Selective gopher disturbance influences plant species effects on nitrogen cycling

Plant species effects on ecosystem processes are mediated by traits such as litter quality and exudation. These same traits also influence the activity and distribution of animals that play key roles in regulating ecosystem dynamics. We planted monocultures of eight plant species commonly found in California grasslands to investigate the relative importance of plant species direct effects on nitrogen cycling, versus their indirect effects mediated by plant interactions with gophers. Plant species differed in their litter C:N ratio, which closely related to species effects on rates of net mineralization and nitrification in undisturbed soil. However, the effect of selective gopher disturbance on N cycling greatly altered these species effects.
Plant species differed in their effects on the type and timing of gopher disturbance. Small feeding holes were formed in late spring in plots containing species with high tissue quality. These feeding holes minimally disturbed the soil and did not alter N cycling rates over the short term. Large gopher mounds were formed in the winter and early spring, primarily in plots containing the grass, Aegilops triuncialis , and to a lesser extent in plots containing Avena barbata . These large mounds significantly disturbed the soil and greatly increased net nitrification rates, but had no consistent effects on net N mineralization. In undisturbed soil, Aegilops had the highest litter C:N ratio and one of the lowest rates of net nitrification. However, gophers preferentially built large mounds in Aegilops plots. Once the effects of gopher burrowing were considered, Aegilops had one of the highest rates of net nitrification, indicating that the indirect effects of plant species on N cycling can be more important than the direct effects alone. This experiment indicates that it is vital to consider interactions between plants and other organisms in order to predict the ecosystem effects of plant communities.     

Community and population dynamics of serpentine grassland annuals in relation to gopher disturbance

Summary This study examines the effects of soil disturbance by gophers on patterns of species abundance in an annual grassland community on serpentine soil. We assessed production, dispersal and storage of seed, germination, survivorship and growth of the most abundant species in undisturbed vegetation and on gopher mounds. Fewer seeds of the dominant species were dispersed onto gopher mounds due to the limited movement of seeds from within the closed vegetation. Species with taller flowering stalks were more likely to colonise gopher mounds. The timing of gopher disturbance in relation to the timing of seed fall determined which species could colonise mounds. Lower numbers of seeds falling onto gopher mounds resulted in lower seedling densities of several species compared with undisturbed areas. Survivorship of the commonest species differed between undisturbed areas and gopher mounds formed at different times of year. This resulted in characteristic spectra of species abundance on the different microhabitats, giving rise to distinct spatial patterning in the community. Plants growing on gopher mounds were generally larger and produced more seed than plants in undisturbed vegetation. We suggest that continued gopher disturbance is a factor allowing several species, including perennial grasses, to persist in this community.     

Pocket Gophers and the Invasion and Restoration of Native Bunchgrass Communities

More than 7 million hectares of California native plant communities are now dominated by exotic annual species, a biological invasion that has made native bunchgrass ecosystems in this region one of the most endangered ecosystems in North America. Many land use and environmental factors have contributed to the conversion of bunchgrass areas to annual grassland, but the role of gopher disturbance remains understudied. Here I report observational evidence that suggests gopher foraging is nonrandomly concentrated in the open spaces between clumps of bunchgrass and as a result may inhibit the recruitment of bunchgrass into these areas. Understanding patterns of direct gopher impact and coincident soil disturbance in bunchgrass versus annual grassland is important for successful restoration of bunchgrass habitats. While the prospect of excluding gophers is daunting, even short-term reductions in gopher populations would help to promote a restored community structure with a mature native bunchgrass and annual forb association.     

Disturbance intensity,timing and history interact to affect pasture weed invasion

Past disturbance events shape future community trajectories if their ecological legacies interact with contemporary disturbance regimes to affect recruitment and reproduction. In pastures, large-scale disturbance events can become undetectable in aboveground community components but persist as augmented weed seedbanks, suggesting disturbance history may be a predictor of weed invasion magnitude following subsequent disturbance. Using a randomized block factorial design, we investigated how soil disturbance intensity, timing and history interact to affect weed recruitment, diversity and reproduction by implementing a range of disturbance intensities in April and June 2004 in pastures of differing disturbance history. Response variables were followed for two years. Disturbance timing interacted with disturbance intensity and history to affect recruitment, with some June treatment combinations having ≥48% higher weed abundance than those in April. Relative to undisturbed controls, low to intermediate disturbance intensity facilitated recruitment by ≥117% but high intensity disturbance had highest recruitment, particularly in previously disturbed pastures (≥543% increase). In both years, weed species richness was highest in intense disturbances in previously disturbed pastures. Importantly, weed reproduction was nearly one order of magnitude higher in intense disturbance patches, especially in previously disturbed pastures, and increased through time by ≥243%. These findings indicate weed recruitment, diversity and reproductive output are seed- and microsite-limited, and that intense soil disturbance may result in high long-term weed abundance. Although moderate disturbance facilitated recruitment, reproduction here was low, suggesting modest disturbance will not appreciably increase weed abundance. The most common species were Taraxacum officinale and Poa pratensis, both of which can benefit forage-livestock production, but most establishing species were of low palatability. We recommend grassland managers explicitly integrate disturbance history into dynamic management planning, consider augmenting seedbanks with ‘desirable’ weed species to help communities recover following inevitable disturbance events and do not rely exclusively on aboveground characters to evaluate their system's vulnerability to undesirable plant establishment and persistence.     

Impact of pocket gopher disturbance on plant species diversity in a shortgrass prairie community

Summary We examined the impact of pocket gopher disturbances on the dynamics of a shortgrass prairie community. Through their burrowing activity, pocket gophers (Thomomys bottae) cast up mounds of soil which both kill existing vegetation and create sites for colonization by competitively-inferior plant species. Three major patterns emerge from these disturbances: First, we show that 10 of the most common herbaceous perennial dicots benefit from pocket gopher disturbance; that is, a greater proportion of seedlings are found in the open space created by pocket gopher disturbance than would be expected based on the availability of disturbed habitat. Additionally, these seedlings exhibited higher growth rates than adjacent seedlings of the same species growing in undisturbed habitat. Second, we tested two predictions of the Intermediate Disturbance Hypothesis and found that species diversity was greatest for plots characterized by disturbances of intermediate age. However, we did not detect significant differences in diversity between plots characterized by intermediate and high levels of disturbance, indicating that many species are adapted to or at least tolerant of high levels of disturbance. Third, we noted that the abundance of grasses decreased with increasing disturbance, while the abundance of dicots increased with increasing disturbance.     

Frequent fire promotes diversity and cover of biological soil crusts in a derived temperate grassland

The intermediate disturbance hypothesis (IDH) predicts that species diversity is maximized at moderate disturbance levels. This model is often applied to grassy ecosystems, where disturbance can be important for maintaining vascular plant composition and diversity. However, effects of disturbance type and frequency on cover and diversity of non-vascular plants comprising biological soil crusts are poorly known, despite their potentially important role in ecosystem function. We established replicated disturbance regimes of different type (fire vs. mowing) and frequency (2, 4, 8 yearly and unburnt) in a high-quality, representative Themeda australis–Poa sieberiana derived grassland in south-eastern Australia. Effects on soil crust bryophytes and lichens (hereafter cryptogams) were measured after 12 years. Consistent with expectations under IDH, cryptogam richness and abundance declined under no disturbance, likely due to competitive exclusion by vascular plants as well as high soil turnover by soil invertebrates beneath thick grass. Disturbance type was also significant, with burning enhancing richness and abundance more than mowing. Contrary to expectations, however, cryptogam richness increased most dramatically under our most frequent and recent (2 year) burning regime, even when changes in abundance were accounted for by rarefaction analysis. Thus, from the perspective of cryptogams, 2-year burning was not an adequately severe disturbance regime to reduce diversity, highlighting the difficulty associated with expression of disturbance gradients in the application of IDH. Indeed, significant correlations with grassland structure suggest that cryptogam abundance and diversity in this relatively mesic (600 mm annual rainfall) grassland is maximised by frequent fires that reduce vegetation and litter cover, providing light, open areas and stable soil surfaces for colonisation. This contrasts with detrimental effects of 2-year burning on native perennial grasses, indicating that this proliferation of cryptogams has potentially high functional significance for situations where vegetation cover is depleted, particularly for reducing soil erosion.     

Resilience of a high‐conservation‐value,semi‐arid grassland on fertile clay soils to burning,mowing and ploughing

In grassland reserves, managed disturbance is often necessary to maintain plant species diversity. We carried out experiments to determine the impact of fire, kangaroo grazing, mowing and disc ploughing on grassland species richness and composition in a nature reserve in semi‐arid eastern Australia. Vegetation response was influenced by winter–spring drought after establishment of the experiments, but moderate rainfall followed in late summer–autumn. Species composition varied greatly between sampling times, and the variability due to rainfall differences between seasons and years was greater than the effects of fire, kangaroo grazing, mowing or disc ploughing. In the fire experiment, species richness and composition recovered more rapidly after spring than autumn burning. Species richness and composition were similar to control sites within 12 months of burning and mowing, suggesting that removal of the dominant grass canopy is unnecessary to enhance plant diversity. Two fires (separated by 3 years) and post‐fire kangaroo grazing had only minor influence on species richness and composition. Even disc ploughing caused only a small reduction in native richness. The minor impact of ploughing was explained by the small areas that were ploughed, the once‐off nature of the treatment, and the high degree of natural movement and cracking in these shrink‐swell soils. Recovery of the composition and richness of these grasslands was rapid because of the high proportion of perennial species that resprout vegetatively after fire and mowing. There appears to be little conservation benefit from fire, mowing or ploughing ungrazed areas, as we could identify no native plant species dependent on frequent disturbance for persistence in this grassland community. However, the ability of the Astrebla‐ and Dichanthium‐dominated grasslands to recover quickly after disturbance, given favourable seasonal conditions, suggests that they are well adapted to natural disturbances (e.g. droughts, fire, flooding and native grazing).     

Manipulating the species composition of permanent grasslands—A new approach to biodiversity experiments

The relationship between biodiversity and ecosystem functions of grasslands has received increasing attention in recent years. So far, experiments were mostly conducted in experimental grasslands. We used a different approach on permanent grassland by applying herbicides selective against either dicots or monocots. This allowed us to alter plant species composition and evenness and to obtain altered constellations of functional group abundances without deliberate introduction of new species or continued disturbance by weeding. The resulting swards were subjected to different management intensities in terms of cutting regime and fertilization. Compared to the baseline data before herbicide application, within one year, the combination of treatments, especially the herbicide application, led to a broad variety of swards with the herbicide treatment alone accounting for more than 25% of the variance in composition. We conclude that the application of specific herbicides is a method highly suitable for creating different sward types, because the swards differed significantly in species number, evenness and composition of functional groups without showing signs of disturbance, as neither the area of open soil nor the proportion of annual colonizer species increased.     

Soil Disturbance as a Grassland Restoration Measure—Effects on Plant Species Composition and Plant Functional Traits

Soil disturbance is recognized as an important driver of biodiversity in dry grasslands, and can therefore be implemented as a restoration measure. However, because community re-assembly following disturbance includes stochastic processes, a focus only on species richness or establishment success of particular species will not inform on how plant communities respond ecologically to disturbance. We therefore evaluated vegetation development following disturbance by quantifying species richness, species composition and functional trait composition. Degraded calcareous sandy grassland was subjected to experimental disturbance treatments (ploughing or rotavation), and the vegetation was surveyed during four subsequent years of succession. Treated plots were compared with control plots representing untreated grassland, as well as nearby plots characterized by plant communities representing the restoration target.Species richness and functional diversity both increased in response to soil disturbance, and rotavation, but not ploughing, had a persistent positive effect on the occurrence of specialist species of calcareous sandy grassland. However, no type of soil disturbance caused the plant species composition to develop towards the target vegetation. The disturbance had an immediate and large impact on the vegetation, but the vegetation developed rapidly back towards the control sites. Plant functional composition analysis indicated that the treatments created habitats different both from control sites and target sites. Community-weighted mean Ellenberg indicator values suggested that the observed plant community response was at least partially due to an increase in nitrogen and water availability following disturbance. This study shows that a mild type of disturbance, such as rotavation, may be most successful in promoting specialist species in calcareous sandy grassland, but that further treatments are needed to reduce nutrient availability. We conclude that a functional trait based analysis provides additional information of the vegetation response and the abiotic conditions created, complementing the information from the species composition.     

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

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

Disturbance persistence in managed grasslands: shifts in aboveground community structure and the weed seed bank

The length of time and form in which disturbances persist in systems depends on the intensity and frequency of disturbance and on the abilities of resident species to recover from such events. In grazed grasslands, trampling by large mammalian herbivores can periodically facilitate weed establishment by exposing patches of bare ground but whether an intense soil disturbance event results in a temporary increase in weed abundance or a persistent weed problem remains unclear. In May 2002, cattle trampling following heavy rain caused severe damage to nine-month old, rotationally grazed, cool-season pastures (Midwest USA). In September 2002, we compared the aboveground composition of paddocks (i.e., fenced pasture sections) that were heavily disturbed to those that received no damage. Relative to undisturbed paddocks, forage species relative cover was 17% lower in disturbed paddocks, and weed species and bare ground relative cover was 61% and 100% higher, respectively. By September 2004, paddock types did not differ in all aboveground community components. However, the abundance and species richness of weed seeds in the soil seed bank averaged respectively 82% and 30% higher in disturbed paddocks between 2003 and 2004. These findings indicate that a spatially extensive, intense soil disturbance event may soon become undetectable in components of aboveground pasture structure but can persist as an augmented weed seed bank. Because of high weed seed bank longevity, disturbances to formerly disturbed pastures would likely result in higher weed recruitment, with more species represented, than in those which lack previous disturbance. Disturbance history may thus be a useful predictor of weed community composition following subsequent disturbance. Based on empirical data supporting this proposition, we recommend that grassland managers explicitly incorporate disturbance history into dynamic management planning and do not rely exclusively on aboveground characters to evaluate the invasion status or colonization potential of an area by undesirable plants. We emphasize that the ecological legacies of past soil disturbance events cannot only influence the contemporary patterns and processes of grasslands, but importantly, affect their compositional trajectories following subsequent perturbation.     

Invasibility and compositional stability in a grassland community: relationships to diversity and extrinsic factors

We present results from an ongoing field study conducted in Kansas grassland to examine correlates of invasibility and community stability along a natural gradient of plant diversity. Invasibility was evaluated by sowing seeds of 34 plant species into 40 experimental plots and then measuring colonization success after two growing seasons. Compositional stability, defined as resistance to change in species relative abundances over two growing seasons and in response to experimental disturbance, was measured in a separate set of 40 plots.
We found that community susceptibility to invasion was greatest in high diversity microsites within this grassland. Multiple regression analyses suggested that the positive correlation between invasibility and plant diversity was due to the direct influences of the extrinsic factors that contribute to spatial variation in diversity (soil disturbances; light availability), not to any direct impact of diversity. In addition, we found that compositional stability in response to disturbance was greatest within low diversity microsites and was strongly related to the dominance (evenness) component of diversity.     

Light distribution in mesic grasslands: Spatial patterns and temporal dynamics

Abstract. Spatial patterns and temporal dynamics of light distribution were investigated using lacunarity analysis, a multi‐scale measure of spatial heterogeneity, in three mesic grasslands with different disturbance regimes. Frequency distributions of relative light intensity (RLI) were similar for the two non‐disturbed grasslands, despite different composition (forbs vs. caespitose grass) resulting from different historical disturbance regimes prior to 1985, and different from the annually disturbed grassland. Spatial heterogeneity of light distribution was greater at all scales in the native, annually disturbed grassland than in the two non‐disturbed grasslands. The disturbance regime affected temporal dynamics of the spatial patterns of light distribution in each grassland. The annually disturbed grassland exhibited a dramatic decrease in lacunarity (heterogeneity) from early to late April, likely the result of considerable growth of a cool‐season grass. A general decrease in lacunarity occurred in the native, non‐disturbed grassland, although the magnitude was much less than in the annually disturbed grassland. The reverted, non‐disturbed grassland did not exhibit an appreciable change in lacunarity until later in the growing season, and then only at smaller scales. Combining the frequency distribution of RLI and the lacunarity curves provided an effective approach to assess relationships between the dynamics of spatial pattern of light distribution and ecological processes as influenced by different disturbance regimes. Integrating lacunarity analysis with more traditional measurements of grassland ecosystems (plant spatial distribution and arrangement and plant species composition and architecture) may be an effective way to assess functional consequences of structural changes in grassland ecosystems.     

Development of microtopography in a semi-arid grassland: Effects of disturbance size and soil texture

Our objective was to evaluate effects of disturbance size and soil texture on the development of microtopography for a shortgrass plant community in north central Colorado USA. Disturbances, defined as the death of individual plants, were created in 1984 and 1985 to evaluate development through time of the small-scale pattern of perennial bunchgrasses and bare soil openings that characterize this semiarid grassland. Disturbed plots of three sizes (50, 100, 150 cm-diameter) comparable in size to naturally-occurring disturbances were produced by killing plants at two sites differing in soil texture (sandy loam, clay loam). Disturbed plots were not manipulated after being created. In 1993, a laser surveying instrument was used to measure heights of crowns of individual plants of the dominant species, the perennial bunchgrass Bouteloua gracilis ([H.B.K.] Lag. ex Griffiths), and bare soil openings between plants for two locations: within each disturbance and in the surrounding undisturbed landscape.Differences between crown heights of plants and bare soil openings were comparable for both the undisturbed landscape and inside disturbances indicating that small-scale microtopography had recovered within nine years after disturbance occurred. However, complete recovery to the undisturbed state had not occurred since crown heights of plants relative to bare soil openings were significantly less on disturbed than undisturbed locations. Differences in height between plant crowns and bare soil openings on disturbed plots increased as disturbance size increased, indicating greater soil redistribution with increasing plot size. Larger differences in height were also found on plots on the fine- than the coarse-textured soil, indicating the importance of soil particle size and plant cover type to the development of microtopography. Differences in height between microsites on disturbed plots were positively related to total plant cover and negatively related to cover of B. gracilis indicating the importance of this species to reducing erosion on disturbed areas.In this semiarid grassland, patterns in microtopography were heterogeneous, likely as a result of the small-scale redistribution of soil between bare soil openings and B. gracilis plants through time. Our results indicate that this redistribution of soil is affected by disturbance size, soil texture, and patchy plant cover. The major effect of small-scale disturbances on patterns in microtopography of the shortgrass steppe are causing plant death and exposing soil to erosional and depositional processes.     

The role of the seed bank,seed rain and the timing of disturbance in gap regeneration

Abstract. Question: Does the degree and timing of disturbance contribute significantly to the pattern and process of regeneration in plant communities as a consequence of the availability and number of species of propagules present? Location: Acid grassland at 230 m a.s.l., eastern Scotland, UK. Methods : Plots were surface disturbed or had their soil profile inverted at monthly intervals at 12 dates during a year. Seed bank and seed rain were assessed at each treatment time. The effect of disturbance intensity and timing on the regenerating vegetation was assessed. Results: Removing the seed bank significantly slowed regeneration, as it contributed 43 % of developing cover after one year where it was present. At an individual seed level, seed in the seed rain had a much higher likelihood of contributing to the regenerating vegetation than a seed in the seed bank. Some species showed a reliance on the seed bank for regeneration, and hence there was a significant difference in the vegetation that developed between plots with the seed bank intact and those where it was removed. Winter disturbed plots (little seed rain) had slower rates of re‐vegetation than summer disturbed plots. Timing had little effect on species composition, though a significantly higher cover of perennial forb species developed on the winter disturbed plots. Conclusion: Removing the contribution of the seed bank had a greater effect on the composition of regenerating vegetation than the effect of seasonal variation on the seed rain.     

Response of floodplain grassland plant communities to altered water regimes

Floodplain grasslands are often composed of a mosaic of plant communities controlled by hydrological regime. This article examines the sensitivity of floodplain grassland plant communities to water regime using reciprocal transplantation of an inundation grassland and a flood-meadow within an English floodplain. Experimental treatments comprised control, transplanted and lifted plots; the last treatment, in order to elucidate any disturbance effects of transplantation. Plant community response was analysed using species abundance and their ecological traits. Results from both communities showed substantial annual variations related to hydrology, including significant species changes, but generally, vegetation seemed to be responding to drier conditions following a major flood event. This ‘drying’ trend was characterised by increased species diversity, a greater abundance of competitive species and fewer typical wetland plants. Transplanted community composition increasingly resembled receptor sites and transplant effects were most pronounced the first year after treatment for both vegetation types. Differential responses to water regime were detected for the two plant communities. The inundation grassland community was particularly dynamic with a composition that rapidly reflected drying conditions following the major flood, but transplantation into a drier flood-meadow site prompted little additional change. The flood-meadow community appeared more resistant to post-inundation drying, but was sensitive to increased wetness caused by transplantation into inundation grassland, which significantly reduced six species while none were significantly favoured. The effects of disturbance caused by lifting the transplants were limited in both communities, although five species showed significant annual fluctuations. The study shows that small alterations in water regime can prompt rapid vegetation changes and significant plant species responses in floodplain grasslands, with effects probably magnified through competitive interactions. The dynamic properties of floodplain vegetation demonstrated by this study suggest that its classification, management and monitoring are challenging and ideally should be based on long-term studies.     

Harvester ant foraging and plant species distribution in annual grassland

Summary The harvester antVeromessor andrei Mayr is a major seed predator on annual grassland growing on serpentine soil at Jasper Ridge, N. California. Ants forage intensively during morning and evening sessions in areas surrounding nests. Activity is at its most intense in early summer, coinciding with peak seed release for most annual plant species. Ants show strong preferences for seeds of non-dominant species, notablyMicroseris douglasii (DC.) Sch.-Bip., but preferences alter over the season in response to seed availability. Seeds of the dominant annual species,Lasthenia californica DC ex Lindley are not foraged until later in the summer when seeds of other species are less abundant.Seedling densities and species compositions on ant nests differ markedly from surrounding areas with species relative abundances being similar to those found on gopher mounds. An exclosure experiment in areas adjacent to nests indicated that ants significantly reduced the densities of species with preferred seeds. Ants may therefore significantly affect plant distribution and abundance within the serpentine grassland.     

Temporal patterns in seedling establishment on pocket gopher disturbances

Disturbances often facilitate seedling establishment, and can change the species composition of a community by increasing recruitment of disturbance-adapted species. To understand the effects of pocket gopher disturbances on alpine seedling dynamics, we examined the gopher disturbances effects on seedling emergence and survival on gopher disturbances 0 to 5 years old. In contrast to results from most other ecosystems, these recently created gopher mounds had lower seedling emergence and survival rates than undisturbed areas. A lack of correlation between species abundances on gopher mounds and undisturbed sites in one of the two communities studied suggested that a suite of disturbance-adapted species recruited onto the mounds. To explain low seedling emergence on recent gopher mounds, we quantified gopher mound seed banks and studied recruitment in a site with mounds that ranged from 0 to >20 years old. Seed numbers in first-year gopher mound soils were extremely low relative to undisturbed soils, and this pattern was mirrored in seedling establishment patterns over the long term. Gopher disturbance depressed seedling emergence density for the first 5 years. Subsequently, emergence density increased until at least 20 years following the disturbance. Emergence on disturbances more than 20 years old was higher than on undisturbed sites. Therefore, gopher disturbances probably facilitate seedling establishment in alpine dry and moist meadow; however, this process takes place over decades.     

Bryophyte Mats Inhibit Germination of Non-native Species in Burnt Temperate Native Grassland Remnants

Species-rich native grasslands in western Victoria, Australia, are often small, have a high perimeter to area ratio and are surrounded by non-native species. Few non-native species, however, have invaded them. A feature of species-rich grasslands is the presence of a bryophyte mat (composed of mosses and liverworts) that carpets the intertussock spaces. I assessed the role of these mats in plant invasions by sowing three non-native species (Briza maxima, Hypochoeris radicata, Plantago lanceolata) in replicated disturbed (mats removed) and undisturbed (mats intact) microsites at three grassland remnants (two recently burnt, one unburnt for 3 years) and followed seedling emergence, survival and growth for 5 months. Three native species were also sown for comparison. The rate of germination and total percent germination of non-native species were significantly enhanced at both burnt sites when the mat was disturbed. The large-seeded Briza maxima failed to germinate at both burnt sites in the absence of soil disturbance. The native species generally did not show a strong germination or growth response to soil disturbance in burnt areas. At the unburnt site, where monthly percent soil moisture was highest, final percent germination of the non-native and native species was greatest of any site in both microsites, and germination was not significantly affected by soil disturbance. Differences in the seed morphology of native and non-native species may play an important role in their ability to establish on bryophyte mats in moisture-limiting environments. Any activity that disrupts the mats in the frequently burnt, species-rich grassland remnants is likely to significantly enhance the germination and subsequent growth by non-natives. However, where burning is infrequent, germination of some non-native species may be expected, regardless of disturbance, although growth will likely be favoured in disturbed areas.     

青藏高原多年冻土区不同草地生态系统恢复能力评价

草地生态系统恢复能力是评价人类工程活动对青藏高原多年冻土生态系统影响的重要组分.分析了不同草地生态系统干扰带和非干扰带群落特征、植物多样性、草地初级生产力和经济类群,综合评价了青藏高原多年冻土区地上植被在受工程活动干扰后的综合恢复能力.结果表明:经过近20多年的自然恢复,青藏苔草草原、紫花针茅草原、扇穗茅草原、高山嵩草草甸、矮蒿草草甸和藏蒿草沼泽化草甸6种草地的盖度和物种组成均有一定程度的恢复,且草原群落的恢复程度好于草甸群落,但干扰群落仍低于未干扰群落;紫花针茅草原分布区物种多样性恢复好于其他草地类型分布区;干扰带由最初的地上植物生物量全部为0恢复到148.8～489.6 g·m^-2,其中藏嵩草沼泽化草甸干扰带恢复最好,生物量达489.6 g·m^-2;除藏嵩草恢复群落的饲用植物类群组成相对稳定外,干扰后的其他5种草地类型饲用价值降低.高寒草原生态系统的植被综合恢复能力显著高于草甸生态系统.