Leaf nitrogen productivity as a mechanism driving the success of invasive annual grasses under low and high nitrogen supply

Invasion of the historically perennial-dominated landscapes in the Great Basin by exotic winter annual grasses is one of the most serious plant invasions in North America. Evidence suggests invasive annuals outperform native perennials under N-poor and N-rich conditions. The objective of this study was to identify key traits contributing to the success of invasive annual grasses in these environments. Three invasive annual grasses, two native perennial grasses and one introduced perennial grass were exposed to three levels of N supply. Root biomass, root length, root N uptake rate, root and leaf morphology, leaf nitrogen productivity (leaf NP) as well as biomass and N allocation were quantified over four harvests. Path analysis indicated that leaf NP was the key trait contributing to variation in N capture among the species. Species with a higher leaf NP produced more root length and consequently captured more N under a range of soil N availability. This suggests variation in leaf NP may be one critical trait determining the ability of the resident plant community to resist establishment of these invaders. Restoration programs may be able to increase weed resistance by specifically selecting for this trait in revegetation efforts.     

Role of competition in restoring resource poor arid systems dominated by invasive grasses

Understanding the role competition intensity and importance play in directing vegetation dynamics is central to developing restoration strategies, especially in resource poor environments. We hypothesized 1) competition would be intense among invasive and native species, but 2) competition would be unimportant in explaining variation in target plant biomass and survivorship relative to other factors driving these variables. We performed a two year addition series field experiment to quantify competition intensity and importance. Densities of two invasive (cheatgrass and medusahead) and two native (Sandberg’s bluegrass and bluebunch wheatgrass) species were arranged in monocultures and mixtures of two, three and four species, producing varying total densities and species proportions. Multiple linear regression models predicting individual plant biomass and survivorship were developed. Based on biomass, competition intensity coefficients ranged from −0.38 to 0.63 with R² < 0.06. All survivorship data produced poor fitting regression models (R² < 0.05). Our results suggest neither competition intensity nor importance influenced plant dominance in resource poor environments during the two years of establishment. Land managers may be more successful at restoration of resource poor ecosystems by overcoming abiotic barriers to plant establishment rather than focusing on plant-plant interactions.     

The influence of plant removal on succession in Wyoming big sagebrush

Restoration treatments are based on the largely untested notion that desired recovery of plant communities following disturbance wouldn’t occur in the absence of active intervention. We identified rate of short-term (10 year) floristic changes following removal of plant functional groups in Wyoming big sagebrush plant communities in 1999-2005 and 2008. Treatments imposed on 6 × 6 m plots were: 1) removal of all plant functional groups, 2) perennial grass removal, 3) shrub removal and 4) control. Our data suggest recovery of the shrub component on shrub removal plots could take decades. Similarly, perennial grass cover and density on perennial grass removal plots was less than half that of unaltered plots 10 years after treatment. When all functional groups were removed, cover of annual forbs, annual grasses, and shrubs returned to unaltered levels within ten years or less. Perennial forbs were unaffected (p > 0.05) by treatment. The fact that natural recovery of some components occurred within a relatively short post-disturbance time interval (i.e. <10 years) suggests that intervention may not be necessary for some functional groups. Restoring shrubs in areas dominated by perennial grasses may require targeted reductions of competing perennial grasses. Conversely, shrub dominance may limit perennial grass re-establishment.     

Competition as a barrier to establishment of a native perennial grass (Elymus elymoides) in alien annual grass (Bromus tectorum) communities

The alien grass Bromus tectorum dominates stable annual-plant communities that have replaced native shrub-perennial grass communities over much of the semi-arid western United States. We conducted field competition experiments between B. tectorum and a native grass, Elymus elymoides, on two sites to determine the effects of B. tectorum competition on perennial grasses, and the role of B. tectorum competition in the stability of B. tectorum-dominated communities. B. tectorum competition acting on seedling-stage E. elymoides plants greatly reduced first-year relative growth rates and biomass which, in turn, reduced second-year survival, biomass, and flowering. However, B. tectorum competition acting on older E. elymoides plants had much less effect, which may help to explain why intact perennial-plant communities are resistant to B. tectorum invasion. At the drier site, direct effects of B. tectorum competition were less, but competition and drier habitat combined to produce greater E. elymoides mortality.     

Is exotic plant invasion enhanced by a traditional wildlife habitat management technique?

Soil disturbance is a wildlife habitat management tool that retards succession and promotes early seral vegetation. Our objective was to determine responses of two invasive herbaceous species (Pennisetum ciliare and Salsola iberica) and native perennial grasses to disking on different soils. Two 10 ×40 m plots were delineated within each of 4 blocks on Ramadero loams and 4 blocks on Delmita fine sandy loams. On Delmita soils, canopy cover of P. ciliare, S. iberica, and native perennial grasses averaged across all years was not affected by disking (ANOVA, P>0.05). On Ramadero soils, P. ciliare canopy cover was similar (Tukey's, P>0.05) on control and disked plots for the first 4 years post-disturbance, but P. ciliare cover was 10-fold greater (Tukey's, P=0.02) the 5th year after disking on disked versus control plots. On Ramadero soils, S. iberica canopy cover averaged across all years was 221 times greater (ANOVA, P=0.05) on disked plots than on control plots. Disking did not affect native perennial grass canopy cover. Land managers should consider soil series when disking for wildlife management, as disking disturbance may exacerbate exotic plant ingress and establishment on certain soils.     

Effects of overgrazing and rainfall variability on the dynamics of semiarid banded vegetation patterns: A simulation study with cellular automata

The effects of factors acting at small scale (grass/grass and grass/shrub biological interactions) and landscape scale (overgrazing and rainfall changes) on the development and permanence of banded vegetation patterns (VB) are assessed with a spatially explicit cellular automaton model. In particular, the influence of two environmental factors that are changing in many VB arid lands (rainfall and grazing pressures) is studied. Three rainfall regimes (representing 0.5, 1.0, and 2.0 times the long-term rainfall average), five grazing intensities, two types (reversible and irreversible) of grazing disturbance, two grazing periodicities (chronic and pulse), two levels of grass colonization ability, positive and negative interactions between shrubs and grasses, and the efficiency of endozoochorous seed dispersal are simulated.The results show that the permanence of undisturbed VB depends on the interaction of two factors, rainfall regime and grass colonization ability. Type and intensity of grazing also modify VB cover and permanence; furthermore, long-term overgrazing may convert mixed grass/shrub plant communities to pure scrublands dominated by endozoochorous-dispersed shrubs due to competitive interactions between shrubs and grasses.Besides providing an adequate representation of the system’s dynamics, the model is a useful tool that may be used to explore the consequences of climate change on management scenarios.     

Effects of fire, grazing, and the presence of shrubs on Chihuahuan desert grasslands

Responses of herbaceous and suffrutescent species to fire, grazing, and presence of Prosopis glandulosa were examined in a Chihuahuan desert grassland in south-central New Mexico. Treatments were assigned randomly to eight 12×8 m plots within each of two blocks. Following fires in June 1995, unfenced plots were exposed to livestock grazing over 4 years. Plots were established that either included or excluded P. glandulosa. Perennial grass cover, primarilyBouteloua eriopoda , decreased by 13% in burned plots but increased 5% in unburned areas. Conversely, perennial forb cover was 4% greater after fire. Perennial grass frequency decreased 30% more and perennial forb frequency increased 10% more following burning. Further, increases in evenness after fire resulted in a 225% increase in species diversity. Grazing also resulted in a decrease in perennial grass cover while frequency decreased 22% more in grazed than ungrazed plots. Only frequency and not cover of perennial forbs and annual grasses increased more following grazing. Presence of P. glandulosa had no differential effect on responses of non-shrub species. Fires were conducted during near drought conditions while grazing occurred during years of precipitation equivalent to the long-term average. Precipitation immediately following fire may be critical for recovery of B. eriopoda -dominated desert grasslands; relationships between fire and post-fire precipitation patterns require future investigation.     

Studying the post-fire performance of tussock grasses in Patagonia: Survival, biomass production and early competition

Patagonia grasslands are subjected to two main disturbances, fire and grazing, but little information is available about its effects on vegetation. We studied post-fire survival and resprouting ability of two dominant grass species, Stipa speciosa and Festuca pallescens, for four years; evaluated the effect of early post-fire defoliation on both species; and tested whether competition is important in post-fire recovery in San Ramón Ranch, NW Patagonia (Argentina). To simulate grazing, a clipping treatment was applied at the beginning and end of growing seasons. Survival rates were high (>60%) and, after three years, biomass of both species in the burned area was similar to the unburned area. Competition seems to play an important role in the early post-fire recovery of both species, particularly in the case of F. pallescens that increased 87% the biomass production without competition. Fire may improve forage quality by eliminating the standing dead material, but early post-fire grazing might endanger the persistence of F. pallescens. We suggest requiring a resting period before livestock introduction and controlling grazing intensity.     

Response of the shortgrass steppe plant community to fire

Fire is an important driver of ecological pattern and process in grasslands worldwide, although its role in semi-arid systems is less well known. We used published studies and new experimental research to 1) provide a synthesis of existing knowledge of fire in the semi-arid grasslands of the North American Great Plains, and 2) assess the degree of similarity in semi-arid and mesic grassland responses to fire in this region. Based on published studies, burning has neutral to negative effects on aboveground productivity in semi-arid grasslands and variable effects on plant communities. To more rigorously assess fire effects, replicated experimental plots were established in ungrazed shortgrass steppe in northern Colorado and prescribed spring fire was applied in 2006 and 2007, 2006 only, or not at all. Aboveground net primary productivity decreased or remained unchanged with burning. Plant community changes included increases in perennial forbs, decreases in annual grasses and a positive response in annual forbs to the combination of fire and a wet spring in 2007. Combined, these results indicate that post-fire changes in productivity in semi-arid grasslands are neutral to negative, in contrast to positive responses in mesic grasslands, and not strongly negative as previously assumed.     

Spatial and temporal patterns of wildfires in the Mojave Desert, 1980–2004

Fire has been historically infrequent in the Mojave Desert, and its increased prevalence caused by the invasion of non-native annual grasses is a major concern for land managers there. The most dramatic changes have occurred in middle elevation shrublands dominated by creosotebush (Larrea tridentata), Joshua tree (Yucca brevifolia), and/or blackbrush (Coleogyne ramossissima), where most of the fires occurred between 1980 and 2004. This zone is more susceptible than other areas of the Mojave Desert to increased fire size following years of high rainfall. Increases in fire size are likely related to the flush of non-native annual grasses, Bromus rubens in particular, that produces continuous fuelbeds following years of high rainfall. This dynamic also has occurred to some degree at lower elevations, but the background cover of native perennial fuels there is already very low, muting the effects of the ephemeral fuels. At elevations above the middle elevation shrublands, fire size does not vary with rainfall, indicating that native woody fuels dictate fire regimes. These results suggest that an invasive plant/fire regime cycle is currently establishing in the middle and possibly the low elevation shrublands of the Mojave Desert, but not at higher elevations.     

Seedbank responses to spring and fall prescribed fire in mountain big sagebrush ecosystems of differing ecological condition at Lava Beds National Monument,California

Land use change in the American West has altered sagebrush dominated ecosystems, facilitating exotic grass invasion, increased woody species, altered fire regimes, and native species losses. Restoring fire regimes may be integral to the restoration of sagebrush ecosystems. However, little is known about how the timing of prescribed fire differentially impacts the soil seedbank. Further, sites of different ecological condition may variably respond to fire. To address these uncertainties, we quantified germination of invasive cheatgrass and plant functional groups (native dicots, exotic dicots, native bunchgrasses, and native shrubs) following spring and fall prescribed burns in three mountain big sagebrush communities at Lava Beds National Monument, California. At cheatgrass and native-dominated sites, there were fewer cheatgrass seedlings (91% and 86%, respectively) immediately following spring fire than in unburned controls, but this reduction did not persist one year later. Fall burns did not significantly impact cheatgrass or exotic dicot germination. At a native dominated site, native dicots increased one year following spring (1620%) and fall burns (67%), suggesting adaptation to a natural fire regime. This research shows that the prefire condition of an ecosystem greatly influences the postfire germination response, and that fire-adapted ecosystems can benefit from the natural disturbance regime.     

Changes in rainfall amount and frequency do not affect the outcome of the interaction between the shrub Retama sphaerocarpa and its neighbouring grasses in two semiarid communities

We evaluated the net outcome of the interaction between the shrub Retama sphaerocarpa, our target plant, and different herbaceous neighbours in response to changes in the magnitude and frequency of rainfall events during three years. The experiment was conducted in natural and anthropogenic grasslands dominated by a perennial stress-tolerator and ruderal annual species, respectively. In spite of the neutral or positive effects of neighbours on water availability, neighbouring plants reduced the performance of Retama juveniles, suggesting competition for resources other than water. The negative effects of grasses on the photochemical efficiency of Retama juveniles decreased with higher water availabilities or heavier irrigation pulses, depending on the grassland studied; however, these effects did not extent to the survival and growth of Retama juveniles. Our findings show the prevalence of competitive interactions among the studied plants, regardless of the water availability and its temporal pattern. These results suggest that positive interactions may not prevail under harsher conditions when shade-intolerant species are involved. This study could be used to further refine our predictions of how plant–plant interactions will respond to changes in rainfall, either natural or increased by the ongoing climatic change, in ecosystems where grass–shrubs interactions are prevalent.     

Mechanical shaking and soil water affect the growth of Psammochloa villosa in the Mu Us Sandland

The Mu Us Sandland is basically characterized by water shortage and high wind. Thus, wind-induced mechanical perturbation (MP) and soil water availability are likely to interact to affect plant growth. Since high water availability and MP can induce responses that are in the opposite direction, we hypothesized that MP effects on perennial grasses might be mitigated by increased soil water availability in the Mu Us Sandland. We counducted an experiment in which seedlings of Psammochloa villosa were subjected to two levels of MP (non-MP vs. MP 1 min d⁻¹) and two levels of water availability (200 ml d⁻¹vs. 400 ml d⁻¹) and measured three plant traits. MP significantly decreased plant height, total biomass, and root/shoot ratio. There were significant interactions between MP and soil water availability on plant height and root/shoot ratio. These findings imply that MP alone is a stressful factor for P. villosa and MP effects on its growth can be partially mitigated by increased soil water availability, and also suggest that P. villosa may respond to MP in a way that allows plants to survive in the windy semiarid environments.     

Soil morphology and organic matter dynamics under cheatgrass and sagebrush-steppe plant communities

Widespread cheatgrass (Bromus tectorum L.) invasion represents a major shift in species dominance that may alter ecosystem processes across much of the western US. To investigate differences following such conversion, soil morphology and organic matter under cheatgrass-dominated and native shrub-steppe vegetation were compared by standard soil analysis procedures at seven paired sites in Idaho and Utah. Results suggest that, following conversion to cheatgrass dominance, increased porosity and labile organic inputs enhance microbial decomposition in near-surface horizons beneath cheatgrass compared to adjacent soils under native vegetation. Enhanced decomposition could result in depletion of long-term SOM, leading to impoverished sites difficult to restore to native perennial vegetation.     

Restoration of palatable grasses: A study case in degraded rangelands of central Argentina

Restoration of palatable grasses on degraded rangelands dominated by unpalatable grasses in central Argentina is limited by low availability of seeds and safe sites for seedling establishment. The objective of our study was to determine how mechanical disturbance of unpalatable grasses (Stipa spp.) in combination with seeding of a palatable grass (Poa ligularis) influenced species composition in a degraded rangeland excluded from livestock grazing. In April 2001 10 blocks were uniformly distributed on a previously burned site dominated by unpalatable grasses, and treatments applied in 8 m×8 m experimental plots. Treatments were ‘disked and seeded’ and control (no disking, no seeding). Perennial plant cover and end-of-season standing crop, at species or species group level, were assessed in December 2002/2003 and in December 2004/2006, respectively. P. ligularis out-competed both tillers of unpalatable grasses that survived mechanical disturbance and seedlings of unpalatable grasses established after mechanical disturbance. The cover and end-of-season standing crop of unpalatable grasses was higher in the control than in the ‘disked and seeded’ treatment, whereas the cover and end-of-season standing crop of P. ligularis was higher in the latter than in the former treatment. Our results suggest that a rapid transition from a state dominated by unpalatable grasses to a state dominated by palatable grasses can be achieved by mechanical disturbance of unpalatable grasses in combination with seeding of palatable grasses in semiarid rangelands of central Argentina.     

An assessment of revegetation treatments following removal of invasive Pennisetum ciliare (buffelgrass)

In semi-arid regions of North America and Australia, Pennisetum ciliare (L.) Link (syn. Cenchrus ciliaris; buffelgrass) is highly invasive and has the potential to introduce fire to fire-intolerant ecosystems. Major efforts to remove P. ciliare continue and it is essential that P. ciliare be prevented from recolonizing. This study investigated potential methods to revegetate with native herbaceous plants: sowing seeds; sowing seeds and mulching; sowing seeds and transplanting seedlings; and relying on natural revegetation from the seedbank. The treatments were applied in 2009 and 2010 at sites in the Sonoran Desert which had undergone P. ciliare removal. Monsoon precipitation was below average each year and seedling emergence and establishment rates were low. There are indications that soil disturbance associated with planting seedlings promoted P. ciliare emergence and increased mortality of brittlebush (Encelia farinosa A. Gray ex Torr.), a common native perennial forb. Addition of mulch may have promoted P. ciliare over native grasses, and seeding had no effect. We did not find competition between herbaceous seedlings. Rather, native and exotic grass seedling densities were positively correlated across sites. Under prevailing conditions, low precipitation appeared to limit herbaceous plant establishment and none of the treatments reduced P. ciliare abundance.     

科尔沁沙地草地植被对围封和放牧的响应

研究了科尔沁沙地疏林草地、针茅草原和草甸植被盖度、地上和地下生物量、物种多样性对围封和放牧的响应。结果表明：（1）围封与放牧草地的优势植物不同,围封草地植物群落优势植物为多年生禾本科植物,放牧草地中一年生植物和小半灌木优势明显。（2）围封和放牧草地的植物盖度、凋落物量、地上生物量和物种丰富度存在明显差异（P<0.05）;围封显著提高了植被盖度和地上生物量,由疏林草地、针茅草原到草甸,植物盖度和地上生物量逐渐增加,而3种草地植被凋落物量大小顺序为针茅草原> 疏林草地> 草甸;放牧条件下植物盖度、凋落物量和物种丰富度差异不显著（P>0.05）。（3）3种草地之间的地下生物量无显著差异（P>0.05）,但围封与放牧之间、不同土壤层次之间地下生物量存在明显差异（P<0.05）;围封显著提高草地的地下生物量（P<0.05）;草地地下生物量随着土壤深度表现出下降趋势（P<0.05）。长期放牧增加了草地一年生植物和小半灌木植物的优势,消除了不同草地之间植被盖度和物种丰富度的差异;而围封能提高草地多年生禾本科植物的优势、增加其物种丰富度,对于草地质量和植物多样性的恢复和保育具有积极作用。     

Absence of interactions between perennial bunchgrasses in a semi-arid temperate savanna: a 5-year field experiment

There is widespread recognition of the importance of long-term experimental studies of plant interactions, but such experiments are rarely conducted. Pairwise interactions between three species of perennial grasses were assessed and described over a 5-year period. Experimental treatments in which some or all neighbors were removed were maintained and plant basal area was monitored throughout the experiment. In addition, above-ground biomass was collected over a 3-year period. Repeated-measures analysis of variance revealed no evidence of interactions between plants.     

Effects of fire on rangeland vegetation in south-western Mendoza plains (Argentina): composition,frequency, biomass,productivity and carrying capacity

The effect of both a non-prescribed summer fire and grazing at high stocking rate following fire on plant community composition, the frequency of occurrence of bare soil, grasses and shrubs, species diversity and biomass of herbaceous forage were evaluated for three growing seasons after fire. Changes in community composition occurred as a consequence of both fire and grazing. Communities were dominated by unpalatable shrubs and grasses in unburned sites as a result of a long history of overgrazing. Fire contributed to a conversion of those shrublands to communities with a more favourable balance between woody and herbaceous species. The frequency of palatable grasses and herbaceous forage biomass increased by a factor ofc. 3 at the expense of woody vegetation. Grazing after fire had significant positive (i.e. decrease in undesirable grasses) and negative (i.e. increase in bare soil) effects.     

Restoring habitat for riparian birds in the lower Colorado River watershed: An example from the Las Vegas Wash, Nevada

The success of riparian restoration projects in the arid southwestern U.S. is often measured in terms of vegetation characteristics such as growth, cover, and structure. Among low-elevation riverine environments within the Colorado River watershed, restoration is typically conducted to improve degraded habitats for birds of conservation concern by replacing the exotic tamarisk (Tamarix ramosissima) with native cottonwoods (Populus spp.) and willows (Salix spp.). The working assumption for many restoration practitioners is that replacing exotic plants with native plants will improve habitat quality and will, therefore, benefit birds. Based on data collected at exotic and restored (i.e., native) sites along the Las Vegas Wash, Nevada, not all birds benefit from restoration. Broad measures of community benefit, including benefits to birds of conservation concern and riparian obligate/dependent birds, were not detected. There were, however, some species-specific benefits. Some environmental variables that were associated with exotic and native sites were significant in explaining the composition of the bird community. For example, the richness of forbs and grass-like plants (a proxy of soil moisture), invertebrate mass, and percent shade (a proxy for canopy characteristics) were important. Considering our results and depending on restoration goals, tamarisk replacement projects may not inherently provide benefits to birds.