Spatial and temporal variability in propagule limitation of California native grasses

Community composition and diversity arise from limitations in propagule supply (i.e. propagule or seed limitation) and propagule establishment after arriving at a site (i.e. establishment or microsite limitation). Recent meta‐analyses suggest that the degree of propagule limitation depends on local abiotic and biotic conditions, which in turn are likely to vary spatially and temporally. Nevertheless, seed addition studies testing propagule limitation are rarely replicated in multiple locations and years and often lack experimental manipulations of critical determinants of propagule limitation, such as the density and species richness of the propagule pool and disturbance and resource supply of recipient site. The invasion of California (USA) grasslands by exotic annual species from the Mediterranean region is unique in its scope (over 9 million ha) and persistence (more than 150 years). This invasion provides an exciting context in which to test the role of spatial and temporal variability in mediating propagule limitation and ultimately the potential for restoration. Here I present the results of native‐grass, seed‐addition trials conducted along a 500 km gradient in California in three consecutive years spanning a wide range of environmental conditions and initial conditions (e.g. rainfall, seeding density and disturbance). While native grasses were able to establish at many locations, per capita seed survival was low suggesting that the fate of these species is governed by interplay between propagule and establishment limitation. Establishment primarily varied spatially, with the most successful establishment occurring at fertile locations with low resident species richness. While recruitment was highly variable among years initially, there was no difference among seedling trials after three years.     

Mechanisms of resistance to invasion in a California grassland: the roles of competitor identity, resource availability, and environmental gradients

Resistance to the invasion of exotic plants may sometimes result from the strong effects of a relatively small number of resident species. Understanding the mechanisms by which such species resist invasion could provide important insights for the management of invaded ecosystems. Furthermore, the individualistic responses of community members to resource availability and environmental gradients could drive spatial variation in resistance at the local to landscape scales. We tested the resistance of monoculture plots of three native perennial grasses from the California coastal prairie to the invasion of the European perennial grass Holcus lanatus. We also used a watering treatment that increased early summer water availability and a natural elevational gradient in resource availability and soil texture to evaluate how resident identity interacted with abiotic resistance to affect Holcus establishment. Two native species, Festuca rubra and Calamagrostis nutkaensis , exhibited strong resistance, correlated with their negative effects on light availability. A third native grass, Bromus carinatus var. maritimus , had either no effect or a weakly facultative effect on Holcus performance relative to bare plots. Water addition did not alter the resistance of these species, but the elevation gradient did. Holcus invasion increased with improving abiotic conditions towards the slope bottom in bare and Bromus plots, but invasion decreased towards the bottom in Calamagrostis plots, where better conditions favored competitive residents. These results support the idea that resistance to invasion can sometimes be due to a subset of native species, and that the resistance provided by even a single species is likely to vary across the landscape. Identifying the mechanisms by which species resist invasion could facilitate the selection of management strategies that at best increase, or at worst do not decrease, natural resistance.     

Antagonistic effects of large- and small-scale disturbances on exotic tree invasion in a native tussock grassland relict

It is generally accepted that disturbances increase community invasibility. Yet the role of disturbance in plant invasions may be less predictable than often assumed, due to the influence of environmental stochasticity and interactions between disturbance regimes. We evaluated the single and interactive effects of prescribed burning (large-scale, infrequent event) and animal diggings (small-scale, frequent events) on the invasion success of Gleditsia triacanthos L. in a tussock grassland relict of the Inland Pampa, Argentina. Tree seedling emergence and survival were monitored over 4 years, after adjusting for propagule pressure through copious seed addition to all disturbance treatments. Burning altered community structure by suppressing tussock grasses and promoting exotic forbs, whereas simulated, armadillo-like diggings had little impact on herbaceous composition. Overall, seedling emergence rather than survival represented the main demographic bottleneck for tree invasion. Tree establishment success varied among seedling cohorts emerged in different climatic years. In a dry year, emergence was only slightly affected by disturbances. In contrast, for two consecutive wet years, initial burning and armadillo-like diggings exerted strong, antagonistic effects on tree recruitment. Whereas fire alone increased recruitment, the simulated burrowing regime prevented seedling emergence in both burned and unburned plots. The latter effect might be explained by reduced soil moisture, and increased seed burial and predation in excavated patches. Thus, the impact of a single, large-scale perturbation promoting woody plant invasion was overridden by a regime of small-scale, frequent disturbances. Our results show that grassland invasibility was contingent on inter-annual climatic variation as well as unexpected interactions between natural and anthropogenic disturbance agents.     

Carbon,fire and seed addition favour native over exotic species in a grassy woodland

Cumberland Plain grassy woodland in western Sydney has been reduced to less than 12% of its pre‐settlement distribution; efforts to restore it on cleared and grazed sites within its former distribution have met with mixed success. Elevated soil nitrate levels, coupled with propagule and establishment limitation, have been identified as barriers to restoration in other grasslands. Our study used a factorial combination of carbon addition, fire and native seed addition to test whether these barriers operated on a former Cumberland plain woodland site dominated by exotic perennial grasses. Replicate field plots were established in November 2004; fire plots were burnt in December 2004; carbon was then added as sugar every 3 months until September 2005; and seeds of five native grasses were added in January 2005. Carbon addition significantly reduced soil nitrate, the effect appearing in October 2005. Carbon addition and fire each reduced the total abundance of exotics; when combined, they halved the abundance of the two dominant exotic grasses. Total abundance of native species responded positively to carbon and seed addition, but significant responses to carbon were not detected for individual species. Abundance of two native grasses responded positively to fire; after treatment the native proportion of total abundance rose from 26% on controls to 44–65% on carbon and/or fire plots. Exotic species richness was decreased independently by carbon addition and fire. Native species richness was increased independently by fire and seed addition. All five native grasses established sporadically, but only on carbon and/or fire plots. The three treatments each significantly and independently affected species composition, which showed the greatest change when all three were applied. The three treatments collectively increased the proportion of natives in measures of both plant abundance and species richness. The study confirmed that elevated soil nitrate, plus propagule and recruitment limitation are barriers to restoring this grassy woodland on cleared and grazed sites.     

Native Consumers and Seed Limitation Constrain the Restoration of a Native Perennial Grass in Exotic Habitats

Native consumers and seed limitation may be particularly important in the restoration of native plants where they have been displaced by exotic plants. We used experimental exclosures and seed additions to examine the role of native mammalian consumers and seeding density (500 or 1,000 seeds/m²) in affecting the establishment of a native perennial grass, Purple needlegrass ( Nassella pulchra ), in the grasslands of California. To focus solely on consumers and propagule density, experimental areas were tilled and weeded. Consumers were important determinants of restoration success: averaged across propagule density, consumers reduced N. pulchra seedling recruitment by nearly 30%, reduced seedling height by 44%, reduced plant establishment by 52%, and reduced reproductive tiller length by 43%. Small rodents affected seedling establishment, especially where seeding density was high but did not affect seedling height. Plots accessible by squirrels and rabbits exhibited significantly decreased seedling height and plant establishment, whereas there was no additional impact of allowing large consumers (i.e., deer) access. Despite strong, spatially variable effects of consumers, doubling seed density led to nearly doubled N. pulchra establishment on average. Consumer effects were persistent, shaping N. pulchra abundance in the subsequent growing season and remaining evident over 18 months after the experiment was initiated. Our work suggests that, despite strong consumer effects, seed addition may be a viable strategy for restoration of N. pulchra in invaded areas where it has been displaced by exotic plants, especially when combined with restoration strategies that reduce competition with exotic plants.     

Effects of resource availability and propagule supply on native species recruitment in sagebrush ecosystems invaded by Bromus tectorum

Resource availability and propagule supply are major factors influencing establishment and persistence of both native and invasive species. Increased soil nitrogen (N) availability and high propagule inputs contribute to the ability of annual invasive grasses to dominate disturbed ecosystems. Nitrogen reduction through carbon (C) additions can potentially immobilize soil N and reduce the competitiveness of annual invasive grasses. Native perennial species are more tolerant of resource limiting conditions and may benefit if N reduction decreases the competitive advantage of annual invaders and if sufficient propagules are available for their establishment. Bromus tectorum, an exotic annual grass in the sagebrush steppe of western North America, is rapidly displacing native plant species and causing widespread changes in ecosystem processes. We tested whether nitrogen reduction would negatively affect B. tectorum while creating an opportunity for establishment of native perennial species. A C source, sucrose, was added to the soil, and then plots were seeded with different densities of both B. tectorum (0, 150, 300, 600, and 1,200 viable seeds m⁻²) and native species (0, 150, 300, and 600 viable seeds m⁻²). Adding sucrose had short-term (1 year) negative effects on available nitrogen and B. tectorum density, biomass and seed numbers, but did not increase establishment of native species. Increasing propagule availability increased both B. tectorum and native species establishment. Effects of B. tectorum on native species were density dependent and native establishment increased as B. tectorum propagule availability decreased. Survival of native seedlings was low indicating that recruitment is governed by the seedling stage.     

Propagule pressure and environmental conditions interact to determine establishment success of an invasive plant species,glossy buckthorn (<Emphasis Type="Italic">Frangula alnus</Emphasis>), across five different wetland habitat types

Many invasive plant species are able to establish within a wide range of community types. This establishment success depends on high propagule pressure and successful recruitment of seedlings following propagule dispersal into receptive environments. This study aimed to investigate interactions between propagule pressure and environmental resistance to seedling recruitment of the invasive shrub, glossy buckthorn (Frangula alnus Mill.), over a range of wetland habitat types. We measured propagule deposition using seed traps and recruitment success using sown plots, while characterizing vegetation and abiotic environmental conditions in five adjacent wetland habitat types. Drier habitats, which included Cedar Swamp, Shrub Carr, and String, had lower resistance to buckthorn establishment than the wetter Flark and Cattail Marsh. The drier habitats supported more woody species and provided more raised hummock surfaces essential for successful buckthorn recruitment and establishment. Propagule pressure was also higher in dry habitats that supported higher densities of adult glossy buckthorn, while long-distance dispersal into areas with low adult density was uncommon. Natural recruitment was highest in sites with intense propagule pressure, but experimental sowing of seeds demonstrated that buckthorn establishes in wet sites with higher resistance if propagule pressure is increased and seeds are deposited on hummocks. This study demonstrates the affinity of glossy buckthorn for drier wetland sites, and provides empirical evidence that environmental resistance can be overcome by higher propagule pressure.     

Establishment of native grasses and their impact on exotic annuals in degraded box gum woodlands

Restoration goals often involve the addition of new species to resident, degraded communities but in box gum woodlands such restoration is often constrained by competition from persistent exotic annuals that control critical ecological processes. Nutrient reduction (via carbon addition) and seed bank depletion are two approaches to reduce competition from exotic annuals but to be effective these treatments must allow establishment of species such as native grasses. This experiment was conducted in two degraded Austrostipa understoreys in the box gum woodlands of south‐east Australia. It compares the effects of carbon addition (sugar), seed depletion (spring burning or spring grazing) and combinations of carbon addition and seed depletion treatments on the establishment of C3 and C4 native grasses, and measured the effects of their establishment on soil nitrate concentration and exotic annuals. Treatments that reduced exotic annual abundance did not increase initial germination of the C4 native grasses, Bothriochloa or Themeda. However, sugar increased seedling survival of Themeda and Bothriochloa and grazing increased seedling survival of Bothriochloa, presumably by reducing effects of exotic annuals. Poa and Rytidosperma (C3 native grasses) failed to establish. Although we were unable to detect any reduction in soil nitrate concentration, swards with successful recruitment of C4 grasses suppressed exotic annuals more than the Austrostipa‐only swards at one site (the other was affected by wildfire). Further, Austrostipa‐Themeda swards were more effective than Austrostipa‐Bothriochloa for suppressing exotics, pointing to a role for both functional and species identity in the degree of resistance conferred.     

Propagule pressure and native community connectivity interact to influence invasion success in metacommunities

Mechanistic insights from invasion biology indicate that propagule pressure of exotic species and native community structure can independently influence establishment success. The role of native community connectivity via species dispersal and its potential interaction with propagule pressure on invasion success in metacommunities, however, remains unknown. Native community connectivity may increase biotic resistance to invasion by enhancing species richness and evenness, but the effects could depend upon the level of propagule pressure. In this study, a mesocosm experiment was used to evaluate the independent and combined effects of exotic propagule pressure and native community connectivity on invasion success. The effects of three levels of exotic Daphnia lumholtzi propagule pressure on establishment success, community structure and ecosystem attributes were evaluated in native zooplankton communities connected by species dispersal versus unconnected communities, and relative to a control without native species. Establishment of the exotic species exhibited a propagule dose‐dependent relationship with high levels of propagule pressure resulting in the greatest establishment success. Native community connectivity, however, effectively reduced establishment at the low level of propagule pressure and further augmented native species richness across propagule pressure treatments. Propagule pressure largely determined the negative impacts of the exotic species on native species richness, native biomass and edible producer biomass. The results highlight that native community connectivity can reduce invasion success at a low propagule dose and decrease extinction risk of native competitors, but high propagule pressure can overcome connectivity‐mediated biotic resistance to influence establishment and impact of the exotic species. Together, the results emphasize the importance of the interaction of propagule pressure and community connectivity as a regulator of invasion success, and argue for the maintenance of metacommunity connectivity to confer invasion resistance.     

Restoration of Themeda australis swards suppresses soil nitrate and enhances ecological resistance to invasion by exotic annuals

Understanding processes that underlie ecological resistance to weed invasion is critical for sustainable restoration of invaded plant communities. Experimental studies have demonstrated that invasive nitrophilic annuals can be controlled by addition of carbon to reduce soil nitrate concentrations, sometimes leading to enhanced establishment of native plants. However, effects of carbon supplements on soil nitrate are temporary, and the longer-term value of carbon supplementation as a restoration tool is dependent on the resistance of the re-established ecosystem to repeat invasion. We investigated whether re-established swards of the tussock grass Themeda australis (R.Br.) Stapf (a natural understorey dominant in mesic grassy woodlands of SE Australia) could suppress soil nitrate concentrations, and through this or other means, could impart ongoing resistance to exotic invasion in restored woodlands. In a remnant invaded by exotic annuals, we applied three plot treatments (carbon supplements, annual spring burns and untreated control) and two seed treatments (± Themeda seed) in a replicated, factorial design. Within 3 years, successful establishment of Themeda swards on burnt and carbon-supplemented plots was associated with a reduction in soil nitrate to levels comparable with non-invaded, Themeda-dominated reference sites in the region (<3 mg/kg), and significantly reduced exotic cover compared with unseeded plots. By contrast, on plots not seeded with Themeda, soil nitrate increased after cessation of carbon addition and exotic cover returned to levels comparable with untreated control plots, despite a high cover of other native perennial grasses. Few persistent effects of carbon supplements or spring burning on soil nutrients were evident 9–19 months after cessation of these treatments. Results suggest that Themeda is a keystone species that regulates nitrate cycling, thereby imparting ecological resistance to invasion by nitrophilic annuals.     

Invasibility of a Nutrient-Poor Pasture through Resident and Non-Resident Herbs Is Controlled by Litter, Gap Size and Propagule Pressure

Since inference concerning the relative effects of propagule pressure, biotic interactions, site conditions and species traits on the invasibility of plant communities is limited, we carried out a field experiment to study the role of these factors for absolute and relative seedling emergence in three resident and three non-resident confamilial herb species on a nutrient-poor temperate pasture. We set up a factorial field experiment with two levels each of the factors litter cover (0 and 400 g m(-2)), gap size (0.01 and 0.1 m(2)) and propagule pressure (5 and 50 seeds) and documented soil temperature, soil water content and relative light availability. Recruitment was recorded in spring and autumn 2010 and in spring 2011 to cover initial seedling emergence, establishment after summer drought and final establishment after the first winter. Litter alleviated temperature and moisture conditions and had positive effects on proportional and absolute seedling emergence during all phases of recruitment. Large gaps presented competition-free space with high light availability but showed higher temperature amplitudes and lower soil moisture. Proportional and absolute seedling recruitment was significantly higher in large than in small gaps. In contrast, propagule pressure facilitated absolute seedling emergence but had no effects on proportional emergence or the chance for successful colonisation. Despite significantly higher initial seedling emergence of resident than non-resident species, seed mass and other species-specific traits may be better predictors for idiosyncratic variation in seedling establishment than status. Our data support the fluctuating resource hypothesis and demonstrate that the reserve effect of seeds may facilitate seedling emergence. The direct comparison of propagule pressure with other environmental factors showed that propagule pressure affects absolute seedling abundance, which may be crucial for species that depend on other individuals for sexual reproduction. However, propagule batch size did not significantly affect the chance for successful colonisation of disturbed plots.     

Establishment, competition and the distribution of native grasses among Michigan old-fields

1 In this study the potential role of competition in influencing the distribution of three displaced native perennial grasses across complex gradients of plant productivity and species composition was investigated in Michigan old-fields. To do this plant removal and propagule addition experiments were conducted at nine old-field sites to examine the effects of living plant neighbours and litter on seedling establishment and growth of target species in relation to community biomass.
2 For two target species, Andropogon gerardi and Schizachyrium scoparium , living plant neighbours suppressed establishment from seed at most sites, and suppressed the growth of transplants at all sites.
3 Plant litter strongly inhibited the seedling establishment of both Andropogon and Schizachyrium at sites of high community biomass and litter accumulation, but had little impact on the growth rate of transplants at any of the sites.
4 The total suppressive effect of the plant community on seedling establishment and transplant growth of both Andropogon and Schizachyrium increased in magnitude in a non-linear fashion with community biomass. These effects increased in magnitude more rapidly across sites of low to medium biomass than sites of medium to high biomass.
5 The results suggest that these native grasses may be restricted to low productivity habitats within this landscape because of strong competitive interference with establishment by the existing vegetation in the most productive sites.     

Summer irrigation,grazing and seed addition differentially influence community composition in an invaded serpentine grassland

Community assembly theory predicts that resource availability, biotic interactions, and dispersal dynamics will determine community composition. Recent work has demonstrated that manipulating these processes or “filters” to exclude exotic species may assist in restoring invaded plant communities. In this study, we began by manipulating an abiotic filter, summer water availability, on the theory that irrigation prior to the growing season could trigger the germination of exotic species during unfavorable environmental conditions. First, we performed a greenhouse experiment to assess the germination traits of 23 native and exotic species at low (16°C, spring) and high (30°C, summer) temperatures. At summer temperatures, we found high emergence of many exotic and native grasses and low emergence of native forbs suggesting that summer irrigation may help deplete the exotic seed bank. In a second experiment, we established field plots to test the efficacy of summer irrigation and simultaneously manipulated a biotic and a dispersal filter, subjecting some plots to grazing and/or native seed addition. Summer irrigation and seed addition had no effect on percent cover or species richness while grazing reduced native cover but increased native species richness and soil nitrogen content. Our data suggest that manipulating grazing (a biotic filter) may be more effective than altering abiotic or dispersal filters when restoring invaded serpentine grassland. However, summer irrigation may also be effective, if applied at lower temperatures or for longer periods.     

Germination and establishment of bioenergy grasses outside cultivation: a multi-region seed addition experiment

Many invasive plants originate as cultivated species. The growing demand for renewable energy has stimulated agricultural production of native and non-native perennial grasses, but little is known about their potential to become invasive outside cultivation, particularly at the early establishment phase. We evaluated effects of propagule pressure and establishment limitations for early establishment of four potential bioenergy grasses in agricultural field margins and forest understory across a 6.3° latitudinal gradient (Ontario, Canada; Illinois and Virginia, USA). We used multiple seed introductions in different years and followed their fate for up to three growing seasons. High interannual variability in establishment indicates that the frequency of propagule introduction is important for invasion outside cultivation. Establishment limitations were stronger in forest than field margins; of 328,800 seeds added, only 1 of 505 persisting seedlings occurred in forest. Removal of competing vegetation had small and variable effects on establishment among sites and species. Unlike previous short-term experiments, our results indicate the potential for the persistence of these bioenergy grasses in both vegetation and seed bank, and highlight the importance of long-term experiments in evaluating invasion risk.     

Slash Pile Burning Effects on Soil Biotic and Chemical Properties and Plant Establishment: Recommendations for Amelioration

Ponderosa pine forest restoration consists of thinning trees and reintroducing prescribed fire to reduce unnaturally high tree densities and fuel loads to restore ecosystem structure and function. A current issue in ponderosa pine restoration is what to do with the large quantity of slash that is created from thinning dense forest stands. Slash piling burning is currently the preferred method of slash removal because it allows land managers to burn large quantities of slash in a more controlled environment in comparison with broadcast burning slash. However burning slash piles is known to have adverse effects such as soil sterilization and exotic species establishment. This study investigated the effects of slash pile burning on soil biotic and chemical variables and early herbaceous succession on burned slash pile areas. Slash piles were created following tree thinning in two adjacent approximately 20‐ha ponderosa pine (Pinus ponderosa) restoration treatments in the Coconino National Forest near Flagstaff, Arizona. We selected 30 burned slash pile areas and sampled across a gradient of the burned piles for arbuscular mycorrhizal (AM) propagule densities, the soil seed bank, and soil chemical properties. In addition, we established five 1‐m² plots in each burned pile to quantify the effect of living soil (AM inoculum) and seeding amendments on early herbaceous succession in burned slash pile areas. The five treatments consisted of a control (no treatment), living soil (AM inoculum) amendment, sterilized soil (no AM inoculum) amendment, seed amendment, and a seed/soil (AM inoculum) amendment. Slash pile burning nearly eliminated populations of viable seeds and AM propagules and altered soil chemical properties. Amending scars with native seeds increased the cover of native forbs and grasses. Furthermore adding both seed and living soil more than doubled total native plant cover and decreased ruderal and exotic plant cover. These results indicate that seed/soil amendments that increase native forbs and grasses may enhance the rate of succession in burned slash pile areas by allowing these species to outcompete exotic and ruderal species also establishing at the site through natural regeneration.     

Mechanisms of resistance of Mediterranean annual communities to invasion by Conyza bonariensis: effects of native functional composition

Recent studies have shown that a high species or functional group richness may not always lead to a greater resistance of plant communities to invasion, whereas species and/or functional group composition can more reliably predict invasion resistance. The aim of this study was to understand the mechanisms through which functional group composition can influence the resistance of Mediterranean annual communities to invasion by the exotic Conyza bonariensis . To analyse the effects of functional composition on the performance of individuals introduced as seedlings we first examined the relationships between the demographic and vegetative parameters of C. bonariensis and the biomass achieved by each functional group (grasses, legumes and Asteraceae rosettes) in synthetic communities. As a further step to approach the mechanisms involved in community resistance to invasion, we included in the analyses measurements of functional variables taken within the synthetic communities.
In agreement with earlier results and theory suggesting that high nutrient availability can favour invasions, an abundant legume biomass in communities increased the final biomass and net fecundity of C. bonariensis , due to positive effects on soil nitrate concentration. Survival and establishment of C. bonariensis were mainly favoured by a high biomass of Asteraceae. Additional results from measurements of herbivory suggested that C. bonariensis survival wasn't related to abiotic conditions but may be owed to a protection against herbivores in plots with abundant Asteraceae . Establishment was on the other hand likely to be hindered by the effects of abundant grass and legume foliage on light quality, and therefore easier within an Asteraceae canopy.
We conclude that invasion of Mediterranean old fields by species with biologies similar to C. bonariensis could be limited by favouring communities dominated by annual grasses.     

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.     

Exotic grasses and feces deposition by an exotic herbivore combine to reduce the relative abundance of native forbs

Increased resource availability can facilitate establishment of exotic plant species, especially when coincident with propagule supply. Following establishment, increased resource availability may also facilitate the spread of exotic plant species if it enhances their competitive abilities relative to native species. Exotic Canada geese (Branta canadensis) introduce both exotic grass seed and nutrients to an endangered plant community on the Gulf Islands of southwestern British Columbia, Canada. I used greenhouse experiments to assess the competitive advantage of the exotic grasses relative to native and exotic forbs in this community and to test the impacts of nutrient addition from goose feces on competitive outcomes. I grew experimental communities varying in their proportion of forbs versus exotic grasses, and added goose feces as a nutrient source. I found that both native and exotic forbs produced significantly more biomass in competition with conspecifics than in competition with the grasses, and that the proportional abundance of two out of three native forbs was lowest in the combined presence of exotic grasses and nutrient addition. In a second experiment, I found that in monoculture all species of forbs and grasses showed equal growth responses to nutrients. The exotic species did not convert additional nutrients into additional biomass at a higher rate, but did germinate earlier and grow larger than the native species regardless of nutrient availability. This suggests that the exotic species may have achieved their competitive advantage partly by pre-empting resources in community mixtures. Small and late-germinating native forbs may be particularly vulnerable to competitive suppression from exotic grasses and forbs and may be at an even greater disadvantage if their competitors are benefiting from early access to additional nutrients. In combination, the input of exotic propagules and additional nutrients by nesting geese may compromise efforts to maintain native community composition in this system.     

Leaf litter depth as an important factor inhibiting seedling establishment of an exotic palm in tropical secondary forest patches

Intact tropical rainforests on continents and continental islands are considered to be relatively resistant to invasions by introduced plant species, but fragmentation and degradation may render them susceptible, especially to species from predominantly shade-tolerant families with centres of diversity in the tropics, such as palms. We investigated the seedling establishment patterns of the most common exotic palm species in Singapore’s secondary forest patches, the Macarthur palm (Ptychosperma macarthurii), in relation to landscape-level planting intensity, abiotic conditions, and recipient community composition. We first used conditional inference forests to narrow down the set of possible explanatory variables, followed by fitting generalised linear models with the forest patch and individual plots as random intercepts, to account for the nesting of plots within patches and overdispersion, respectively. The number of cultivated adults in the vicinity was not an important variable. Instead, leaf litter was the most important predictor of seedling density. Thick leaf litter in the disturbed and younger secondary forest matrix that surrounds old growth forest patches may therefore serve as a buffer against invasions, especially by small-seeded exotics. However, if adults of exotic species are allowed to establish unchecked, for example along forest trails that lead into the interior of the forested landscape, the seed rain may eventually reach old growth forest where leaf litter is typically thin. Further studies are required to determine if second-generation adults within invaded habitats contribute disproportionately more to propagule pressure than first-generation cultivated plants outside the invaded habitat.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.