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.     

Promoting Native Vegetation and Diversity in Exotic Annual Grass Infestations

Exotic plant invasions are especially problematic because reestablishment of native perennial vegetation is rarely successful. It may be more appropriate to treat exotic plant infestations that still have some remaining native vegetation. We evaluated this restoration strategy by measuring the effects of spring burning, fall burning, fall applied imazapic, spring burning with fall applied imazapic, and fall burning with fall applied imazapic on the exotic annual grass, medusahead (Taeniatherum caput‐medusae (L.) Nevski), and native vegetation at six sites in Oregon for 2 years post‐treatment. Medusahead infestations included in this study had some residual native perennial bunchgrasses and forbs. Burning followed by imazapic application provided the best control of medusahead and resulted in the greatest increases in native perennial vegetation. However, imazapic application decreased native annual forb cover the first year post‐treatment and density the first and second year post‐treatment. The spring burn followed by imazapic application produced an almost 2‐fold increase in plant species diversity compared to the control. The fall burn followed by imazapic application also increased diversity compared to the control. Results of this study indicate that native plants can be promoted in medusahead invasions; however, responses vary by plant functional group and treatment. Our results compared to previous research suggest that restoration of plant communities invaded by exotic annual grass may be more successful if efforts focus on areas with some residual native perennial vegetation. Thus, invasive plant infestations with some native vegetation remaining should receive priority for restoration efforts over near monocultures of invasive plant species.     

Optimal prescribed burn frequency to manage foundation California perennial grass species and enhance native flora

Grasslands can be diverse assemblages of grasses and forbs but not much is known how perennial grass species management affects native plant diversity except in a few instances. We studied the use of late-spring prescribed burns over a span of 11 years where the perennial grass Poa secunda was the foundation species, with four additional years of measurements after the final burn. We evaluated burn effects on P. secunda, the rare native annual forb Amsinckia grandiflora and local native and exotic species. Annual burning maintained P. secunda number, resulted in significant expansion, the lowest thatch and exotic grass cover, the highest percentage of bare ground, but also the lowest native forb and highest exotic forb cover. Burning approximately every 3 years maintained a lower number of P. secunda plants, allowed for expansion, and resulted in the highest native forb cover with a low exotic grass cover. Burning approximately every 5 years and the control (burned once from a wildfire) resulted in a decline in P. secunda number, the highest exotic grass and thatch cover and the lowest percentage of bare ground. P. secunda numbers were maintained up to 4 years after the final burn. While local native forbs benefited from burning approximately every 3 years, planted A. grandiflora performed best in the control treatment. A. grandiflora did not occur naturally at the site; therefore, no seed bank was present to provide across-year protection from the effects of the burns. Thus, perennial grass species management must also consider other native species life history and phenology to enhance native flora diversity.     

Seedling Recruitment in the Long‐Lived Perennial,Echinacea angustifolia: A 10‐Year Experiment

Establishment of persistent plant populations may be restricted by limitations on the numbers of seeds, emergence of seedlings, or survival to reproductive maturity. The relative importance of these phases in establishment of new populations, particularly restorations, is poorly understood. In an experiment to quantify seedling emergence and juvenile survival of Echinacea angustifolia during its reintroduction to previously agricultural sites, we evaluated effects of two types of vegetation and prescribed burning at four times relative to sowing. We collected achenes from prairie remnants in western Minnesota, United States, and, each October 2000–2002, overseeded them into nearby study plots either in recently planted stands of native grasses or in oldfields abandoned 40 years earlier. For each cohort, we determined germinability of achenes in the laboratory and, in the field, monitored seedling emergence the following spring and subsequent survival in annual censuses through summer 2009. Germinability ranged from 20 to 37%, varying significantly among collection years. Seedlings emerged in every treatment combination, but emergence rarely exceeded 8% of achenes sown. Burns during the spring prior to sowing tended to enhance emergence, but to differing degrees depending on the year and vegetation. Burning in the spring after sowing reduced emergence. Burning enhanced juvenile survival in restored plots but not in oldfields. Strategies to reintroduce this species should include burning in the spring before sowing, sowing large quantities of seed, avoiding burning in the spring following sowing, and burning at least once within the first 6 years.     

Ecological Consequences of Shifting the Timing of Burning Tallgrass Prairie

In the Kansas Flint Hills, grassland burning is conducted during a relatively narrow window because management recommendations for the past 40 years have been to burn only in late spring. Widespread prescribed burning within this restricted time frame frequently creates smoke management issues downwind. A potential remedy for the concentrated smoke production in late spring is to expand burning to times earlier in the year. Yet, previous research suggested that burning in winter or early spring reduces plant productivity and cattle weight gain while increasing the proportion of undesirable plant species. In order to better understand the ecological consequences of burning at different times of the year, plant production and species abundance were measured for 20 years on ungrazed watersheds burned annually in autumn, winter, or spring. We found that there were no significant differences in total grass production among the burns on either upland or lowland topographic positions, although spring burned watersheds had higher grass culm production and lower forb biomass than autumn and winter burned watersheds. Burning in autumn or winter broadened the window of grass productivity response to precipitation, which reduces susceptibility to mid-season drought. Burning in autumn or winter also increased the phenological range of species by promoting cool-season graminoids without a concomitant decrease in warm-season grasses, potentially widening the seasonal window of high-quality forage. Incorporating autumn and winter burns into the overall portfolio of tallgrass prairie management should increase the flexibility in managing grasslands, promote biodiversity, and minimize air quality issues caused by en masse late-spring burning with little negative consequences for cattle production.     

California native and exotic perennial grasses differ in their response to soil nitrogen,exotic annual grass density,and order of emergence

Early emergence of plant seedlings can offer strong competitive advantages over later-germinating neighbors through the preemption of limiting resources. This phenomenon may have contributed to the persistent dominance of European annual grasses over native perennial grasses in California grasslands, since the former species typically germinate earlier in the growing season than the latter and grow rapidly after establishing. Recently, European perennial grasses have been spreading into both non-native annual and native perennial coastal grass stands in California. These exotic perennials appear to be less affected by the priority effects arising from earlier germination by European annual grasses. In addition, these species interactions in California grasslands may be mediated by increasing anthropogenic or natural soil nitrogen inputs. We conducted a greenhouse experiment to test the effects of order of emergence and annual grass seedling density on native and exotic perennial grass seedling performance across different levels of nitrogen availability. We manipulated the order of emergence and density of an exotic annual grass (Bromus diandrus) grown with either Nassella pulchra (native perennial grass), Festuca rubra (native perennial grass), or Holcus lanatus (exotic perennial grass), with and without added nitrogen. Earlier B. diandrus emergence and higher B. diandrus density resulted in greater reduction in the aboveground productivity of the perennial grasses. However, B. diandrus suppressed both native perennials to a greater extent than it did H. lanatus. Nitrogen addition had no effect on the productivity of native perennials, but greatly increased the growth of the exotic perennial H. lanatus, grown with B. diandrus. These results suggest that the order of emergence of exotic annual versus native perennial grass seedlings could play an important role in the continued dominance of exotic annual grasses in California. The expansion of the exotic perennial grass H. lanatus in coastal California may be linked to its higher tolerance of earlier-emerging annual grasses and its ability to access soil resources amidst high densities of annual grasses.     

The Role of Seed and Vegetative Reproduction in Plant Recruitment and Demography in Tallgrass Prairie

Recruitment, establishment and survivorship of seed- and vegetatively-derived shoots were quantified biweekly in annually burned and infrequently burned tallgrass prairie to investigate the contributions of seed and vegetative reproduction to the maintenance and dynamics of tallgrass prairie plant populations, the demography of seedlings and ramets, and the influence of fire on the demography of grasses and forbs. Clonally produced grass and forb ramets comprised >99%of all established shoots present at the end of the growing season, whereas established seedlings accounted for <1%,emphasizing the rarity of successful seedling establishment and the importance of vegetative reproduction in driving the annual regeneration and dynamics of aboveground plant populations in tallgrass prairie. Most recruitment from vegetative reproduction occurred early in the growing season and was higher in annually burned than infrequently burned sites, although low levels of new stem recruitment occurred continuously throughout the growing season. Peak recruitment on annually burned prairie coincided with peak recruitment of the dominant C₄ grasses Andropogon gerardii and Sorghastrum nutans prior to prescribed spring fire, with a second peak in recruitment occurring following fire. On infrequently burned prairie, grass and forb recruitment was highest in early April and declined steadily through May. The naturalized C₃ grass, Poa pratensis, was responsible for most of the early recruitment on unburned sites, whereas A. gerardii contributed most to recruitment later in May. Infrequently burned prairie was dominated by these two grasses and contained a larger forb component than annually burned prairie. The principal demographic effect of fire was on ramet natality rather than mortality. Fire regime, plant functional group, or timing of cohort emergence before or after fire did not affect ramet survivorship. C₄ grass shoots that emerged early and were damaged by fire showed similar survivorship patterns to tillers that emerged after fire. Differences in species composition between annually burned and infrequently burned prairie are driven by fire effects on vegetative reproduction and appear to be related principally to the effect of fire and detritus accumulation on the development of belowground vegetative meristems of C₄ grasses and their emergence dynamics.     

The effects of fire frequency and grazing on tallgrass prairie productivity and plant composition are mediated through bud bank demography

Periodic fire, grazing, and a variable climate are considered the most important drivers of tallgrass prairie ecosystems, having large impacts on the component species and on ecosystem structure and function. We used long-term experiments at Konza Prairie Biological Station to explore the underlying demographic mechanisms responsible for tallgrass prairie responses to two key ecological drivers: fire and grazing. Our data indicate that belowground bud banks (populations of meristems associated with rhizomes or other perennating organs) mediate tallgrass prairie plant response. Fire and grazing altered rates of belowground bud natality, tiller emergence from the bud bank, and both short-term (fire cycle) and long-term (>15 year) changes in bud bank density. Annual burning increased grass bud banks by 25% and decreased forb bud banks by 125% compared to burning every 4 years. Grazing increased the rate of emergence from the grass bud bank resulting in increased grass stem densities while decreasing grass bud banks compared to ungrazed prairie. By contrast, grazing increased both bud and stem density of forbs in annually burned prairie but grazing had no effect on forb bud or stem density in the 4-year burn frequency treatment. Lastly, the size of the reserve grass bud bank is an excellent predictor of long-term ANPP in tallgrass prairie and also of short-term interannual variation in ANPP associated with fire cycles, supporting our hypothesis that ANPP is strongly regulated by belowground demographic processes. Meristem limitation due to management practices such as different fire frequencies or grazing regimes may constrain tallgrass prairie responses to interannual changes in resource availability. An important consequence is that grasslands with a large bud bank may be the most responsive to future climatic change or other global change phenomena such as nutrient enrichment, and may be most resistant to exotic species invasions.     

Effectiveness of repeated autumn and spring fires for understorey restoration in weed‐invaded temperate eucalypt woodlands

Question. Can strategic burning, targeting differing ecological characteristics of native and exotic species, facilitate restoration of native understorey in weed‐invaded temperate grassy eucalypt woodlands? Location. Gippsland Plains, eastern Victoria, Australia. Methods. In a replicated, 5‐year experimental trial, the effects of repeated spring or autumn burning were evaluated for native and exotic plants in a representative, degraded Eucalyptus tereticornis grassy woodland. Treatments aimed to reduce seed banks and modify establishment conditions of exotic annual grasses, and to exhaust vegetative reserves of exotic perennial grasses. Treatments were applied to three grassland patch types, dominated by the native grass Austrodanthonia caespitosa, ubiquitous exotic annuals, or the common exotic perennial grass Paspalum dilatatum. Results. The dominant native grass Austrodanthonia caespitosa and native forbs were resilient to repeated fires, and target exotic annuals and perennials were suppressed differentially by autumn and spring fires. Exotic annuals were also suppressed by drought, reducing the overall treatment effects but indicating important opportunities for restoration. The initially sparse exotic geophyte Romulea rosea increased in cover with fire and the impact of this species on native forbs requires further investigation. There was minimal increase in diversity of subsidiary natives with fire, probably owing to lack of propagules. Conclusions. While fire is often considered to increase ecosystem invasibility, our study showed that strategic use of fire, informed by the relative responses of available native and exotic taxa, is potentially an effective step towards restoration of weed‐invaded temperate eucalypt woodlands.     

Aboveground Biomass Removal by Burning and Raking Increases Diversity in a Reconstructed Prairie

Prescribed spring burning often contributes to a predominance of C₄ grasses and low forb abundance and is impractical at many sites, especially near development. We tested raking after mowing as an alternative to prescribed burning in a reconstructed Minnesota prairie. We also tested mowing without raking as a possible means of maintaining prairie communities. Frequency, flowering stem abundance, and cover were measured for all plant species and native functional groups (C₄ grasses, C₃ graminoids, forbs, legumes, and annual or biennial forbs). Mowing alone did not differ from the control in its effect on any functional groups of plants. Round‐headed bush clover (Lespedeza capitata), a legume, and Black‐eyed Susan (Rudbeckia hirta), a biennial, increased in frequency with treatments that removed biomass (i.e., fire or raking), but they did not have significantly more flowering stems. Thus, new plants established well from seed, whereas the vitality of mature plants did not change. Raking had similar effects to burning on most functional groups, although flowering stems of C₄ grasses were significantly more abundant after fire than after raking. Burning reduced some C₃ forbs and grasses and favored the dominance of C₄ grasses. Therefore, raking after mowing in the spring provides an alternative to prescribed burning that has many of the same positive aspects as fire but does not promote aggressive C₄ grasses to the same extent.     

Competitive suppression of Quercus douglasii (Fagaceae) seedling emergence and growth

Reduced recruitment of blue oak (Quercus douglasii) seedlings in California grasslands and woodlands may result from shifts in seasonal soil water availability coincident with replacement of the native perennial herbaceous community by Mediterranean annuals. We used a combination of container and field experiments to examine the interrelationships between soil water potential, herbaceous neighborhood composition, and blue oak seedling shoot emergence and growth. Neighborhoods of exotic annuals depleted soil moisture more rapidly than neighborhoods of a perennial grass or "no-neighbor" controls. Although effects of neighborhood composition on oak seedling root elongation were not statistically significant, seedling shoot emergence was significantly inhibited in the annual neighborhoods where soil water was rapidly depleted. Seedling water status directly reflected soil water potential, which also determined the extent and duration of oak seedling growth during the first year. End-of-season seedling height significantly influenced survival and growth in subsequent years. While growth and survival of blue oak seedlings may be initially constrained by competition with herbaceous species, subsequent competition with adult blue oak trees may further contribute to reduced sapling recruitment.     

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.     

Comparison of damage to native and exotic tallgrass prairie plants by natural enemies

We surveyed the prevalence and amount of leaf damage related to herbivory and pathogens on 12 pairs of exotic (invasive and noninvasive) and ecologically similar native plant species in tallgrass prairie to examine whether patterns of damage match predictions from the enemy release hypothesis. We also assessed whether natural enemy impacts differed in response to key environmental factors in tallgrass prairie by surveying the prevalence of rust on the dominant C₄ grass, Andropogon gerardii, and its congeneric invasive exotic C₄ grass, A. bladhii, in response to fire and nitrogen fertilization treatments. Overall, we found that the native species sustain 56.4% more overall leaf damage and 83.6% more herbivore-related leaf damage when compared to the exotic species. Moreover, we found that the invasive exotic species sustained less damage from enemies relative to their corresponding native species than the noninvasive exotic species. Finally, we found that burning and nitrogen fertilization both significantly increased the prevalence of rust fungi in the native grass, while rust fungi rarely occurred on the exotic grass. These results indicate that reduced damage from enemies may in part explain the successful naturalization of exotic species and the spread of invasive exotic species in tallgrass prairie.     

Emergence and survival of herbaceous seedlings in temperate grassy woodlands: Recruitment limitations and regeneration niche

Abstract Field experiments examined herbaceous seedling emergence and survival in temperate grassy woodlands on the New England Tablelands of New South Wales. Effects of intensity of previous grazing, removal of ground cover by fire or clearing, burial of seeds, grazing and seed theft by ants on seedling emergence and survival were studied in two field experiments. Thirteen species with a range of traits were used in the experiments and their cumulative emergence was compared with laboratory germination studies. Field emergence correlated to laboratory germination but all species had lower emergence in the field. Little natural emergence of native species was observed in the field in unsown treatments. Short‐lived forbs had the highest emergence, followed by perennial grasses; rhizomatous graminoids and perennial forbs had the lowest emergence. Soil surface and cover treatments did not markedly enhance emergence suggesting that intertussock spaces were not prerequisites for forb emergence. No consistent pattern of enhanced emergence was found for any treatment combination across all species. Seedling survival varied among species, with perennial grasses and short‐lived forbs having the highest seedling mortality. Low mortality rates in the graminoids and rhizomatous forbs appeared partially to compensate for lower seedling emergence. All perennial grasses and some short‐lived forbs showed increased risk of mortality with grazing. Differences in emergence and survival of species were related to ground cover heterogeneity, soil surfaces and, to some extent, herbivory. The complexity of these patterns when superimposed on temporal variability suggests that no generalizations can be made about the regeneration niche of herbaceous species groups. Strong recruitment limitation and partitioning of resources in the regeneration niche may reduce competition among native species and explain the high species richness of the herbaceous layer in the temperate grassy communities of eastern Australia.     

Plant demographic responses to mycorrhizal symbiosis in tallgrass prairie

The effects of mycorrhizal symbiosis on seedling emergence, flowering and densities of several grasses and forbs were assessed in native tallgrass prairie and in sown garden populations at the Konza Prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed with the fungicide benomyl. Flowering and stem densities of the cool-season grass, Dichanthelium oligosanthes, sedges (Carex spp.), and the forb Aster ericoides were higher in non-mycorrhizal (benomyl-treated) than in mycorrhizal plots and the magnitude of these differences was significantly affected by burning. Mycorrhizae significantly enhanced flowering of the warmseason grasses Andropogon gerardii and Sorghastrum nutans in burned prairie, but not in unburned sites. These patterns suggest that mycorrhizal effects on the dynamics of cool-season graminoid and forb populations are likely to be mediated indirectly through effects of the symbiosis on the competitive dominance of their neighbors. Seedling emergence rates of the cool-season C₃ grasses Elymus canadensis and Koeleria cristata were significantly reduced in the benomyl-treated plots, whereas benomyl treatment had no significant effect on seedling emergence of the warm-season C₄ grasses A. gerardii and Panicum virgatum. The forbs showed variable responses. Seedling emergence of Liatris aspera was greater under mycorrhizal conditions, but that of Dalea purpurea was unaffected by mycorrhizal treatment. These results show that effects of mycorrhizal symbiosis on the population dynamics of co-occurring prairie plants vary significantly both among species and among different life history stages within species. The results also indicate that mycorrhizas and fire interact to influence competitive interactions and demographic patterns of tallgrass prairie plant populations.     

Effects of Frequent Mowing on Survival and Persistence of Forbs Seeded into a Species-Poor Grassland

Many early attempts at tallgrass prairie reconstruction failed to achieve the high species diversity of remnant prairies, and instead consist primarily of C₄ grasses. We hypothesized that frequent mowing of established prairie grasses could create sufficient gaps in the aboveground and belowground environment to allow for the establishment of native forbs from seed. We studied forb seedling establishment in a 25‐year‐old prairie planting in northern Iowa that was dominated by native warm‐season grasses. In winter 1999, 23 species of native forbs were broadcast into the recently burned sod at a rate of 350 viable seeds/m². Treatment plots were mowed weekly for either one or two growing seasons, and control plots were unmowed. Mowed plots had greater light availability than controls, especially when warm‐season grasses began to flower. Overwinter seedling mortality was 3% in mowed treatments compared to 29% in the controls. Forbs in mowed plots had significantly greater root and shoot mass than those in control plots in the first and second growing seasons but were not significantly more abundant. By the fourth growing season, however, forbs were twice as abundant in the mowed treatments. No lasting negative impacts of frequent mowing on the grass population were observed. Mowing a second year influenced species composition but did not change total seedling establishment. Experimental evidence is consistent with the idea that mowing reduced competition for light from large established grasses, allowing forb seedlings the opportunity to reach sufficient size to establish, survive, and flower in the second and subsequent years.     

The effects of burning on the understorey composition of 11–13 year-old rehabilitated bauxite mines in Western Australia – Vegetation characteristics

Alcoa of Australia Limited has been rehabilitating bauxite mines in the jarrah (Eucalyptus marginata) forest of Western Australia for more than 30 years. Mines rehabilitated in the early 1980s using out-dated methods have built up substantial fuel loads that may be reduced through prescribed burning. The vegetation response of 11–13 year-old rehabilitated bauxite mines to fire regimes differing in intensity and season over the first two years of post-burn succession is compared to the native jarrah forest. A total of 243 species from 137 genera and 56 families were identified in the native forest reference sites and in the 11–13 year-old rehabilitated areas before and after burning. The vegetation of the pre-burn rehabilitated areas was very different to that of the native jarrah forest. While total live plant cover, Acacia density, non-native eucalypt seedling density, weed density and the evenness index were similar between the two areas, total plant density, live Acacia cover, the proportion of weeds, native species numbers and diversity were significantly different. However, the greatest difference between the vegetation of the pre-burn rehabilitated sites and the native jarrah forest was the higher dominance of seeding species (plants killed by fire) in rehabilitated areas. In contrast, native jarrah forest was dominated by resprouting species (plants that survive fire). Burning the rehabilitated sites was successful in making the areas more similar to the forest in terms of total plant density, live Acacia cover and native species numbers but decreased their similarity in terms of live plant cover, Acacia density, non-native eucalypt seedling density, weed density and evenness. The vegetation response of the rehabilitated areas to different seasons of burning showed that autumn burning led to a greater increase in plant establishment than spring burning. Autumn burning also resulted in an undesirable increase in the density of non-native eucalypt seedlings that was not observed following spring burning. Although burning these 11–13 year-old rehabilitated sites will increase similarity to the native forest, it is unlikely that they will resemble the native jarrah forest without further management intervention.     

The effect of soil nitrogen on competition between native and exotic perennial grasses from northern coastal California

The invasion of European perennial grasses represents a new threat to the native coastal prairie of northern California. Many coastal prairie sites also experience anthropogenic nitrogen (N) deposition or increased N availability as a result of invasion by N-fixing shrubs. We tested the hypothesis that greater seedling competitive ability and greater responsiveness to high N availability of exotic perennial grasses facilitates their invasion in coastal prairie. We evaluated pairwise competitive responses and effects, and the occurrence of asymmetrical competition, among three common native perennial grasses (Agrostis oregonensis, Festuca rubra, and Nassella pulchra) and three exotic perennial grasses (Holcus lanatus, Phalaris aquatica, and Festuca arundinacea), at two levels of soil N. We also compared the root and shoot biomass and response to fertilization of singly-grown plants, so we could evaluate how performance in competition related to innate plant traits. Competitive effects and responses were negatively correlated and in general varied continuously across native and exotic species. Two exceptions were the exotic species Holcus, which had large effects on neighbors and small responses to them, and competed asymmetrically with all other species in the experiment, and the native grass Nassella, which had strong responses to but little effect on neighbors, and was out-competed by all but one other species in the experiment. High allocation to roots and high early relative growth rate appear to explain Holcus’s competitive dominance, but its shoot biomass when grown alone was not significantly greater than those of the species it out-competed. Competitive dynamics were unaffected by fertilization. Therefore, we conclude that seedling competitive ability alone does not explain the increasing dominance of exotic perennial grasses in California coastal prairie. Furthermore, since native and exotic species responded individualistically, grouping species as ‘natives’ and ‘exotics’ obscured underlying variation within the two categories. Finally, elevated soil N does not appear to influence competition among the native and exotic perennial grasses studied, so reducing soil N pools may not be a critical step for the restoration of California coastal prairie.     

Postwildfire seeding to restore native vegetation and limit exotic annuals: an evaluation in juniper‐dominated sagebrush steppe

Reestablishment of perennial vegetation is often needed after wildfires to limit exotic species and restore ecosystem services. However, there is a growing body of evidence that questions if seeding after wildfires increases perennial vegetation and reduces exotic plants. The concern that seeding may not meet restoration goals is even more prevalent when native perennial vegetation is seeded after fire. We evaluated vegetation cover and density responses to broadcast seeding native perennial grasses and mountain big sagebrush (Artemisia tridentata Nutt. spp. vaseyana [Rydb.] Beetle) after wildfires in the western United States in six juniper (Juniperus occidentalis ssp. occidentalis Hook)‐dominated mountain big sagebrush communities for 3 years postfire. Seeding native perennial species compared to not seeding increased perennial grass and sagebrush cover and density. Perennial grass cover was 4.3 times greater in seeded compared to nonseeded areas. Sagebrush cover averaged 24 and less than 0.1% in seeded and nonseeded areas at the conclusion of the study, respectively. Seeding perennial species reduced exotic annual grass and annual forb cover and density. Exotic annual grass cover was 8.6 times greater in nonseeded compared to seeded areas 3 years postfire. Exotic annual grass cover increased over time in nonseeded areas but decreased in seeded areas by the third‐year postfire. Seeded areas were perennial‐dominated and nonseeded areas were annual‐dominated at the end of the study. Establishing perennial vegetation may be critical after wildfires in juniper‐dominated sagebrush steppe to prevent the development of annual‐dominated communities. Postwildfire seeding increased perennial vegetation and reduced exotic plants and justifies its use.     

Mid-Spring Burning Reduces Spotted Knapweed and Increases Native Grasses during a Michigan Experimental Grassland Establishment

Infestations of the exotic perennial Spotted knapweed (Centaurea maculosa Lam.) hinder the restoration and management of native ecosystems on droughty, infertile sites throughout the Midwestern United States. We studied the effects of annual burning on knapweed persistence on degraded, knapweed‐infested gravel mine spoils in western Michigan. Our experiment included 48, 4‐m² plots seeded to native warm‐season grasses in 1999 using a factorial arrangement of initial herbicide and fertility treatments. Beginning in 2003, we incorporated fire as an additional factor and burned half of the plots in late April or May for 3 years (2003–2005). Burning increased the dominance of warm‐season grasses and decreased both biomass and dominance of knapweed in most years. Burning reduced adult knapweed densities in all 3 years of the study, reduced seedling densities in the first 2 years, and reduced juvenile densities in the last 2 years. Knapweed density and biomass also declined on the unburned plots through time, suggesting that warm‐season grasses may effectively compete with knapweed even in the absence of fire. By the end of the study, mean adult knapweed densities on both burned (0.4‐m²) and unburned (1.3‐m²) plots were reduced to levels where the seeded grasses should persist with normal management, including the use of prescribed fire. These results support the use of carefully timed burning to help establish and maintain fire‐adapted native plant communities on knapweed‐infested sites in the Midwest by substantially reducing knapweed density, biomass, and seedling recruitment and by further shifting the competitive balance toward native warm‐season grasses.