Establishing Native Grasses in a Big Sagebrush–Dominated Site: An Intermediate Restoration Step

Many semiarid rangelands in the Great Basin, U.S.A., are shifting dominance to woody species as a consequence of land degradation including intense livestock grazing and fire suppression. Whereas past rehabilitation efforts in Big sagebrush (Artemisia tridentata) steppes removed the shrub and added introduced forage grasses to successfully shift communities from shrublands to grasslands, current consensus is that native species should be included in restoration projects and that retention of some woody plants is desirable. We examined the potential for interseeding grasses into dense shrub communities as a precursor to thinning shrubs and releasing grasses from shrub interference. We compared seedling establishment of the native grass, Bluebunch wheatgrass (Pseudoroegneria spicata), with that of the Eurasia grass, Crested wheatgrass (Agropyron desertorum), in dense Ar. tridentata stands. Shrubs may play an important role as nurse plants for seedling establishment (reduced solar radiation, “island of fertility” effect) but result in highly contrasting light environments and root interference for seedlings. In experimental plots, we examined effects of Ar. tridentata shade levels (0, 40, 70, and 90% reduction of solar radiation) and initial root exclusion (present/absent) on the establishment and growth of P. spicata and Ag. desertorum seedlings. With this design we evaluated the interference effects of Ar. tridentata on the two grasses and identified the most beneficial microsites for grass restoration in Ar. tridentata–dominated communities. We predicted seedling survival and growth to be greater under moderate shade (40% reduction) and limited root competition than under no or strong shade conditions (0 and 90%) and unrestricted root interactions. Fifty to 85% of the P. spicata and Ag. desertorum seedlings survived the dry summer months of 1995 and 1996 and the intervening winter. Neither shading nor root exclusion from Ar. tridentata affected final seedling survival of either species. Seedling biomass of both grass species was negatively affected by initial root interactions with Ar. tridentata. However, the analysis of seedling biomass variability (coefficient of variation) indicated that in all shade and root‐exclusion treatments, some seedlings of both species developed to large individuals to survive in Ar. tridentata–dominated rangelands. Thus, the use of interseeding techniques shows promise for restoring herbaceous species in dense Ar. tridentata stands and should be given further consideration when shrub retention is an important consideration.     

Comparative demography of co-occurring introduced and native tussock grasses: persistence and potential expansion

Summary Demographic characteristics associated with the maintenance and growth of populations, such as seed dynamics, seedling emergence, survival, and tiller dynamics were examined for two tussock grasses, the native Agropyron spicatum and the introduced Agropyron desertorum in a 30-month field study. The introduced grass was aerially sown onto a native grassland site. Seed production of the introduced grass was greater than the native grass in both above- and below-average precipitation years. Seeds of A. spicatum were dispersed when they mature, while A. desertorum retained some seeds in inflorescences, and dispersed them slowly throughout the year. This seed retention allowed some seeds of the introduced grass to escape peak periods of seed predation during the summer and allowed seeds to be deposited constantly into the seed bank. Carryover of seeds in the seed bank beyond one year occurred in the introduced grass but not in the native species. For both species, seedling emergence occurred in both autumn or spring. Survival rates for A. desertorum were higher than A. spicatum when seedlings emerged between November and March. Survival rates of cohorts emerging before November favored A. spicatum whereas survival rates did not differ between species for cohorts emerging after March. Individuals of both species emerging after April were unable to survive the summer drought. Demographic factors associated with seeds of A. desertorum seemed to favor the maintenance and spread of this introduced grass into native stands formerly dominated by A. spicatum.     

The timing and degree of root proliferation in fertile-soil microsites for three cold-desert perennials

Summary Root proliferation in nutrient-rich soil patches is an important mechanism facilitating nutrient capture by plants. Although the phenomenon of root proliferation is well documented, the specific timing of this proliferation has not been investigated. We studied the timing and degree of root proliferation for three perennial species common to the Great Basin region of North America: a shrub, Artemisia tridentata, a native tussock grass, Agropyron spicatum, and an introduced tussock grass, Agropyron desertorum. One day after we applied nutrient solution to small soil patches, the mean relative growth rate of Agropyron desertorum roots in these soil patches was two to four times greater than for roots of the same plants in soil patches reated with distilled water. Most of the increased root growth came from thin, laterally branching roots within the patches. This rapid and striking root proliferation by Agropyron desertorum occurred in response to N-P-K enrichment as well as to P or N enrichment alone. A less competitive bunchgrass, Agrophyron spicatum, showed no tendency to proliferate roots in enriched soil patches during these two-week experiments. The shrub Artemisia tridentata proliferated roots within one day of initial solution injection in the N-enrichment experiment, but root proliferation of this species was more gradual and less consistent in the N-P-K and P-enrichment experiments, respectively. The ability of Agropyron desertorum to proliferate roots rapidly may partly explain both its general competitive success and its superior ability to exploit soil nutrients compared to Agropyron spicatum in Great Basin rangelands of North America.     

