Annual plant community responses to density of small-scale soil disturbances in the Negev Desert of Israel

We investigated whether plant diversity and productivity in small-scale soil disturbances, which is known to be higher than in undisturbed soil, decreases as the density of the disturbances increases. We studied this in an experiment with soil diggings (15 cm diameter and 15 cm depth) dug at a range of densities, on a north- and a south-facing slope of a watershed in the central Negev Desert of Israel. The diggings were similar to the commonly occurring pits made by porcupines (Hystrix indica) as they forage for below-ground plant parts. We used four levels of digging density, within the naturally occurring range in the region, represented by a rectangular plot with rows of diggings dug at four distances between diggings. The plots were laid out in a blocked design with three replications on both slopes, with each block containing all four levels of digging density. In the spring of 1992, 1994 and 1995 we measured plant density, species richness and plant productivity in the diggings, and in adjacent equal-sized undisturbed control areas (“soil matrix”) and on the mounds made by the removed excess soil. Plant density, species richness and productivity of annual plants were higher in the diggings than in the undisturbed matrix, while these responses were very low on the mounds. Plant density, species richness and productivity in the diggings, but not in the matrix or mounds, decreased as digging density increased. This effect varied slightly with location within a watershed and with annual rainfall. The density of seeds captured in the diggings from outside the digging during the 1995 dispersal season decreased with increasing digging density, but only on one of the slopes. At the highest digging density, plant density and species number in the diggings did not decrease down the slope, as expected if interference between diggings in runoff water capture were the cause of the digging density effect. There was a weak decrease in biomass production in 1994–1995 down the slope. We used a simple mathematical model to estimate whether the distribution of rainfall intensities that occurred during the winter of 1994–1995 could result in differences between digging densities in the amount of water captured by the diggings, and whether this could explain the observed effect of digging density. The model showed that there were four events during which less water was captured by the diggings at high digging densities, except in the topmost row of diggings. Soil moisture measurements, however, showed very little difference between diggings at different digging densities. We explain our findings as the result of the interaction between the properties of the disturbance patch with its surroundings, as the diggings capture resources in the form of runoff water, and seeds moved primarily by wind. The additional resources and seeds captured in diggings increase plant density, species richness and productivity relative to the undisturbed matrix. However, the contrast in plant responses between the disturbed patches and undisturbed soil diminishes at higher digging densities. We explain this as interference among diggings at close proximity. As we did not detect a decrease in plant responses down the slopes, we conclude that interference is due to interception of the wind-driven, non-directional flow of seeds. Interception of the down-slope flow of runoff water by upslope diggings is insufficient to affect plant density, determined at the beginning of the season. Later in the season, runoff interception may become important for biomass production. Received: 16 August 1996 / Accepted: 26 October 1997     

Associations between vegetation patterns and soil texture in the shortgrass steppe

A recent conceptual model of controls on vegetation structure in semiarid regions includes the hypothesis that the balance between the dominance of woody and herbaceous species is partly controlled by soil texture. The model predicts that the dominance of woody plants is associated with coarse textured soils, and that ecotones between woody and herbaceous plant functional types are associated with soil textural changes. We analyzed vegetation and soil data (from US Soil Conservation Service maps) for an area of shortgrass steppe in Northern Weld County, Colorado, in a canonical correlation procedure to test the hypothesis at a regional scale. In support of the model, we found significant correlations between (a) a canonical vegetation variable correlated with C₃ grass biomass and shrub biomass, and a canonical soil variable correlated with sandy topsoils, and (b) a canonical vegetation variable correlated with succulent biomass, and a canonical soil variable correlated with clay soils. Relatively sharp transitions between shrub- and grass-dominated vegetation types occur in a number of areas in the shortgrass steppe of northeastern Colorado and southeastern Wyoming, and we selected four sites to test the above hypothesis at a local scale. We gathered data on vegetation cover and soil texture from transects (50 m long) positioned across the transition zones from grassland to shrubland. We conducted a further canonical correlation analysis of the vegetation and soil data to test for the relationships between vegetation structure and soil texture, and a performed regression analyses on individual site data to describe site-specific relationships between vegetation and soil texture variables. Vegetation structure along the transects, at the level of plant functional types, was similar at all four sites. The transition from grassland to shrubland encompassed a change from a C₄ grass/half-shrub complex to a shrub/C₃ grass/succulent complex. At two of the sites these transitions were associated with a change to coarser-textured soils in the shrubland zone. Within the context of the shortgrass steppe, our overall findings support the predictions of the conceptual at a regional scale, but indicate that soil texture is only one factor that can influence vegetation structure at the local scale.     

Using plant functional types to compare vegetation structure of alien-invaded and uninvaded Acacia nilotica savannas

Four plant functional types (PFTs) were used to compare the vegetation structure of an alien-invaded Acacia nilotica savanna with one of negligible invasions. Heights, canopy covers and species richness of three native PFTs (woody plants, grasses and herbs) and one alien PFT (woody plants) were measured in 14, 1-m² quadrats sampled in a stratified-random pattern in a 400-m² plot demarcated in each savanna. In the uninvaded plot, mean heights of native PFTs were stratified. In the invaded plot, the mean height of aliens extended into the native woody stratum with the lower range of native woody PFT heights reduced to the grass stratum. Discriminant analysis of canopy covers and species richness of the four PFTs revealed significant differences in composition between plots with the alien PFT being the most important variable correlated with these differences. Univariate analysis confirmed the dominance of alien woody plants in the invaded plot but also showed significant reductions in the canopy covers and species richness of native herbs and grasses compared to those in the uninvaded plot. These results suggest that PFTs can rapidly measure small-scale, spatial differences in the physiognomy, composition and species richness of A. nilotica savannas when invaded by alien woody plants.     

Effect of long-term cutting versus abandonment on the vegetation of a mountain hay meadow (Polygono-Trisetion) in Central Europe

The aim of this study was to identify changes in the plant species richness, diversity and composition of a mountain hay meadow (alliance Polygono bistortae-Trisetion flavescentis) after abandonment in comparison with a control cut once per year. The experiment was carried out from 1999 to 2008 in a mountain hay meadow in the Bukovec nature reserve in the north-eastern part of the Jizera Mountains (Jizerské hory, Góry Izerskie, Isergebirge), Czech Republic.The number of vascular plants species with cover greater than or equal to 1% remained almost the same throughout the study period; however, the total number of vascular plants was higher in the cut treatment after the first four years of the study. The cover of Festuca rubra, Agrostis capillaris, Anthoxanthum odoratum, Briza media and Trifolium repens was positively affected by cutting. On the other hand, the cover of Cirsium heterophyllum, Geranium sylvaticum, Hypericum maculatum, Trisetum flavescens and Luzula luzuloides was higher on the abandoned treatment plots.The main effect of the abandonment on plant species composition was the shift in cover of the dominant species. Despite ten years of contrasting management, changes in the vegetation were relatively small with no shift to a different plant community. Therefore the cutting regime combined with several years of no management may be a suitable management strategy for the maintenance of Polygono-Trisetion grasslands and will not be detrimental to the preservation of the target vegetation.     

Short-term responses of rehabilitating coastal dune forest ground vegetation to livestock grazing

We investigated the responses of the ground vegetation in a 17‐year‐old coastal dune forest plant community to four levels of experimentally applied livestock grazing (three grazing levels and one ungrazed control) from May 1994 to March 1996. The effects of grazing were apparently subordinate to site‐specific intrinsic vegetation change and there were some indications that rainfall interacted with grazing level. Grazing had some apparent but no significant effects on plant species composition, significantly affected plant species richness over time, and significantly increased the range of species richness and vegetation cover values as well as the relative abundance and numbers of plant species with erect growth forms. Vegetation cover changed significantly over time, independently of grazing. Our results point to two important, easily measured mechanisms for the conservation management of coastal dune forests – the interaction of disturbance type with plant growth form and the increase of variation in community structural variables under disturbance. These mechanisms, although they potentially have wide application and predictive power, have not been studied adequately.     

The influence of soil texture on the soil water dynamics and vegetation structure of a shortgrass steppe ecosystem

We analyzed soil water data from three sites with different soil textures in the shortgrass steppe of northeastern Colorado, USA. Our objective was to evaluate the relationship between the occurrence of plant functional types and the effect of soil texture on soil water availability. Soil water availability was greatest in the upper soil layers at all three sites, but the loamy sand site had significantly greater soil water availability than the sandy clay loam and sandy clay sites in wetter years at depths below 60 cm. Calculations of proportional water availability by layer using both field data and fifty-year soil water model simulations, showed that the sandy clay loam and sandy clay soils on average had greater water availability in layers 30 cm and above, but that the loamy sand had the greatest water availability in layers beneath this, particularly at 105 cm. This observation can be linked to the occurrence of a fine textured subsoil at this site. The textural pattern in the loamy sand profile effectively creates two water resources: a shallow pool accessible to all plants; and a deep pool accessible only to deep-rooted plants. This is offered as an explanation for the co-dominance of the two main plant functional types at the loamy sand site. At the other two sites, shallow-rooted shortgrass vegetation dominated, being more consistent with the general pattern for the area. Thus the patterns of vegetation structure at the three sites were consistent with the hypothesis. Aboveground net primary production data for the three sites, along with transpiration estimates from the model simulations, indicated that the additional water availability in the coarse textured soil was associated with higher overall plant productivity.Nomenclature: Taxonomic nomenclature follows R. L. McGregor & T. M. Barkley (1986) Flora of the Great Plains. Great Plains Flora Association. University Press of Kansas, Lawrence.     

Nutrient relations in calcareous grassland under elevated CO2

Plant nutrient responses to 4 years of CO₂ enrichment were investigated in situ in calcareous grassland. Beginning in year 2, plant aboveground C:N ratios were increased by 9% to 22% at elevated CO₂ (P < 0.01), depending on year. Total amounts of N removed in biomass harvests during the first 4 years were not affected by elevated CO₂ (19.9 ± 1.3 and 21.1 ± 1.3 g N m⁻² at ambient and elevated CO₂), indicating that the observed plant biomass increases were solely attained by dilution of nutrients. Total aboveground P and tissue N:P ratios also were not altered by CO₂ enrichment (12.5 ± 2 g N g⁻¹ P in both treatments). In contrast to non-legumes (>98% of community aboveground biomass), legume C/N was not reduced at elevated CO₂ and legume N:P was slightly increased. We attribute the less reduced N concentration in legumes at elevated CO₂ to the fact that virtually all legume N originated from symbiotic N₂ fixation (%N_dfa ≈ 90%), and thus legume growth was not limited by soil N. While total plant N was not affected by elevated CO₂, microbial N pools increased by +18% under CO₂ enrichment (P = 0.04) and plant available soil N decreased. Hence, there was a net increase in the overall biotic N pool, largely due increases in the microbial N pool. In order to assess the effects of legumes for ecosystem CO₂ responses and to estimate the degree to which plant growth was P-limited, two greenhouse experiments were conducted, using firstly undisturbed grassland monoliths from the field site, and secondly designed `microcosm' communities on natural soil. Half the microcosms were planted with legumes and half were planted without. Both monoliths and microcosms were exposed to elevated CO₂ and P fertilization in a factored design. After two seasons, plant N pools in both unfertilized monoliths and microcosm communities were unaffected by CO₂ enrichment, similar to what was found in the field. However, when P was added total plant N pools increased at elevated CO₂. This community-level effect originated almost solely from legume stimulation. The results suggest a complex interaction between atmospheric CO₂ concentrations, N and P supply. Overall ecosystem productivity is N-limited, whereas CO₂ effects on legume growth and their N₂ fixation are limited by P. Received: 12 July 1997 / Accepted: 15 April 1998     

Plant diversity effects on soil heterotrophic activity in experimental grassland ecosystems

The loss of plant species from terrestrial ecosystems may cause changes in soil decomposer communities and in decomposition of organic material with potential further consequences for other ecosystem processes. This was tested in experimental communities of 1, 2, 4, 8, 32 plant species and of 1, 2 or 3 functional groups (grasses, legumes and non-leguminous forbs). As plant species richness was reduced from the highest species richness to monocultures, mean aboveground plant biomass decreased by 150%, but microbial biomass (measured by substrate induced respiration) decreased by only 15% (P = 0.05). Irrespective of plant species richness, the absence of legumes (across diversity levels) caused microbial biomass to decrease by 15% (P = 0.02). No effect of plant species richness or composition was detected on the microbial metabolic quotient (qCO₂) and no plant species richness effect was found on feeding activity of the mesofauna (assessed with a bait-lamina-test). Decomposition of cellulose and birchwood sticks was also not affected by plant species richness, but when legumes were absent, cellulose samples were decomposed more slowly (16% in 1996, 27% in 1997, P = 0.006). A significant decrease in earthworm population density of 63% and in total earthworm biomass by 84% was the single most prominent response to the reduction of plant species richness, largely due to a 50% reduction in biomass of the dominant `anecic' earthworms. Voles (Arvicola terrestris L.) also had a clear preference for high-diversity plots. Soil moisture during the growing season was unaffected by plant species richness or the number of functional groups present. In contrast, soil temperature was 2 K higher in monocultures compared with the most diverse mixtures on a bright day at peak season. We conclude that the lower abundance and activity of decomposers with reduced plant species richness was related to altered substrate quantity, a signal which is not reflected in rates of decomposition of standard test material. The presence of nitrogen fixers seemed to be the most important component of the plant diversity manipulation for soil heterotrophs. The reduction in plant biomass due to the simulated loss of plant species had more pronounced effects on voles and earthworms than on microbes, suggesting that higher trophic levels are more strongly affected than lower trophic levels.     

Trophic-level differences in functional composition of the Nardus grassland vegetation

In this study, we examined floristic and functional composition of Nardus grassland of the highlands of NE Slovenia. The data-set included 55 relevés, 59 plant species, and 17 plant functional traits (PFT). The TWINSPAN classification resulted in two plant communities; calcifuge species (G1_oligotr) and another group of species characteristic of mesotrophic meadow (G2_mesotr). On the basis of selected PFT 11 out of 17 differ significantly between the groups. Group G1_oligotr had higher community-weighted mean trait values for chamaephytes (G1 = 0.09; G2 = 0.02), geophytes (G1 = 0.03; G2= 0.01), competitors (G1 = 0.43; G2 = 0.41) and plants that start flowering later (G1 = 142.84; G2 = 136.69). On the other side was G2_mesotr represented with more plants that are therophytes (G1 = 0.04; G2 = 0.09), creeping (G1 = 0.01; G2 = 0.12) and short-lived (G1 = 0.04; G2 = 0.11) and have longer flowering period (G1 = 3.24; G2 = 3.60). Differences may reflect the stronger effect of disturbance and eutrophication in G2_mesotr, probably due to intensification of grassland management (grazing) in the region. Our findings are significant in understanding the relative influence of environmental stress and disturbance within Nardus grasslands, and this may have important implications for their conservation management.     

Assessing facilitative responses to a nurse shrub at the community level: the example of Potentilla fruticosa in a sub‐alpine grassland of northwest China

Biotic interaction studies have revealed a large discrepancy among experiments in target responses to the effects of neighbours, which may in part be due to both high species‐specificity of plant responses and low number of target species used in experiments. Our aim was to assess facilitative responses at the community level and the role of both functional groups and ecological attributes of target species. In a sub‐alpine grassland on the eastern Tibet plateau, we assessed growth responses of all species in the community to removal of a dominant shrub. We also measured changes in the main environmental variables. Species responses were analysed by functional group and in relation to their mean regional altitudinal distribution. All significant interactions were positive and affected one‐third of the total species richness of the community. All functional groups were facilitated but forbs were less strongly facilitated than in the two other groups. High‐alpine species were less strongly facilitated than low‐sub‐alpine species, but the strength of this relationship was weaker than that reported in previous work. There was evidence of a decrease in extreme temperatures below the canopy of the shrub but no variations in soil moisture. We conclude that the highly stressful conditions induced by the dry continental climate of the eastern Tibet plateau are a main driver of the exclusive dominance of positive interactions. Assessing interactive responses at the community level is likely to provide a useful tool to better understand the role of biotic interactions in community responses to environmental changes.     

The role of landuse and natural determinants for grassland vegetation composition in the Swiss Alps

The Alps provide a high habitat diversity for plant species, structured by broad- and fine-scale abiotic site conditions. In man-made grasslands, vegetation composition is additionally affected by the type of landuse. We recorded vegetation composition in 216 parcels of grassland in 12 municipalities representing an area of 170×70 km in the south-eastern part of the Swiss Alps. Each parcel was characterized by a combination of altitudinal level (valley, intermediate, alp), traditional landuse (mown, grazed), current management (mown, grazed, abandoned), and fertilization (unfertilized, fertilized). For each parcel we also assessed the abiotic factors aspect, slope, pH value, and geographic coordinates, and for each municipality annual precipitation and its cultural tradition. We analysed vegetation composition using (i) variation partitioning in RDA, (ii) cover of graminoids, non-legume forbs, and legumes, and (iii) dominance and frequency of species. Species composition was determined by, in decreasing order of variation explained, landuse, broad-scale abiotic factors, fine-scale abiotic factors, and cultural tradition. Current socio-economically motivated landuse changes, such as grazing of unfertilized former meadows or their abandonment, strongly affect vegetation composition. In our study, the frequency of characteristic meadow species was significantly smaller in grazed and even smaller in abandoned parcels than in still mown ones, suggesting less severe consequences of grazing for vegetation composition than of abandonment. Therefore, low-intensity grazing and mowing every few years should be considered valuable conservation alternatives to abandonment. Furthermore, because each landuse type was characterized by different species, a high variety of landuse types should be promoted to preserve plant species diversity in Alpine grasslands.     

General relations of stomatal responses to xylem sap abscisic acid under stress in the rooting zone – A global perspective

A sensitivity factor that quantifies the responsiveness of stomata to xylem sap abscisic acid concentration ([ABA]_xyl) is described, using the relation between [ABA]_xyl and maximum leaf conductance (g_max). Plotting g_max against this factor results in a common linear relationship for woody and herbaceous species from boreal to (semi-) arid climates. The global distribution of the sensitivity factor reveals an unexpected pattern which is inverse to rainfall, i.e., plants in humid climates respond more sensitively to ABA than plants in arid areas. The implications for the response of natural vegetation and consequences for agriculture are discussed.     

Community diversity and invasion resistance: An experimental test in a grassland ecosystem and a review of comparable studies

The relationship between community diversity and invasion resistance in a grassland was examined using experimental plant assemblages that varied in species richness and composition. The assemblages were weeded for three seasons to remove unsown species and we used the number of weeded seedlings, their total biomass and the number of species removed as indicators of community resistance and susceptibility to invasion. In general, we found a positive relationship between invasion resistance and increasing community diversity. Similar patterns of establishment were observed at the end of the fourth field season after several months without weeding. Increased invasion resistance with higher diversity appears to come through reduced levels of several above- and belowground resources, although these did not fully explain the effects of species richness in the studys analyses. Experimental increases and reductions of litter biomass within the studys experimental plant assemblages did not modify these patterns significantly. A review of comparable studies of invasion across directly manipulated diversity gradients revealed similar patterns. Positive effects of species diversity on invasion resistance were found in experimental manipulations of plant diversity conducted in the field and in the glasshouse, from studies with aquatic microcosms and in a marine system. Although some exceptions to this pattern were found in both terrestrial plant systems and aquatic microcosms, it was concluded that in biodiversity manipulation experiments more diverse communities are generally more resistant to invasion.     

Immediate responses in soil chemistry,vegetation and ground beetles to soil perturbation when implemented as a restoration measure in decalcified sandy grassland

The species-rich calcareous grassland communities in Europe are gradually disappearing due to lack of management such as grazing or cultivation, resulting in decalcification and reduction of gaps in the vegetation. In this study, experimental soil perturbation (deep and shallow) was performed in degenerated sandy grassland in plots with a size of 8 × 8 m, using a randomised block design. The hypothesis that soil perturbation that inverts the soil layers decreases nutrient availability, creates vegetation gaps and thereby selects for desirable species was tested through comparisons with untreated controls as well as with nearby target habitats. The deep perturbation was designed to bring CaCO₃ up to the surface, whereas the shallow perturbation tested the effect of disturbance alone. The effects of soil perturbation on soil chemistry, vegetation and beetle communities were analysed for the 2 years following the treatments. Increased pH and calcium concentration, and decreased nitrogen and phosphorus availability, showed that deep perturbation was successful in restoring the soil chemistry to levels similar to those of the target habitat. Perturbated plots were rapidly colonised by the acid tolerant grass Corynephorus canescens, but the slow colonisation of the threatened calcicole species Koeleria glauca was an indication that the vegetation could be evolving towards the target vegetation. Six red-listed beetle species associated with open, dry grasslands were found, out of which four were found only in perturbated plots, although this could not be statistically tested. In conclusion, it may take many years or even decades for the establishment of desirable flora, and seeding could therefore be a suitable method of increasing the rate of succession.     

Large-scale manipulation of plant litter and fertilizer in a managed successional temperate grassland

Plant litter may play an important role in herbaceous plant communities by limiting primary production and influencing plant species richness. However, it is not known how the effect of litter interacts with fertilization. We tested for the role of litter and fertilization in a large-scale experiment to investigate effects on diversity and biomass of plant species, growth forms, native vs. non-native groups, and abiotic ecosystem components (e.g., soil moisture, PAR). We manipulated plant litter (removed vs. left in situ) and nutrient availability (NPK-fertilized vs. unfertilized) for 4 years in 314-m² plots, replicated six times, in an old-field grassland. While many of our species-level results supported previously published studies and theory, our plant group results generally did not. Specifically, grass species richness and forb biomass was not affected by either fertilization or plant litter. Moreover, plant litter removal significantly increased non-native plant species richness. Relative to native plant species, all of our experimental manipulations significantly increased both the biomass and the species richness of non-native plant species. Thus, this grassland system was sensitive to management treatments through the facilitation of non-native plant species. We coupled biotic and abiotic components within a nonmetric multidimensional scaling (NMS) analysis to investigate treatment effects, which revealed that specific treatments altered ecosystem development. These results suggest that fertilization and plant litter may have larger impacts on plant communities and on ecosystem properties than previously understood, underscoring the need for larger-scale and longer-term experiments.     

Plant and soil microbial responses to defoliation in temperate semi-natural grassland

There is much interest in understanding the nature of feedback mechanisms between plants and soil organisms in grazed ecosystems. In this study, we examine the effects of different intensities of defoliation on the growth of three dominant grass species, and observe how these plant responses relate to the biomass and activity of the microbial community in the root zone. Our data show that grassland plants with varying tolerances to grazing have markedly different growth responses to defoliation, and that these responses vary with the intensity of cutting. Defoliation of grasses which are tolerant to grazing, namely Festuca rubra and Cynosurus cristatus, leads to a reduction in root mass and an increase in the allocation of resources to shoots. In contrast, defoliation of a grass with low tolerance to grazing, Anthoxanthum odoratum, had little effect on root mass, but increased the relative allocation of resources below-ground. In all plant species, defoliation led to an increase in soil microbial biomass and C use efficiency in the root zone. This response was greatest in the root zone of A. odoratum and is likely to be related to changes in root exudation pattern following defoliation. The significance of these changes in relation to soil nutrient dynamics and plant nutrient uptake during regrowth require further exploration. This revised version was published online in June 2006 with corrections to the Cover Date.     

Distinguishing between signal and noise in faunal responses to environmental change

Aim We tested whether variation in snapshots of butterfly species composition and species richness taken from one to six years apart could be interpreted as an ecologically meaningful trend or whether they might merely reflect stochasticity. Location Field research was conducted in the Toquima Range and Shoshone Mountains, Lander and Nye counties, Nevada, USA. Methods We obtained data for 49 sites in the Toquima Range from 1996 to 2002 and 39 sites in the Shoshone Mountains from 2000 to 2002. Sites spanned the gradient of local topographic and climatic conditions in those mountain ranges. Data on species composition and species richness were based on comprehensive field inventories. We calculated similarity of species composition using the Jaccard index. We conducted one‐factor repeated‐measures analyses of variance to test whether the distribution of similarity of species composition and the distribution of mean species richness depended on the number of years between inventories. Results In both mountain ranges, much less of the difference in species composition was attributable to turnover of species composition within sites over time than to spatial differences among sites. Annual species richness in the Toquima Range was more variable than in the Shoshone Mountains, but again far less of the variation in species richness was attributable to year than to differences among sites. Main conclusions Despite the fact that desert ecosystems are not expected to be highly resilient to global environmental change, there may be a time lag between deterministic environmental changes and a detectable faunal response, even in taxonomic groups that are known to be sensitive to changes in climate and vegetation. Although information on species richness and similarity of species composition are among the most practical data to collect in managed landscapes, these measures may not be highly sensitive to environmental changes over the short to moderate term.