摘要:

Element dynamics (C, N, P, Ca, Mg, Mn, Fe, and Al) were examined along a decay continuum from freshly fallen litter to soil organic matter in a red spruce ecosystem in Maine. The continuum was defined using previously reported data on litterfall chemistry and the early stages (i.e., first 24 mo) of decay, combined with new studies on an additional 33 mo of decay and an evaluation of forest floor organic matter at the same site. Carbon concentrations decreased over time as fresh litter was transformed into soil organic matter; N, Fe, and Al concentrations increased; and P, Ca, Mg, and Mn concentrations showed variable patterns of increased and decrease. In general, homogeneity of the litter increased as fresh litter of different initial chemistries was transformed into chemically more uniform soil organic matter. Ecosystem budgets indicated that after 57 mo of decay, litter had lost 63% of its original mass and had released C (65%),N (17%), P (79%), Ca (50%), Mg (80%), and Mn (74%), and had accumulated Al (431%) and Fe (353%). Evidence is presented indicating that the retention of Al (and Fe) is primarily controlled by abiotic adsorption onto litter exchanges sites. Overall results are consistent with a two—phase model of decay: in the first phase (0 to 3—4 yr), mass loss fits a negative exponential model of decay, an inverse linear relationship exists between N concentration and mass loss, and rapid changes are observed in litter chemistry; the second phase (3—4+ yr) is characterized by markedly slower changes in mass loss and litter chemistry.

ISSN:0012-9658    

DOI:10.2307/1939412

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We conducted a 456—d laboratory incubation of an old—growth coniferous forest soil to aid in the elucidation of C controls on N cycling processes in forest soils. Gross rates of N mineralization, immobilization, and nitrification were measured by15N isotope dilution, and net rates N mineralization and nitrification were calculated from changes in KCl—extractable inorganic N and NO3—@ON pool sizes, respectively. Changes in the availability of C were assessed by monitoring rates of CO2evolution and the sizes of extractable organic C and microbial biomass pools. Net and gross rates of N mineralization (r2= 0.038, P = .676) and nitrification (r2= 0.403, P = .125) were not significantly correlated over the course of the incubation, suggesting that the factors controlling N consumptive and productive processes do not equally affect these processes. A significant increase in the NO3—pool size (net nitrification) only occurred after 140 d, when the NO3—pool size increased suddenly and massively. However, gross nitrification rates were substantial throughout the entire incubation and were poorly correlated with these changes in NO3—pool sizes. Concurrent decreases in the microbial biomass suggest that large increases in NO3—pool sizes after prolonged incubation of coniferous forest soil may arise from reductions in the rate of microbial immobilization of NO3—, rather than from one of the mechanisms proposed previously (e.g., sequestering of NH4+by microbial heterotrophs, the deactivation of allelopathic compounds, or large increases in autotrophic nitrifier populations). Strong correlations were found between rates of CO2evolution and gross N mineralization (r2= 0.974, P<.0001) and immobilization (r2= 0.980, P<.0001), but not between CO2evolution and net N mineralization rates. Microbial growth efficiency, determined by combining estimates of gross N immobilization, CO2evolution, and microbial biomass C and N pool sizes, declined exponentially over the incubation. These results suggest the utilization of lower quality substrates as C availability declined during incubation. Results from this research indicate the measurement of gross rates of N transformations in soil provides a powerful tool for assessing C and N cycling relationships in forests.