Characteristics of successful competitors: an evaluation of potential growth rate in two cold desert tussock grasses

Summary Within the first few weeks after seedling emergence, Agropyron desertorum, a more competitive tussock grass, had a much higher mean relative growth rate (RGR) than Agropyron spicatum, a very similar, but less competitive species. However, beyond the early seedling stage, the two grasses had a remarkably similar whole-plant RGR in hydroponic culture and aboveground RGR in glasshouse soil, if root temperatures were above approximately 12°C. At soil temperatures between 5 and 12°C, A. desertorum exhibited a 66% greater aboveground RGR than A. spicatum (P<0.05). Both species responded similarly to warming soil temperatures. In the field, however, tiller growth rates were generally similar. Neither species showed marked tiller elongation until a couple of weeks after snowmelt, by which time soil temperatures, at least to a depth of 10 cm, were above 12°C for a significant portion of the day. Aboveground biomass accumulation over a three-year period indicated that both grasses had similar potential growth rates whereas Artemisia tridentata ssp. vaseyana, a common neighbor planted in the same plots, had a much greater potential growth rate. The greater competitive ability of adult A. desertorum, as compared to A. spicatum, cannot be attributed to appreciable differences in potential growth rates.     

Competitive ability is linked to rates of water extraction

Summary The relative competitive abilities of Agropyron desertorum and Agropyron spicatum under rangeland conditions were compared using Artemisia tridentata ssp. wyomingensis transplants as indicator plants. We found A. desertorum to have substantially greater competitive ability than A. spicatum as manifested by the responses of Artemisia shrubs that were transplanted into nearly monospecific stands of these grass species. The Artemisia indicator plants had lower survival, growth, reproduction, and late-season water potential in the neighborhoods dominated by A. desertorum than in those dominated by A. spicatum. In similar, essentially monospecific grass stands, neutron probe soil moisture measurements showed that stands of A. desertorum extracted water more rapidly from the soil profile than did those of A. spicatum. These differences in extraction rates correlate clearly with the differences in indicator plant success in the respective grass stands. Nitrogen and phosphorus concentrations in Artemisia tissues suggested these nutrients were not limiting indicator plant growth and survival in the A. desertorum plots.     

Effects of adult neighbour and gap size on seedling emergence and early growth of <Emphasis Type="Italic">Bromus inermis</Emphasis> Leyss

A field study was conducted to determine the effects of neighbour root exclusion and gap size on the seedling emergence and early growth of Bromus inermis. Seeds of B. inermis were added to artificially created gaps in an improved shortgrass steppe in northern China. Neighbour root exclusion was accomplished using PVC tubes sunk in the soil of gaps. Emergence and survival of seedlings were greater in all gaps than in the control (0-cm diameter), but better growth performance (height of seedling, leaves, tillers and dry weight per seedling) was observed only in large gaps (20- and 40-cm diameter) with neighbour root present and gaps without neighbour root presence. Seedling growth performance was improved by reducing root and shoot competition. Neighbour root exclusion provided a favourable environment for seedling establishment. Differences between gaps in light levels and topsoil temperature can explain the patterns of germination. These results confirm that B. inermis is a gap-enhanced species. Our study strongly suggests that asymmetric competition by adult neighbour plants negatively influences the establishment of B. inermis.     

