摘要:

Primary production, plant biomass, plant element content, and various measures of turnover and element cycling were compared among four contrasting tundra vegetation types near Toolik Lake, Alaska. The study sites were selected to represent extreme examples of the wide variation in plant growth form composition that is typical of northern ecosystems. The aim of the research was to determine whether vegetation types that differ in their dominant plant growth form also differ in their production: biomass relationships and overall patterns of element use. The four sites included tussock tundra, a deciduous shrub—dominated riparian tundra, an evergreen heath tundra, and wet sedge tundra. Biomass and element content (N, P, K, Ca, and Mg) were determined for both vascular and nonvascular plants, and production estimates were obtained for vascular plants. Production and biomass of most tissues were determined by quadrat harvest methods, with additional, separate determinations of stem secondary growth and belowground rhizome growth as components of net primary production (NPP). Production, biomass, and element content of roots were not determined. Vascular plant biomass (excluding roots) varied by nearly 9 × among sites, from 217 to 1877 g/m2. At least 50% of the vascular biomass at all four sites was belowground stems, but the relative allocation to leaves vs. aboveground stems varied greatly. When mosses and lichens were included, total biomass varied by only 5 × among sites, and lichens were in fact the most abundant plant form at the evergreen heath site. The element content in vascular biomass of these communities varied by 8—21 ×, depending upon the element; including mosses and lichens, element content varied by 6—12 ×. Primary productivity of vascular plants also varied sharply among sites, from 32 to 305 g · m—2 · yr—1(not including root production). Leaves were the largest single component of NPP at all sites, but the relative importance of above— and belowground stem growth varied considerably. The element requirements of vascular NPP varied by 9—17 ×, depending upon the element. Despite these order—of—magnitude differences among sites in biomass, production, and element requirements, and the dramatic variation in allocation patterns, there was remarkably little difference among sites in the overall production: biomass relationships and element turnover. These overall similarities were due to the dominant effect of stems and rhizomes in the whole—plant and whole—vegetation biomass and element budgets. The wide range of leaf turnover rates and leaf production efficiencies was compensated at the whole—plant level by stem processes, especially element storage. Thus, we conclude that plant growth form composition is not a good indicator of whole—vegetation biomass turnover rates and patterns of element use, although growth form composition is related to total production and biomass.

ISSN:0012-9615    

DOI:10.2307/1942997

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

In order to develop a model of the carbon cycle for mature slash pine (Pinus elliottii) stands in north Florida, we studied seasonal variation in leaf area index (LAI, allsided), aboveground biomass increment and litterfall, and light penetration through the forest canopy, over a 3—yr period. The primary approach to establishing monthly LAI included annual destructive analyses and monthly measurements of needle fall and elongation. Imagery from the Landsat Thematic Mapper (TM) and pattern of light penetration were also used in attempts to derive less ardouous estimates; the TM imagery was most promising. LAIs ranged from 3.0 to 6.5 on control plots over the 3 yr, with repeated fertilization increasing maximum LAI by>40%. Seasonal variation was high (40%), as was variation from year to year. An average of 31% of the incident photosynthetically active radiation (PAR) penetrated the canopies annually, ranging from 18 to 42% seasonally. Seasonal light penetration could not be described using a simple application of the Beer—Lambert law, perhaps due to the highly aggregated nature of the canopies. Models incorporating more information on canopy structure are necessary to predict light penetration through slash pine stands accurately. A model of needle litterfall was derived that could account for much of the seasonal and annual variation using stand basal area and climate conditions from the spring of the previous year; this model may be useful for developing climate—driven predictions of LAI. Efficiencies of use of incoming and intercepted PAR were low compared to other forest types. Low light interception and high nutrient—use efficiencies (demonstrated in earlier studies) are important adaptive characteristics of slash pine stands to these relatively warm and nutrient—poor sites.

ISSN:0012-9615    

DOI:10.2307/1942998

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Field experiments in ecological and environmental research usually do not meet the criteria for modern experimental design. Subsampling is often mistakenly substituted for true replication, and sample sizes are too small for adequate power in tests of significance. In many cases, field—study objectives may be better served by various kinds of sampling procedures, even though the resulting inferences will be weaker than those obtainable through controlled experimentation. The present paper provides a classification and description of methods for designing environmental studies, with emphasis on techniques as yet little used in ecology. Eight categories of techniques for field studies are defined in terms of the nature of control exerted by the observer, by the presence or absence of a perturbation, and by the domain of study. The first two categories include classical experimental approaches, replicated and unreplicated. Sampling for modelling provides efficient designs for estimating parameters in a specified model. Intervention analysis measures the effect of a known perturbation in a time series. Observational studies contrast selected groups from a population, while analytical sampling provides comparisons over the entire population. Descriptive survey sampling estimates means or totals over an entire population, while sampling for pattern deals with spatial patterns over a selected region. We propose that the statistical concept of a "superpopulation" may be useful in ecology, and that it may be desirable to approach community and ecosystem studies in a sampling framework, with experimentation used for a fairly narrow range of subsidiary investigations. Much more attention to processes for drawing inferences about cause and effect is needed, in any case.

