Effects of invasive plants on fire regimes and postfire vegetation diversity in an arid ecosystem

We assessed the impacts of co‐occurring invasive plant species on fire regimes and postfire native communities in the Mojave Desert, western USA. We analyzed the distribution and co‐occurrence patterns of three invasive annual grasses (Bromus rubens, Bromus tectorum, and Schismus spp.) known to alter fuel conditions and community structure, and an invasive forb (Erodium cicutarium) which dominates postfire sites. We developed species distribution models (SDMs) for each of the four taxa and analyzed field plot data to assess the relationship between invasives and fire frequency, years postfire, and the impacts on postfire native herbaceous diversity. Most of the Mojave Desert is highly suitable for at least one of the four invasive species, and 76% of the ecoregion is predicted to have high or very high suitability for the joint occurrence of B. rubens and B. tectorum and 42% high or very high suitability for the joint occurrence of the two Bromus species and E. cicutarium. Analysis of cover from plot data indicated two or more of the species occurred in 77% of the plots, with their cover doubling with each additional species. We found invasive cover in burned plots increased for the first 20 years postfire and recorded two to five times more cover in burned than unburned plots. Analysis also indicated that native species diversity and evenness as negatively associated with higher levels of relative cover of the four invasive taxa. Our findings revealed overlapping distributions of the four invasives; a strong relationship between the invasives and fire frequency; and significant negative impacts of invasives on native herbaceous diversity in the Mojave. This suggests predicting the distributions of co‐occurring invasive species, especially transformer species, will provide a better understanding of where native‐dominated communities are most vulnerable to transformations following fire or other disturbances.     

Increases in Desert Shrub Productivity under Elevated Carbon Dioxide Vary with Water Availability

Productivity of aridland plants is predicted to increase substantially with rising atmospheric carbon dioxide (CO₂) concentrations due to enhancement in plant water-use efficiency (WUE). However, to date, there are few detailed analyses of how intact desert vegetation responds to elevated CO₂. From 1998 to 2001, we examined aboveground production, photosynthesis, and water relations within three species exposed to ambient (around 38 Pa) or elevated (55 Pa) CO₂ concentrations at the Nevada Desert Free-Air CO₂ Enrichment (FACE) Facility in southern Nevada, USA. The functional types sampled—evergreen (Larrea tridentata), drought-deciduous (Ambrosia dumosa), and winter-deciduous shrubs (Krameria erecta)—represent potentially different responses to elevated CO₂ in this ecosystem. We found elevated CO₂ significantly increased aboveground production in all three species during an anomalously wet year (1998), with relative production ratios (elevated:ambient CO₂) ranging from 1.59 (Krameria) to 2.31 (Larrea). In three below-average rainfall years (1999–2001), growth was much reduced in all species, with only Ambrosia in 2001 having significantly higher production under elevated CO₂. Integrated photosynthesis (mol CO₂ m⁻² y⁻¹) in the three species was 1.26–2.03-fold higher under elevated CO₂ in the wet year (1998) and 1.32–1.43-fold higher after the third year of reduced rainfall (2001). Instantaneous WUE was also higher in shrubs grown under elevated CO₂. The timing of peak canopy development did not change under elevated CO₂; for example, there was no observed extension of leaf longevity into the dry season in the deciduous species. Similarly, seasonal patterns in CO₂ assimilation did not change, except for Larrea. Therefore, phenological and physiological patterns that characterize Mojave Desert perennials—early-season lags in canopy development behind peak photosynthetic capacity, coupled with reductions in late-season photosynthetic capacity prior to reductions in leaf area—were not significantly affected by elevated CO₂. Together, these findings suggest that elevated CO₂ can enhance the productivity of Mojave Desert shrubs, but this effect is most pronounced during years with abundant rainfall when soil resources are most available.     

Variation in the establishment of a non-native annual grass influences competitive interactions with Mojave Desert perennials

Competition between native and non-native species can change the composition and structure of plant communities, but in deserts, the highly variable timing of resource availability also influences non-native plant establishment, thus modulating their impacts on native species. In a field experiment, we varied densities of the non-native annual grass Bromus madritensis ssp. rubens around individuals of three native Mojave Desert perennials—Larrea tridentata, Achnatherum hymenoides, and Pleuraphis rigida—in either winter or spring. For comparison, additional plots were prepared for the same perennial species and seasons, but with a mixture of native annual species as neighbors. Growth of perennials declined when Bromus was established in winter because Bromus stands had 2–3 months of growth and high water use before perennial growth began. However, water potentials for the perennials were not significantly reduced, suggesting that direct competition for water may not be the major mechanism driving reduced perennial growth. The impact of Bromus on Larrea was lower than for the two perennial grasses, likely because Larrea maintains low growth rates throughout the year, even after Bromus has completed its life cycle. This result contrasts with the perennial grasses, whose phenology completely overlaps with (Achnatherum) or closely follows (Pleuraphis) that of Bromus. In comparison, Bromus plants established in spring were smaller than those established in winter and thus did not effectively reduce growth of the perennials. Growth of perennials with mixed annuals as neighbors also did not differ from those with Bromus neighbors of equivalent biomass, but stands of these native annuals did not achieve the high biomass of Bromus stands that were necessary to reduce perennial growth. Seed dormancy and narrow requirements for seedling survivorship of native annuals produce densities and biomass lower than those achieved by Bromus; thus, impacts of native Mojave Desert annuals on perennials are expected to be lower than those of Bromus.     

Identifying Native Vegetation for Reducing Exotic Species during the Restoration of Desert Ecosystems

There is currently much interest in restoration ecology in identifying native vegetation that can decrease the invasibility by exotic species of environments undergoing restoration. However, uncertainty remains about restoration's ability to limit exotic species, particularly in deserts where facilitative interactions between plants are prevalent. Using candidate native species for restoration in the Mojave Desert of the southwestern U.S.A., we experimentally assembled a range of plant communities from early successional forbs to late‐successional shrubs and assessed which vegetation types reduced the establishment of the priority invasive annuals Bromus rubens (red brome) and Schismus spp. (Mediterranean grass) in control and N‐enriched soils. Compared to early successional grass and shrub and late‐successional shrub communities, an early forb community best resisted invasion, reducing exotic species biomass by 88% (N added) and 97% (no N added) relative to controls (no native plants). In native species monocultures, Sphaeralcea ambigua (desert globemallow), an early successional forb, was the least invasible, reducing exotic biomass by 91%. However, the least‐invaded vegetation types did not reduce soil N or P relative to other vegetation types nor was native plant cover linked to invasibility, suggesting that other traits influenced native‐exotic species interactions. This study provides experimental field evidence that native vegetation types exist that may reduce exotic grass establishment in the Mojave Desert, and that these candidates for restoration are not necessarily late‐successional communities. More generally, results indicate the importance of careful native species selection when exotic species invasions must be constrained for restoration to be successful.     

Winter cold tolerance and the geographic range separation of Bromus tectorum and Bromus rubens,two severe invasive species in North America

The invasive grasses Bromus rubens and Bromus tectorum are responsible for widespread damage to semiarid biomes of western North America. Bromus. tectorum dominates higher and more northern landscapes than its sister species B. rubens, which is a severe invader in the Mojave desert region of the American Southwest. To assess climate thresholds controlling their distinct geographic ranges, we evaluated the winter cold tolerance of B. tectorum and B. rubens. Freezing tolerance thresholds were determined using electrolyte leakage and whole‐plant mortality. The responses of the two species to winter cold and artificial freezing treatments were similar in 2007–2008 and 2009–2010. When grown at minimum temperatures of 10 °C, plants of both species had cold tolerance thresholds near ?10 °C, while plants acclimated to a daily minimum of ?10 to ?30 °C survived temperatures down to ?31 °C. In the winter of 2010–2011, a sudden severe cold event on December 9, 2010 killed all B. rubens populations, while B. tectorum was not harmed; all tested plants were 7–8 weeks old. Controlled acclimation experiments demonstrated that 8‐week‐old plants of B. rubens had a slower acclimation rate to subzero temperatures than B. tectorum and could not survive a rapid temperature drop from 1 to ?14 °C. Four‐month‐old B. rubens populations were as cold tolerant as B. tectorum. Our results show that severe and sudden freeze events in late autumn can kill young plants of B. rubens but not B. tectorum. Such events could exclude B. rubens from the relatively cold, Intermountain steppe biome of western North America where B. tectorum predominates.     

The effects of parental CO2 environment on seed quality and subsequent seedling performance in Bromusrubens

Seeds were collected and compared from parent plants of Bromusrubens L. (Poaceae), an exotic Mojave Desert annual grass, grown in ambient (360 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂ to determine if parental CO₂ growth conditions affected seed quality. Performance of seeds developed on the above plants was evaluated to determine the influence of parental CO₂ growth conditions on germination, growth rate, and leaf production. Seeds of B. rubens developed on parents grown in elevated CO₂ had a larger pericarp surface area, higher C:N ratio, and less total mass than ambient-developed seeds. Parental CO₂ environment did not have an effect on germination percentage or mean germination time, as determined by radicle emergence. Seedlings from elevated-CO₂-developed seeds had a reduced relative growth rate and achieved smaller final mass over the same growth period. Elevated-CO₂-developed seeds had smaller seed reserves than ambient seeds, as determined by growing seedlings in sterile media and monitoring senescence. It appears that increased seed C:N ratios associated with plants grown under elevated CO₂ may have a major effect on seed quality (morphology, nutrition) and seedling performance (e.g., growth rate and leaf production). Since the invasive success of B. rubens is primarily due to its ability to rapidly germinate, increase leaf area and maintain a relatively high growth rate compared to native annuals and perennial grasses, reductions in seed quality and seedling performance in elevated CO₂ may have significant impacts on future community composition in the Mojave Desert. Received: 11 April 1997 / Accepted: 20 November 1997     

Pre-mission invasion of Erodium cicutarium in California

Abstract. The California grassland is dominated by alien plant species. It is generally assumed that the invasion of aliens began with the initial introduction of livestock by Spanish missionaries in 1769. In this paper we present pollen evidence which indicates that Erodium cicutarium, a Mediterranean annual, was well established in the Santa Barbara region several years before the founding of the first California mission at San Diego in 1769. Historical evidence shows that it took the Spanish nearly a decade to develop a livestock base in California. Disturbance by livestock was therefore not a necessary prerequisite for invasion by alien plants. Historical and macrofossil evidence indicates that E. cicutarium invaded California from Baja California.     

Contemporary evolution of an invasive grass in response to elevated atmospheric CO2 at a Mojave Desert FACE site

Elevated atmospheric CO₂ has been shown to rapidly alter plant physiology and ecosystem productivity, but contemporary evolutionary responses to increased CO₂ have yet to be demonstrated in the field. At a Mojave Desert FACE (free‐air CO₂ enrichment) facility, we tested whether an annual grass weed (Bromus madritensis ssp. rubens) has evolved in response to elevated atmospheric CO₂. Within 7 years, field populations exposed to elevated CO₂ evolved lower rates of leaf stomatal conductance; a physiological adaptation known to conserve water in other desert or water‐limited ecosystems. Evolution of lower conductance was accompanied by reduced plasticity in upregulating conductance when CO₂ was more limiting; this reduction in conductance plasticity suggests that genetic assimilation may be ongoing. Reproductive fitness costs associated with this reduction in phenotypic plasticity were demonstrated under ambient levels of CO₂. Our findings suggest that contemporary evolution may facilitate this invasive species' spread in this desert ecosystem.     

Life histories of desert geophytes —the demographic consequences of reproductive biomass partitioning patterns

Summary During five consecutive growing seasons (winters) ca. 110 plants of the desert geophyte Bellevalia desertorum were marked individually in a 25 m² plot on a south-facing slope in the central Negev Desert of Israel. The number of rosette leaves of each plant was recorded, as well as whether it flowered and produced seeds. Multiple regression analysis of the data on the B. desertorum individuals showed that the reproductive state of a plant was determined by 1) its previous size and 2) the current conditions (rainfall), but not by previous conditions, nor by previous reproductive activity. Plant surveys supported these findings. These demographic results were consistent with the current understanding of the reproductive resource allocation pattern of B. desertorum. Flowering was most affected by rainfall until January of the same season, the number of leaves by rainfall until March and seed set was by the total annual amount. Surveys in populations ofB. eigii, a species of more mesic habitats in the Negev Desert suggested that in this species there is a negative effect of previous reproduction in combination with the previous and current conditions, which is also expected from its biomass partitioning pattern.The significance of demographic studies of individual plants in natural populations for the interpretation of experimentally determined resource partitioning patterns was discussed.     

Species‐specific influences of shrubs on the non‐dormant soil seed bank of native and exotic plant species in central‐northern Monte Desert

Deserts shrubs are well known to facilitate vegetation aggregation, mostly through seed trapping, and stress amelioration during and after plant establishment. Because vegetation aggregation effects are a by‐product of shrub presence, beneficiary species may not only be native, but also exotic. However, despite the high risk that exotic invasive species pose to ecosystem services, little is known of the role of desert shrubs on plant invasions. We assessed the influence of two shrub species on the non‐dormant soil seed bank (i.e. the number of seeds that readily germinate with sufficient water availability) of an invasive annual grass (Schismus barbatus) and of coexisting native species in a central‐northern Monte Desert (Argentina). Soil samples were collected beneath the canopies of two dominant shrub species (Bulnesia retama and Larrea divaricata) and in open spaces (i.e. intercanopies) in May 2001. Overall, the density of germinated seedlings of Schismus and that of the native species were negatively associated across microsite types. Schismus density was similar to that of all native species pooled together (mostly annuals), and was highest in Larrea samples (with no significant differences between Bulnesia and intercanopies). On the contrary, the density of all native species pooled together was highest in Bulnesia samples. Our results suggest that shrubs may contribute to plant invasions in our study system but, most importantly, they further illustrate that this influence can be species specific. Further research is needed to assess the relative importance of in situ seed production (and survival) and seed redistribution on soil seed bank spatial patterns.     

Photosynthetic down-regulation in Larrea tridentata exposed to elevated atmospheric CO2: interaction with drought under glasshouse and field (FACE) exposure

The photosynthetic response of Larrea tridentata Cav., an evergreen Mojave Desert shrub, to elevated atmospheric CO₂ and drought was examined to assist in the understanding of how plants from water-limited ecosystems will respond to rising CO₂. We hypothesized that photosynthetic down-regulation would disappear during periods of water limitation, and would, therefore, likely be a seasonally transient event. To test this we measured photosynthetic, water relations and fluorescence responses during periods of increased and decreased water availability in two different treatment implementations: (1) from seedlings exposed to 360, 550, and 700 μmol mol^–1 CO₂ in a glasshouse; and (2) from intact adults exposed to 360 and 550 μmol mol^–1 CO₂ at the Nevada Desert FACE (Free Air CO₂ Enrichment) Facility. FACE and glasshouse well-watered Larrea significantly down-regulated photosynthesis at elevated CO₂, reducing maximum photosynthetic rate (A_max), carboxylation efficiency (CE), and Rubisco catalytic sites, whereas droughted Larrea showed a differing response depending on treatment technique. A_max and CE were lower in droughted Larrea compared with well-watered plants, and CO₂ had no effect on these reduced photosynthetic parameters. However, Rubisco catalytic sites decreased in droughted Larrea at elevated CO₂. Operating C_i increased at elevated CO₂ in droughted plants, resulting in greater photosynthetic rates at elevated CO₂ as compared with ambient CO₂. In well-watered plants, the changes in operating C_i, CE and A_max resulted in similar photosynthetic rates across CO₂ treatments. Our results suggest that drought can diminish photosynthetic down-regulation to elevated CO₂ in Larrea, resulting in seasonally transient patterns of enhanced carbon gain. These results suggest that water status may ultimately control the photosynthetic response of desert systems to rising CO₂.     

Are Mojave Desert annual species equal? Resource acquisition and allocation for the invasive grass Bromus madritensis subsp. rubens (Poaceae) and two native species

Abundance of invasive plants is often attributed to their ability ot outcompete native species. We compared resource acquisition and allocation of the invasive annual grass Bromus madritensis subsp. rubens with that of two native Mojave Desert annuals, Vulpia octoflora and Descurainia pinnata, in a glasshouse experiment. Each species was grown in monoculture at two densities and two levels of N availability to compare how these annuals capture resources and to understand their relative sensitivities to environmental change. During >4 mo of growth, Bromus used water more rapidly and had greater biomass and N content than the natives, partly because of its greater root-surface area and its exploitation of deep soils. Bromus also had greater N uptake, net assimilation and transpiration rates, and canopy area than Vulpia. Resource use by Bromus was less sensitive to changes in N availability or density than were the natives. The two native species in this study produced numerous small seeds that tended to remain dormant, thus ensuring escape of offspring from unfavorable germination conditions; Bromus produced fewer but larger seeds that readily germinated. Collectively, these traits give Bromus the potential to rapidly establish in diverse habitats of the Mojave Desert, thereby gaining an advantage over coexisting native species.     

The effects of mowing and fertilization on carbohydrate reserves and regrowth of grasses: do they promote plant coexistence in species-rich meadows?

After the cessation of regular management and after fertilization a single clonal species tends to dominate in many types of grasslands, whereas in regularly managed meadows these potential dominants usually attain a low cover. It has been hypothesized that plants reaching a high dominance in abandoned and fertilized meadows are selectively suppressed by mowing so that a balanced competition is maintained and competitive exclusion is postponed. We compared regeneration capacity and carbohydrate reserves accumulated by three species of clonal grasses, which markedly increase their dominance in irregularly mown, un-mown or fertilized meadows. Above-ground biomass and the amount of storage carbohydrates of the two largest species, (Molinia arundinacea, Calamagrostis epigejos) were reduced in a mown meadow. This effect was weaker in Bromus erectus, which produces smaller shoots. Shoots of Molinia were most impacted by mowing but their regeneration was efficient due to the large carbohydrate reserves in the shoot bases. Fertilization did not affect Bromus and Calamagrostis. In contrast, fertilized plants of Molinia produced larger storage organs and accumulated more carbohydrates. We conclude that plant size and growth form are important features promoting the ability of potential dominants to tolerate mowing and regenerate after it. Our results confirm that taller plants are selectively suppressed by this type of disturbance, thereby potentially promoting plant coexistence.     

Effects of growing conditions on the growth of and interactions between salt marsh plants: implications for invasibility of habitats

A common but often less tested explanation for the successful invasion of alien species is that invasive alien species outcompete their co-occurring natives, which may not always be the case. In this study, we established artificial environmental gradients in a series of pot experiments with controlled environments to investigate the effects of salinity, sediment type and waterlogging on the performance of and interactions between Phragmites australis (native) and Spartina alterniflora (alien), which generally co-exist in the saline intertidal zones of Chinese and American coasts. Significant effects of salinity and waterlogging were detected on biomass production and morphological characteristics of S. alterniflora and P. australis, and the competitive interactions between the two species were found to vary with all three environmental factors in our experiments. Relative Neighbor Effect (RNE) analyses indicate that competitive dominance of S. alterniflora occurred under the conditions of high salinity, sandy sediment and full immersion, whereas P. australis showed competitive dominance under the conditions of low salinity and non-immersion. Our results suggest that S. alterniflora might outcompete P. australis under conditions present in early salt marsh succession, which support the viewpoint that the outcomes of competition between co-occurring native and invasive alien plants depend on the growing conditions. The implication of this study is that in response to the environmental changes expected from seawater intrusion and sea-level rise, the range of S. alterniflora is expected to expand further in the Yangtze River estuary in the future.     

Effects of Biological Soil Crusts on Seed Bank, Germination and Establishment of Two Annual Plant Species in the Tengger Desert (N China)

The presence of biological soil crusts can affect the germination and survival of vascular plants, but the reasons are not well investigated. We have conducted a field investigation and greenhouse experiments to test the effect of crusts on two desert annual plants, which occur on the stabilized dunes of the Tengger Desert in N China. The results showed that biological soil crusts negatively influenced the seed bank of Eragrostis poaeoides and Bassia dasyphylla. The important effect of biological soil crusts on seed germination and establishment were performed indirectly through reducing the amount of germinating seeds. Field investigation and experimental results with regard to the seed bank indicated that higher seedling density was found in disturbed crust soil and bare soil surface than in intact crust soils. Greenhouse experiments showed that the effects of biological soil crusts on germination and establishment of the two plants are not obvious in moist condition, while disturbed crusts are more favorable to seed germination in dry treatment. Significant differences in biomass were found between disturbed crust soil and bare soil. Survival and growth of the two annual plants were enhanced in both algal and moss crusts during the season of rainfall or in moist environment, but crusts did not affect seedling survival in the dry period. The small seeded E. poaeoides has higher germination than larger-seeded B. dasyphylla in crust soils, but B. dasyphylla has a relatively higher survival rate than E. poaeoides in crust soils.     

Interactions among unrelated species: Granivorous rodents,a parasitic fungus,and a shared prey species

Summary Granivorous rodents and a parasitic fungus in the Sonoran Desert utilize a common prey species, Erodium cicutarium, a desert annual plant. Experimental removal of rodents from field exclosures resulted in significantly higher densities of E. cicutarium. Fungal infection was significantly higher in the absence of rodents, suggesting that, while they do not interact directly, rodents and the fungus affect each other's densities by their use of a common prey species.     

The mycorrhizal status of Welwitschia mirabilis

This is the first report of the mycorrhizal status of Welwitschia mirabilis, a gymnosperm endemic to the Namib Desert. Like all other gymnosperms except the Pinaceae and Gnetaceae, W. mirabilis is associated with vesicular-arbuscular mycorrhizal (VAM) fungi. Mycorrhizal colonization of roots and the diversity and abundance of VAM species were determined at seven sites. Six sites received annual rainfall of 0–100 mm, varying widely from year to year. The seventh site experienced more predictable annual rainfall of 150–200 mm. Perennial vegetation was sparse at the six low-rainfall sites. Dry annual grasses from previous rain events were present at only three of these six sites and mean mycorrhizal colonization levels of W. mirabilis at these three sites were as high as 18%. W. mirabilis was not mycorrhizal at sites where grasses were absent. The seventh site, receiving higher rainfall, supported small trees and annual grasses in addition to W. mirabilis. Mycorrhizal colonization levels of W. mirabilis at this site were significantly higher than at the other six sites, closely paralleling those of the surrounding annual grasses. The mycorrhizal flora of W. mirabilis consisted of four Glomus species. These taxa were not unique to W. mirabilis, having been found with Stipagrostis and Cladoraphis grasses throughout the Namib and Kalahari deserts.     

中国北部不同地点樟子松人工林径向生长对气候响应的差异

通过树木年代学方法,测定了毛乌素和塞罕坝相同密度樟子松(Pinus sylvestris var.mongolica)全木(Ringwood,RW)、早材(Earlywood,EW)和晚材(Latewood,LW)宽度,计算胸高断面积增量(Basal area increment,BAI),并建立了樟子松年轮宽度年表,分析其对气候响应的差异。结果显示,毛乌素(Mu Us)樟子松轮宽随树龄呈"升-降"的曲线变化,塞罕坝(Saihanba)呈线性下降,两地樟子松BAI变化相似,呈"升-降"的曲线变化,EW占RW的65%—70%,表明EW对RW贡献较大。生长期间,毛乌素樟子松早晚材比例保持平稳,塞罕坝EW/RW值下降,LW/RW值上升,两地干旱事件均使LW/RW值下降,EW/RW值上升。差值年表(Residual chronology,RES)相关性分析显示,毛乌素樟子松径向生长主要与4、7月平均降雨,7月标准化降水蒸散发指数(Standardized precipitation evapotranspiration index,SPEI),3、8月平均温度及上年12月和当年3月最低温度呈正...     

Short-term soil inorganic N pulse after experimental fire alters invasive and native annual plant production in a Mojave Desert shrubland

Post-fire changes in desert vegetation patterns are known, but the mechanisms are poorly understood. Theory suggests that pulse dynamics of resource availability confer advantages to invasive annual species, and that pulse timing can influence survival and competition among species. Precipitation patterns in the American Southwest are predicted to shift toward a drier climate, potentially altering post-fire resource availability and consequent vegetation dynamics. We quantified post-fire inorganic N dynamics and determined how annual plants respond to soil inorganic nitrogen variability following experimental fires in a Mojave Desert shrub community. Soil inorganic N, soil net N mineralization, and production of annual plants were measured beneath shrubs and in interspaces during 6 months following fire. Soil inorganic N pools in burned plots were up to 1 g m⁻² greater than unburned plots for several weeks and increased under shrubs (0.5–1.0 g m⁻²) more than interspaces (0.1–0.2 g m⁻²). Soil NO₃ ⁻−N (nitrate−N) increased more and persisted longer than soil NH₄ ⁺−N (ammonium−N). Laboratory incubations simulating low soil moisture conditions, and consistent with field moisture during the study, suggest that soil net ammonification and net nitrification were low and mostly unaffected by shrub canopy or burning. After late season rains, and where soil inorganic N pools were elevated after fire, productivity of the predominant invasive Schismus spp. increased and native annuals declined. Results suggest that increased N availability following wildfire can favor invasive annuals over natives. Whether the short-term success of invasive species following fire will direct long-term species composition changes remains to be seen, yet predicted changes in precipitation variability will likely interact with N cycling to affect invasive annual plant dominance following wildfire.     

Epizoic mode of life in Brachionus rubens Ehrenberg as a deterrent against predation by Asplanchna intermedia Hudson

The planktonic rotifer Brachionus rubens has a propensity for an epizoic mode of life, and in nature is often found attached to cladocerans. In this way the rotifer avoids to a certain extent the adverse effects of interference competition with cladocerans. We test the hypothesis that the epizoic habit of B. rubens acts also as a deterrent against invertebrate predation. Using Asplanchna intermedia as predator, we followed the population growth patterns of B. rubens alone and in the presence of the host species Daphnia carinata and Ceriodaphnia rigaudi. In the absence of cladocerans, the prey was eliminated within three days, followed by extinction of the predator due to starvation. With D. carinata in the medium, the prey-predator system persisted much longer, with B. rubens reaching high population densities. With the smaller-sized C. rigaudi, allowing a significantly smaller fraction of B. rubens population to be epizoic, the system persisted longer than in the controls, but both the prey and predator eventually became extinct. We conclude that the epizoic habit of B. rubens, by acting as a prey refugium, helps a portion of the population to escape from predation, and facilitates its coexistence with Asplanchna intermedia.