Plant Community Biomass Research Articles

Effects of plant diversity, plant productivity and habitat parameters on arthropod abundance in montane European grasslands

Arthropod abundance has been hypothesized to be correlated with plant diversity but the results of previous studies have been equivocal. In contrast, plant productivity, vegetation structure, abiotic site conditions, and the physical disturbance of habitats, are factors that interact with plant diversity, and that have been shown to influence arthropod abundance. We studied the combined effect of plant species diversity, productivity and site characteristics on arthropod abundance in 71 managed grasslands in central Germany using multivariate statistics. For each site we determined plant species cover, plant community biomass (productivity), macro‐ and micronutrients in the soil, and characterized the location of sites with respect to orographic parameters as well as the current and historic management regimes. Arthropods were sampled using a suction sampler and classified a priori into functional groups (FGs). We found that arthropod abundance was not correlated with plant species richness, effective diversity or Camargo's evenness, even when influences of environmental variables were taken into account. In contrast, plant community composition was highly correlated with arthropod abundances. Plant community productivity influenced arthropod abundance but explained only a small proportion of the variance. The abundances of the different arthropod FGs were influenced differentially by agricultural management, soil characteristics, vegetation structure and by interactions between different FGs of arthropods. Herbivores, carnivores and detritivores reacted differently to variation in environmental variables in a manner consistent with their feeding mode. Our results show that in natural grassland systems arthropod abundance is not a simple function of plant species richness, and they emphasize the important role of plant community composition for the abundance patterns of the arthropod assemblages.

Effects of invasive macrophytes on littoral-zone productivity and foodweb dynamics in a New Zealand high-country lake

AbstractInvasion of littoral zones by adventive macrophyte species can facilitate major changes in the ecology of lakes. In Lake Wanaka, a large alpine New Zealand lake, the macrophytes Lagarosiphon major and Elodea canadensis (Hydrocharitaceae) have invaded parts of the lake where they form tall dense plant beds throughout mid-depths (2–7 m) of the littoral zone. We investigated differences in plants, benthic invertebrates, fish, and food webs characterizing native and exotic plant beds in mid-depths of the littoral zone. The 3× higher plant biomass and 2× higher plant surface area in exotic than in native plant beds (quillworts, milfoils, and charophytes) contributed to greater standing stocks and productivity of epiphyton in the exotic plant beds. Invertebrate communities were less dense (1890/m2 vs 4030/m2) and less diverse (richness = 9 vs 12) in native than in exotic plant beds because of differences in biomass, productivity, and physical structure of native and exotic plant communities. Invertebrat...

Seasonal changes in the relationship between plant species richness and community biomass in early succession

