Soil physical properties associated with the invasive spotted knapweed and native grasses are similar

Centaurea maculosa Lam. (spotted knapweed), a Eurasian perennial forb, has invaded disturbed and undisturbed semiarid grasslands in the western United States. In the past, success in controlling C. maculosa and restoring invaded areas has been limited. Most research has addressed chemical aspects of invasive species interactions with soils, while potential impacts of altered soil physical properties on C. maculosa's success has not been studied. We hypothesized that the persistence of C. maculosa in semiarid rangelands might reflect an ability to alter site conditions. The objective of this study was to compare selected soil physical properties under C. maculosa-dominated and native perennial grass-dominated areas on semiarid grassland. We used six field sites in western Montana containing adjacent plots dominated by C. maculosa and by native perennial grasses. Soil physical properties including particle size fractions, bulk density, and hydraulic and thermal properties, as well as total organic carbon content, of near-surface soils were measured for each vegetation type. Soil physical properties seldom differed between C. maculosa- and native grass-dominated areas. When soil physical properties differed, the differences were inconsistent within and among sites. Presence of C. maculosa did not alter surface soil characteristics at our six sites, thus its persistence on these semi-arid grasslands cannot be explained by an ability to alter near-surface soil characteristics.     

Response of the invasive Centaurea maculosa and two native grasses to N-pulses

Nitrogen is often a limiting resource on semi-arid grasslands. During the growing season, N is often only available during short-term pulses associated with wetting events. The Eurasian forb Centaurea maculosa Lam. has invaded millions of hectares of semi-arid grasslands in western North America. C. maculosa's success could be attributed to greater use of N-pulses, or more efficient use of N supplied in those pulses compared with native grasses. In a glasshouse, C. maculosa and two native grasses, the caespitose Pseudoroegneria spicata [Scribn. and Smith] A. Love and the rhizomatous Pascopyrum smithii [Rybd.] A. Love, were established in mixed- and monoculture combinations, and then conditioned to weekly N-pulses of 8, 24, or 72 h for 8 weeks. These pulse durations are typical on semi-arid grasslands. At the end of the 8 weeks, plants were exposed to ¹⁵N-labeled nitrate (¹⁵NO₃ ^–) for 8 h and harvested 16 h later to compare short-term root uptake of ¹⁵NO₃ ^–. C. maculosa did not have greater enrichment (atom % ¹⁵N), rate of ¹⁵N-uptake (mol g^–1 h^–1), or ¹⁵N acquired (relative to ¹⁵N applied) than the grasses. C. maculosa's ¹⁵N-uptake per unit mass was relatively consistent across pulse durations, whereas ¹⁵N-uptake was lower at the longer pulse durations for the grasses. In general, C. maculosa acquired more of the applied ¹⁵N than P. spicata but less than P. smithii. ¹⁵N acquired was often influenced by the neighbour's identity. Regarding growth responses, C. maculosa produced more total biomass than the grasses, except for P. smithii plants growing with C. maculosa conditioned to 72 h pulses of N. Root mass ratios varied depending on the neighbor. Overall, C. maculosa used nitrogen less efficiently than the grasses. C. maculosa's success as an invasive species cannot be explained wholly by a greater response to N-pulses or more efficient use of N-pulses compared with native grasses with which it competes.     

Traits of the invasive Centaurea maculosa and two native grasses: effect of N supply

The Eurasian forb Centaurea maculosa (Lam.; spotted knapweed) has invaded millions of hectares of semi-arid grasslands in western North America. It readily colonizes disturbed areas, but also invades pristine grasslands. C. maculosa's success could be attributed to greater use, or more efficient use, of available soil nitrogen (N). Soil N often limits growth on semi-arid grasslands. Greater or more efficient use of soil N by C. maculosa, if this occurred, may inhibit establishment, survival, or reproduction of native grasses. In a glasshouse, C. maculosa and two native grasses, Pseudoroegneria spicata [Scribn. and Smith] A. Love and Pascopyrum smithii [Rybd.] A. Love, were grown in mixed- and monoculture for 8 weeks to determine growth response to two soil N supplies, which mimicked low and high N mineralization rates in semi-arid grasslands. At the end of the 8 weeks, plants were exposed to ¹⁵N-labeled nitrate for 24 h, and harvested to compare uptake of NO₃ ^–. C. maculosa's growth response to N indicated that it was more competitive for N than the tussock grass P. spicata, but less competitive than the rhizomatous grass P. smithii. C. maculosa used nitrogen less efficiently than both of these native grasses. C. maculosa roots took up more ¹⁵N per unit root mass than the grasses, but acquired less N than P. smithii because P. smithii had greater root mass than C. maculosa. Total biomass and ¹⁵N uptake of C. maculosa varied depending on which species it was growing with. C. maculosa's success cannot be explained wholly by greater or more efficient use of soil N than that of the native grasses with which it competes.     

Woody Plant Encroachment by <Emphasis Type="Italic">Juniperus virginiana</Emphasis> in a Mesic Native Grassland Promotes Rapid Carbon and Nitrogen Accrual

The cover and abundance of Juniperus virginiana L. in the U.S. Central Plains are rapidly increasing, largely as a result of changing land-use practices that alter fire regimes in native grassland communities. Little is known about how conversion of native grasslands to Juniperus-dominated forests alters soil nutrient availability and ecosystem storage of carbon (C) and nitrogen (N), although such land-cover changes have important implications for local ecosystem dynamics, as well as regional C and N budgets. Four replicate native grasslands and adjacent areas of recent J. virginiana encroachment were selected to assess potential changes in soil N availability, leaf-level photosynthesis, and major ecosystem C and N pools. Net N mineralization rates were assessed in situ over two years, and changes in labile soil organic pools (potential C and N mineralization rates and microbial biomass C and N) were determined. Photosynthetic nitrogen use efficiencies (PNUE) were used to examine differences in instantaneous leaf-level N use in C uptake. Comparisons of ecosystem C and N stocks revealed significant C and N accrual in both plant biomass and soils in these newly established forests, without changes in labile soil N pools. There were few differences in monthly in situ net N mineralization rates, although cumulative annual net N mineralization was greater in forest soils compared to grasslands. Conversely, potential C mineralization was significantly reduced in forest soils. Encroachment by J. virginiana into grasslands results in rapid accretion of ecosystem C and N in plant and soil pools with little apparent change in N availability. Widespread increases in the cover of woody plants, like J. virginiana, in areas formerly dominated by graminoid species suggest an increasing role of expanding woodlands and forests as regional C sinks in the central U.S.     

Invasion by <Emphasis Type="Italic">Aegilops triuncialis</Emphasis> (Barb Goatgrass) Slows Carbon and Nutrient Cycling in a Serpentine Grassland

Invasive plant species alter plant community composition and ecosystem function. In the United States, California native grasslands have been displaced almost completely by invasive annual grasses, with serpentine grasslands being one of the few remaining refugia for California grasslands. This study examined how the invasive annual grass, Aegilops triuncialis, has altered decomposition processes in a serpentine annual grassland. Our objectives were to (1) assess howA. triuncialis alters primary productivity and litter tissue chemistry, (2) determine whether A. triuncialis litter is more recalcitrant to decomposition than native litter, and (3) evaluate whether differences in the soil microbial community in A. triuncialis-invaded and native-dominated areas result in different decomposition rates of invasive and/or native plant litter. In invaded plant patches, A. triuncialis was approximately 50% of the total plant cover, in contrast to native plant patches in which A. triuncialis was not detected and native plants comprised over 90% of the total plant cover. End-of-season aboveground biomass was 2-fold higher in A. triuncialis dominated plots compared to native plots; however, there was no significant difference in belowground biomass. Both above- and below-ground plant litter from A. triuncialis plots had significantly higher lignin:N and C:N ratios and lower total N, P, and K than litter from native plant plots. Aboveground litter from native plots decomposed more rapidly than litter from A. triuncialis plots, although there was no difference in decomposition of belowground tissues. Soil microbial community composition associated with different soil patch types had no effect on decomposition rates. These data suggest that plant invasion impacts decomposition and nutrient cycling through changes in plant community tissue chemistry and biomass production.     

Oxalate contributes to the resistance of Gaillardia grandiflora and Lupinus sericeus to a phytotoxin produced by Centaurea maculosa

Centaurea maculosa Lam. is a noxious weed in western North America that produces a phytotoxin, (±)-catechin, which is thought to contribute to its invasiveness. Areas invaded by C. maculosa often result in monocultures of the weed, however; in some areas, North American natives stand their ground against C. maculosa and show varying degrees of resistance to its phytotoxin. Two of these resistant native species, Lupinus sericeus Pursh and Gaillardia grandiflora Van Houtte, were found to secrete increased amounts of oxalate in response to catechin exposure. Mechanistically, we found that oxalate works exogenously by blocking generation of reactive oxygen species in susceptible plants and reducing oxidative damage generated in response to catechin. Furthermore, field experiments show that L. sericeus indirectly facilitates native grasses in grasslands invaded by C. maculosa, and this facilitation can be correlated with the presence of oxalate in soil. Addition of exogenous oxalate to native grasses and Arabidopsis thaliana (L.) Heynh grown in vitro alleviated the phytotoxic effects of catechin, supporting the field experiments and suggesting that root-secreted oxalate may also act as a chemical facilitator for plant species that do not secrete the compound.     

Stress responses of native and exotic grasses in a Neotropical savanna predict impacts of global change on invasion spread

Communities subject to stress, including those with low invasibility, may be dominated by exotic generalist species. African grasses are aggressive invasive species in Neotropical savannas, where their response to abiotic stress remains unknown. We assessed the role of waterlogging and canopy closure on the presence, abundance and reproductive tillering of African and native grasses in a Neotropical savanna in southeastern Brazil. We obtained abundance and reproductive tillering data of exotic (Melinis minutiflora, Melinis repens and Urochloa decumbens) and common native grasses in 20 sites. We also determined the groundwater depth, soil surface water potential and canopy cover at these sites. The grass species generally had a low frequency and performed poorly where soil remained waterlogged throughout the year, except for two native species. Most native species were exclusive to either well‐drained savannas or better drained wet grasslands. However, two species (Loudetiopsis chrysothrix and Trachypogon spicatus) occurred in both vegetation types. Two exotic species (M. minutiflora and M. repens) were less common but demonstrated reasonable performance in wet grasslands, possibly due to their root system plasticity. Furthermore, U. decumbens had a lower occurrence, density and reproductive tillering at these sites, but was successful at sites where the groundwater level was slightly deeper. Although the favourable water regime in the savannas increases their invasibility in general, resistance to invasion by African grasses may be greater at microsites with high canopy closure, where these species showed lower performance and did not affect the abundance of co‐occurring native grasses. In summary, the Brazilian savanna becomes more susceptible to the spread of African grasses when disturbances decrease canopy closure or lower rainfall associated with climate change reduces the average groundwater depth and consequently releases invasive species from soil waterlogging in grasslands.     

Water use and water-use efficiency of the invasive Centaurea maculosa and three native grasses

The Eurasian herb Centaurea maculosa Lam. has invaded millions of hectares of semi-arid grasslands in western North America. Its success may reflect that it may be more competitive than native species, it is not grazed by large herbivores, it was introduced without its native enemies, it may interfere with native species via allelopathy, or most likely some combination of these factors. Greater competitive ability could include greater use of limiting soil resources, such as water, or more efficient use of soil water, thereby inhibiting establishment, survival, and reproduction of native species. We measured water use and water-use efficiency of Centaurea and three native grasses, Pseudoroegneria spicata [Scribn. and Smith] A. Love, Pascopyrum smithii [Rybd.] A. Love, and Festuca idahoensis Elmer, in a glasshouse. Water-use efficiency was determined by the traditional measure of biomass produced per mass of water used, and by carbon-isotope discrimination (). Centaurea did not use the most water, or use water more efficiently (based on biomass (g)/ water (kg) and carbon-isotope discrimination) than all three native grasses. We also determined carbon-isotope discrimination of Centaurea and dominant native grasses during the 1999 and 2000 growing seasons at three field sites. Centaurea rosettes had the lowest water-use efficiency (greatest carbon-isotope discrimination), followed by mature plants of Centaurea, and then native grasses. Water-use efficiency of mature Centaurea plants and native grasses was greater in late summer than early summer. Centaurea's success as an invasive species in North America cannot be attributed to greater use of soil water or greater water-use efficiency than native grasses.     

Requirement of vegetation gaps for seedling establishment of two unpalatable grasses in a native grassland of central Argentina

Abstract The objective was to determine the effects of root and shoot competition on seedling establishment of the unpalatable grasses Stipa gynerioides and S. tenuissima in a native grassland of central Argentina dominated by the palatable grass S. clarazii. Seeds of the two unpalatable species were sown in natural occurring microsites with shoot and root competition from the palatable species, and in artificially created microsites without either shoot competition or shoot and root competition. In addition, fresh seeds of the unpalatable species were subjected to daily alternating temperatures under laboratory and field conditions to determine the effect on seed dormancy and germination. Seedling establishment of S. gynerioides and S. tenuissima occurred only in microsites without shoot and root competition. Also, the fluctuation of temperature near the soil surface in these microsites reduced dormancy and promoted rapid germination in both species. Our results support the hypothesis that, in swards dominated by palatable grasses, vegetation gaps of low competitive pressure favour seedling establishment of unpalatable grasses. It is suggested that the creation of these gaps by overgrazing can be an important mechanism in the process of species replacement in native grasslands.     

Biological invasions and climate change amplify each other’s effects on dryland degradation

Climate models predict that, in the coming decades, many arid regions will experience increasingly hot conditions and will be affected more frequently by drought. These regions are also experiencing rapid vegetation change, notably invasion by exotic grasses. Invasive grasses spread rapidly into native desert ecosystems due, in particular, to interannual variability in precipitation and periodic fires. The resultant destruction of non-fire-adapted native shrub and grass communities and of the inherent soil resource heterogeneity can yield invader-dominated grasslands. Moreover, recurrent droughts are expected to cause widespread physiological stress and mortality of both invasive and native plants, as well as the loss of soil resources. However, the magnitude of these effects may differ between invasive and native grasses, especially under warmer conditions, rendering the trajectory of vegetated communities uncertain. Using the Biosphere 2 facility in the Sonoran Desert, we evaluated the viability of these hypothesized relationships by simulating combinations of drought and elevated temperature (+5°C) and assessing the ecophysiological and mortality responses of both a dominant invasive grass (Pennisetum ciliare or buffelgrass) and a dominant native grass (Heteropogan contortus or tanglehead). While both grasses survived protracted drought at ambient temperatures by inducing dormancy, drought under warmed conditions exceeded the tolerance limits of the native species, resulting in greater and more rapid mortality than exhibited by the invasive. Thus, two major drivers of global environmental change, biological invasion and climate change, can be expected to synergistically accelerate ecosystem degradation unless large-scale interventions are enacted.     

Soil modification by invasive plants: effects on native and invasive species of mixed-grass prairies

Invasive plants are capable of modifying attributes of soil to facilitate further invasion by conspecifics and other invasive species. We assessed this capability in three important plant invaders of grasslands in the Great Plains region of North America: leafy spurge (Euphorbia esula), smooth brome (Bromus inermis) and crested wheatgrass (Agropyron cristatum). In a glasshouse, these three invasives or a group of native species were grown separately through three cycles of growth and soil conditioning in both steam-pasteurized and non-pasteurized soils, after which we assessed seedling growth in these soils. Two of the three invasive species, Bromus and Agropyron, exhibited significant self-facilitation via soil modification. Bromus and Agropyron also had significant facilitative effects on other invasives via soil modification, while Euphorbia had significant antagonistic effects on the other invasives. Both Agropyron and Euphorbia consistently suppressed growth of two of three native forbs, while three native grasses were generally less affected. Almost all intra- and interspecific effects of invasive soil conditioning were dependent upon presence of soil biota from field sites where these species were successful invaders. Overall, these results suggest that that invasive modification of soil microbiota can facilitate plant invasion directly or via ‘cross-facilitation’ of other invasive species, and moreover has potential to impede restoration of native communities after removal of an invasive species. However, certain native species that are relatively insensitive to altered soil biota (as we observed in the case of the forb Linum lewisii and the native grasses), may be valuable as ‘nurse’species in restoration efforts.     

Host plant differences in arbuscular mycorrhizae: Extra radical hyphae differences between an invasive forb and a native bunchgrass

Arbuscular mycorrhizae affect grassland plant community composition and host plant nutrient uptake, and can mediate shifts in competitive outcome between plant species. Centaurea maculosa, an invasive forb from Eurasia, dominates more than 4 million hectares in the Rocky Mountain region of North America. We examined the role of AM for phosphorus (P) acquisition from a distant source for C. maculosa and Festuca idahoensis, a native bunchgrass. Plants were grown individually in pots divided by a barrier that either excluded plant roots and AM hyphae, or only plant roots. In the half of the pot without a plant, 1 of 3 P treatments was applied: no P, phosphate rock (PR) or triple superphosphate (TSP), applied at a rate of 144 mg P kg^–1 soil. After 14 weeks of growth, C. maculosa was twice as large as F. idahoensis, and neither species biomass was affected by barrier type. Phosphorus fertilizer, and especially PR, moved across the barrier to the plant side of the pot. Tissue P concentration for C. maculosa was highest with the PR treatment, and was not affected by the barrier type. In contrast, F. idahoensis tissue P concentration did not vary with barrier or P treatments. There was more AM extra radical hyphae (ERH) associated with C. maculosa than F. idahoensis, suggesting that C. maculosa provides more carbon for the AM fungi, resulting in greater ERH production, ERH soil exploration and potential for soil nutrient pool exploitation. Although not tested in this study, differences between host plants may be the result of different physiological characteristics of the host plant or differences in AM fungal species that colonize the invader, with different fungal species accessing P from different distances.     

Invasive plant architecture alters trophic interactions by changing predator abundance and behavior

As primary producers, plants are known to influence higher trophic interactions by initiating food chains. However, as architects, plants may bypass consumers to directly affect predators with important but underappreciated trophic ramifications. Invasion of western North American grasslands by the perennial forb, spotted knapweed (Centaurea maculosa), has fundamentally altered the architecture of native grassland vegetation. Here, I use long-term monitoring, observational studies, and field experiments to document how changes in vegetation architecture have affected native web spider populations and predation rates. Native spiders that use vegetation as web substrates were collectively 38 times more abundant in C. maculosa-invaded grasslands than in uninvaded grasslands. This increase in spider abundance was accompanied by a large shift in web spider community structure, driven primarily by the strong response of Dictyna spiders to C. maculosa invasion. Dictyna densities were 46–74 times higher in C. maculosa-invaded than native grasslands, a pattern that persisted over 6 years of monitoring. C. maculosa also altered Dictyna web building behavior and foraging success. Dictyna webs on C. maculosa were 2.9–4.0 times larger and generated 2.0–2.3 times higher total prey captures than webs on Achillea millefolium, their primary native substrate. Dictyna webs on C. maculosa also captured 4.2 times more large prey items, which are crucial for reproduction. As a result, Dictyna were nearly twice as likely to reproduce on C. maculosa substrates compared to native substrates. The overall outcome of C. maculosa invasion and its transformative effects on vegetation architecture on Dictyna density and web building behavior were to increase Dictyna predation on invertebrate prey ≥89 fold. These results indicate that invasive plants that change the architecture of native vegetation can substantially impact native food webs via nontraditional plant → predator → consumer linkages.     

The invasive plant species Centaurea maculosa alters arbuscular mycorrhizal fungal communities in the field

While several recent studies have described changes in microbial communities associated with exotic plant invasion, how arbuscular mycorrhizal fungi (AMF) communities respond to exotic plant invasion is not well known, despite the salient role of this group in plant interactions. Here, we use molecular methods (terminal restriction fragment length polymorphism analyses based on the large subunit of the rRNA gene) to examine AMF community structure in sites dominated by the invasive mycorrhizal forb, Centaurea maculosa Lam. (spotted knapweed), and in adjacent native grassland sites. Our results indicate that significant AMF community alteration occurs following C. maculosa invasion. Moreover, a significant reduction in the number of restriction fragment sizes was found for samples collected in C. maculosa-dominated areas, suggesting reduced AMF diversity. Extraradical hyphal lengths exhibited a significant, on average 24%, reduction in C. maculosa-versus native grass-dominated sites. As both AMF community composition and abundance were altered by C.maculosa invasion, these data are strongly suggestive of potential impacts on AMF-mediated ecosystem processes. Given that the composition of AMF communities has the potential to differentially influence different plant species, our results may have important implications for site restoration after weed invasion.     

Soil characteristics of Rocky Mountain National Park grasslands invaded by Melilotus officinalis and M. alba

Aim Invasion of nitrogen‐fixing non‐native plant species may alter soil resources and impact native plant communities. Altered soils may be the driving mechanism that provides a suitable environment to facilitate future invasions and decrease native biodiversity. We hypothesized that Melilotus invasion would increase nitrogen availability and produce soil microclimate and biochemical changes, which could in turn alter plant species composition in a montane grassland community. Location Our research addressed the effects of white and yellow sweet clover (Melilotus officinalis and M. alba) invasion on soil characteristics and nitrogen processes in the montane grasslands in Rocky Mountain National Park. Methods We sampled soil in replicate sites of Melilotus‐invaded and control (non‐invaded) patches within disturbed areas in montane grassland habitats. Soil composites were analysed for available nitrogen, net nitrogen mineralization, moisture, carbon/nitrogen (C : N ratio), texture, organic matter and pH. Data were recorded at three sample dates during the growing seasons of 1998 and 1999. Results Contrary to our expectations, we observed lower nitrogen availability and mineralization in invaded patches, and differences in soil moisture content and soil C : N. Soil C : N ratios were higher in invaded plots, in spite of the fact that Melilotus had the lowest C : N ratios of other plant tissue analysed in this study. Main conclusions These findings provide land managers of natural areas with a better perspective on the possibilities of nitrogen‐fixing species impact on soil nutrient levels.     

The response of stocks of C,N, and P to plant invasion in the coastal wetlands of China

The increasing success of invasive plant species in wetland areas can threaten their capacity to store carbon, nitrogen, and phosphorus (C, N, and P). Here, we have investigated the relationships between the different stocks of soil organic carbon (SOC), and total C, N, and P pools in the plant–soil system from eight different wetland areas across the South‐East coast of China, where the invasive tallgrass Spartina alterniflora has replaced the native tall grasses Phragmites australis and the mangrove communities, originally dominated by the native species Kandelia obovata and Avicennia marina. The invasive success of Spartina alterniflora replacing Phragmites australis did not greatly influence soil traits, biomass accumulation or plant–soil C and N storing capacity. However, the resulting higher ability to store P in both soil and standing plant biomass (approximately more than 70 and 15 kg P by ha, respectively) in the invasive than in the native tall grass communities suggesting the possibility of a decrease in the ecosystem N:P ratio with future consequences to below‐ and aboveground trophic chains. The results also showed that a future advance in the native mangrove replacement by Spartina alterniflora could constitute a serious environmental problem. This includes enrichment of sand in the soil, with the consequent loss of nutrient retention capacity, as well as a sharp decrease in the stocks of C (2.6 and 2.2 t C ha^‐1 in soil and stand biomass, respectively), N, and P in the plant–soil system. This should be associated with a worsening of the water quality by aggravating potential eutrophication processes. Moreover, the loss of carbon and nutrient decreases the potential overall fertility of the system, strongly hampering the reestablishment of woody mangrove communities in the future.     

Neighboring plant influences on arbuscular mycorrhizal fungal community composition as assessed by T-RFLP analysis

Controls on root colonization by arbuscular mycorrhizal fungi (AMF) include host nutrient status, identity of symbionts and soil physico-chemical properties. Here we show, in the field, that the subset of the AMF community colonizing the roots of a common grass species, Dactylis glomerata, was strongly controlled by neighboring roots of a different plant species, Centaurea maculosa, an invasive forb, thus adding a biological spatial component to controls on root colonization. Using an AMF-specific, 18s rDNA-based terminal restriction fragment length polymorphism (T-RFLP) analysis method, significant differences were found between AMF community fingerprints of samples derived from roots of grasses with (G_Cm) and without (G₀) neighboring C. maculosa. There were also significant differences between samples derived from C. maculosa roots (C_mac) and both G_Cm and G₀ roots. Sample ordination indicated three generally distinct groups consisting of C_mac, G_Cm and G₀, with G_Cm samples being of intermediate distance between G₀and C_mac. Our results indicate that, with the presence of C. maculosa, AMF communities of D. glomerata shift to reflect community composition associated with C. maculosa roots. These results highlight the importance of complex spatial distributions of AMF communities at the scale of a root system. An additional dimension to our study is that C. maculosa is an aggressively invasive plant in the intermountain West. Viewed in this light, these results suggest that pervasive influences of this plant on AMF communities, specifically in roots of its competitors, may represent a mechanism contributing to its invasive success. However, further work is clearly required to determine the extent to which AMF genotypic alteration by neighboring plants influences competitive relationships.     

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.     

Biotic constraints on the invasion of diffuse knapweed (Centaurea diffusa) in North American grasslands

Knapweeds (Centaurea spp.) are among the most invasive of non-indigenous plant species that have colonized western North America over the last century. We conducted a 4-year experiment in a reconstructed grassland to test hypotheses related to the ability of grasslands to resist the invasion of diffuse knapweed (C. diffusa). We experimentally invaded C. diffusa and three native species into areas where we manipulated soil nitrogen (N) and phosphorus (P) availability and removed extant grasses to reduce competition. We evaluated the growth response of these species to these resources and competitive manipulations. Of the native species that were experimentally added, only one species, Ratibida pinnata (prairie coneflower), established in any numbers. Establishment values in intact vegetation were low for both species, but establishment by C. diffusa (0.02%) clearly outperformed that of R. pinnata (0.001%). Under reduced grass competition, establishment was enhanced, but the values for C. diffusa (0.68%) were not statistically different from those of R. pinnata (0.57%). Neither species performed better under higher soil nutrients in the presence of competing grasses. In plots with both species, biomass of the two planted species was positively correlated, but the biomass of both species was negatively correlated with non-added weedy species. Subsequent harvests of C. diffusa indicated that establishment was enhanced in treatments with higher soil nutrients but that the biomass of these plants could only be enhanced when plant competition was also reduced. These results indicate that C. diffusa can establish in intact grasslands at rates higher than natives, but opportunism rather than competitive ability best describes the invasiveness of C. diffusa. Thus, the mechanisms contributing to the establishment of this knapweed species are different from factors identified as contributing to the dominance of this invader.     

Diminishing Spatial Heterogeneity in Soil Organic Matter across a Prairie Restoration Chronosequence

Habitat restoration resulting in changes in plant community composition or species dominance can affect the spatial pattern and variability of soil nutrients. Questions about how these changes in soil spatial heterogeneity develop over time at restoration sites, however, remain unaddressed. In this study, a geostatistical approach was used to quantify changes over time in the spatial heterogeneity of soil organic matter (SOM) across a 26‐year chronosequence of tallgrass prairie restoration sites at FermiLab, outside of Chicago, Illinois. We used total soil N and C as an index of the quantity of SOM. We also examined changes in C:N ratio, which can influence the turnover of SOM. Specifically, the spatial structure of total N, total C, and C:N ratio in the top 10 cm of soil was quantified at a macroscale (minimum spacing of 1.5 m) and a microscale (minimum spacing of 0.2 m). The magnitude of spatial heterogeneity (MSH) was characterized as the proportion of total sample variation explained by spatially structured variation. At the macroscale, the MSH for total N decreased with time since restoration (r²= 0.99, p < 0.001). The decrease in spatial heterogeneity over time corresponded with a significant increase in the dominance of the C₄ grasses. At the microscale, there was significant spatial structure for total N at the 4‐year‐old, 16‐year‐old, and 26‐year‐old sites, and significant spatial structure for total C at the 16‐year‐old and 26‐year‐old sites. These results suggest that an increase in dominance of C₄ grasses across the chronosequence is homogenizing organic matter variability at the field scale while creating fine‐scale patterns associated with the spacing of vegetation. Areas of higher soil moisture were associated with higher soil N and C at the two oldest restoration sites and at the native prairie site, potentially suggesting patches of increased belowground productivity in areas of higher soil moisture. This study is one of the first to report significant changes over time in the spatial structure of organic matter in response to successional changes initiated by restoration.