ISSN:0012-9615    

DOI:10.2307/1942999

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

In this paper I combine experiment and theory to demonstrate that aggregation of carrion flies ovipositing among carcasses may facilitate coexistence in nature. The paper is divided into an experimental part and a theoretical part. In the experiment part, I present field experiments that show the aggregative pattern of carrion flies ovipositing among carcasses, as well as associated experiments that explore the behavior leading to these patterns. I also use experiments to quantify the strength of larval competition between Phaenicia coeruleiviridis (Calliphoridae) and Sarcophaga bullata (Sarcophagidae). In the theoretical part, I develop two methods that use these data to assess the effect of aggregation on competition in the field. From the first method, I predict that intraspecific aggregation of P. coeruleiviridis females causes a 26% decrease in the expected recruitment of P. coeruleiviridis and a 74% increase in the expected recruitment of S. bullata. As a result, within a single generation, intraspecific aggregation increases intraspecific competition and reduces interspecific competition. The second method for assessing aggregation involves a new mathematical formula that gives conditions under which species may coexist indefinitely. This formula requires estimates of only the degrees of intra— and interspecific aggregation among species; there is no need to estimate adult survival and larval competition. I use this second method to show that the minimum effect of aggregation on coexistence for five carrion fly species is equivalent to reducing the amount of larval competition between pairs of species by an average of 57%. Aggregation may therefore play a major role in explaining the coexistence of carrion fly competitors.

ISSN:0012-9615    

DOI:10.2307/1943000

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

An investigation of the processes that cause succession was performed in an intertidal community on the central Oregon coast. The community was dominated by barnacles and several species of macroalgae. The successional sequence was determined at three different sites by clearing sets of plots in a way that mimicked natural disturbance. Succession at each of these sites followed the same general sequence. A barnacle, Chthamalus dalli, first colonized the plots and was later replaced by a second barnacle, Balanus glandula. The macroalgae Pelvetiopsis limitata, Fucus distichus, and Endocladia muricata colonized the plots only after Balanus was established. While the order of species arrival and departure was consistent, the rate of succession varied greatly among sites. The sequence of community development that was observed at one site over a 36—mo period occurred in<12 mo in a nearby area. Differences in the rate of succession appeared to result from variation in the timing of successful Balanus recruitment. The mechanisms of succession were investigated in a series of field experiments. An experiment with Balanus—removal, Chthamalus—removal, and control plots was used to assess the interactions between barnacles. A direct interaction, competition for space with Balanus, caused Chthamalus to decrease in abundance as succession proceeded. Chthamalus did not affect the establishment of Balanus. Successful Balanus recruitment depended on occasional periods of larval settlement followed by periods of favorable weather. At all three sites, algal colonization was dependent on the presence of barnacles. Balanus greatly facilitated algal colonization, while Chthamalus only weakly facilitated algal colonization. Facilitation of algal colonization by epoxy—filled barnacle tests indicated that facilitation resulted from barnacle tests altering the substrate, rather than the activities of the living animals. A factorial experiment involving manipulations of barnacle and consumer (limpet) abundances demonstrated that the facilitation of algae by barnacles is an indirect interaction that is mediated by limpets. Barnacles decreased limpet foraging activity and thereby increased algal abundance. Succession in this community is complicated by several processes that are not included in traditional views of succession. (1) Not all early successional species have the same effect on the establishment of later successional species. This results in spatial variation in the rate of succession. (2) The model of succession is different in each step in the successional sequence. The Cthamalus—Balanus interaction is an example of the tolerance model, while the barnacle—algae interaction is an example of the facilitation model. (3) Both direct and indirect interactions between species determine the course of succession. The results of this study support a general model that predicts the effect of consumers on the rate of succession. Consumers slowed succession in this community in two ways. First, limpets delayed the establishment of Balanus and the competitive exclusion of Chthamalus. Second, limpets delayed the establishment of macroalgae. Previous studies in marine and terrestrial habitats have found that consumers may slow, accelerate, or have no effect on the rate of succession and these observations are consistent with the predictions of this general model.

ISSN:0012-9615    

DOI:10.2307/1943001

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY