摘要:

It appears that polar regions of the Earth will bear the brunt of global temperature increases. Because of the ecological importance of the sedge Eriophorum vaginatum in the arctic and the large amount of data available on its growth and physiology, we chose this species as a test case to model the potential long—term response of arctic plants to global climate change. Our simulation model utilizes a mechanistic framework and includes the effects of light, temperature, season length, nitrogen availability, and CO2concentration on E. vaginatum growth dynamics. The model was parameterized based on a series of published studies of the growth responses of E. vaginatum to nutrients and validated using (1) field studies on the growth responses of E. vaginatum to temperature and shading, and (2) the effects of elevated CO2and temperature on E. vaginatum photosynthesis. The effect of a 50—yr period of climate change on peak biomass (overwintering biomass plus seasonal production) in E. vaginatum was explored. We use climate change here to refer to linear increases over a 50—yr period in temperature (from 8° to 13°C), season length (from 100 to 120 d), and atmospheric CO2(from 340 to 680 @mL/L). Similarly, a wide range of nitrogen availabilities (from 9 to 18 g°m—2°yr—1) was also examined because of its importance in productivity. The model predicts that a simultaneous increase in the direct effects of temperature, season length, and CO2, with no change in nitrogen availability, will result in a slight decrease in peak biomass. A simulated long—term doubling of nitrogen availability results in an °70% increase in peak biomass, whereas with concurrent changes in climate and nitrogen availability, the model predicts a slight decline in peak biomass compared to increases in nitrogen alone. In essence, the model predicts that climate change will have substantial effects on E. vaginatum only indirectly through changes in nitrogen availability. Simulated peak biomass responds linearly up to a doubling of current nitrogen availabilities. Therefore, at low—to—moderate increases in nitrogen availability, the predicted response of E. vaginatum to climate change is linearly (and almost exclusively) dependent on our ability to predict the effects of climate change on nitrogen cycling. At nitrogen availabilities>2x current availabilities, the relationship flattens out very rapidly because the plant becomes limited by carbon uptake. Thus, if nitrogen availabilities more than double in the future, E. vaginatum may shift from being a nutrient—limited to a carbon—limited system and, consequently, increased season length and elevated CO2concentrations may play an important role in controlling E. vaginatum productivity.

ISSN:1051-0761    

DOI:10.2307/1941868

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The influence of repeated chlorine additions on the resistance and resilience of laboratory stream ecosystems was investigated. Prior to the disturbance, four treatments were imposed on eight laboratory streams in a 2 x 2 factorial design, resulting in two replicate streams per treatment. Treatments included two circulation regimes (once—through or 98% recirculated) and two levels of grazer density (1000 individuals/m2or 0 individuals/m2, using the snail Elimia clavaeformis). Resistance of periphyton biomass to an initial set of chlorine additions was greater in once—through than recirculated streams. Periphyton resistance following a second set of chlorine additions was greater in streams without snails compared to those with snails. This appeared to be a biomass—mediated response; the greater biomass levels in no—snail streams provided a larger buffer to the oxidizing action of chlorine. Resilience of most structural parameters and area—specific carbon—fixation rates was not influenced to a significant degree either by flow regime or snail density. Resilience of chlorophyll—specific carbon fixation and exoproteolytic activity (an index of microbial activity) was greater in recirculated streams compared to once—through streams, but only when snails were present. Over the entire experimental period (15 wk), most structural parameters and area—specific carbon fixation rates were significantly greater in (1) streams without snails compared to those with them and (2) streams with once—through circulation compared to those that were recirculated, but only for those streams with snails. Our chlorine additions appeared to have relatively little influence on snails, and snail growth over the experiment was significantly greater in once—through compared to recirculated streams. These data are consistent with other findings that total residual chlorine concentrations of<1.0 mg/L can significantly affect periphyton structure and function. However, nutrient cycling rates appeared to be little affected by the chlorine perturbation, which allowed recirculated systems to recover faster than predicted. The findings from this study suggest the periphyton mat physiognomy and integrity strongly influence system resistance (due to buffering capacity) and resilience (because of nutrient cycling).

