排序方式: 共有45条查询结果,搜索用时 625 毫秒
31.
L. C. Johnson G. R. Shaver A. E. Giblin K. J. Nadelhoffer E. R. Rastetter J. A. Laundre G. L. Murray 《Oecologia》1996,108(4):737-748
We examined the importance of temperature (7°C or 15°C) and soil moisture regime (saturated or field capacity) on the carbon (C) balance of arctic tussock tundra microcosms (intact blocks of soil and vegetation) in growth chambers over an 81-day simulated growing season. We measured gaseous CO2 exchanges, methane (CH4) emissions, and dissolved C losses on intact blocks of tussock (Eriophorum vaginatum) and intertussock (moss-dominated). We hypothesized that under increased temperature and/or enhanced drainage, C losses from ecosystem respiration (CO2 respired by plants and heterotrophs) would exceed gains from gross photosynthesis causing tussock tundra to become a net source of C to the atmosphere. The field capacity moisture regime caused a decrease in net CO2 storage (NEP) in tussock tundra micrososms. This resulted from a stimulation of ecosystem respiration (probably mostly microbial) with enhanced drainage, rather than a decrease in gross photosynthesis. Elevated temperature alone had no effect on NEP because CO2 losses from increased ecosystem respiration at elevated temperature were compensated by increased CO2 uptake (gross photosynthesis). Although CO2 losses from ecosystem respiration were primarily limited by drainage, CH4 emissions, in contrast, were dependent on temperature. Furthermore, substantial dissolved C losses, especially organic C, and important microhabitat differences must be considered in estimating C balance for the tussock tundra system. As much as 20% of total C fixed in photosynthesis was lost as dissolved organic C. Tussocks stored 2x more C and emitted 5x more methane than intertussocks. In spite of the limitations of this microcosm experiment, this study has further elucidated the critical role of soil moisture regime and dissolved C losses in regulating net C balance of arctic tussock tundra. 相似文献
32.
Species Diversity Across Nutrient Gradients: An Analysis of Resource Competition in Model Ecosystems
Darrell A.?HerbertEmail author Edward B.?Rastetter Laura?Gough Gaius R.?Shaver 《Ecosystems》2004,7(3):296-310
The capture and efficient use of limiting resources influence the competitive success of individual plant species as well as species diversity across resource gradients. In simulations, efficient nutrient acquisition or nutrient retention by species were key predictors of success when nutrients were limiting. Increased nutrient supply favored species with characteristics that improved light interception or light use. Ecological theory suggests that low diversity on fertile sites may be a consequence of competitive exclusion by one or a few species with superior light-interception characteristics. On infertile sites, competitive exclusion may be a function of superior nutrient-acquisition characteristics in species. At intermediate fertility, a shift from single-resource specialization to a balanced effort in the acquisition of multiple resources should allow for greater species diversity. Thus, a unimodal relationship between diversity and nutrient supply, vegetation biomass, or productivity is predicted. However, simulations demonstrated alternate relationships depending on the ecosystem characteristic to which diversity was compared. Diversity was greatest at intermediate total biomass but increased monotonically with net primary production
and nitrogen (N) supply. The highest diversity occurred midrange on a scale of community-level leaf area to fine-root length ratios, which in the context of the model indicates that the vegetation as a whole was simultaneously limited by both N and light and that effort toward the acquisition of both resources is distributed in such a way that both resources are equally exploited. Diversity was lowered by the presence of species with a superior ability to sequester resources. 相似文献
33.
Towards an ecological understanding of biological nitrogen fixation 总被引:28,自引:5,他引:23
Vitousek Peter M. Cassman Ken Cleveland Cory Crews Tim Field Christopher B. Grimm Nancy B. Howarth Robert W. Marino Roxanne Martinelli Luiz Rastetter Edward B. Sprent Janet I. 《Biogeochemistry》2002,(1):1-45
Biogeochemistry - N limitation to primary production and other ecosystem processes is widespread. To understand the causes and distribution of N limitation, we must understand the controls of... 相似文献
34.
Changes in Individual Allometry Can Lead to Species Coexistence without Niche Separation 总被引:1,自引:0,他引:1
The principle of competitive exclusion is a fundamental tenet of ecology. Commonly used competition models predict that at
most only one species per limiting resource can coexist in the same environment at steady state; hence, the upper limit to
species diversity depends only on the number of limiting resources and not on the rates of resource supply. We demonstrate
that such model behavior is the result of both the growth and biomass turnover functions being proportional to the population
biomass. We argue that at least the growth function should be a nonlinear, concave downward function of biomass. This form
for the growth function should arise simply because of changes in the allometry of individuals in the population. With this
change in model structure, we show that any number of species can coexist at an asymptotically stable steady state, even where
there is only one limiting resource. Furthermore, if growth increases nonlinearly with biomass, the steady-state resource
concentration and hence the potential for biodiversity increases as the resource supply rate increases.
Received 31 August 2001; accepted 10 April 2002. 相似文献
35.
Resource partitioning, facilitation, and sampling effect are the three mechanisms behind the biodiversity effect, which is depicted usually as the effect of plant-species richness on aboveground net primary production. These mechanisms operate simultaneously but their relative importance and interactions are difficult to unravel experimentally. Thus, niche differentiation and facilitation have been lumped together and separated from the sampling effect. Here, we propose three hypotheses about interactions among the three mechanisms and test them using a simulation model. The model simulated water movement through soil and vegetation, and net primary production mimicking the Patagonian steppe. Using the model, we created grass and shrub monocultures and mixtures, controlled root overlap and grass water-use efficiency (WUE) to simulate gradients of biodiversity, resource partitioning and facilitation. The presence of shrubs facilitated grass growth by increasing its WUE and in turn increased the sampling effect, whereas root overlap (resource partitioning) had, on average, no effect on sampling effect. Interestingly, resource partitioning and facilitation interacted so the effect of facilitation on sampling effect decreased as resource partitioning increased. Sampling effect was enhanced by the difference between the two functional groups in their efficiency in using resources. Morphological and physiological differences make one group outperform the other; once these differences were established further differences did not enhance the sampling effect. In addition, grass WUE and root overlap positively influence the biodiversity effect but showed no interactions. 相似文献
36.
Ruth D. Yanai John J. Battles Andrew D. Richardson Corrie A. Blodgett Dustin M. Wood Edward B. Rastetter 《Ecosystems》2010,13(2):239-248
Ecosystem nutrient budgets often report values for pools and fluxes without any indication of uncertainty, which makes it
difficult to evaluate the significance of findings or make comparisons across systems. We present an example, implemented
in Excel, of a Monte Carlo approach to estimating error in calculating the N content of vegetation at the Hubbard Brook Experimental
Forest in New Hampshire. The total N content of trees was estimated at 847 kg ha−1 with an uncertainty of 8%, expressed as the standard deviation divided by the mean (the coefficient of variation). The individual
sources of uncertainty were as follows: uncertainty in allometric equations (5%), uncertainty in tissue N concentrations (3%),
uncertainty due to plot variability (6%, based on a sample of 15 plots of 0.05 ha), and uncertainty due to tree diameter measurement
error (0.02%). In addition to allowing estimation of uncertainty in budget estimates, this approach can be used to assess
which measurements should be improved to reduce uncertainty in the calculated values. This exercise was possible because the
uncertainty in the parameters and equations that we used was made available by previous researchers. It is important to provide
the error statistics with regression results if they are to be used in later calculations; archiving the data makes resampling
analyses possible for future researchers. When conducted using a Monte Carlo framework, the analysis of uncertainty in complex
calculations does not have to be difficult and should be standard practice when constructing ecosystem budgets. 相似文献
37.
M. Williams Y. E. Shimabukuro D. A. Herbert S. Pardi Lacruz C. Renno E. B. Rastetter 《Ecosystems》2002,5(7):0692-0704
Transferring fine-scale ecological knowledge into an understanding of earth system processes presents a considerable challenge
to ecologists. Our objective here was to identify and quantify heterogeneity of, and relationships among, vegetation and soil
properties in terra firme rain forest ecosystems in eastern Amazonia and assess implications for generating regional predictions
of carbon (C) exchange. Some of these properties showed considerable variation among sites; soil textures varied from 11%
to 92% clay. But we did not find any significant correlations between soil characteristics (percentage clay, nitrogen [N],
C, organic matter) and vegetation characteristics (leaf area index [LAI], foliar N concentration, basal area, biomass, stem
density). We found some evidence for increased drought stress on the sandier sites: There was a significant correlation between
soil texture and wood δ13C (but not with foliar δ13C); volumetric soil moisture was lower at sandier sites; and some canopy foliage had large, negative dawn water potentials
(ψld), indicating limited water availability in the rooting zone. However, at every site at least one foliage sample indicated
full or nearly full rehydration, suggesting significant interspecific variability in drought vulnerability. There were significant
differences in foliar δ15N among sites, but not in foliar % N, suggesting differences in N cycling but not in plant access to N. We used an ecophysiological
model to examine the sensitivity of gross primary production (GPP) to observed inter- and intrasite variation in key driving
variables—LAI, foliar N, and ψld. The greatest sensitivity was to foliar N; standard errors on foliar N data translated into uncertainty in GPP predictions
up to ±10% on sunny days and ±5% on cloudy days. Local variability in LAI had a minor influence on uncertainty, especially
on sunny days. The largest observed reductions in ψld reduced GPP by 4%–6%. If uncertainty in foliar N estimates is propagated into the model, then GPP estimates are not significantly
different among sites. Our results suggest that water restrictions in the sandier sites are not enough to reduce production
significantly and that texture is not the key control on plant access to N.
Received 28 June 2001; accepted 13 March 2002. 相似文献
38.
Glycoproteins of leukemic cells and 24-hour urinary proteins were subjected to SDS polyacrylamide gel electrophoresis followed by affinity labelling I125 with Concanavalin A, indicating glycoproteins with mannose and/or glucose carbohydrate residues. Among the cellular glycoproteins a 41 000 dalton glycoprotein appeared under induction therapy in close correlation to the reduction of leukemic cells in ALL as well as in AML. 相似文献
39.
40.
Edward B. Rastetter Kevin L. Griffin Bonnie L. Kwiatkowski George W. Kling 《Global Change Biology》2023,29(21):6093-6105
Whole-ecosystem interactions and feedbacks constrain ecosystem responses to environmental change. The effects of these constraints on responses to climate trends and extreme weather events have been well studied. Here we examine how these constraints respond to changes in day-to-day weather variability without changing the long-term mean weather. Although environmental variability is recognized as a critical factor affecting ecological function, the effects of climate change on day-to-day weather variability and the resultant impacts on ecosystem function are still poorly understood. Changes in weather variability can alter the mean rates of individual ecological processes because many processes respond non-linearly to environmental drivers. We assessed how these individual-process responses to changes in day-to-day weather variability interact with one another at an ecosystem level. We examine responses of arctic tundra to changes in weather variability using stochastic simulations of daily temperature, precipitation, and light to drive a biogeochemical model. Changes in weather variability altered ecosystem carbon, nitrogen, and phosphorus stocks and cycling rates in our model. However, responses of some processes (e.g., respiration) were inconsistent with expectations because ecosystem feedbacks can moderate, or even reverse, direct process responses to weather variability. More weather variability led to greater carbon losses from land to atmosphere; less variability led to higher carbon sequestration on land. The magnitude of modeled ecosystem response to weather variability was comparable to that predicted for the effects of climate mean trends by the end of the century. 相似文献