We analysed the relationship between plant species richness and productivity on first-year-old fields at two similar sites in central Europe. At both sites, a wide range of productivity levels was available resulting from different long-term fertilisation. In order to identify underlying mechanisms of the species richness–productivity relationship we included the seasonal dynamics and the number of individuals of each species in our analysis. We sampled 10 and 21 plots, respectively, at the two sites in May, June and July by harvesting all aboveground parts of vascular plants in 0.25 m2 subplots. Species richness, number of individuals of each species and community biomass as a surrogate of productivity were recorded in each sample. At one site, the relationship between species richness and biomass was significantly positive in the May and June harvest. This relationship disappeared in the July harvest due to a reduction in species richness at high productivity levels. The relations between species richness and number of individuals and between number of individuals and biomass paralleled the species richness–productivity relation but the individual number–biomass relationship remained positive until the last harvest. Between-species differences in individual number–community biomass relationships and their seasonal dynamics revealed “interspecific competitive exclusion” even though the species richness–biomass relationships were not negative or hump-shaped. At the second site, species richness was not related to productivity or to number of individuals. Our study demonstrated the importance of temporal dynamics and regional processes in understanding species richness–productivity patterns. Die Beziehung zwischen pflanzlicher Artenvielfalt und Produktivität wurde in einjährigen Brachen an zwei ähnlichen Standorten in Mitteleuropa untersucht. Innerhalb beider Standorte hatte eine langjährig unterschiedliche Düngung zu grossen Differenzen in der Produktivität geführt. Um die der Artenvielfalt-Produktivitäts Beziehung zugrundeliegenden Mechanismen heraus zu arbeiten, wurden die zeitliche Dynamik und die Individuenzahl jeder einzelnen Art in die Analyse einbezogen. Im Mai, Juni und Juli wurden an beiden Standorten die oberirdischen Teile aller Gefässpflanzen in 10 bzw. 21 Teilflächen von je 0.25 m2 abgeerntet. Für jede Probe wurden die Artenzahl, die Individuenzahl pro Art sowie die Biomasse als Maß für die Produktivität bestimmt. An einem der beiden Standorte war die Beziehung zwischen Artenvielfalt und Biomasse in der Mai- und Juni-Ernte signifikant positiv. Aufgrund einer Verringerung der Artenvielfalt bei hoher Produktivität verschwand dieser positive Zusammenhang jedoch in der Juli-Ernte. Die Beziehungen zwischen Artenvielfalt und Individuenzahl sowie zwischen Individuenzahl und Biomasse verliefen parallel zu der Beziehung zwischen Artenvielfalt und Biomasse, die Individuenzahl–Biomasse–Beziehung blieb allerdings bis zur letzten Ernte positiv. Zwischen den Arten traten große Unterschiede in der Beziehung zwischen Individenzahl und Biomasse pro Teilfläche auf. Auch wenn keine negativen oder unimodalen Beziehungen zwischen Artenvielfalt und Biomasse beobachtet wurden, stellen diese artspezifischen Unterschiede und vor allem ihr zeitlicher Verlauf einen Beleg für den Ausschluss von Arten durch interspezifische Konkurrenz dar. Am zweiten Standort war die Artenvielfalt zu keinem Zeitpunkt mit der Biomasse und der Individuenzahl korreliert. Die vorliegende Arbeit veranschaulicht die große Bedeutung von zeitlicher Dynamik und standörtlichen Unterschieden für das Verständnis der Beziehung zwischen Artenvielfalt und Produktivität.

Biomass production, N:P ratio and nutrient limitation in a Caucasian alpine tundra plant community

AbstractQuestions:1. To what extent is biomass production of a Caucasian alpine tundra plant community limited by soil nitrogen and/ or phosphorus? 2. Can the foliar N:P ratio predict the nutrient limitation pattern of alpine vascular plant communities?Location:Lichen‐rich tundra on Mt. Malaya Khatipara in the NW Caucasus, Russia (43°27’N, 41°42’E; alt. 2800 m a.s.l.).Methods:We conducted a 4–year fertilization experiment (N, P, N+P, lime additions and irrigation) on the alpine tundra in the northwestern Caucasus, Russia. We determined responses of biomass, tissue nutrient concentrations and nutrient pools of the above‐ground component of the plant community.Results:Total plant community biomass did not respond to fertilization. However, lichen biomass strongly decreased in response to the N‐ and N+P treatments, whereas vascular plant biomass increased in response to the N‐ and even more to the N+P treatment, but not to P or lime addition or irrigation. P‐concentrations in vascular plant species were very low, but their biomass production was not principally P‐limited, suggesting adaptation to low soil P‐availability. The N‐limitation of vascular plant biomass production in the community, which in lowlands usually occurs at N:P ratios below 16, could not be predicted from the mean foliar N:P mass ratio in the control (N:P = 29).Conclusions:This Caucasian alpine plant community is an example of N‐ and P‐co‐limitation of vascular plant biomass production, with N being the principal and P the secondary limiting nutrient. Critical N:P ratios as determined for lowland communities are not applicable here.

WHAT DETERMINES THE STRENGTH OF A TROPHIC CASCADE?

Trophic cascades have been documented in a diversity of ecological systems and can be important in determining biomass distribution within a community. To date, the literature on trophic cascades has focused on whether and in which systems cascades occur. Many biological (e.g., productivity : biomass ratios) and methodological (e.g., experiment size or duration) factors vary with the ecosystem in which data were collected, but ecosystem type, per se, does not provide mechanistic insights into factors controlling cascade strength. Here, we tested various hypotheses about why trophic cascades occur and what determines their magnitude using data from 114 studies that measured the indirect trophic effects of predators on plant community biomass in seven aquatic and terrestrial ecosystems. Using meta-analysis, we examined the relationship between the indirect effect of predator ma- nipulation on plants and 18 biological and methodological factors quantified from these studies. We found, in contrast to predictions, that high system productivity and low species diversity do not consistently generate larger trophic cascades. A combination of herbivore and predator metabolic factors and predator taxonomy (vertebrate vs. invertebrate) explained 31% of the variation in cascade strength among all 114 studies. Within systems, 18% of the variation in cascade strength was explained with similar predator and herbivore char- acteristics. Within and across all systems, the strongest cascades occurred in association with invertebrate herbivores and endothermic vertebrate predators. These associations may result from a combination of true biological differences among species with different phys- iological requirements and bias among organisms studied in different systems. Thus, al- though cascade strength can be described by biological characteristics of predators and herbivores, future research on indirect trophic effects must further examine biological and methodological differences among studies and systems.

Biomass production, N:P ratio and nutrient limitation in a Caucasian alpine tundra plant community

Abstract Questions: 1. To what extent is biomass production of a Caucasian alpine tundra plant community limited by soil nitrogen and/ or phosphorus? 2. Can the foliar N:P ratio predict the nutrient limitation pattern of alpine vascular plant communities? Location: Lichen-rich tundra on Mt. Malaya Khatipara in the NW Caucasus, Russia (43°27’N, 41°42’E; alt. 2800 m a.s.l.). Methods: We conducted a 4–year fertilization experiment (N, P, N+P, lime additions and irrigation) on the alpine tundra in the northwestern Caucasus, Russia. We determined responses of biomass, tissue nutrient concentrations and nutrient pools of the above-ground component of the plant community. Results: Total plant community biomass did not respond to fertilization. However, lichen biomass strongly decreased in response to the N- and N+P treatments, whereas vascular plant biomass increased in response to the N- and even more to the N+P treatment, but not to P or lime addition or irrigation. P-concentrations in vascular plant species were very low, but their biomass production was not principally P-limited, suggesting adaptation to low soil P-availability. The N-limitation of vascular plant biomass production in the community, which in lowlands usually occurs at N:P ratios below 16, could not be predicted from the mean foliar N:P mass ratio in the control (N:P = 29). Conclusions: This Caucasian alpine plant community is an example of N- and P-co-limitation of vascular plant biomass production, with N being the principal and P the secondary limiting nutrient. Critical N:P ratios as determined for lowland communities are not applicable here.

Functional regularity: a neglected aspect of functional diversity

Functional diversity has been identified as a key to understanding ecosystem and community functioning. However, due to the lack of a sound definition its nature and measurement are still poorly understood. In the same way that species diversity can be split into species richness and species evenness, so functional diversity can be split into functional richness (i.e. the amount of functional trait/character/attribute space filled) and functional evenness (i.e. the evenness of abundance distribution in functional trait space). We propose a functional regularity index (FRO) as a measure of functional evenness for situations where species are represented only by a single functional trait value (e.g. mean, median or mode), and species abundances are known. This new index is based on the Bulla O index of species evenness. When dealing with functional types or categorical functional traits, the Bulla O or any other accepted species evenness index may be used directly to measure functional evenness. The advantage of FRO is that it supplies a measure of functional evenness for continuous trait data. The FRO index presented in this paper fulfils all the a priori criteria required. We demonstrate with two example datasets that a range of FRO values may be obtained for both plant and animal communities. Moreover, FRO was strongly related to ecosystem function as seen in photosynthetic biomass in plant communities, and was able to differentiate sampling stations in a lagoon based on the functional traits of fish. Thus, the FRO index is potentially a highly useful tool for measuring functional diversity in a variety of ecological situations.

Identification of the limiting resource within a semi-arid plant association

Our objective was to determine the resource most limiting to plant growth within a semi-arid plant association. We hypothesized that the addition of nitrogen (N) would elicit an increase in biomass more than additions of water, light, phosphorus (P), potassium (K), or sulfur (S). Treatments included the addition of water, N, P, K, or S, or reduction of ambient light on two sites in southwestern Montana within the Festuca idahoensis/Agropyron spicatum plant association. Nitrogen was the only treatment that increased plant community biomass, specifically native grass biomass (about 50% greater than the control). The addition of water decreased above-ground plant community biomass. Our results suggest that N is the primary limiting resource for the dominant functional group within the F. idahoensis/A. spicatum plant association.

Multivariate control of plant species richness and community biomass in blackland prairie

Recent studies have shown that patterns of plant species richness and community biomass are best understood in a multivariate context. The objective of this study was to develop and evaluate a multivariate hypothesis about how herbaceous biomass and richness relate to gradients in soil conditions and woody plant cover in blackland prairies. Structural equation modeling was used to investigate how soil characteristics and shade by scattered Juniperus virginiana trees relate to standing biomass and species richness in 99 0.25 m2 quadrats collected in eastern Mississippi, USA. Analysis proceeded in two stages. In the first stage, we evaluated the hypothesis that correlations among soil parameters could be represented by two underlying (latent) soil factors, mineral content and organic content. In the second stage, we evaluated the hypothesis that richness and biomass were related to (1) soil properties, (2) tree canopy extent, and (3) each other (i.e. reciprocal effects between richness and biomass). With some modification to the details of the original model, it was found that soil properties could be represented as two latent variables. In the overall model, 51% and 53% of the observed variation in richness and biomass were explained. The order of importance for variables explaining variations in richness was (1) soil organic content, (2) soil mineral content, (3) community biomass, and (4) tree canopy extent. The order of importance for variables explaining biomass was (1) tree canopy and (2) soil organic content, with neither soil mineral content nor species richness explaining significant variation in biomass. Based on these findings, we conclude that variations in richness are uniquely related to both variations in soil conditions and variations in herbaceous biomass. We further conclude that there is no evidence in these data for effects of species richness on biomass.

Are invasive plant species better competitors than native plant species? – evidence from pair‐wise experiments

Invasive plants often appear to be more competitive than native species, but there have been few tests of this hypothesis. We reviewed published pair‐wise experiments between invading and native plant species. Although the designs that have been used allow only limited inferences, the available data suggest that the effect of invasive species on native species is usually stronger than vice versa. Furthermore, mixtures of invasive and native species are generally less productive than monocultures of the native species, but not less than monocultures of the invasive species. However, the selection of invaders and natives for study has not been random, and the data could be biased towards highly competitive invaders and natives that are weaker than average competitors. We attempt to clarify confusion surrounding the concept of competitive superiority in the context of plant invasions, and we discuss the limitations of the methods that have been used to investigate competition between invasive and native species. To rigorously test the generality of the hypothesis that invaders are better competitors than natives we need to compare the effects of closely related native and invasive species on each other. We suggest that the influence of an invading species on total plant community biomass is an important clue in understanding the role of competition in a plant invasion. The role of competition in the establishment and naturalization stages of the invasion process may be very different from its role in the “outbreak” stage.

Identification of the limiting resource within a semi-arid plant association

Our objective was to determine the resource most limiting to plant growth within a semi-arid plant association. We hypothesized that the addition of nitrogen (N) would elicit an increase in biomass more than additions of water, light, phosphorus (P), potassium (K), or sulfur (S). Treatments included the addition of water, N, P, K, or S, or reduction of ambient light on two sites in southwestern Montana within the Festuca idahoensis/ Agropyron spicatum plant association. Nitrogen was the only treatment that increased plant community biomass, specifically native grass biomass (about 50% greater than the control). The addition of water decreased above-ground plant community biomass. Our results suggest that N is the primary limiting resource for the dominant functional group within the F. idahoensis/ A. spicatum plant association.

Dietary shift and lowered biomass gain of a generalist herbivore in species-poor experimental plant communities.

Species loss of primary producers is likely to affect processes on other trophic levels. We studied consumption and individual performance of the generalist herbivore Parapleurus alliaceus (Orthoptera) in relation to the species richness of primary producers. Adult grasshoppers were caged and left to feed for 2 weeks on experimental grassland communities ranging in plant species richness from one, two, four, eight to 32 species. Low plant diversity had a negative effect on both plant community biomass and on biomass gain of female grasshoppers, feeding to produce eggs (male grasshoppers did not gain biomass during the feeding period). This was surprising because plots with high plant diversity had a low proportion of grass biomass and grasshoppers preferentially selected grasses, leading to a greater exploitation of grasses in experimental communities of higher diversity. Thus, the concurrent increase in non-grass species in the diet from these high-diversity communities must have been beneficial to the generalist herbivore. In addition to the positive effects of plant diversity, the presence of legumes in a mixture with grasses further enhanced the biomass gain of grasshoppers at a given level of diversity. These findings suggest that plant species loss may lead to shifts in herbivore population sizes, reducing those of generalists and benefiting specialists of the remaining plant species. Our results further suggest that generalist herbivores, by having feeding preferences, can also change the relative abundances of plant species with different functional characteristics. This may feedback on both composition and diversity of plant communities.

Does the Fretwell–Oksanen model apply to invertebrates?

Despite the myriads of herbivores in terrestrial ecosystems, the world is green (Hairston et al. 1960). Nevertheless, all these animals consume plant material and they should therefore have some influence on plant individuals, populations and communites. Hence, in order to understand community and ecosystem processes, we need a quantitative understanding of the impact of herbivores on plant communities and of its variability across ecosystems. Many ecologists will agree, that invertebrates have a lower impact on plant communities than vertebrates (Crawley 1989). However, exclusion experiments revealed that invertebrates influence biomass, species richness, competition regimes and nutrient cycling in plant communities (Bach 1994, Brown 1994, Brown and Gange 1989, Carson and Root 1999, Fraser and Grime 1997, Gibson et al. 1990). Furthermore, the results of these experiments showed considerable variability of the herbivore effect across plant communities (Hendrix et al. 1988, Fraser and Grime 1997). One explanation for this variability is the so-called Fretwell–Oksanen model. Fretwell (1977, 1987), Oksanen (1990) and Oksanen et al. (1981) suggested that the relative importance of top-down and bottom-up forces within communities changes with productivity. A low primary productivity should not be sufficient to sustain appreciable populations of herbivores (bottomup control of herbivores), whereas at high levels of productivity predators control herbivores (top-down control of herbivores). Hence, on the community level herbivore impact should show a hump at intermediate levels of productivity. Oksanen et al. (1981) and Oksanen (1990) emphasised that their model applies only to vertebrates. Invertebrates should have limited possibilities to respond on primary productivity, due to their low mobility, narrow dietary niche breadth and low costs of dormancy. Furthermore, invertebrate carnivores exploit also detritus-feeding prey and need little energy to run their physiological machinery throughout the year. Nevertheless, the Fretwell–Oksanen model has been applied to invertebrate grazing systems. The results of some recent studies provided some support for a hump-shaped pattern along nutrient gradients (Fraser 1998, Fraser and Grime 1997). Therefore, the objective of our study was to approach the following question: Do the available experimental results suggest a general hump-shaped relationship between productivity and the impact of invertebrate phytophages on plant communities?

Plant diversity and tree responses following contrasting disturbances in boreal forest

We determined the abundance and diversity of vascular plants in seven types of disturbance in mixed-wood boreal forest. Disturbance treatments included wildfire, natural regeneration after harvest and several methods of silvicultural site preparation. Relative to undisturbed forest, all disturbance treatments increased plant diversity to about the same extent. The abundance of plant growth-forms differed significantly between disturbance treatments. Silvicultural treatments involving soil disturbance (disk-trenching, drum-chopping and blading) had higher cover of grasses and annual forbs; naturally regenerated and Bräcke-cultivated treatments contained more perennial forbs and shrubs. Thus, different post-disturbance plant communities established following contrasting types of disturbance. Plant community biomass and tree growth varied among disturbance treatments. Shoot mass of aspen ( Populus tremuloides Michx.) and the root mass of all species declined significantly with increasing soil disturbance intensity. Aspen and white spruce ( Picea glauca (Moench) Voss) differed in their response to disturbance. Aspen growth was similar among disturbance treatments. In contrast, aspen density was significantly lower in disk-trenched and bladed treatments than in burned or naturally regenerated treatments, and aspen basal area was significantly lower only in drum-chopped treatments. White spruce grew fastest in drum-chopped sites. Burned treatments had the highest recruitment of volunteer spruce seedlings (up to 3200 ha −1), but not significantly higher than in other disturbance treatments. Taken together these results suggest that the most intensive silvicultural treatments had the expected effects of reducing aspen abundance and increasing the growth of spruce, but also contained more grasses and forbs and had lower total root mass than burned or naturally regenerating sites. Further work is needed to examine long-term productivity and the persistence of both native and persistent weedy species following contrasting types of disturbance.

Understory development in young Cordia alliodora plantations

The biomass and composition of theunderstory plant community associated with Cordiaalliodora planted at 2 m × 2 m spacing arecharacterized for the first (1992) and fifth (1996)years after stand establishment. The study plantationscomprise three replicates each of C. alliodoramonocultures and C. alliodora interplanted withperennial monocots (polycultures), part of anexperiment located at the La Selva Biological Stationin northern Costa Rica. In the first year, there wasno significant difference in the mean abovegroundunderstory biomass between the monocultures of C.alliodora (7975 kg/ha) and the polycultures of treesand monocots (5427 kg/ha). After 5 years, theunderstory biomass in the monocultures increased(16,918 kg/ha), while in the polycultures it decreased(116 kg/ha). In contrast to the first year, therefore, by the fifth year the understory biomass differed significantly between the two types of plantations (p = 0.005, R2 = 0.78, LSD = 5200 kg).

Biomass and carbon pool of two bogs in the Experimental Lakes Area, northwestern Ontario

This study was undertaken to estimate the aboveground biomass and carbon pool of plant communities in two bogs: one to be experimentally flooded to simulate a hydroelectric reservoir and the other left unflooded as a reference. The biomass of trees, saplings, and tall shrubs was estimated using allometric regression equations, while that of tree and tall shrub seedlings, low shrubs, herbs, bryophytes, and litter was harvested from 0.25-m2 plots. The eight communities were open or treed in low to high densities with Picea mariana (Mill.) BSP. and sparse Larix laricina (Du Roi) K. Koch. Aboveground community biomass (excluding litter) ranged from 7189 ± 924 (mean ± SE) to 72 909 ± 20 627 kg·ha-1, and litter ranged from 1181 ± 548 to 7664 ± 2848 kg·ha-1. Mean carbon concentration in the plant material analyzed ranged from 430 to 540 mg C·g-1. Carbon concentration of leaves in woody species was higher than in branches and stems. Carbon pool estimates (excluding litter) for the communities ranged from 3140 ± 480 to 35 010 ± 8890 kg C·ha-1. Community carbon per unit area as a percentage of biomass per unit area ranged from 44 to 48%. The available carbon pool (including litter and excluding tree branches and stems) of communities in the experimental bog ranged from 6386 to 9907 kg C·ha-1. Low shrubs, dominated by Chamaedaphne calyculata (L.) Moench in wetter, more open communities and Ledum groenlandicum Oeder in drier, more densely treed communities, composed 34-53% of the available carbon pool. Litter was the largest contributor (33%) to the available carbon pool in the most densely treed community, and bryophytes (mostly Sphagnum angustifolium (C. Jens. ex Russ.) C. Jens. in Tolf and Sphagnum fallax (Klinggr.) Klinggr., 46%) in the sedge community. The available carbon pool in aboveground vegetation of the experimental bog was estimated to be 1.17 × 105 kg C, approximately 12% of the estimated carbon pool in the peat.Key words: biomass, bog, carbon, experimental reservoir.

A field study of the effects of elevated CO 2 on plant biomass and community structure in a calcareous grassland

The effects of elevated CO(2) on plant biomass and community structure have been studied for four seasons in a calcareous grassland in northwest Switzerland. This highly diverse, semi-natural plant community is dominated by the perennial grass Bromus erectus and is mown twice a year to maintain species composition. Plots of 1.3 m(2) were exposed to ambient or elevated CO(2) concentrations (n = 8) using a novel CO(2) exposure technique, screen-aided CO(2) control (SACC) starting in March 1994. In the 1st year of treatment, the annual harvested biomass (sum of aboveground biomass from mowings in June and October) was not significantly affected by elevated CO(2). However, biomass increased significantly at elevated CO(2) in the 2nd (+20%, P = 0.05), 3rd (+21%, P = 0.02) and 4th years (+29%, P = 0.02). There were no detectable differences in root biomass in the top 8 cm of soil between CO(2) treatments on eight out of nine sampling dates. There were significant differences in CO(2) responsiveness between functional groups (legumes, non-leguminous forbs, graminoids) in the 2nd (P = 0.07) and 3rd (P < 0.001) years of the study. The order of CO(2) responsiveness among functional groups changed substantially from the 2nd to the 3rd year; for example, non-leguminous forbs had the smallest relative response in the 2nd year and the largest in the 3rd year. By the 3rd year of CO(2) exposure, large species-specific differences in CO(2) response had developed. For five important species or genera the order of responsiveness was Lotus corniculatus (+271%), Carex flacca (+249%), Bromus erectus (+33%), Sanguisorba minor (no significant CO(2) effect), and six Trifolium species (a negative response that was not significant). The positive CO(2) responses in Bromus and Carex were most closely related to increases in tiller number. Species richness was not affected by CO(2) treatment, but species evenness increased under elevated CO(2) (modified Hill ratio; P = 0.03) in June of the 3rd year, resulting in a marginally significant increase in species diversity (Simpson's index; P = 0.09). This and other experiments with calcareous grassland plants show that elevated atmospheric CO(2) concentrations can substantially alter the structure of calcareous grassland communities and may increase plant community biomass.

Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: a unified conceptual model

In semiarid landscapes, the ratio of herbaceous to woody plant biomass is a major determinant of ecosystem properties. This ratio depends to a large extent on the amount and spatial distribution of soil moisture that is available to plants, and these variables, in turn, are determined primarily by climate and land use. Current conceptual models for determining the ratio of herbaceous to woody plant biomass in semiarid plant communities are based either on differences in soil moisture with depth (vertical heterogeneity) from one site to another (Walter's two-layer model) or on differences in soil moisture between canopy and intercanopy patches at the same site (horizontal heterogeneity) that result from disturbances associated with land use (Schlesinger et al.'s model of desertification). We developed a model that unifies these two perspectives by relaxing two assumptions of Walter's two-layer model. First, our model recognizes that soil moisture varies horizontally between canopy and intercanopy patches, not only due to land-use disturbance, a general assumption of the Schlesinger et al. model, but also due to the physical nature of the canopy itself. Second, while retaining the general assumption of Walter that woody plants obtain moisture from deeper soil layers than do herbaceous plants, our model recognizes the existence of two types of woody plants: those that extract a substantial proportion of their moisture from deeper layers and those that extract mainly from shallower layers. By modifying the two-layer hypothesis to include four soil compartments and distinguishing between shallow- and deeper-rooted woody species, our model integrates three key concepts in semiarid ecology: (1) the proportion of woody cover increases as moisture in the deeper soil layers increases (Walter's two-layer hypothesis for coexistence of herbaceous and woody plants); (2) land use practices that cause a reduction in herbaceous vegetation and compaction of intercanopy soils lead to a long-term increase in the proportion of woody plants (Schlesinger et al.'s concept, or more generally, that at a given site multiple variations in the proportions of herbaceous and woody plant biomass are possible); and (3) changes in the ratios of herbaceous to woody plant biomass exhibit complex behavior (changes can happen quickly and are not directly reversible without intensive management). This integration of concepts results because rather than assuming a simple, one-way dependence of plant functional types on soil moisture heterogeneity, our model assumes an interdependence between the two: soil moisture heterogeneity constrains the composition of the plant community, which in turn modifies soil moisture heterogeneity. The four-compartment model that we propose enables, for the first time, an integrated picture of both dimensions of soil moisture heterogeneity – horizontal and vertical – and of the interdependence between soil moisture heterogeneity and the proportions of the plant functional types that make up a given plant community. This unified conceptual model can be applied to provide insight into the individual and the combined effects of climate and land use on semiarid plant communities within the grassland/forest continuum, which vary in the proportions of canopy and intercanopy patches.

