Plant Community Richness Research Articles

Browsing‐mediated shrub canopy changes drive composition and species richness in forest‐tundra ecosystems

Changes in climate and in browsing pressure are expected to alter the abundance of tundra shrubs thereby influencing the composition and species richness of plant communities. We investigated the associations between browsing, tundra shrub canopies and their understory vegetation by utilizing a long‐term (10–13 seasons) experiment controlling reindeer and ptarmigan herbivory in the subarctic forest tundra ecotone in northwestern Fennoscandia. In this area, there has also been a consistent increase in the yearly thermal sum and precipitation during the study period. The cover of shrubs increased 2.8–7.8 fold in exclosures and these contrasted with browsed control areas creating a sharp gradient of canopy cover of tundra shrubs across a variety of vegetation types. Browsing exclusions caused significant shifts in more productive vegetation types, whereas little or no shift occurred in low‐productive tundra communities. The increased deciduous shrub cover was associated with significant losses of understory plant species and shifts in functional composition, the latter being clearest in the most productive plant community types. The total cover of understory vegetation decreased along with increasing shrub cover, while the cover of litter showed the opposite response. The cover of cryptogams decreased along with increasing shrub cover, while the cover of forbs was favoured by a shrub cover. Increasing shrub cover decreased species richness of understory vegetation, which was mainly due to the decrease in the cryptogam species. The effects were consistent across different types of forest tundra vegetation indicating that shrub increase may have broad impacts on arctic vegetation diversity. Deciduous shrub cover is strongly regulated by reindeer browsing pressure and altered browsing pressure may result in a profound shrub expansion over the next one or two decades. Results suggest that the impact of an increase in shrubs on tundra plant richness is strong and browsing pressure effectively counteracts the effects of climate warming‐driven shrub expansion and hence maintains species richness.

Recovery of understory vegetation in clear-cut stone pine (Pinus pinea L.) plantations

The recovery of understory vegetation after clear-cutting of stone pine (Pinus pineaL.) plantations is crucial for biodiversity conservation. The development of plant cover, and the richness, diversity and composition of understory plant communities were investigated within five years from clear-cutting. After three years, plant cover ranged from 48 to 74%, and was formed by over 90% of shrubs and graminoids. Diversity decreased in cut plots and the index of similarity showed marked shifts in species composition. Richness increased and around 50 new species per plot were inventoried over three years. New species showed a high proportion of annuals with broad ecological amplitude and aliens, which changed life form and chorology spectra. Most new species had low frequency, but some were still present after five years. Extinction of resident species was low, and their frequency increased with time from clear-cutting. Most of them were perennials with sprouting ability. Common features of plant communities developing in clear-cut stone pine plantations were: (1) the linear increase of plant cover during the first three growing seasons, (2) increased species richness, (3) decreased diversity, (4) invasion by annuals, and (5) invasion by Alien and Cosmopolitan species.

Native plant diversity in tropical savannas decreases when exotic pasture grass cover increases

The invasion of exotic plants into savanna ecosystems can disrupt the natural pattern of vegetation. Indian couch Bothriochloa pertusa was introduced into Australia as a species for rehabilitation of degraded grazing land. In this study the effect of increasing B. pertusa cover on native plant diversity and possible mechanisms of its spread were examined. Forty sites were sampled in uncleared Queensland rangelands with a range of B. pertusa and Bothriochloa ewartiana (a native species) cover. The mean number of native species per quadrat declined with increasing B. pertusa cover but remained stable over sites with increasing B. ewartiana cover. Mean species richness accumulated at a significantly lower rate for sites with B. pertusa present. Canonical analysis of principle coordinates suggested that three groups of sites, ranging from low to high species richness and cover of native plants, were correlated along gradients of B. pertusa cover, grazing intensity and basal area of dead trees. Generalised linear modelling indicated significant negative relationships between B. pertusa cover and total ground cover, forbs and perennial grass richness and cover, and cover of nine native perennial tussock grasses (Aristida leptopoda, B. decipiens, B. ewartiana, Chrysopogon fallax, Dichanthium fecundum, D. sericeum, Heteropogon contortus, H. triticeus and Themeda triandra). This study suggests that an increase in B. pertusa is associated with a substantial change in the cover and species richness of native plant communities. This was considered to occur via competitive effects as well as a result of cattle grazing, rainfall deficit and tree death. Regardless of the mechanism, continued spread of exotic pasture species has the potential to cause significant changes to rangeland biodiversity.

Grassland plant species diversity decreases invasion by increasing resource use.

Species richness of plant communities has been demonstrated to provide resistance to invasion by unsown species, though the relationship with resource availability varies between studies. The present work involved five grassland species grown in monocultures and in four-species mixtures sown in accordance with a simplex design. The species used represented different functional groups (i.e. grasses, legumes and non-N(2)-fixing species), each of which differed internally in terms of competitiveness. I hypothesized that sown diversity would negatively affect invader performance by decreasing the availability of light and soil nitrogen (N) for invading species, and that functional composition of the sown diversity would affect the functional composition of the invading flora. The experimental plots were harvested for two years, and were fertilized with 100 kg N ha(-1) each year. The number of unsown species (classified into four functional groups) invading each plot and their proportion of the biomass harvested were recorded. The penetration of incoming light through the canopy, the apparent N uptake by the sown species from the soil, and the mineral N content in the soil were measured. I found that diverse communities captured more resources both above- and belowground, and the number of invading species and their biomass production were smaller in mixed than in monoculture plots. However, the sampling effect of one grass was also strong. These results suggest that increased resource use in diverse communities can reduce invasion.

Equivalence in the strength of deer herbivory on above and below ground communities

Herbivores exert a strong influence on the species composition and richness of plant communities, but the magnitude of their effect on belowground communities remains poorly understood. While an increasing number of studies acknowledge the importance of documenting belowground effects of herbivores, very few of these evaluate variation in the strength of the response from aboveground to belowground communities. Our study documents the long-term consequences of sustained deer herbivory for plant and arthropod communities adjacent to 15 exclosures that have been in place since 1996. We hypothesized that herbivory would alter the composition and diversity of communities, but the strength of the effects of herbivory would weaken from plants, to leaf-litter invertebrates, and to belowground microarthropod communities. First, we found that herbivory negatively impacted plant seedling and sapling abundance and performance, reduced the abundance of ants and the taxonomic richness of arthropods in the litter layer and reduced the richness of soil microarthropod communities. Second, in contrast to our hypothesis, the magnitude of effect size did not vary among trophic levels, indicating that effects of deer herbivory cascade from plants to the leaf-litter and soil arthropod communities with equal strength. While much recent research has focused on how specific traits of plants may mediate the effects of herbivory on associated species, our results suggest that indirect effects of herbivory might influence many components of belowground communities.

Plant species richness belowground: higher richness and new patterns revealed by next‐generation sequencing

Variation in plant species richness has been described using only aboveground vegetation. The species richness of roots and rhizomes has never been compared with aboveground richness in natural plant communities. We made direct comparisons of grassland plant richness in identical volumes (0.1 × 0.1 × 0.1 m) above and below the soil surface, using conventional species identification to measure aboveground richness and 454 sequencing of the chloroplast trnL(UAA) intron to measure belowground richness. We described above- and belowground richness at multiple spatial scales (from a neighbourhood scale of centimetres to a community scale of hundreds of metres), and related variation in richness to soil fertility. Tests using reference material indicated that 454 sequencing captured patterns of species composition and abundance with acceptable accuracy. At neighbourhood scales, belowground richness was up to two times greater than aboveground richness. The relationship between above- and belowground richness was significantly different from linear: beyond a certain level of belowground richness, aboveground richness did not increase further. Belowground richness also exceeded that of aboveground at the community scale, indicating that some species are temporarily dormant and absent aboveground. Similar to other grassland studies, aboveground richness declined with increasing soil fertility; in contrast, the number of species found only belowground increased significantly with fertility. These results indicate that conventional aboveground studies of plant richness may overlook many coexisting species, and that belowground richness becomes relatively more important in conditions where aboveground richness decreases. Measuring plant belowground richness can considerably alter perceptions of biodiversity and its responses to natural and anthropogenic factors.

The Goldilocks effect: intermittent streams sustain more plant species than those with perennial or ephemeral flow

Summary1. Drylands worldwide are typified by extreme variability in hydrologic processes, which structures riparian communities at various temporal and spatial scales. One key question is how underlying differences in hydrology over the length of interrupted perennial rivers influence spatial and temporal patterns in species richness and species composition.2. We examined effects of differences in dry season hydrology on species richness, composition and cover of herbaceous plant communities in the streamside zone (the zone influenced directly by low flows in the channel). Data were collected at ephemeral, intermittent and perennial flow reaches on three rivers of the desert Southwest (Arizona, U.S.A.): Lower Cienega Creek, Hassayampa River and Lower San Pedro River.3. Patterns of species richness varied with temporal scale of analysis, that is between single‐year and multi‐year time frames. At the annual timescale, quadrat species richness (m−2) and herbaceous cover were higher at sites with perennial flow than at either intermittent or ephemeral sites. In contrast to this single‐year pattern, the highest long‐term richness occurred at intermittent sites.4. Quadrat species richness, total species richness at a site (per 18 1‐m2 plots) and cover were more variable year to year at non‐perennial sites than at perennial flow sites. On two of the three rivers, ephemeral sites had the highest inter‐annual compositional variance, while the perennial sites had the lowest.5. Compositional differences between the hydrologic site types were dominated by species turnover, not nestedness. The perennial sites had more wetland and perennial species than the other two site types. The intermittent sites had more annual species than did the other two types.6. High long‐term species richness and distinct species composition of intermittent sites are probably sustained by pronounced temporal variability in environmental conditions (i.e. frequent and persistent flow events, and dry periods). Plants at these sites take advantage of greater moisture than those at ephemeral sites and also experience less competition from resident species than those at perennial sites.7. Conservation of desert riparian diversity depends upon the protection of consistently wet conditions at perennial flow sites, as well as the maintenance of the processes that cause fluctuations in environmental conditions at non‐perennial sites.

Species-Specific Effects of Woody Litter on Seedling Emergence and Growth of Herbaceous Plants

The effect of litter on seedling establishment can influence species richness in plant communities. The effect of litter depends on amount, and also on litter type, but relatively little is known about the species-specific effects of litter. We conducted a factorial greenhouse experiment to examine the effect of litter type, using two woody species that commonly co-occur in boreonemoral forest—evergreen spruce (Picea abies), deciduous hazel (Corylus avellana), and a mixture of the two species—and litter amount—shallow (4 mm), deep (12 mm) and leachate—on seedling emergence and biomass of three understorey species. The effect of litter amount on seedling emergence was highly dependent on litter type; while spruce needle litter had a significant negative effect that increased with depth, seedling emergence in the presence of hazel broadleaf litter did not differ from control pots containing no litter. Mixed litter of both species also had a negative effect on seedling emergence that was intermediate compared to the single-species treatments. Spruce litter had a marginally positive (shallow) or neutral effect (deep) on seedling biomass, while hazel and mixed litter treatments had significant positive effects on biomass that increased with depth. We found non-additive effects of litter mixtures on seedling biomass indicating that high quality hazel litter can reduce the negative effects of spruce. Hazel litter does not inhibit seedling emergence; it increases seedling growth, and creates better conditions for seedling growth in mixtures by reducing the suppressive effect of spruce litter, having a positive effect on understorey species richness.

Road effects on vegetation composition in a saline environment

Aims Road effects from maintenance and traffic have the potential to alter plant communities, but the exact relationships between these effects and changes in plant community composition have not often been studied in diverse environments. To determine the direction and level of community composition changes in saline environment due to road effects, we conducted a study along roads of different ages and in nearby non-road (i.e. natural) areas in the Yellow River Delta, China. Additionally, to potentially elucidate the mechanisms underlying the changes in the richness and composition of plant communities along roads, we evaluated physiochemical changes in soil of roadside and non-road areas. Methods Floristic and environmental data were collected along roadside of different ages and nearby non-road areas. To evaluate plant communities at each site, six 2 m 3 2 m quadrats were placed at 3-m intervals along roads and six quadrats were arranged randomly in non-road areas. To determine the difference in plant community composition between roadside and non-road areas, we measured species richness and the abundance of each species, examined species turnover and floristic dissimilarity between the two areas and positioned plant species and sites in an abstract multivariate space. Plant community (species richness, percentage of halophytes) and soil physicochemical properties (pH, salinity, moisture content, bulk density, nitrate and ammonium nitrogen concentration) were compared between roadside and non-road areas (young roadside vs. corresponding non-road areas, old roadside vs. corresponding non-road areas) by using t-tests. Classification and ordination techniques were used to examine the relationship between vegetation and related environmental variables in both roadside and non-road areas. Important Findings For both the young and old roadside areas, species richness in roadside areas was significantly higher than in non-road areas and high floristic dissimilarity values indicated that roadside and non-road areas differed greatly in community composition. In both the young and old roadside areas, the plant communities in roadside areas had lower percentages of halophytes than non-road communities. Correspondence analysis and two-way indicator species analysis showed that halophytes dominated in the non-road areas, while a number of typical non-salt-tolerant species dominated in the roadside areas. Compared to non-road areas, activities associated with roads significantly decreased soil moisture, bulk density and salinity and increased soil pH and nitrate content. Forward selection for the environmental variables in canonical correspondence analysis showed that soil salinity was the most important factor related to the variation of species composition between roadside and non-road areas. Our study demonstrates that road effects have a significant impact on the associated vegetation and soil, and these changes are consistent across roads of different ages in our system.

Does phosphorus limitation promote species-rich plant communities?

It is known that the number of limiting nutrients may affect the species richness of plant communities, but it is unclear whether the type of nutrient limitation is also important. I place the results from a study in Patagonia (elsewhere in this issue) in the context of the number and types of nutrients that are limiting. I present four mechanisms through which N or P limitation may potentially influence species richness. These mechanisms are related to: (i) the number of forms in which P or N are present in soil and the plant traits needed to acquire them, (ii) the mechanisms and traits that control species competition and coexistence under N or P limitation, (iii) the regional species pools of plants capable of growing under N- and P-limited conditions, and (iv) the interaction between the type of nutrient limitation and community productivity. It appears likely that P limitation can favour a higher species richness than N limitation, in at least in a variety of low productive plant communities, but evidence to support this conclusion is so far lacking. The four mechanisms proposed here offer a framework for exploring whether the type of nutrient limitation per se, or an interaction with productivity, is a potential driver for variation in species diversity.

Vegetation of Mediterranean temporary pools: a fading jewel?

Although Mediterranean temporary pools are of great value for conservation, they are in rapid decline under the impact of various forms of anthropogenic pressure. Their disappearance from the landscape may result in a weakening of the biological connections between pools due to increasing isolation and the impoverishment of their communities. In Western Morocco (province of Benslimane), temporary pools have undergone severe regression over the past decades. The quantification of these losses and the impact on the richness of plant communities remain, however, unstudied. Since this is of vital importance for the conservation of the biodiversity of these habitats, a study has been undertaken associating (1) an assessment of the pool losses (both in density and surface area) between 1955 and 2001 using remote sensing, (2) surveys of vegetation and water depth (in 2006) in 48 pools, and (3) an assessment of the density and surface area of pools occurring within a 3 km radius around each of the sampled pools. The results show a loss of 23% in number and 61% in surface area of pools in the province over a period of 47 years. This decline, promoted by their small size and shallowness, is probably related to socio-economic changes (intensification of agricultural practices and population growth). The richness in characteristic and rare species of the pools was related to both local (water depth) and regional features (land use, pool density and total water surface area in the surrounding landscape). The significant impact of the current density of pools and their total surface area on the conservation value of the studied pools suggests a weakening of the metacommunity dynamics between pools. Given the rapid socio-economic changes in the province and the current rate of pool disappearance (0.5% per year) we predict a continuing reduction in pool density with a high risk of the widespread loss of their unique flora in the long term.

The roles of biotic resistance and nitrogen deposition in regulating non-native understory plant diversity

Understanding the mechanisms that allow for plant invasions is important for both ecologists and land managers, due to both the environmental and economic impacts of native biodiversity losses. We conducted an observational field study in 2008 to examine the relationship between native and non-native forest understory plant species and to investigate the influence of soil nitrogen (N) on plant community richness and diversity. In 2009, we conducted a companion fertilization experiment to investigate how various forms of N deposition (inorganic and organic) influenced native and non-native species richness and diversity. We found that native species richness and diversity were negatively correlated with 1) non-native species richness and diversity and 2) higher total soil inorganic N. In the deposition experiment, adding organic N fertilizers decreased native richness and diversity compared to inorganic N fertilizers. Together, these results indicate that increasing soil N can be detrimental to native species; however, native species richness and diversity may counteract the N-stimulation of non-native species. Furthermore, the negative effects of organic N deposition on native plants may be just as strong, if not stronger, than the effects of inorganic N deposition.

Altitudinal distribution of native and alien plant species in roadside communities from central Argentina

AbstractRoadsides may homogenize the distribution of native species and act as corridors for the spread of alien taxa. We examined the variation in native and alien plant species richness and composition at two spatial scales defined by altitude and habitat type (edges and fill slopes), as well as the relationship between native and exotic species richness in roadside plant communities in mountains from central Argentina. Following a gradient from 1100 to 2200 m a.s.l. along a mountain road, plant species cover was recorded within sample plots of 30 m × 10 m systematically located at 100‐m altitude intervals on both roadside habitats. Although native species richness decreased with altitude and composition changed accordingly, the number of alien species peaked at both extremes of the elevation gradient and did not reflect an altitudinal replacement of chorological groups. The number of both native and alien species was higher in roadside edges, but a negative association between the richness of native and alien species occurred only on fill slopes, suggesting that roadside habitats differ in their susceptibility to plant species colonization and in the mechanisms driving native and alien species richness. Our results highlight the importance of altitude and roadside habitat as factors controlling plant species richness and composition along roadside communities in central Argentina. Although altitude acts as a filter for native plants, it apparently did not constrain the establishment of alien species along the studied roadsides, indicating that the influence of this road as a plant species corridor may increase with time, promoting the opportunities for aliens to expand their current distribution.

Grazing impact on plant spatial distribution and community composition

Background and aims Re-introduction of large grazers in the few remaining natural and semi-natural grasslands are thought to be an effective management tool to prevent dominance of late successional plant species and restoration of plant biodiversity. The main objective of this study was to test whether the introduction of large herbivores retard the succession by reducing the abundance of highly competitive tall species and whether it is accompanied with changes in plant community composition and spatial distribution of plant species. Methods In order to test this hypothesis, we studied the effect of grazing by large herbivores on vegetation at three hierarchical levels: individual plant species, emergent groups of functionally similar herbaceous plant species, and the main gradients of plant community composition. Study sites were thirteen spatially separated, dry coastal dune grasslands in western Belgium and north-western France. Key results Grazing had a predominantly negative effect on high competitive dominant species and led to changes in composition of emergent groups toward less competitive plant species. Additionally, these changes in plant community composition were accompanied with changes in spatial distribution patterns of individual plant species and community richness. Conclusions Our results suggest that the current grazing management applied in these nature reserves is able to prevent the expansion of dominant highly competitive species and establishment of functionally different plant species.

Is plant community richness regulated over time? Contrasting results from experiments and long-term observations

There is considerable debate among ecologists as to whether or not communities are saturated. In saturated communities, species richness should remain relatively constant over time, despite compositional turnover, because richness is negatively correlated with colonization and positively correlated with local extinction. Few studies have tested for saturation using temporal observational data as well as diversity-perturbation experiments. We analyzed 10 years of data for plant species richness at 71 sites on contrasting serpentine and non-serpentine soils within Californian (USA) grasslands. We also manipulated local richness and measured its effects on immigration and extinction. Consistent with saturation, we observed that richness was positively correlated with extinction rates and negatively correlated with colonization rates, and randomization tests confirmed that diversity fluctuated less than expected by chance. However, experimental species additions and removals did not affect extinction or colonization, suggesting that richness is not regulated by local species interactions. Instead, we propose three reasons why richness may fluctuate within narrow limits causing the appearance of saturation in temporal observational data sets: negatively autocorrelated patterns of biotic response to yearly conditions, differential affinities of particular species for local conditions, or stochastic abundance-dependent colonization and extinction rates. We illustrate the latter using a metacommunity model.

Species richness and evenness as a function of biomass in arable plant communities

Poggio SL & Ghersa CM (2011). Species richness and evenness as a function of biomass in arable plant communities. Weed Research51, 241–249.SummaryWe evaluated the hypothesis that competitive dominants change the species richness and evenness of arable plant communities. Three field experiments including cool‐season crops and unsown short fallows were carried out in Buenos Aires, Argentina. Ground cover, biomass and species richness of crops and weeds were assessed. Evenness was calculated by using species biomass data. Richness and evenness were correlated with total community biomass, and with the biomass of either weeds or the dominant species in the community. Crops, through growth and shading effects, affected the species richness and evenness of arable plant communities. Conversely, the dominant weed in fallows was not as suppressive as crops. Species richness and evenness were constrained by community biomass. Species richness of understory weeds decreased as crops suppressed weed growth. Evenness also decreased as the dominant species became increasingly productive, regardless of their identity (weeds or crops). Our findings provide valuable models to characterise the trajectories that species richness and evenness may follow in different farming scenarios. Community biomass is a major constraint on the maximum diversity of local communities and, consequently, of substantial ecological importance for both biodiversity conservation and weed management purposes.

Community succession analysis and environmental biological processes of naturally colonized vegetation on abandoned hilly lands and implications for vegetation restoration strategy in Shanxi, China

Data were collected simultaneously at different succession stages using a space-for-time substitution, and were analyzed using the quantitative classification method (TWINSPAN) and the ordination technique (DCA). The community succession of natural colonized plants on abandoned hilly lands in Shanxi are below: Assoc. Potentilla chinensis + Setaria viridis -> Assoc. Artemisia sacrorum + S. viridis + Oxytropis caoraloa -> Assoc. A. sacrorum + Artemisia capillaries -> Assoc. A. capillaries + Pedivularis shansiensis + Echinops pseudosetifer -> Assoc. Hippophae rhamnoides- A. sacrorum + Cleistogenes squarrosa -> Assoc. H. rhamnoides + Ostryopsis davidiana-A. sacrorum -> Assoc. O. davidiana-A. sacrorum + Dendranthema chanetii -> Assoc. Populus davidiana-Caragana korshinskii-A. sacrorum -> Assoc. Larix principis-rupprechtii-H. rhamnoides-A. sacrorum. This established a recovery model of natural vegetation on abandoned hilly lands in Shanxi. The structure, composition and life-forms changed significantly during succession. Four indices of species diversity were used to analyze changes in the heterogeneity, dominance, richness and evenness of species during the succession process. The species richness and heterogeneity of plant communities increased significantly, the dominance decreased obviously and the evenness decreased slightly. The analysis of variance (ANOVA) also proved the significance of these five indices. Pioneer species of S. viridis, A. sacrorum, O. caoraloa, A. capillaries, P. shansiensis, E. pseudosetifer, C. squarrosa, H. rhamnoides, O. davidia, C. korshinskii, P. davidiana and L. principis-rupprechtii, etc. colonize successfully and play important roles on the vegetation restoration of abandoned hilly lands.

Floristic and structural analysis of the woodland vegetation around Dello Menna, southeast Ethiopia

Floristic composition and vegetation structure were described for the woodland vegetation around Dello Menna, Bale zone, southeast Ethiopia. A total 50 (20 m × 20 m) quadrats were sampled to identify and describe plant community types, species diversity, richness and evenness and to relate the identified plant community types with some environmental factors and describe the population structure of woody plant species. In each quadrat, data on species identity, abundance, height and Diameter at Breast Height (DBH) of woody plant species, altitude and slope were recorded. Vegetation classification was performed using PC — ORD software package. Sorensen’s similarity coefficient was used to detect dissimilarities among communities. Shannon — Wiener diversity index, species richness and Shannon’s evenness were computed to describe species diversity of the plant community types. Results show that a total of 171 vascular plant species representing 53 families were recorded. Fabaceae is the dominant family represented by 13 genera and 26 species (15%) followed by Asteraceae, Lamiaceae and Anacardiaceae with eight species each (4.6%). Based on the results of vegetation classification, three plant communities (Dalbergia microphylla community, Grewia bicolar- Acacia brevispica community, and Combretum molle-Combretum collinum community) are recognized and described. Species richness, diversity and evenness varied among the plant communities. Species richness and diversity exhibit a bell — shaped pattern along altitude. Species turn over among communities more or less follow altitudinal gradients. Tukey’s pairwise comparison of means among the plant community types shows significant variations in altitude, implying that altitude is one of the most important factors determining the distribution of plant communities. The community Dalbergia microphylla type exhibits the highest species richness and diversity. Analysis of population structure of the dominant species reveals various patterns. Future research directions and recommendations are suggested for the sustainable utilization of the vegetation.

Low-dimensional trade-offs fail to explain richness and structure in species-rich plant communities

Mathematical models and ecological theory suggest that low-dimensional life history trade-offs (i.e. negative correlation between two life history traits such as competition vs. colonisation) may potentially explain the maintenance of species diversity and community structure. In the absence of trade-offs, we would expect communities to be dominated by ‘super-types’ characterised by mainly positive trait expressions. However, it has proven difficult to find strong empirical evidence for such trade-offs in species-rich communities. We developed a spatially explicit, rule-based and individual-based stochastic model to explore the importance of low-dimensional trade-offs. This model simulates the community dynamics of 288 virtual plant functional types (PFTs), each of which is described by seven life history traits. We consider trait combinations that fit into the trade-off concept, as well as super-types with little or no energy constraints or resource limitations, and weak PFTs, which do not exploit resources efficiently. The model is parameterised using data from a fire-prone, species-rich Mediterranean-type shrubland in southwestern Australia. We performed an exclusion experiment, where we sequentially removed the strongest PFT in the simulation and studied the remaining communities. We analysed the impact of traits on performance of PFTs in the exclusion experiment with standard and boosted regression trees. Regression tree analysis of the simulation results showed that the trade-off concept is necessary for PFT viability in the case of weak trait expression combinations such as low seed production or small seeds. However, species richness and diversity can be high despite the presence of super-types. Furthermore, the exclusion of super-types does not necessarily lead to a large increase in PFT richness and diversity. We conclude that low-dimensional trade-offs do not provide explanations for multi-species co-existence contrary to the prediction of many conceptual models.

Nitrate leaching as a function of plant community richness and composition, and the scaling of soil nutrients, in a restored temperate grassland

Two, two-factor experiments manipulated species and functional form plant richness and the spatial scaling of either nitrogen (N) or phosphorous (P) in restored tallgrass prairie in North Dakota, USA. Nitrate (NO3−) leaching was measured in these plots and analyzed for its response to the treatment factors and measured plant community parameters. Nitrate extracted from anion exchange resin was regressed against the first principal component of species and functional form richness, the spatial scaling of N or P, the measured biomass of the functional forms used and the plot values for plant parameters based on weighted averages by species biomass. The treatments applied in the N and P experiments were 1, 2, 5, 10, or 20 plant species taxa, and the application of fertilizer in a random fractal pattern with either fine-scale or coarse-scale heterogeneity. Nitrate leaching decreased with plant diversity and increased by a factor of two going from fine-scale to coarse-scale N. It was also related to a number of plant functional parameters, and was positively correlated with the biomass of late successional C3 grasses (Koeleria cristata (Lam.) Beauv., Poa pratensis L., Stipa comata Trin. & Rupr., and Stipa viridula Trin.), which are known from previous studies to have negative mycorrhizal responsiveness and are characterized by high root lateral spread per unit of root biomass. Our results show that while plant diversity has a highly significant influence on plant community uptake of NO3−, this effect is mediated by the scaling of soil N and the functional traits of the species comprising the plant assemblage.