ISSN:0012-9658    

DOI:10.2307/1939413

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The Serengeti ecosystem contains some of Africa's most geobotanically diverse landscapes and supports some of the highest primary and secondary production on Earth. In an attempt to characterize landscape patterns in soil microbial processes across the Serengeti, 17 study sites from nine landscape regions were sampled for soil physical/chemical characteristics and laboratory determination of soil microbial biomass, 20—d net turnover rates of carbon and nitrogen, and respiratory and nitrogen mineralization responses to carbon and nitrogen amendments. A large variation in soil physical/chemical characteristics across landscapes and a high degree of intercorrelation among these soil properties were found. Soil microbial biomass carbon ranged from 587 to 8971 μg/g soil dry mass, constituting between 3.4 and 9.4% of the total soil carbon at the Central Hills and Southern Plains landscape sites, respectively. Soil respiration rates (as carbon loss per unit soil dry mass) ranged from 9 μ—1.d—1in the Northwest to 57 mg.g—1.d—1on the Southern Plains and were positively correlated with soil microbial biomass. Regression models incorporating percent water—holding capacity and total organic carbon were highly predictive of levels of microbial biomass and soil respiration across all landscapes. Net nitrogen mineralization rates per unit soil dry mass, averaging between ‐0.48 and 1.09 mg°g—1°d—1, were positively correlated with soil respiration rates, but unrelated to soil mineral nitrogen pools or soil microbial biomass. (NH4)2SO4additions significantly reduced both soil respiration rates and net nitrogen mineralization rates, but significantly increased net nitrate production, suggesting that nitrification is limited, in part, by ammonium availability. Low phosphorus availability may not only restrict nitrate production, but also limit ammonium production, thus having fundamental impacts on the nitrogen economy in this ecosystem. The interaction between N and P cycling is likely most significant in the tallgrass Northwest and Northeast landscapes, where granitic, P—deficient parent materials predominate, and nitrogen competition between plants and soil microbes limits aboveground nitrogen flow. On the Southern, Southeast, and Northwest Plains, where grazing intensity is greatest, soils have the highest levels of soil microbial biomass and lower C:P ratios, and microbial growth and nitrogen turnover rates appeared to be more C than N limited. The Eastern and Western Corridor, which support lower levels of sustained, intensive grazing, showed intermediate levels of soil microbial biomass and carbon and nitrogen turnover rates. The observation that mean annual rainfall is positively correlated with net primary production but negatively correlated with soil fertility across African savannas is supported by our data, which generally show that regional patterns of soil fertility and soil microbial processes are negatively correlated with mean annual rainfall across the Serengeti. We found soil microbial processes to be closely correlated with landscape patterns of herbivore use and intensity of herbivory. Herbivores track plant growth, which is highly variable both temporally and spatially, and in doing so, profoundly impact nutrient cycling processes where they feed. Thus, the bursts in soil microbial processes and plant growth that follow rainfall events are accompanied by the direct impact of herbivores on these processes.

ISSN:0012-9658    

DOI:10.2307/1939414

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Beaver (Castor canadensis) affect biogeochemical cycles and the accumulation and distribution of chemical elements over time and space by altering the hydrologic regime. Aerial photograph analyses of beaver activities on the 298—km2Kabetogama Peninsula, Minnesota, were coupled with site—specific studies of soil and pore water concentrations of nutrients (nitrogen, phosphorus) and other ions (potassium, calcium, magnesium, iron, sulfate, chloride), nitrogen cycling processes (nitrogen fixation and denitrification), and biophysical environment variables (vegetation, temperature, organic matter, soil structure, pH, and oxidation—reduction potential). Our analysis demonstrate that beaver influence the distribution, standing stocks, and availability of chemical elements by hydrologically induced alteration of biogeochemical pathways and by shifting element storage from forest vegetation to sediments and soils. Over the 63 yr of aerial photo records (1927—1988), beaver converted 13% of the peninsula to meadows and ponds. Elemental concentrations in soils (in micrograms per cubic centimetre) and in pore water (in milligrams per litre) revealed complex patterns within and among the principal hydrologic zones (e.g., forest, moist meadow, wet meadow, pond, stream). Principal components analysis (PCA) suggested that anaerobic conditions caused by saturation of soil by water was the fundamental control over subsequent alterations of biogeochemical pathways. Although few clear statistical trends were detected for mass— or volume—specific elemental concentrations among habitats, organic horizon (O and A) depths were greatest in the wet meadows and ponds (@>15 cm), causing the standing stocks of chemical elements to be greatest there. We argue that the net effect of beaver activities has been to translocate chemical elements from the originally inundated upland forest vegetation to downstream communities and to pond sediments. As the upland vegetation dies and decays after dam construction, only a portion of the chemical elements are exported downstream (except for calcium and magnesium) or returned to the atmosphere (C and N only ). Consequently, the organic horizons of pond sediments accumulate substantial standing stocks of chemical elements that are available for vegetative growth when dams fail, the ponds drain, and meadows are formed. Since 1927 beaver activities have augmented the standing stock of chemical elements in the organic horizons by 20—295%, depending on the element. These influences are spatially extensive and long lasting, affecting fundamental environmental characteristics of boreal forest drainage networks for decades to centuries.

ISSN:0012-9658    

DOI:10.2307/1939415

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

To determine why herbaceous productivity in tropical and subtropical savannas is often significantly higher under crowns of isolated trees than in adjacent grass—lands, experimental plots were established in three concentric zones, crown, tree—root, and grassland, surrounding isolated trees of Acacia tortilis in low—rainfall and high—rainfall savannas in Tsavo National Park, Kenya. Plots were fertilized (to determine the importance of nutrient enrichment by trees), shaded (to determine the importance of crown shade), fertilized and shaded (to identify fertilizer x shade interactions), or trenched (tree roots entering plots were severed to determine the importance of belowground competition between overstory trees and understory herbaceous plants). In addition, vertical root distributions of trees and herbaceous species were determined, and root systems of A. tortilis saplings were excavated. At both sites fertilization significantly increased herbaceous productivity in tree—root and grassland zones, but not in canopy zones; artificial shade had no effect on productivity at the low—rainfall site but increased productivity in the tree—root zone at the high—rainfall site; and severing tree roots had no effect on herbaceous productivity at the low—rainfall site, but increased productivity in the crown and tree—root zones at the high—rainfall site. Roots of herbaceous and woody species co—occurred within the same soil horizons, but tree roots extended farther into grasslands at the low—rainfall site than at the high—rainfall site. These studies suggest that savanna trees completed more intensely with understory plants at wetters sites, where their roots terminated in or near crown zones, than at drier sites, where their roots extended farther into open grassland. Nutrients added by trees to crown zones in the form of tree litter and animal droppings increased understory productivity by fertilizing nutrient—limited soils. Shade contributed more to regrowth after severe defoliation than to growth under more normal conditions.

ISSN:0012-9658    

DOI:10.2307/1939416

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Some clonal plant species decrease rhizome or stolon internode lengths and/or increase the frequency of branching when they grow in favorable environments. Thisforaging response is thought to be beneficial since it should allow ramets to concentrate in areas of favorable habitat. However, there have been few critical tests of the effectiveness with which ramets are placed in favorable habitat as a result of the foraging response. In this paper, I use empirically calibrated stochastic simulation and diffusion models to compare the growth of clones in favorable and unfavorable habitat. I ask whether observed changes in rhizome lengths and clonal branching patterns are likely to decrease significantly the distance clones move, and thus, to enable mets to remain for longer periods of time in favorable habitat. For the empirical data used in this study, results from the models indicate that the effectiveness of the foraging response is likely to be variable. In some cases, such as results from models based on the response of Glechoma hederacea to nutrients, there is no significant difference in the distance clones move in favorable and unfavorable habitat. Thus, even through rhizome lengths may be significantly shorter in favorable patches, this does not guarantee that clones disperse significantly less far and thereby remain longer in favorable habitat. The effectiveness of the foraging response depends strongly on the distribution of clonal growth angles, the pattern of clonal branching, and the variance in rhizome or stolon internode length. These results, particularly the importance of growth angles and the relatively limited effectiveness of the foraging response, differ from those in previously published models of foraging in clonal plant species. I conclude with a discussion of reasons that may underlie these differences.

ISSN:0012-9658    

DOI:10.2307/1939417

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The aim of this study was (1) to evaluate the importance of dormant buds for the bud demography of the mountain birch Betula pubesecens ssp. tortuosa near the tree line and (2) to study whether sexual reproduction leads to costs for bud production rates. A bud population census was taken in two consecutive years for six branches of each of 90 mountain birch trees. The trees were growing in a common garden and belonged to 10 different progenies originating from different parts of Finnish Lapland. The data were analyzed with matrix population models. The most important transformations of the bud populations were between vegetative short and long shoots. However, if most apparently dead buds are actually latent dormants, they make an even more important contribution to the bud population growth rate than vegetative long and short shoots. Dormant buds may have considerable importance especially after events such as herbivore outbreaks, in which short and long shoots are damaged. Generative long shoots (with male catkins) and short shoots (with female catkins) had approximately the same bud production rate as the corresponding vegetative shoots, i.e., bud populations did not show any major costs due to sexual reproduction. Meristem costs, i.e., a decrease in the number of buds due to sexual reproduction, may be relatively low in mountain birch, because new axillary buds develop and compensate for lost shoot apices. This compensation capacity may be especially well developed under suboptimal conditions, where canopy expansion is limited by the harsh environment rather than by the availability of meristems. The resource cost of reproduction (e.g., in terms of carbon or mineral nutrients) may also be partly compensated especially when flowering intensity is sufficiently low.