The plant communities of a foredune in southeastern Brazil

Species composition, biomass and environmental characteristics of a coastal plant community of southeastern Brazil, was studied using data from 110 quadrats collected on the foredune of Barra de Maricá, Rio de Janeiro, Brazil. Indirect and direct gradient analyses were used to describe the foredune community. Ordination by detrended correspondence analysis (DCA) indicated the existence of three vegetation groups: pioneer, embryo dunes and a zone of fixed dunes with blow out areas. The gaps observed between groups were attributed to discontinuities in environmental factors. Most of the variation in species composition was expressed by the second axis of the DCA ordination, which was correlated with the distance from the beach bank. The second axis is interpreted as a stress gradient, with high stress in the pioneer zone decreasing to fixed dunes. The coenocline analyzed by direct gradient ordination, was divided into three segments equivalent to Groups detected by DCA ordination. The important species were: Althernanthera maritima St. Hil. in the pioneer zone, and Sporobolus virginicus (L.) Kunth, Mariscus pedunculatus (R. Brown) T. Koyama, and Mitracarpus frigidum K. Sch. in the embryo dunes. The grassPanicum racemosum (Spreng) was associated with areas of fixed dune. that were also characterized by Mollugo verciticillata L. Both biomass and species richness were lower at the high stress disturbance ends of the gradient, with their maximum value in different portions of the middle range of the gradient.Key words: detrended correspondence analysis, direct gradient analysis, foredune communities, vegetation zonation.

Ecosystem structure in the Brazilian Cerrado: a vegetation gradient of aboveground biomass, root mass and consumption by fire

Conversion to permanent agriculture is rapidly occurring over vast areas of the 1.8 million km2; Brazilian Cerrado; a region that is naturally a mosaic of grasslands, savannas and evergreen tropical woodlands. Yet, few studies have quantified total biomass of plant communities in this ecosystem, particularly the belowground component; a C pool of potential global significance. Total biomass (aboveground and belowground), and the quantity of biomass consumed by fires were measured in four plant communities comprising a vegetation gradient from pure grassland (campo limpo) to a woodland with a closed canopy of tall shrubs and scattered trees (cerrado denso) near Brasilia, DF, Brazil. Total aboveground biomass (TAGB) increased along this gradient from 5.5 Mg ha-1 in campo limpo to 29.4 Mg ha-1 in cerrado denso. Vegetation structure varied among communities; trees were nonexistent in campo limpo, but were at a density of 1000 ha-1 and a biomass of 12.9 Mg ha-1 in cerrado denso. Fires consumed 92 and 84% of the TAGB in campo limpo (pure grassland) and campo sujo (savanna), respectively. In cerrado aberto and cerrado denso, trees and tall shrubs were little affected by fire. Combustion factors of the TAGB in these communities was 54 and 33%, respectively. The total biomass consumed by fire ranged from 5.0 Mg ha-1 in campo limpo to 13.5 Mg ha-1 in cerrado aberto. Compared to other widespread Brazilian ecosystems (tropical dry forest and evergreen forest), the Cerrado has a lower aboveground biomass. The TAGB of cerrado denso is &gt; 9% of that of Amazonian tropical evergreen forest. The total quantity of biomass consumed by fire, and hence emissions to the atmosphere is lower in intact Cerrado communities compared to fires in slashed tropical forest.