ISSN:1051-0761    

DOI:10.2307/1941869

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Ecological theory predicts that diversity should decline as disturbance frequency increases beyond an intermediate level. However, few data exist on the responses of deciduous forests to high—frequency disturbance. We compared regenerating woody plant communities in northern Kentucky power—line corridors to adjacent second—growth forests to determine if high—frequency disturbance (5—6 yr mechanical or manual cutting cycle) modifies forest development. Black locust (Robinia pseudoacacia) and white ash (Fraxinus americana) dominated corridor communities. Still, many species present in adjacent forests persisted in corridors, with the result that 15 of 20 corridor samples were classified by TWINSPAN close to samples taken from adjacent forests. Of the most important tree species found in the study sites, sugar maple (Acer saccharum) was least favored by high—frequency cutting. In general, high—frequency cutting did not totally inhibit regeneration of a single species, diversity showed a small but nonsignificant increase, and several early—successional, shade—intolerant species established anew. When forests are repeatedly cut before stems reach the stem exclusion stage, individualistic patterns of sprouting and carbon allocation will likely determine dominance. White ash and black locust will assume greater importance in fragmented forests as disturbance frequencies increase.

ISSN:1051-0761    

DOI:10.2307/1941870

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Five years after planting and initial treatment, we examined the response of a microbial community to three intensive silvicultural practices: soil surface modification (scarification), fertilization, and control of competing vegetation by herbicide. We correlated microbial response with changes in environmental conditions following treatment, including soil temperature and moisture, total and available nutrients in soil, and light intensity in the tree canopy. The microbial biomass C (2.1—5.3 mg/g in the F/H horizon and 0.14—0.62 mg/g in the surface mineral soil) as determined by the fumigation—extraction method was reduced by vegetation control and fertilization. The ratio of microbial to total organic carbon (Cmic/Corg) was also reduced by vegetation control, and tended to increase in the new organic horizon developed during the 5 yr after soil surface scarification. Microbial biomass N (0.15—0.40 mg/g in the F/H horizon and 0.014 to 0.057 mg/g in the mineral soil) was not affected by the treatments. The microbial community structure (relative volumes of bacteria and fungi) was evaluated on glass slides placed in the litter bags of pine and aspen litter. The treatment effects on the relative volumes of microorganisms on slides were similar to effects noted for microbial biomass C in the soil. Five years of vegetation succession resulted in conservative N cycling and N limitation of the plant community in control plots. In response to silvicultural treatments this state may either remain relatively unchanged after fertilization, the N limitation may be increased (scarification), or apparent C limitation induced (vegetation control). Reducing the nutrient pool by scarification caused an apparent nutrient limitation, and the microbial community tended to widen the C/N ratio. Increasing the nutrient pool by fertilization or vegetation control tended to narrow the C/N ratio of microbial biomass.

ISSN:1051-0761    

DOI:10.2307/1941871

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

We examined potential effects of herbivory on temperate forest ecosystems using FORET, a forest succession simulation model with the capacity for treating various hierarchical levels for long time periods. Two species of trees were chosen for evaluating herbivory effects: white oak (Quercus alba), a relatively slow—growing shade—tolerant species, and tulip poplar (Liriodendron tulipifera), a fast—growing shade—intolerant species. Both are dominants in the Southeastern U.S. forest system selected as a basis for this study. The study focused on four hierarchical levels, covering individual, phenotypic, interspecific, and community interactions. Simulations of herbivory were arrived at by stochastically varying annual incidence that herbivores were present on each simulation plot over a 500—yr period, and by modifying the proportion of energy allocated to either tree growth, or to its defense from herbivore attack. Two hypothetical tree phenotypes were considered, one that allocated specified amounts of energy to herbivore defense mechanisms at all times, and the other that made the allocation only when herbivores were present according to the stochastic determination made for a specific study period. Thus, one phenotype was fixed in its life history strategy; the other was given a facultative strategy where it switched its growth and defense tactics as a function of variation in herbivore presence. The results of the simulations suggest how deciduous forests may respond to long—term variations in the intensity of herbivore stress on two dominant tree species, and show the importance of hierarchical relationships within the community. White oak tended to show a greater sensitivity to interspecific interactions; tulip poplar showed a higher sensitivity to intraspecific interactions. Changes in growth rates associated with the switching strategies (an ultimate factor) were more important in answering variations in productivity than was impact imparted by annual changes in incidence of herbivore presence (a proximate factor) for these two species, although there were important differences in several statistical interactions. While our results suggest that herbivore stress can explain a larger degree of the variation in long—term community dynamics, ecological interactions between herbivore and climate effects must be more closely linked in such long—term studies.

ISSN:1051-0761    

DOI:10.2307/1941872

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Forest edges created by dispersed—patch clear—cutting have become a conspicuous landscape feature in western North America, but the effects of edge on forest structure and function are still poorly understood. In this paper we describe responses of stocking density, growth, mortality, and regeneration for three conifer species from the clear—cut edge into the interior of old—growth forest patches adjacent to 10—15 yr old clearcuts in southern Washington and central Oregon. The significance of edge effects for each variable was tested through a single—factor (distance) analysis of variance (F test). Relationships between these variables and depth—of—edge influence (i.e., edge width) on old—growth forest were characterized by nonlinear regression models. Near the edge (forest—clearcut boundary line), the old—growth forest has (1) reduced stocking density, as measured by canopy cover, number of stems per hectare, and basal area; (2) increased growth rates of dominant Douglas—fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla), as calculated by an index of relative growth rate; (3) elevated rates of tree mortality, as measured by standing dead and down trees (snags and logs); and (4) greater numbers of Douglas—fir and western hemlock seedlings (@<100 cm tall) and saplings (101—200 cm) but fewer of Pacific silver fir (Abies amabilis). The depth—of—edge influence, when calculated as the point along the clearcut—forest gradient at which a given variable has returned to a condition representing 2/3 of the interior forest environment, ranged from 16 to 137 m for variables related to distance from the edge. The amount of a square forest patch affected by edge decreased as patch size increased and varied greatly with the depth—of—edge influence. With increasing concerns about organisms and processes that require interior forest habitat, determining the area of residual forest influenced by adjacent clearcut is critical to current and future resource management. Responses of additional biological variables must be explored and information on edge phenomena should be extended to the scale of landscapes.

ISSN:1051-0761    

DOI:10.2307/1941873

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

We evaluated the relationship between annual forage production and annual and seasonal precipitation and temperature at a shortgrass steppe site in north—central Colorado using a long—term data set (52 yr). We also constructed a relationship between forage production and aboveground net primary production (ANPP). Precipitation fluctuated randomly, but temperature had clear warming and cooling trends including a 17—yr warming trend from 1974 to 1990. Forage production was significantly related to both annual and seasonal precipitation but not temperature. Precipitation events between 15 and 30 mm accounted for most of the variability in production because they accounted for most of the variability in precipitation and because they wetted the soil layers that have the largest effect on production. Forage production amplified variability in annual precipitation. Production showed time lags of several years in responding to increases in precipitation. Change in vegetation structure has a characteristic response time, which contrains production responses in wet years. Constraint caused by vegetation structure is the reason why regional ANPP—precipitation models have a steeper slope than long—term models and point out a weakness of exchanging space for time in predicting production patterns.

ISSN:1051-0761    

DOI:10.2307/1941874

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Among the numerous site—ranking or reserve selection approaches is to select reserves based on the current distribution of a suite of species. This approach only requires distribution data for the species in the "focal taxon." To what extent is the proposed reserve network dependent on the species included in the focal taxon? Can a suite of species act as an umbrella or indicator for other species whose distribution was not directly considered? A simple reserve selection algorithm was applied to species lists from sites where multiple taxa were sampled. The algorithm selected sites so that each species in the focal taxon was represented in at least one reserve. The species lists were complied for birds, mammals, reptiles, and plants on islands in the Gulf of California, and for birds, mammals, and plants in canyons in San Diego County, California. Within both data sets, there were significant rank correlations (P<.01) between the number of species in different taxa per potential reserve. Yet the number of proposed reserves that satisfied the selection algorithm differed strongly between taxa. For example, 2 of 25 canyons were in the San Diego bird reserve network, but 10 of 25 canyons were in the native plant reserve network. The area of the proposed reserves selected by the algorithm ranged from a minimum of 3.2% of the total area for San Diego bird reserves to a maximum of 98.3% of the total area for the Gulf of California plant reserves. The bird reserves included only 45—78% of the species in the other taxa. Conversely, the plant reserve network would contain 88—100% of the vertebrate species. Thus, plants were better indicators than birds for reserve establishment. Plant occurrences were less ordered than bird occurrences, and there were 10 or 20 times more plant species than bird species. The better umbrella taxon was more speciose or had less pattern to species occurrences. Rather than a literal application of plants as reserve indicators, a better approach would be to collect some preliminary distribution data for a number of taxa in the potential reserve network and use these preliminary data to define the focal taxon (or taxa).

ISSN:1051-0761    

DOI:10.2307/1941875

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Howellia aquatilis is an annual aquatic plant of ephemeral ponds. It is considered extirpated or endangered throughout its range in the United States Pacific Northwest. I studied populations in the Swan Valley of Montana to determine life history traits and ecological attributed in order to influence planning decisions on lands managed for multiple use. Germination trials demonstrated that seeds of H. aquatilis require an aerobic environment and cool temperatures to germinate. Germination was highest with daily alternating temperatures and total darkness, and was unaffected by different light regime and aeration retreatment conditions of 50— and 100—d durations. Natural seed banks were largest immediately following seed dispersal and pond drawdown, but were reduced 82—90% by the following spring. Seeds remaining in the seed bank showed reduced germinability and vigor. Aquarium experiments indicate that growth of H. aquatilis is best in the peat substrate in which it occurs naturally, and is reduced by 45% in a more fine—textured organic soil and by 85% in a mineral soil. Multiple regression analyses of 12 environmental variables from 23 H. aquatilis ponds and comparisons between ponds with and without H. aquatilis suggest that pond depth, amount of dissolved solids in pond water, depth and composition of pond substrate, and degree of human—caused disturbance have significant effects on the abundance of H. aquatilis in the Swan Valley. Taken together, these results suggest that Howellia aquatilis is rare because it can persist in only a small subset of wetlands—freshwater ephemeral ponds with a shallow, coarse textured organic surface horizon. Thus, H. aquatilis will be sensitive to disturbances that alter pond water quality or substrate composition. Lack of a persistent seed bank makes H. aquatilis prone to large fluctuations in population size due to environmental fluctuations. Both habitat specificity and large variations in population size are often associated with a high risk of extinction. Howellia aquatilis can only persist during periods of climatic fluctuation as a metapopulation in an area that contains large numbers of appropriate ponds of various depths. Consequently, long—term protection of this rare species will require habitat protection in large wetland complexes throughout its range.

ISSN:1051-0761    

DOI:10.2307/1941876

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The urgent need for an effective monitoring scheme for grizzly bear (Ursus arctos) populations led us to investigate the effort required to detect changes in populations of low—density dispersed animals, using sign (mainly scats and tracks) they leave on trails. We surveyed trails in Glacier National Park for bear tracks and scats during five consecutive years. Using these data, we modeled the occurrence of bear sign on trails, then estimated the power of various sampling schemes. Specifically, we explored the power of bear sign surveys to detect a 20% decline in sign occurrence. Realistic sampling schemes appear feasible if the density of sign is high enough, and we provide guidelines for designs with adequate replication to monitor long—term trends of dispersed populations using sign occurrences on trails.

ISSN:1051-0761    

DOI:10.2307/1941877

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY