome which is contingent on the quantitative interaction between host resistance and beetle numbers. At low attack densities, trees respond by confining beetles and their associated fungi within necrotic lesions containing toxic or inhibitory compounds. Beyond a critical "threshold of attack," however, the defensive capacity of the tree is exhausted, and mortality occurs. This threshold occurred at °40 galleries/m2in our experimental stands. The reproductive success of the bark beetle is directly related to the depletion of host defenses through concentrated attack. At sufficiently high attack densities, °80 galleries/m2, the potential suppressive effect of the host on brood development is not manifested. Increased beetle numbers, however, cause an exponential decline in brood production due to intraspecific competition. Consequently, there is an optimal density, °62 attacks/m2at which reproduction and survival of the beetle is maximized. Dendroctonus ponderosae utilize defensive secretions as precursors and synergists of aggregation pheromones, and thereby, elicit maximum responses from flying beetles while the host's production of toxins is at its maximum. This mechanism also ensures that the attack will terminate once the tree has been rendered suitable for brood development and, thus, minimizes the deleterious effect of intraspecific competition. Such an interaction, in which all trees could theoretically be colonized regardless of their resistance capacity, cannot result in a stable host—parasite system. Conifers, however, seem to be able to interfere with bark beetle communication. Pheromone synthesis within the hindgut and emission from the entrance site are distinct events, and beetles contained within a copious flow of resin seem unable to elicit attraction. This ability to interrupt the beetles' communication sequence, however, is inversely related to the number of beetles initiating localized attacks. Consequently, the colonization sequence reflects the outcome of a dynamic interaction between the tree and the initial "pioneer" beetles.

ISSN:0012-9615    

DOI:10.2307/1942586

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Storage and fluxes on N, P, Ca, Mg, and K in aboveground detritus were measured in six contrasting lodgepole pine (Pinus contorta ssp. latifolia) stands in southeastern Wyoming. Litterfall was predominantly leaves (67—80%) in 80—100 yr old stands, while woody litter was more important in an older stand (240 yr old). Leaf litter nutrient concentrations were very low compared with other pine forests, particularly for N (0.40% dry mass). Dry mass loss from decomposing leaf litter was slow (15%/yr in first 2 yr), and summer rates did not differ significantly from winter rates beneath the insulating snowpack. Significant amounts of N, P, and Ca were added to decomposing leaves during the first winter, and N and Ca addition continued for 2 yr. Potassium and magnesium were rapidly lost from decomposing leaves. Rates of mass and nutrient loss from decomposing bark, twigs, and cones were comparable to those observed in other studies of temperate—zone forests. Mass loss from decaying bole wood appeared to be exponential through 40 yr, with an average decay coefficient (k) of 0.016, which is comparable to that in other cold temperate forests. Nitrogen content of decaying boles doubled between 30 and 55 yr following tree death, while smaller additions of P, Ca, K, and Mg also were noted. Relatively large accumulations of organic matter and nutrients were observed in the forest floor, leading to very high steady—state residence times for dry mass (mean = 18 yr), N (54 yr), P (39 yr), Ca (35 yr), Mg (21 yr), and K (18 yr). Deadfall contributed by the present forest generation was a minor component of the aboveground detritus except in an old—age stand and in a dense, self—thinning forest site. In contrast, dead wood inherited from the previous forest generation (killed by fire) was a major detrital storage component, exceeding forest floor mass by several—fold in 80—100 yr old stands. High nutrient immobilization in the dead wood led to storage values which were similar to those of the forest floor in these stands.

ISSN:0012-9615    

DOI:10.2307/1942587

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

The level and diversity of metabolism in lotic ecosystems are largely functions of channel geomorphology and hydrology, making site—specific studies difficult to extrapolate to other parts of the watershed. This paper describes the pattern and distribution of aquatic oxygen metabolism for undisturbed boreal forest watersheds in eastern Quebec, Canada. Metabolism by periphyton, mosses, macrophytes, fine particulate organic matter (FPOM: 0.5 μm—1mm), an coarse particulate organic matter (CPOM:>1mm) was examined during the ice—free period (April to November) of 1979 and 1980 in First Choice Creek (first order; watershed area: 0.25 km2), Beaver Creek (second order; 1.83 km2), Muskrat River (fifth order; 207 km2), matamek River (sixth order; 673 km2), and the Moisie River (ninth order; 19 871 km2). As watershed area increases, primary production and the number of autotrophic groups are augmented with moss and macrophyte communities. Total O2metabolism increases downstream, ranging from.76 in most cases). These data are combined with a geomorphological analysis of the watershed to discern the spatial distribution of aquatic metabolism, and to estimate total aquatic metabolism in the Moisie River drainage network. Mosses, occurring only in streams of fourth or higher order, are the most productive autotrophic component in the watershed (3.9 x 1010g/yr); by comparison, periphyton produce only 2.1 x 1010g/yr. FPOM is the most active detrital component (6.6 x 1010g/yr). Geomorphic analyses show that streams of fourth or higher order comprise only 1.2% of the total number and 12.7% of the total length, but have 76.8% of the lotic surface area, and are responsible for 86.3% of the gross production by the entire lotic drainage network. The surface area of lakes is approximately six times that of streams, but their contribution (phytoplankton) to total aquatic ecosystem metabolism is comparable only to that of lotic periphyton. Results demonstrate that the River Continuum Concept can be extended to the watershed level by combining biological measurement with geomorphological analyses of the drainage network, thus allowing a detailed description of spatial and temporal patterns for specific metabolic components and total ecosystem metabolism to be constructed.

ISSN:0012-9615    

DOI:10.2307/1942588

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Four co—occurring species of goldenrod bloom at different times with varying degrees of overlap; in order of peak flowering they are Solidago juncea, S. graminifolia, S. canadensis, and S. nemoralis. All four species are self—incompatible and require an insect vector for successful seed—set. First, the relationship between flowering time and seed—set of individual plants of these four species was determined. All four species of Solidago had significant differences in both the percentage of filled seeds and the total seed—set of clones that flowered at different times. Early—flowering clones had lower seed—set than did late—flowering clones in S. canadensis, S. graminifolia, and S. nemoralis. In contrast, early—flowering ramets of S. juncea had significantly greater seed—set than did late—flowering ramets. Secondly, the underlying factors limiting seed—set were investigated by observational and experimental techniques, in order to lend insight into the processes which can select for flowering time in natural plant populations. The abundance of pollinators on goldenrods and other plant species and the abundance of flower predators were monitored over the season. Experimental hand—pollinations were performed on individual plants of each species over the entire flowering season, to determine if seed—set was limited by the amount of pollen reaching stigmas or by factors intrinsic to individual plants which flowered at different times. Apis mellifera, the introduced honeybee, is the major pollinator of goldenrods in this system. The major factor influencing the relative abundances of honeybees on these four species of goldenrod seemed to be overlap among the flowering periods of the goldenrods and those of several introduced plant species. Apis began visiting goldenrods at a point when the abundance of flowers in the weedy flora had declined greatly. The flowering period of S. juncea overlapped almost entirely with the flowering periods of several species in the weedy summer flora. This appears to explain the lack of Apis visits to S. juncea, which was visited only by small, native bees and beetles. However, the results from experimental pollinations suggest that seed production of S. juncea was at its potential maximum throughout the flowering season. In early—flowering clones of S. graminifolia, the natural seed—set was significantly lower (by 57%) than the maximum potential seed—set determined from experimental pollinations, but in late—flowering clones there was no differences between actual and potential seed—set. Of the reduction from potential seed—set in early—flowering clones, the low frequency of honeybee visits was estimated to account for most of the loss, and flower predation by the blister beetles, Epicauta pennsylvanica, accounted for the remainder. In S. canadensis, the seed—set of both hand—pollinated and control flowers was greater in late—relative to early—flowering clones of S. canadensis (i.e., late—flowering clones had greater physiological potential for seed production). The differences in the seed—set of clones flowering at were different times were due to physiological or microenvironmental differences among clones. There was also some degree of pollen limitation of seed—set (17—34% of the potential maximum seed—set) at all times. Again, pollen limitation of seed—set was due mostly to pollinators, as opposed to flower predators. In S. nemoralis, the late—flowering clones had significantly higher seed—set than early—flowering clones, but maximum potential seed—set was not determined for this species. Such differences in seed—set could select for flowering time, given the right combination of heritability and morphological or life history tradeoffs. The selection would not necessarily lead to further divergence in flowering phenology among these congeners, but rather for later flowering in S. graminifolia, S. canadensis, and S. nemoralis and for earlier flowering in S. juncea.

ISSN:0012-9615    

DOI:10.2307/1942589

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY