Manipulating disturbance regimes and seeding to restore mesic Mediterranean grasslands

Question: Can managing disturbance regimes alone or in combination with seeding native species serve to shift the balance from exotic towards native species? Location: Central coast of California, USA. Methods: We measured vegetation composition for 10 yr in a manipulative experiment replicated at three sites. Treatments included no disturbance, grazing and clipping at three frequencies with and without litter removal. We seeded eight native species into clipped plots and compared cover in comparable plots with no seeding. Results: Regardless of frequency, clipping generally shifted community dominance from exotic annual grasses to exotic annual forbs, rather than consistently favoring native species. At one site, perennial grass cover decreased in no‐disturbance plots, but only after 4 yr. Litter removal had minimal impact on litter depth and plant community composition. Grazing had a highly variable effect on the abundance of different plant guilds across sites and years. Seeding increased abundance of only two of eight native species. Conclusions: Managing disturbance regimes alone is insufficient to restore native species guilds in highly‐invaded grasslands and seeding native species has highly variable success.     

Stronger effect of gastropods than rodents on seedling establishment,irrespective of exotic or native plant species origin

Experimental evidence about how generalist consumers affect exotic plant invasions is equivocal, but most tests have been limited to few plant species, single herbivore guilds, and single locations. Using a seed‐addition experiment, we studied effects of gastropods and rodents on recruitment success of 37 exotic and 37 native plant species affiliated to three different functional groups (i.e. grasses, legumes and non‐legume herbs). We replicated our seed addition x herbivore exclusion experiment at multiple grassland sites, located within a few km of each other in two regions, coastal central California (USA) and southern Saxony–Anhalt (Germany). The two study regions differed in climate, land‐use, invasion history and species pools which allowed us to disentangle general from context‐specific effects. In both regions, herbivory by gastropods had a stronger impact on the proportion of recruited seedlings and the proportion of recruited species than rodent herbivory, but this effect was much more pronounced in California than in Germany. Especially, seedling recruitment of non‐legume herbs and legumes suffered from gastropod herbivory. Contrastingly, the effect of rodents was negative at the German sites and positive at the Californian sites, likely driven by context‐specific differences in the rodent assemblages. Across both study regions, exotics had higher seedling recruitment than natives, indicating that higher recruitment success constitutes an inherent feature of exotic species. After two years, more exotic than native species established at grassland sites in California while the opposite was true for the German grassland sites. Consistently across regions, native and exotic species did, however, not differ in their response to herbivory, suggesting that generalist consumers suppress recruitment and colonization of plant species irrespective of their origin. Our results demonstrate the importance of a multi‐species, multi‐site approach to separate general responses of exotic and native plants to generalist herbivory from local, regional or species‐specific peculiarities.     

A conceptual model of plant community changes following cessation of cultivation in semi‐arid grassland

Question: Can vegetation changes that occur following cessation of cultivation for cereal crop production in semi‐arid native grasslands be described using a conceptual model that explains plant community dynamics following disturbance? Location: Eighteen native grasslands with varying time‐since‐last cultivation across northern Victoria, Australia. Methods: We examined recovery of native grasslands after cessation of cultivation along a space for‐ time chronosequence. By documenting floristic composition and soil properties of grasslands with known cultivation histories, we established a conceptual model of the vegetation states that occur following cessation of cultivation and inferred transition pathways for community recovery. Results: Succession from an exotic‐dominated grassland to native grassland followed a linear trajectory. These changes represent an increase in richness and cover of native forbs, a decrease in cover of exotic annual species and little change in native perennial graminoids and exotic perennial forbs. Using a state‐and‐transition model, two distinct vegetation states were evident: (1) an unstable, recently cultivated state, dominated by exotic annuals, and (2) a more diverse, stable state. The last‐mentioned state can be divided into two further states based on species composition: (1) a never‐cultivated state dominated by native perennial shrubs and grasses, and (2) a long‐uncultivated state dominated by a small number of native perennial and native and exotic annual species that is best described as a subset of the never‐cultivated state. Transitions between these states are hypothesized to be dependent upon landscape context, seed availability and soil recovery. Conclusions: Legacies of past land use on soils and vegetation of semi‐arid grasslands are not as persistent as in other Australian communities. Recovery appears to follow a linear, directional model of post‐disturbance regeneration which may be advanced by overcoming dispersal barriers hypothesised to restrict recovery.     

Effects of the litter layer of Pteridium aquilinum on seed banks under experimental restoration

Questions: Does the litter layer of Pteridium aquilinum (bracken) act as a barrier to certain species in the seed bank? Does bracken control/restoration treatment affect seed transfer through the litter layer? Location: Five experiments at three sites across the UK covering two major vegetation types; acid‐grassland and heath‐land. Methods: At each experiment a range of bracken control and vegetation restoration treatments were applied for about ten years. The seed bank was sampled in both the bracken litter and the soil. The cover (%) of each species in the vegetation and the bracken litter abundance (cover and depth) was also estimated. Results: The bracken litter layer acts as an inert barrier as it contained a large proportion of seeds available in the litter‐soil profile (38%– 67% of the total). Bracken litter depth and cover also influenced significantly the seed bank composition in both the bracken litter and the soil. These effects were site‐specific, and species‐specific. The application of treatments changed significantly the balance between seed inputs and outputs in the bracken litter layer for some species. This was either a positive or negative response relative to the untreated control plots. Conclusion: For heathland and acid‐grassland restoration, the bracken litter layer may be an important seed source, but it must be disturbed particularly before seed addition.     

Fire and Litter Effects on Seedling Establishment in Western Oregon Upland Prairies

Prescribed burning is an important tool for managing and restoring prairies and other ecosystems. One effect of fire is plant litter removal, which can influence seedling establishment. Four experimental treatments (burned, clipped and raked to remove litter, burned with litter reapplied, and unmanipulated) were applied to 2 × 2.5–m plots in three western Oregon, United States, upland prairies to determine how burning affects seedling establishment. Seeds of common exotic and native prairie species were sowed into the experimental plots after treatments. Seedlings were censused the following spring. The experiment was repeated on each of the three sites, representing three common types of prairie vegetation: an Annual Exotic Grass site, a Perennial Exotic Grass site, and a Native Bunchgrass site. In both the Annual Exotic Grass and the Perennial Exotic Grass sites, burning significantly improved native, but not exotic, seedling establishment over those on unburned plots. Litter removal was a significant component of this burn effect, particularly on the Perennial Exotic Grass site. In these winter‐moist systems, the net effect of litter is to inhibit seedling establishment. Burning treatments on the Native Bunchgrass site significantly increased seedling establishment only of short‐lived exotic species. These results suggest that in prairie ecosystems similar to the Annual and Perennial Exotic Grass sites, prescribed burning followed by sowing native seeds can be an effective restoration technique. Burning alone or sowing alone would be counter‐productive, in the first case because increased establishment would come from exotic species and in the second case because establishment rates are low in unburned plots.     

Differentiation between native and exotic plant species in a dry grassland: realized responses to perturbation,and comparison with fundamental responses

Abstract An area of dry grassland in New Zealand, comprising an equal mixture of native and exotic species, was subject to perturbations of irrigation, fertilization and cessation of grazing. The vegetation response was recorded for 3 years. Total cover, and the contribution of native species to that cover, fluctuated between years even in the control plots. Irrigation increased total cover, but decreased the cover of native species. Fertilization produced the same effects, only less strongly, and also reduced species richness, the loss being in native species. In spite of overall effects of treatments on native and exotic cover, when individual species’ responses to irrigation, fertilization or exclosure were calculated, there was no significant difference between the native and exotic plant guilds. Species differed in their responses, but the native and exotic guilds overlapped. When grouped by morphology, the only significant difference between the responses to perturbation was that forbs and graminoids responded more positively to irrigation than woody and cryptogamic species. The realized responses of the species to the perturbations described here showed little correlation with their physiological responses as determined in previous greenhouse experiments. It is suggested that the realized responses are strongly, and currently unpredictably, influenced by competition from the other species present. Soil nutrients and soil water were both important controls on the community. The relative similarity in the nature of the response to these two factors – nutrients and water – suggests that they affect species in similar ways, possibly because the greater growth rate of the exotic species mediates the short‐term response to both. Grazing has less effect on the current community than either nutrients or water, although it may have been historically important in shaping the species pool. From the poor predictability of field responses from morphological guilds or from ecophysiological responses, it is suggested that the ‘functional types’ approach, although conceptually attractive, lacks experimental support in these grasslands. It is concluded that the exotic species have invaded by being pre‐adapted to the environment with the same environmental responses as the natives, but with the advantage of generally higher growth rates.     

Biogeochemical and Microbial Legacies of Non‐Native Grasses Can Affect Restoration Success

The restoration of disturbed ecosystems is challenging and often unsuccessful, particularly when non‐native plants are abundant. Ecosystem restoration may be hindered by the effects of non‐native plants on soil biogeochemical characteristics and microbial communities that persist even after plants are removed. To examine the importance of soil legacy effects, we used experimental restorations of Florida shrubland habitat that had been degraded by the introduction of non‐native grasses coupled with either mechanical disturbance or pasture conversion. We removed non‐native grasses and inoculated soils with native microbial communities at each degraded site, then examined how habitat structure, soil nitrogen, soil microbial abundances, and native seed germination responded over two years compared to undisturbed native sites. Grass removal treatments effectively restored some aspects of native habitat structure, including decreased exotic grass cover, increased bare ground, and reduced litter cover. Soil fungal abundance was also somewhat restored by grass removals, but soil algal abundance was unaffected. In addition, grass removal and microbial inoculation improved seed germination rates in degraded sites, but these remained quite low compared to native sites. High soil nitrogen persisted throughout the experiment regardless of treatment. Many treatment effects were site‐specific, however, with legacies in the more degraded vegetation type tending to be more difficult to overcome. These results support the need for context‐dependent restoration approaches and suggest that the degree of soil legacy effects may be a good indicator of restoration potential.     

Rapid biodiversity declines in both ungrazed and intensely grazed exotic grasslands

Exotic-dominated ecosystems with low diversity are becoming increasingly common. It remains unclear, though, whether differences between native and exotic species (driver model), or changes in disturbances or resources (passenger model), allow exotics to become competitive dominants. In our field experiment, plant species origin (native or exotic), cattle grazing (ungrazed or intensely grazed once), and species composition treatments were fully crossed and randomly assigned to four-species mixtures and monocultures of grassland plants. We found that biodiversity declined more rapidly in exotic than in native species mixtures, regardless of our grazing disturbance treatment. Early declines in species evenness (i.e., increases in dominance) led to subsequent declines in species richness (i.e., local extinctions) in exotic mixtures. Specifically, Simpson’s diversity was 29% lower after 1 year, and species richness was 15% lower after 3 years, in exotic than in native mixtures. These rapid biodiversity declines in exotic mixtures were partly explained by decreased complementarity (i.e., niche partitioning and facilitation), presumably because exotic species lack the coevolutionary history that can lead to complementarity and coexistence in native communities. Thus, our results suggest that exotic species can drive biodiversity declines in the presence or absence of a grazing disturbance, partly because exotic species interactions differ from native species interactions. This implies that restoring plant biodiversity in grasslands may require removal of exotic species, in addition to disturbance management.     

Promotion of exotic weed establishment by endangered giant kangaroo rats (Dipodomys ingens) in a California grassland

Giant kangaroo rats (Dipodomys ingens) continually modify their burrow precincts by digging tunnels, clipping plants, and other activities. In the valley grasslands of the Carrizo Plain Natural Area (San Luis Obispo County, California), this chronic disturbance to soil and vegetation promoted the establishment of exotic ruderal and early successional plant species. Erodium cicutarium, Bromus madritensis ssp. rubens, and other Mediterranean annuals were found to constitute a very large proportion of the vegetation on giant kangaroo rat precincts. When vegetation on precincts was compared with the vegetation in less disturbed intermediate areas located between precincts, species richness, cover and frequency of exotic plants were significantly greater on precincts. The reverse was found for native species. In addition, exotic species encountered in this study had significantly larger seeds than did native species, suggesting that these granivorous kangaroo rats preferentially cache large weed seeds on their precincts. Since the kangaroo rats depend on exotic plants for food and the exotic plants depend upon the kangaroo rats to disturb their habitat continually, the weed-kangaroo rat relationship is mutualistic. This strong relationship may also inhibit population growth of native grassland plants which occupy disturbed habitats but have difficulty competing with exotic weeds for resources. From a conservation perspective, this mutualism presents an intractable management dilemma. Restoration of valley grasslands where endangered giant kangaroo rats occur, to conditions where native species dominate, may be impossible.     

Fine‐scale variables associated with the presence of native forbs in natural temperate grassland

Broad‐scale threats to floristic diversity in native temperate grasslands are well‐documented and include elevated soil nutrients, changes in disturbance regimes and exotic species. However, fine‐scale variables associated with the presence of native forbs, such as gap size and biomass cover, have received relatively little attention. We conducted a case–control study to determine the relative influence of physical structural dimensions and other fine‐scale variables associated with the presence of native forbs in a modified temperate grassland previously used for domestic grazing. We matched 145 case plots centred on 27 different species of native forbs with 290 control plots not centred on a native forb. For each percentage increase in ground litter cover, dead biomass cover, grass cover or exotic forb cover, or the area of bare ground within 30 cm, the relative odds that a native forb was present vs absent declined by a mean of 10–13%. Living and dead biomass reduces light availability, and the former can also reduce nutrient and water availability. Declines in the presence of native forbs associated with increasing total bare ground may suggest that gap sizes were too small or the soil surface condition too degraded. Our results add to a body of evidence suggesting that native forbs in temperate native grassland are likely to benefit from periodic removal of living and dead grass biomass and a reduction in the cover of exotic forbs.     

Ecosystem response to removal of exotic riparian shrubs and a transition to upland vegetation

Understanding plant community change over time is essential for managing important ecosystems such as riparian areas. This study analyzed historic vegetation using soil seed banks and the effects of riparian shrub removal treatments and channel incision on ecosystem and plant community dynamics in Canyon de Chelly National Monument, Arizona. We focused on how seeds, nutrients, and ground water influence the floristic composition of post-treatment vegetation and addressed three questions: (1) How does pre-treatment soil seed bank composition reflect post-treatment vegetation composition? (2) How does shrub removal affect post-treatment riparian vegetation composition, seed rain inputs, and ground water dynamics? and (3) Is available soil nitrogen increased near dead Russian olive plants following removal and does this influence post-treatment vegetation? We analyzed seed bank composition across the study area, analyzed differences in vegetation, ground water levels, and seed rain between control, cut-stump and whole-plant removal areas, and compared soil nitrogen and vegetation near removed Russian olive to areas lacking Russian olive. The soil seed bank contained more riparian plants, more native and fewer exotic plants than the extant vegetation. Both shrub removal methods decreased exotic plant cover, decreased tamarisk and Russian olive seed inputs, and increased native plant cover after 2 years. Neither method increased ground water levels. Soil near dead Russian olive trees indicated a short-term increase in soil nitrogen following plant removal but did not influence vegetation composition compared to areas without Russian olive. Following tamarisk and Russian olive removal, our study sites were colonized by upland plant species. Many western North American rivers have tamarisk and Russian olive on floodplains abandoned by channel incision, river regulation or both. Our results are widely applicable to sites where drying has occurred and vegetation establishment following shrub removal is likely to be by upland species.     

Species‐specific disturbance tolerance,competition and positive interactions along an anthropogenic disturbance gradient

Question: As it has been found that stress promotes positive interactions mediated by physical amelioration of the environment, is it possible that interactions may turn positive with increasing chronic anthropogenic disturbance (CAD) intensity? Also, is it possible that species that do not tolerate disturbance may require environmental amelioration by their neighbours in disturbed areas, whereas tolerant species may not? Location: The semi‐arid grassland in Concepción Buenavista, Oaxaca, southern Mexico. Methods: We assessed interaction intensity and importance through a neighbour removal experiment along a CAD gradient for three species differing in disturbance tolerance. Water potential was monitored on vegetated and bare soil. Results: A shift from competitive effects in low CAD sites to positive interactions in degraded sites was found. The disturbance‐tolerant species did not respond to CAD, whereas the less tolerant species changed its interactions drastically in terms of growth and reproduction. The species with medium tolerance had an intermediate response. Neighbours promoted germination in all species. Vegetation removal reduced soil humidity. Conclusions: Positive interactions seemingly resulted from the amelioration of the abiotic stresses induced by vegetation removal. The dependence on neighbours to germinate, grow, or reproduce suggests that if CAD eliminates the plant cover, vegetation will hardly recover. Irreversible changes are known to occur in communities where positive interactions predominate, but CAD may set the conditions for irreversible shifts even in communities where interactions are normally competitive.     

Reciprocal effects of litter from exotic and congeneric native plant species via soil nutrients

Invasive exotic plant species are often expected to benefit exclusively from legacy effects of their litter inputs on soil processes and nutrient availability. However, there are relatively few experimental tests determining how litter of exotic plants affects their own growth conditions compared to congeneric native plant species. Here, we test how the legacy of litter from three exotic plant species affects their own performance in comparison to their congeneric natives that co-occur in the invaded habitat. We also analyzed litter effects on soil processes. In all three comparisons, soil with litter from exotic plant species had the highest respiration rates. In two out of the three exotic-native species comparisons, soil with litter from exotic plant species had higher inorganic nitrogen concentrations than their native congener, which was likely due to higher initial litter quality of the exotics. When litter from an exotic plant species had a positive effect on itself, it also had a positive effect on its native congener. We conclude that exotic plant species develop a legacy effect in soil from the invaded range through their litter inputs. This litter legacy effect results in altered soil processes that can promote both the exotic plant species and their native congener.     

Spatial relationships between plant litter,gopher disturbance and vegetation at different stages of old‐field succession

Abstract. Fine‐scale spatial patterns of small mammal disturbances and local accumulation of plant litter were studied together with the spatial pattern of vegetation in different stages of old‐field succession at Cedar Creek Natural History Area, Minnesota, USA. Seven stands from one to 66 years old were sampled. Presence of living plant species, local soil disturbances by pocket gophers (Geomys bursarius) and plant litter accumulation were recorded in 10 cm × 10 cm contiguous microquadrats along elliptical transects. Spatial patterns and associations were analyzed using information theory models. Dominant grasses were spatially independent, while subordinate functional groups were strongly dependent on the existing patchwork of dominant species, plant litter and gopher disturbances. Litter had consistent negative associations with subordinate functional groups in all but the initial years. Gopher disturbances were abundant but had weak and variable associations with vegetation. These results suggest that gopher disturbance does not facilitate the colonization of native prairie species and that diversity can be improved by controlling litter accumulation in Minnesota old‐fields.     

Effects of fertilisation and irrigation on ‘foliar afterlife’ in alpine tundra

Question: How do increases in soil nutrient and water availability alter the nutrient fluxes through the resorption and litter decomposition pathways and how do they affect litter nutrient pools in a low‐productive alpine tundra ecosystem? Location: An alpine lichen‐rich tundra on Mt. Malaya Khati‐para in the NW Caucasus, Russia (43°27’ N, 41°42’ E; altitude 2800 m a.s.l.). Methods: We conducted a 4‐year fertilisation (N, P, N+P, lime) and irrigation experiment, and analysed the responses of nutrient resorption from senescing leaves, leaf litter quality and decomposability of six pre‐dominant vascular plant species, total plant community litter production and litter (nutrient) accumulation. Results: Vascular plant litter [N] and [P] increased 1.5 and 10 fold in response to N and P additions, due to increased concentrations of the nutrients in fresh leaves and unchanged or reduced resorption efficiency. Litter decomposability was not affected by nutrient amendments. Fertilisation enhanced litter production (180%; N+P treatment) and litter accumulation (80%; N+P), owing to tremendously increased production and low decomposability of graminoids. Together with increased litter [N] and [P] this led to great increases in total litter nutrient pools. Conclusions: Due to increased production of graminoids, nutrients added to the alpine tundra soil were mostly immobilised in recalcitrant, nutrient‐rich litter. This suggests that changing species composition in low productive ecosystems may act as an internal buffer mechanism, which under increased soil nutrient availability prevents the community from rapidly acquiring features typical of a high productive ecosystem such as high decomposability and high nutrient availability.     

Exotic plant species in a C4-dominated grassland: invasibility, disturbance, and community structure

We used data from a 15-year experiment in a C₄-dominated grassland to address the effects of community structure (i.e., plant species richness, dominance) and disturbance on invasibility, as measured by abundance and richness of exotic species. Our specific objectives were to assess the temporal and spatial patterns of exotic plant species in a native grassland in Kansas (USA) and to determine the factors that control exotic species abundance and richness (i.e., invasibility). Exotic species (90% C₃ plants) comprised approximately 10% of the flora, and their turnover was relatively high (30%) over the 15-year period. We found that disturbances significantly affected the abundance and richness of exotic species. In particular, long-term annually burned watersheds had lower cover of exotic species than unburned watersheds, and fire reduced exotic species richness by 80–90%. Exotic and native species richness were positively correlated across sites subjected to different fire (r = 0.72) and grazing (r = 0.67) treatments, and the number of exotic species was lowest on sites with the highest productivity of C₄ grasses (i.e., high dominance). These results provide strong evidence for the role of community structure, as affected by disturbance, in determining invasibility of this grassland. Moreover, a significant positive relationship between exotic and native species richness was observed within a disturbance regime (annually burned sites, r = 0.51; unburned sites, r = 0.59). Thus, invasibility of this C₄-dominated grassland can also be directly related to community structure independent of disturbance. Received: 9 February 1999 / Accepted: 12 May 1999     

Responses of riparian plants to accumulation of silt and plant litter: the importance of plant traits

Abstract. A 2‐yr field experiment was used to determine the response of riparian plants to accumulation of litter or silt in a river flood‐plain meadow in northern Sweden. Such disturbances occur regularly in free‐flowing rivers but are likely to change as a result of global changes in land use or climate. We anticipated that plants with different traits would differ in their response to litter and silt accumulation. We quantified plant response as relative change in above‐ground biomass, and regressed it on either litter mass or silt depth, and on plant traits such as lateral spread, plant height, relative growth rate, seed mass and seed persistence in soil. The relative changes in riparian plant biomass following litter or silt accumulation were negatively related to litter mass and silt depth, and positively related to most examined plant traits such as seed mass, seed persistence and lateral spread. The vegetation recovery in the second season was largely determined by plant traits; litter or silt accumulation had no significant effect. Litter accumulation selected for large‐seeded species, but silt accumulation selected for species with strong ability of lateral spread. Seed persistence was a useful variable in predicting species recovery from both litter and silt accumulation. Plant height was negatively related to plant recovery, but relative growth rate was not significantly related to relative change in plant biomass after silt or litter accumulation. Our results imply that plant traits are important variables to consider for predicting the responses of riparian vegetation to deposition of organic and inorganic matter.     

Differentiation between native and exotic plant species from a dry grassland: fundamental responses to resource availability,and growth rates

Abstract Species from a mixed native/exotic dry grassland community in New Zealand were experimentally assessed for three ecophysiological parameters: nutrient response, water response and maximum relative growth rate (RGR_max). These are parameters that relate to factors proposed as important in structuring plant communities in dry environments. Native and exotic species did not differ consistently in water response. Exotic species tended to have a greater response to nutrients, but there was considerable overlap between native and exotic guilds. However, exotic species did have a higher intrinsic growth rate, and this effect was not attributable to differences in life histories. The results suggest that the exotic species are more competitive and more generalist than the native species. These traits are compatible with the concept of the ‘ideal invader’, and suggest the C‐R strategy of Grime’s theory. The native species showed characteristics consistent with stress tolerance (sensu Grime). The paucity of evidence for ecophysiological differentiation between the native and exotic guilds, except in intrinsic growth rate, indicates that the exotic species were able to invade not because they had superior adaptation to the physical environment, but because they possessed, by pre‐adaptation, the same ways of coping with that environment as the existing species. However, their ability to invade can be related to their growth rates.     

Recovery of Foredune and Blowout Habitats in a Freshwater Dune Following Removal of Invasive Austrian Pine (Pinus nigra)

Invasive species removal is an important first step toward restoring invaded ecosystems; however, restoration following removal may be hindered by (1) unintended consequences of management, such as habitat destabilization, and/or (2) legacy effects of the invader, such as persistent alterations of soil structure or plant community composition. During 1956–1972, approximately 26,000 individuals of the non‐native pine, Pinus nigra, were planted into multiple freshwater sand dune habitats as a stabilization measure on the eastern shore of Lake Michigan in Allegan County, MI, U.S.A. From 2004 to 2010, we evaluated the recovery of foredune and blowout habitats following P. nigra removal in 2003–2005. We compared sand movement and plant community structure, composition, and richness between removal and control sites over the 6 years following pine removal. In addition, we evaluated the impact of litter removal on recolonization of native graminoids in foredunes. Sand movement patterns never differed between removal and control sites in foredunes; however, accumulation was more common in removal sites in blowouts 1 and 6 years following pine removal. Vegetation cover in removal sites became indistinguishable from control sites in both foredunes and blowouts, but species richness for both forb and woody species was higher in removal sites in blowouts. Removal sites in both foredunes and blowouts had higher cover by forbs and lower cover by graminoids. Pine litter did not inhibit recolonization of foredunes by native graminoids. These results suggest that high disturbance habitats, such as sand dunes, have the potential to recover from invasion if the mechanism of disturbance is restored and pioneer species are present to recolonize the system.     

Effects of exotic annual grass litter and local environmental gradients on annual plant community structure

Exotic annual grasses have been introduced into many semi-arid ecosystems worldwide, often to the detriment of native plant communities. The accumulation of litter from these grasses (i.e. residual dry biomass) has been demonstrated to negatively impact native plant communities and promote positive feedbacks to exotic grass persistence. More targeted experiments are needed, however, to determine the relative impact of exotic grass litter on plant community structure across local environmental gradients. We experimentally added exotic grass litter to annual forb-dominated open woodland communities positioned along natural canopy cover gradients in southwest Western Australia. These communities are an important component of this region’s plant biodiversity hotspot and are documented to be under threat from exotic annual grasses. After a one-year treatment period, we measured the effects of exotic grass litter, soil properties, and canopy cover on native and exotic species richness and abundance, as well as common species’ biomass and abundances. Plant community structure was more strongly influenced by soil properties and canopy cover than by grass litter. Total plant abundances per plot, however, were significantly lower in litter addition plots than control plots, a trend driven by native species. Exotic grass litter was also associated with lower abundances of one very common native species: Waitzia acuminata. Our results suggest that exotic grass litter limits the establishment of some native species in this system. Over multiple years, these subtle impacts may contribute substantially to the successful advancement of exotic species into this system, particularly in certain microenvironments.