ISSN:0012-9658    

DOI:10.2307/1939418

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The widely advocated statement that ecotone movement may be useful in studies of the impacts of global warming implies not only the ecotones exist but that they can be delineated spatially. With sampled field data, the accuracy of the detected ecotone is related to the data type and its spatiotemporal resolution. In the present study, I introduce two edge detection algorithms for regularly (lattice—wombling) and irregularly (triangulation—wombling) two—dimensional sampled data. I investigate the reliability of these algorithms in detecting potential ecotones using simulated vegetation data that follow the individualistic, continuum—gradient, and community—type patterns. Ecotones were defined quantitatively as long narrow regions of high rates of change. Under this definition, significant ecotones were found mostly in the community—type patterns using either of the edge detection algorithms (lattice—wombling or triangulation—wombling) and a systematic or random sampling design, respectively.

ISSN:0012-9658    

DOI:10.2307/1939419

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

In order to understand the implications of changes in global CO2concentrations and temperature for the growth and fitness of individual plants, performance must be investigated in relation to the performance of other plants within a population. In this study we examined patterns of recruitment, mortality, and size structure of monospecific stands in response to ambient (400 μL/L) and elevated CO2concentrations (700 μL/L) across three temperature regimes; 18°, 28°, and 38°C. We created experimental populations of two annual plants that differ in their photosynthetic pathway and water use patterns: Abutilon theophrasti (C3) and Amaranthus retroflexus (C4). The effects of CO2, temperature, and their interactions on population structure were complex and species dependent. For both species increasing temperature resulted in higher germination and faster initial growth rates. These initial temperature responses increased the intensity and role of competition in determining stand size and structure. Postemergence responses to elevated CO2differed markedly between the two species. For Abutilon, the C3species, self—thinning and the mean biomass of the survivors increased under elevated CO2. For Amaranthus, survivorship, but not growth, increased under elevated CO2conditions. We attribute differences in response between species not only to photosynthetic pathway, but also to differences in the onset of competition mediated through differences in plant form and in resource uptake and deployment. The patterns of stand development in response to CO2and temperature suggest that the effects of changing CO2and temperature may be understood within mechanistically based models of resource use. Temperature regulates the rate of resource use and the onset of interference among plants, while CO2functions both as a resource and a resource regulator. Although mortality was concentrated later in stand development for Abutilon than Amaranthus, overall patterns of stand size and structure were similar for both species; mortality and size inequalities increased with increasing temperature and CO2. Because size is often correlated with fecundity, and increase in size hierarchies in response to elevated CO2, in conjunction with a decrease in survivorship, may result in a smaller effective population size. Our ability to predict changes in effective population size due to changing size due to changing size hierarchies alone, however, should also consider developmental shifts in response to elevated CO2that may result in, as in this study, a decrease in the minimum size at the onset of flowering.

ISSN:0012-9658    

DOI:10.2307/1939420

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The woody C3Prosopis glandulosa (honey mesquite) and C4perennial grass Schizachyrium scoparium (little bluestem) were grown along a gradient of daytime carbon dioxide concentrations from near 340 to 200 μmol/mol air in a 38 m long controlled environment chamber. We sought to determine effects of historical and prehistorical increases in atmospheric CO2concentration on growth, resource use, and competitive interactions of a species representative of C4—concentration of C4—dominated grassland in the southwestern United States and the invasive legume P. glandulosa. Increasing CO2concentration stimulated N2fixation by individually grown P. glandulosa and elicited in C3seedlings a similar relative increase in leaf intercellular CO2concentration, net assimilation rate, and intrinsic water use efficiency (leaf net assimilation rate/stomatal conductance). Aboveground biomass of P. glandulosa was not altered by CO2concentration, but belowground biomass and whole—plant water and nitrogen use efficiencies increased linearly with CO2concentration in seedlings that were grown alone. Biomass produced by P. glandulosa that was grown with S. scoparium was not affected by CO2concentration. Stomatal conductance declined and leaf assimilation rates of S. scoparium at near maximum incident light increased at higher CO2concentration, but there was no effect of CO2concentration on biomass production or whole—plant water use efficiency of the C4grass. Rising CO2concentration, especially the 27% increase since the beginning of the 19th century, may have contributed to more abundant P. glandulosa on C4grasslands by stimulating the shrub's growth or reducing the amount of resources that the C3required. Much of the potential response of P. glandulosa to CO2concentration, however, appears to be contingent on the shrub's escaping competition with neighboring grasses.

ISSN:0012-9658    

DOI:10.2307/1939421

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY