Changes in vegetation states in grazed and ungrazed Mackenzie Basin grasslands, New Zealand, 1990-2000

Changes in vegetation from 1990 to 2000 were examined at 10 high country localities, representing four grassland types: fescue tussock (Festuca novae-zelandiae), snow tussock (Chionochloa rigida), red tussock (C. rubra), and silver tussock (Poa cita). At each locality, three treatments were established: ambient sheep+rabbit grazing, rabbit grazing only, and no grazing. The mutivariate methods of classification and ordination were used on individual-quadrat cover data to define vegetation states and to examine transitions between them over time. Vegetation states in quadrats already dominated by Hieracium pilosella(> 50% cover) in 1990 showed little change in species composition regardless of grassland type and grazing treatment. In fescue tussock grassland, H. pilosellaincreased regardless of grazing treatment in states with low initial H. pilosellacover (< 5%), while the cover of Carex colensoi, Aira caryophyllea and Rumex acetosella decreased. In the single silver tussock locality, Poa citadecreased markedly in the ungrazed treatment as adventive species such as Dactylis glomerataand Echium vulgare increased. However, Poa citaalso decreased, probably due to drought, in the grazed treatment. Snow tussock and red tussock grassland states were more stable than those in short tussock grasslands, but there was also a general trend towards increasing H. pilosellacover in intertussock vegetation regardless of treatment. However, at one snow tussock locality, transitions from H. piloselladominated to C. rigida-dominated states occurred in ungrazed quadrats, while the reverse occurred in grazed vegetation. Implications for the management of tussock grasslands for conservation are discussed.     

Distribution and diet of chamois (

Changes in Hieracium abundance in Eastern Otago tussock grassland were examined by sampling 163 sites in 1982 and again in 1992. For Hieracium pilosella, H. praealtum and H. lepidulum, as well as Agrostis capillaris for comparison, colonisation of new sites was recorded, as well as extinction of species from sites over the 10 years, and changes in cover. H. pilosella colonised the majority of sites from which it had been absent in 1982; it disappeared from only a few sites where it had been present at very low cover. In sites where it remained over the decade, its cover increased by 50%. Its rate of increase was very similar to that recorded for North Canterbury, but with no indication of the 34% stabilisation point that has been hypothesised for the latter area. H. praealtum and H. lepidulum, in contrast, remained absent from most sites, and colonised only a few; in very few sites was their cover greater than 1%. Agrostis capillaris was widespread and abundant, but increased little over the period. The sites where H. pilosella colonised tended to be degraded, with a greater, and increasing, percentage of bare ground and low tussock cover. The few sites where it disappeared were ones where pasture improvement had occurred, as seen in the increase of species indicative of pasture development. Several explanations are considered for the increase in H. pilosella in recent decades. The increase of this species in Eastern Otago has been greater than that of most other exotic species. Changes in land management have contributed to its increase, but probably changes caused indirectly by government policy, not gradual degradation. Genetic changes in the species, and metapopulation dynamics, have not been investigated, but they could well have contributed to Hieracium invasion.     

Rapid short-tussock grassland decline with and without grazing, Marlborough, New Zealand

Species abundance, species richness, and ground cover were measured over 10 years on nine paired grazed and exclosure plots in short-tussock grassland in the early stages of invasion by Hieracium species. With and without grazing, H. pilosella and H. caespitosum increased markedly and H. lepidulum increased locally. In contrast, 50% of all other common species and species groups, and total, native, and exotic species richness declined significantly. Exclusion increased or had no effect on rates of increase in Hieracium species and rates of decline in short tussocks, and did not reduce rates of decline in other species. Exclusion had no effect on decline in native species richness, but mainly accelerated declines in total and exotic richness. Declines in 13 key vegetation variables were significantly predicted by increase in Hieracium abundance, suggesting competitive exclusion. With or without grazing, Hieracium species will become more dominant and other species will continue to decline. The effects of large herbivores on plant species diversity can often be predicted from site productivity. Our results indicate the need also to account for species origin, spatial scale, time, and exotic invasion.     

Growth and tillering patterns within tussocks of Eriophorum vaginatum

Growth patterns were investigated for Eriophorum vaginatum tussocks from disturbed and undisturbed tussock tundra at two sites in Alaska. Total basal area of tussocks decreased with increased cryoturbation but mean basal area per tussock did not. Flowering was observed in tussocks of significantly smaller size on disturbed compared to undisturbed tundra. For tussocks with < 10% cover by shrubs and moss, number of tillers per tussock was linearly related to tussock diameter in most disturbed and undisturbed sites. Exceptions occurred in an area that had been bladed with a bulldozer 7 yr before our survey where tiller number increased as the square of diameter and in an area with much frost activity where tiller number was not emulated with diameter. The ratio of daughter tillers to adult tillers decreased with diameter in disturbed tundra, whereas the trend was less pronounced in undisturbed tundra.
Microsuccession in undisturbed tussock tundra was investigated by sampling tussocks with different amounts of shrub and moss cover for number of daughter tillers per adult tiller, weight per tiller, percent nitrogen, and percent phosphorus. Small tussocks without cover by other species and large, partially covered tussocks were not significantly different by any measures, but tussocks that were almost completely buried had significantly (p < 0.05) lower values of tillering index, weight per tiller, and percent phosphorus.     

Patterns of abundance and demography: Collembola in a habitat patch gradient

The demography and abundance of Collembola in relation to a gradient of increasingly isolated tussocks of Carex ursina were investigated near Ny-Alesund, Svalbard The study area was divided into three zones according to tussock density Ten tussocks were sampled in each zone In addition, samples were taken between tussocks, which consisted of ground covered with a layer of cyanobacteria A total of ten Collembola species were found, five of which were chosen for further studies The Carex tussocks were the preferred habitat for the majority of these species Only one species, Hypogastrura viatica, was found regularly between tussocks, although at low density The gradient in tussock distribution was probably an important factor in determining the distribution, abundance and the underlying demographic processes of most species This was indicated by an increase in demographic heterogeneity with patch isolation The different species were affected differently, however Whereas one species appeared to be unaffected by the gradient (H viatica), two species (H longispina) and (Folsomia sexoculata) were somewhat surprisingly found to have their highest density where tussocks were furthest apart Factors other than the spatial configuration of the habitat are probably important in determining the distribution of these two species, indicated by a positive correlation at tussock level between them In accordance with general hypotheses on the effect of patchiness on population dynamics the remaining two species, F quadrioculata and Onvchiurus groenlandicus, occurred in very low numbers or not at all, respectively, in the zone where tussocks were furthest apart Their response is probably dependent on their ability to successfully colonise isolated tussocks We predict that different species specific demographic strategies, and in particular dispersal rates may account for the observed patterns     

Factors predisposing short-tussock grasslands to

The effects of environment and management on the composition of short-tussock grasslands and the abundance of the invasive weed Hieracium pilosella were investigated in two small catchments. Species composition and site factors were recorded on a total of 182 plots and the management history of each catchment was reviewed. H. pilosella was present on >80% of all plots, but was at an early stage of invasion in one catchment (<5% cover) and dominant in the other (25% cover). Classification and ordination revealed strong between-catchment differences in community composition that reflected differences in environment (soil fertility and rainfall), disturbance history (animal populations and burning), and the stage of invasion by H. pilosella. In both catchments H. pilosella tended to be least abundant on the wettest, driest, and most fertile soils. However, such relationships were weak. Generalised additive models and regression showed that in the earlier stage of invasion individual site factors explained less than 20% of the variation in H. pilosella cover. Topographic position and slope (both indicative of soil moisture) were the most significant combined predictors, but together explained only 32% of the variation. In the later stage of invasion individual factors explained up to 33% of the variation. Topsoil sulphur, slope, and topsoil calcium were the most significant combined predictors, but together explained only 53% of the variation. Between-catchment comparisons highlighted the inter-related roles of environment, disturbance history, geographic location, availability of H. pilosella propagules, and stage of invasion in more fully explaining the abundance of H. pilosella. Of five models that have been proposed for Hieracium invasion, the: "grassland decline" model best incorporated the inter-related factors that influence spatial and temporal variation in H. pilosella abundance in the study area. This model concentrates on identifying predisposing and trigger factors that increase the likelihood of invasion and accounts for multiple causes and interactions by specifying five key factors that influence the ability of a plant species to invade existing vegetation: environment, disturbance, vegetation structure and composition, life history attributes of the invader, and the availability of invading propagules. The model potentially provides a comprehensive framework for evaluating the causes of Hieracium invasion, targeting research effort, and developing sustainable management strategies.     

Local interactions, dispersal, and native and exotic plant diversity along a California stream

Although the species pool, dispersal, and local interactions all influence species diversity, their relative importance is debated. I examined their importance in controlling the number of native and exotic plant species occupying tussocks formed by the sedge Carex nudata along a California stream. Of particular interest were the factors underlying a downstream increase in plant diversity and biological invasions. I conducted seed addition experiments and manipulated local diversity and cover to evaluate the degree to which tussocks saturate with species, and to examine the roles of local competitive processes, abiotic factors, and seed supply in controlling the system‐wide patterns.
Seeds of three native and three exotic plants sown onto experimentally assembled tussock communities less successfully established on tussocks with a greater richness of resident plants. Nonetheless, even the most diverse tussocks were somewhat colonized, suggesting that tussocks are not completely saturated with species. Similarly, in an experiment where I sowed seeds onto natural tussocks along the river, colonization increased two‐ to three‐fold when I removed the resident species. Even on intact tussocks, however, seed addition increased diversity, indicating that the tussock assemblages are seed limited. Colonization success on cleared and uncleared tussocks increased downstream from km 0 to km 3 of the study site, but showed no trends from km 3 to km 8. This suggests that while abiotic and biotic features of the tussocks may control the increase in diversity and invasions from km 0 to km 3, similar increases from km 3 to km 8 are more likely explained by potential downstream increases in seed supply. The effective water dispersal of seed mimics and prevailingly downstream winds indicated that dispersal most likely occurs in a downstream direction. These results suggest that resident species diversity, competitive interactions, and seed supply similarly influence the colonization of native and exotic species.     

Multiclonal tussocks in the grass Achnatherum splendens (Trinius) Nevskia (Poaceae)

Clonal plant species often form genetically diverse populations, even when sexual reproduction in a population is rarely observed. Here we test whether the spatially discrete clusters of plants (tussocks of graminoids) formed within populations of some clonal species can likewise be multiclonal. We sampled leaves of ramets (shoots) within 20 tussocks of the grass Achnatherum splendens in the Otindag Sandland in Inner Mongolia, China, and genotyped the ramets using standard molecular protocols. The 20 tussocks were allocated to three classes: (i) small, circular, (ii) large, circular and (iii) large, irregular. Most tussocks (80%) were multiclonal and some contained at least eight different clones. Irregularly shaped tussocks contained twice as many clones as circular tussocks; neither size nor cover within a tussock affected number of clones per tussock, and the smaller clones in a tussock showed no tendency to occur on the edge or near the center of a tussock. These patterns seem more consistent with formation of multiclonal tussocks by coalescence than by colonization. Therefore, individual tussocks, especially large, irregular ones, cannot a priori be treated as genetic individuals without assessing their genetic information in, e.g., population demography, genetics and evolution studies.     

Effects of sedge and cottongrass tussocks on plant establishment patterns in a post-mined peatland,northern Japan

Facilitation (positive inter-specific interaction) plays an important role in promoting succession in harsh environments. To examine whether tussocks facilitate the establishment of other species, after peat mining, investigations were carried out in a formerly Sphagnum-dominated wetland (Sarobetsu mire, northern Japan). Two tussock-forming species, Carex middendorffii and Eriophorum vaginatum, have established in sparsely vegetated areas, with a dry ground surface, since peat extraction ended. The following factors were examined, in three microhabitats created by tussocks (center = raised tussock center, edge = tussock edge covered with litter, and flat = flat areas without tussocks): (1) relationships between tussock microhabitats and plant distributions, and (2) the effects of tussocks on survival, growth, flowering and seed immigration of common species. Two (1 × 10 m) plots were established, in each of three sparsely vegetated sites, in September 2005. Tussocks were mapped in each plot, and species, location, flowering, growth stage (seedling, juvenile and fertile) and size of all plants were recorded, during snow-free periods from September 2005 to September 2006. Seed traps were used to investigate seed dispersal from June to October 2006. Four native species, Drosera rotundifolia, Lobelia sessilifolia, Moliniopsis japonica, Solidago virgaurea, and an exotic species, Hypochaeris radicata, were most common. During seedling and juvenile stages, these species were distributed more densely at the tussock edge than in the flat areas, but were less common at the center. H. radicata had a higher survival rate at the edge than in the flat during the winter. The annual growth of H. radicata, L. sessilifolia and S. virgaurea was higher at the edge. Seed traps detected that D. rotundifolia seeds accumulate more at the edge. In conclusion, tussocks facilitated plant establishment in the edge microhabitat by providing litter cover, enhancing seed accumulation, germination and survival, and thus promoted revegetation. However, Sphagnum mosses have not established in the study sites, and the vegetation differs strongly from the areas where no peat mining had taken place.     

The effects of spatial habitat configuration on recruitment, growth and population structure in arctic Collembola

The population density and demography of five species of arctic Collembola were studied in a naturally patchy habitat, consisting of Carex ursinae tussocks with varying degrees of isolation. Focal predictor variables were those describing the spatial configuration of tussocks, including tussock size and isolation and the amount of habitat (cover) at a 1-m² scale surrounding each tussock population. The Collembola populations were heavily influenced by environmental stochasticity in the form of winter mortality and summer drought, and the influence of patchiness on population characteristics was evaluated in this context. The five species showed very different responses to the structuring effect of the habitat, depending on life history characteristics, mobility and habitat requirements. Population density was highly variable in both time and space. Spring densities indicated larger winter mortality compared to observations from a previous study, and the snow- and ice-free season from June to August only resulted in population growth for Folsomia sexoculata. In the other species, adult mortality must have been high as there was no net population growth despite observed reproduction. The exception was Hypogastrura viatica, whose population decline was more likely to have been the result of migration out of the study area. Cover was the most important variable explaining density. No pure area or isolation effects at the tussock level were detected, even in areas with very low habitat cover. Drought was probably an important mortality factor, as July was particularly warm and dry. Due to qualitative differences in the tussocks and the matrix substrate, desiccation risk would be higher during dispersal between tussocks. We suggest that increased dispersal mortality gave the observed pattern of increased density in relation to cover, both in general and in F. quadrioculata, an opportunistic species otherwise known for rapid population growth. Onychiurus groenlandicus, which had a similar density response to cover, may also be influenced by a rescue effect sustaining densities in areas with high cover. The cover effect can be viewed as a large-scale factor which encompasses the general spatial neighbourhood of each tussock, where inter-population processes are important, as opposed to internal patch dynamics. Received: 15 March 1999 / Accepted: 22 March 2000     

Regeneration of native vascular plants facilitated by Ischaemum aristatum var. glaucum tussocks: an experimental demonstration

Facilitation of a perennial tussock grass, Ischaemum aristatum var. glaucum, was hypothesized to be the provision of germination safe sites against inundation. We conducted field census and seed addition experiments to test this hypothesis. Facilitation effects of I. aristatum var. glaucum tussocks were obvious; in contrast to 280 seedlings/m² of ten native species observed on tussocks, seedlings hardly emerged on bare ground, even after seed addition. Although effects of moss occurrence at tussocks were not significant on the total number or species richness of emerged seedlings, significantly positive effects were observed on the seedling survival of some species, including endangered species. Conservation of facilitators will efficiently ensure the regeneration success of native vascular plants.     

Sexual and vegetative reproduction of Hieracium pilosella L. under competition and disturbance: a grid-based simulation model

We used a spatially explicit simulation model to examine the relative importance of vegetative and sexual reproduction in Hieracium pilosella L. Based on an understanding of the complex life cycle of this species and on data from in situ population dynamics in a calcareous grassland in NW Switzerland, we simulated growth and the relative contribution of clonal reproduction by stolons and reproduction by seeds across a gradient of increasing soil fertility. Competition by a clonal grass resulted in nearly complete exclusion of H. pilosella from the more fertile part of the simulation plot. Under low soil fertility, when grass could not survive, H. pilosella largely persisted by vegetatively produced rosettes. This pattern of a sharp separation of both species was shifted slightly in favour of H. pilosella by introducing random disturbances. Only by adding: (1) long-distance seed dispersal, and (2) facilitation of seedling establishment in the vicinity of grass tussocks in vegetation gaps was a more realistic representation of field observations realised, with rosettes of H. pilosella grown from seeds occasionally distributed within dense grass vegetation. Phenotypic plasticity of stolon length was a decisive factor for the maintenance of H. pilosella populations. We conclude that a mixed strategy of clonal growth and reproduction by seeds in H. pilosella is necessary to maintain populations of this species in the presence of high interspecific competition and a shortage of open space.     

Effects of agri-environment management for cirl buntings on other biodiversity

Agri-environment scheme (AES) management has increased populations of cirl buntings (Emberiza cirlus) in South Devon, England, and might be expected to provide benefits for other declining biodiversity, due to less intensive farm management. Fields managed under AES for cirl buntings (low-input spring barley or permanent pasture without inputs) were contrasted with control fields under conventional management (spring barley without management restrictions and winter cereals, or grazed without management restrictions) to identify such benefits for vascular plants, butterflies, bumblebees, carabid beetles, foliar invertebrates and bats. Activity-density and species richness of carabid beetles were both higher in AES spring barley fields than in control spring barley and winter cereal fields. Forb cover and abundance of butterflies and bumblebees were higher in AES spring barley fields than in winter cereals, but did not differ between AES and control spring barley. No difference was observed in plant species richness between any of the arable field types. Plant species richness and butterfly abundance were higher in AES pasture fields than in controls. Abundance, activity-density and/or species richness of other taxa did not differ between AES and control pastures. Benefits observed in AES spring barley fields arise from management specific to AES agreements, and also, we suggest, from the maintenance of spring-sown barley in the landscape. Benefits in AES pasture fields are ascribed to the absence of fertiliser and pesticide inputs, and reductions in stocking arising from this; there is also likely to have been some pre-selection for older pastures to be entered into AES management agreements. Agri-environment measures for cirl buntings have benefits for a range of taxa beyond the target species, and therefore, largely through reduction of management intensity and maintenance of land-use diversity, improve the overall biodiversity of the farmed landscape where they are present.     

Negative Plant–Soil Feedback and Positive Species Interaction in a Herbaceous Plant Community

Increasing evidence shows that facilitative interaction and negative plant–soil feedback are driving factors of plant population dynamics and community processes. We studied the intensity and the relative impact of negative feedback on clonal growth and seed germination of Scirpus holoschoenus, a ‘ring’ forming sedge dominant in grazed grassland, and the consequences for species coexistence. The structure of aboveground tussocks was described. A Lithium tracer assessed belowground distribution of functional roots. Seed rain and seedling emergence were compared for different positions in relation to Scirpus tussocks. Soil bioassays were used to compare growth on soil taken from inside and outside Scirpus tussocks of four coexisting species (Mentha acquatica, Pulicaria dysenterica, Scirpus holoschoenus and Dittrichia viscosa). We also compared plant performance of dominant plant species inside and outside Scirpus tussocks in the field. The ‘ring’ shaped tussocks of S. holoschoenus were generated by centrifugal rhizome development. Roots were functional and abundant under the tillers and extending outside the tussocks. The large roots mats that were present in the inner tussock zone were almost all dead. Seedling emergence and growth both showed a strong negative feedback of Scirpus in the inner tussock zone. Scirpus clonal development strongly reduced grass biomass. In the degenerated tussock zone, Pulicaria and Mentha mortality was lower, and biomass of individual plants and seed production were higher. This positive indirect interaction could be related to species-specific affinity to soil conditions generated by Scirpus, and interspecific competitive release in the degenerated tussock zone. We conclude that Scirpus negative feedback affects its seedling emergence and growth contributing to the development of the degenerated inner tussock zone. Moreover, this enhances species coexistence through facilitative interaction because the colonization of the inner tussock zone is highly species-specific.     

Distribution and abundance of small insects and arachnids in relation to structural heterogeneity of grazed, indigenous grasslands

1. The species composition and spatial distribution of small insects (Hemiptera, Coleoptera, Lepidoptera) and arachnids (Araneae, Opiliones, and Pseudoscorpiones) were investigated in three indigenous, upland grasslands identified as the National Vegetation Classification Festuca–Agrostis–Galium typical subcommunity (code U4a), Festuca–Agrostis–Galium, Vaccinium–Deschampsia subcommunity (code U4e), and Nardus stricta species-poor sub-community (code U5a), on which grazing management was manipulated experimentally. 2. Two hypotheses were tested that predicted arthropod diversity in upland grasslands. The habitat heterogeneity hypothesis predicts that the species number and abundance of arthropods will have an asymptotic relationship with increasing numbers of plant species and greater structural heterogeneity in the vegetation. The symbiosis between patches hypothesis states that the species number and abundance of arthropods will express a unimodal relationship with the grain size of sward patches created by grazing. The sward patches must be large enough to be apparent to, and support populations of, arthropods, but small enough that interspersed tussocks provide shelter from weather and a deterrent to disturbance by grazers. 3. The hypotheses were tested by sampling arthropods from the geometrical patterns represented by the individual tussocks and intermediate sward components of three indigenous grasslands produced by different grazing treatments. Paired samples of arthropods were taken by motorized suction sampler, the first of the pair from the grazed sward and the second, the accumulated samples from the surrounding triad of tussocks (U4a and U5a grasslands) or hummocks (U4e grassland). The paired samples were taken from six randomly-selected locations across both replicates of each of the grazing treatments. 4. Arthropod species composition and abundance were compared between the paired sward and tussock samples and in turn with measures of the vertical and horizontal components of vegetation structure, i.e. the variance in vegetation height per unit area and the area covered by tussock compared with sward. 5. There were consistently more species and a greater abundance of arthropods associated with tussocks than with swards and the average species number and abundance for the combined pair of samples declined with increased grazing pressure. The relationship between vertical and horizontal components of vegetation structure and the species number and abundance of selected arthropods was asymptotic as opposed to unimodal, supporting the habitat heterogeneity hypothesis, rather than the symbiosis between patches hypothesis. 6. Small and relatively sedentary insects and arachnids are more sensitive to grazing intensity and species of grazer in these upland, indigenous grasslands than are larger Coleoptera and Araneae, which respond less directly to varied grazing management. The overall linear reduction of small herbivorous and predatory arthropods with increased grazing intensity was buffered in grasslands with substantial tussock patches.     

Carex sempervirens tussocks induce spatial heterogeneity in litter decomposition, but not in soil properties, in a subalpine grassland in the Central Alps

Tussocks of graminoids can induce spatial heterogeneity in soil properties in dry areas with discontinuous vegetation cover, but little is known about the situation in areas with continuous vegetation and no study has tested whether tussocks can induce spatial heterogeneity in litter decomposition. In a subalpine grassland in the Central Alps where vegetation cover is continuous, we measured soil properties [concentration of N, C, organic matter (OM) and pH] and monitored litter decomposition traits (dry mass loss, loss of C, N, P and K) inside and outside tussocks of Carex sempervirens. Soil C, N, OM concentrations or pH inside tussocks did not differ significantly from those outside tussocks. After 1 year of decomposition, litter dry mass loss, C and K loss were significantly smaller inside than outside tussocks. The slower litter decomposition inside tussocks was likely caused by the elevated tussock base, which made environmental conditions inside tussocks much dryer than those outside in early spring when snow melts. Our results suggest that in areas with continuous vegetation cover tussocks induce spatial heterogeneity in litter decomposition but not in soil properties.     

Influence of a dominant macrophyte, <Emphasis Type="Italic">Juncus effusus</Emphasis>, on wetland plant species richness,diversity, and community composition

Few studies have experimentally investigated the influence of competition for light on structure and composition of wetland vascular plant communities, despite the well-documented high productivity in such systems. Influences of the dominant emergent wetland plant Juncus effusus on the surrounding macrophyte community were evaluated in a shallow freshwater wetland through two consecutive growing seasons. Permanent transects were constructed along diagonals of randomly oriented 1 m² plots centered around isolated, colonizing J. effusus tussocks. Percent areal cover was measured for each species or identifiable taxon group in 400 cm² sub-plots centered 10, 20, 30, and 40 cm from the tussock edge, to evaluate species richness, diversity, and dominance indices. Observational studies during the first growing season indicated that plots having larger, more heavily shading tussocks yielded significantly lower cover and species richness in the surrounding plant community than less shaded plots. Shading by J. effusus was reduced experimentally during the second growing season by holding culms in a vertical position in half of the J. effusus-occupied study plots in order to assess directly the influence of shading by J. effusus. Manipulated plots became more diverse and species-rich and developed higher total percent areal cover. Within shaded reference plots, reductions in cover, richness, and diversity were correlated with intensity of shading; each of these measures was lowest in sub-plots centered 10 cm from the tussock, where measured light reduction was highest. Neighborhood analyses of biomass, species composition, and light reduction indicated that individual species biomass varied with distance from J. effusus tussocks and shading intensity, an indication of the potential for shifts in community composition as populations of this dominant macrophyte expand to fill a wetland area. A mathematical model is presented to illustrate shading effects of J. effusus as a population grows from isolated, colonizing tussocks to an interacting system of tussocks in an established population of this dominant macrophyte.     

Kobresia tibetica tussocks facilitate plant species inside them and increase diversity and reproduction

Many plant species grow inside tussocks of some graminoids, but the underlying mechanisms remain unclear. We address whether some species occur and flower mainly inside tussocks so that species diversity and sexual reproduction are higher inside than outside tussocks, and whether relieving biological and physical stress is the mechanism associated with the facilitative process. In a heavily grazed grassland on the eastern Tibetan Plateau, where both physical (due to high altitude) and biological conditions (due to heavy grazing) are extremely harsh, we investigated vegetation in paired plots inside and outside 150 Kobresia tibetica tussocks and measured tussock basal area (=plot area). We also measured temperatures at soil surface, 5 and 10 cm depth and the number of animals (yaks, sheep and horses) grazing inside and outside tussocks. Sixty-seven percent of the species occurred and 42% flowered more frequently inside than outside tussocks, but none less frequently. Inside tussocks 78% species flowered, but outside tussocks only 31% did. Consequently, number of species, number of flowering species and number of inflorescences were all markedly larger inside than outside tussocks. Differences in number of species, number of flowering species and number of inflorescences inside and outside tussocks increased with increasing tussock basal area. Soil temperatures were lower inside than outside tussocks, but grazing intensity was much larger outside tussocks. Therefore, tussocks of K. tibetica facilitated the species inside them likely by grazing prevention, but not by increasing warmth. This study provides evidence that plant species colonizing tussocks of graminoids can be facilitated by the tussock species, and facilitation by grazing prevention may be one mechanism causing the coexistence of the species inside tussocks.     

Soil cellulase, chitinase, and protease activity in Eriophorum vaginatum tussock tundra at Eagle Summit, Alaska

Cellulase, chitinase, and protease activities were examined in Eriophorum vaginatum tussocks at Eagle Summit, Alaska. Changes in the activities were recorded for tussocks of differing relative plant composition. Highest total activity was found in tussocks with at least 50% (relative surface) cover by mycorrhizal colonized shrubs. Total enzyme activity declined in tussocks as the variety of plants decreased. The lowest enzyme activity was found in the moss dominated tussock. Changes in the nature of cellulose hydrolysis suggested that decomposition may be primarily regulated by litter composition in some cases and soil environment in others.