Coping with herbivory: Photosynthetic capacity and resource allocation in two semiarid Agropyron bunchgrasses

Summary Agropyron desertorum, a grazing-tolerant bunchgrass introduced to the western U.S. from Eurasia, and Agropyron spicatum, a grazing-sensitive bunchgrass native to North America, were examined in the field for photosynthetic capacity, growth, resource allocation, and tiller dynamics. These observations allowed identification of physiological characteristics that may contribute to grazing tolerance in semiarid environments. A uniform matrix of sagebrush, Artemisia tridentata, provided an ecologically relevant competitive environment for both bunch-grass species. Physiological activity, growth, and allocation were also followed during recovery from a severe defoliation treatment and were correlated with tiller dynamics.Potential photosynthetic carbon uptake of both species was dominated by stems and leaf sheaths during June, when maximum uptake rates occurred. For both species, water use efficiency of stems and sheaths was similar to that of leaf blades, but nitrogen investment per photosynthetic surface area was less than in blades. In addition, soluble carbohydrates in stems and sheaths of both species constituted the major labile carbon pools in control plants. Contrary to current theory, these findings suggest that culms from which leaf blades have been removed should be of considerable value to defoliated bunchgrasses, and in the case of partial defoliation could provide important supplies of organic nutrients for regrowth. These interpretations, based on total pool sizes, differ markedly from previous interpretations based on carbohydrate concentrations alone, which suggested that crowns contain large carbohydrate reserves. In this study, crowns of both species contained a minor component of the total plant carbohydrate pool.Following defoliation, A. desertorum plants rapidly reestablished a canopy with 3 to 5 times the photosynthetic surface of A. spicatum plants. This difference was primarily due to the greater number of quickly growing new tillers produced following defoliation. Agropyron spicatum produced few new tillers following defoliation despite adequate moisture, and carbohydrate pools that were equivalent to those in A. desertorum.Leaf blades of regrowing tillers had higher photosynthetic capacity than blades on unclipped plants of both species, but the relative increase, considered on a unit mass, area, or nitrogen basis, was greater for A. desertorum than for A. spicatum. Agropyron desertorum also had lower investment of nitrogen and biomass per unit area of photosynthetic tissues, more tillers and leaves per bunch, and shorter lived stems, all of which can contribute to greater tolerance of partial defoliation.Greater flexibility of resource allocation following defoliation was demonstrated by A. desertorum for both nitrogen and carbohydrates. Relatively more allocation to the shoot system and curtailed root growth in A. desertorum resulted in more rapid approach to the preclipping balance between the root and shoot systems, whereas root growth in A. spicatum continued unabated following defoliation. Nitrogen required for regrowth in both species was apparently supplied by uptake rather than reserve depletion. Carbohydrate pools in the shoot system of both species remained very low following severe defoliation and were approximately equivalent to carbon fixed in one day by photosynthesis of the whole canopy.Dedicated to Drs. Michael Evenari and Konrad Springer     

<Emphasis Type="Italic">Parthenium hysterophorus</Emphasis> L. (Asteraceae) invasion had limited impact on major soil nutrients and enzyme activity: Is the null effect real or reflects data insensitivity?

Background and aims

Competition from the annual grass Bromus tectorum threatens aridland perennial bunchgrass communities. Unlike annuals, perennials must allocate part of their first year nitrogen (N) budget to storage rather than growth, potentially placing them at a competitive disadvantage.

Methods

We evaluated N acquisition and conservation for two perennial bunchgrasses, Agropyron desertorum and Pseudoroegneria spicata, at the seedling stage to investigate potential trade-offs between storage and growth when grown with and without B. tectorum under two levels of soil N.

Results

Agropyron desertorum had higher growth rates, N uptake, and N productivity than P. spicata when grown without B. tectorum, but trait values were similarly low for both species under competition. Without competition, N resorption was poor under high soil N, but it was equally proficient among species under competition.

Conclusions

A. desertorum had higher growth rates and N productivity than P. spicata without competition, suggesting these traits may in part promote its greater success in restoration programs. However, B. tectorum neighbors reduced its trait advantage. As plant traits become more integral to restoration ecology, understanding how N capture and conservation traits vary across candidate species and under competition may improve our ability to select species with the highest likelihood of establishing in arid, nutrient-limited systems.

     

Nitrogen acquisition by annual and perennial grass seedlings: testing the roles of performance and plasticity to explain plant invasion

Differences in resource acquisition between native and exotic plants is one hypothesis to explain invasive plant success. Mechanisms include greater resource acquisition rates and greater plasticity in resource acquisition by invasive exotic species compared to non-invasive natives. We assess the support for these mechanisms by comparing nitrate acquisition and growth of invasive annual and perennial grass seedlings in western North America. Two invasive exotic grasses (Bromus tectorum and Taeniatherum caput-medusae) and three perennial native and exotic grasses (Pseudoroegneria spicata, Elymus elymoides, and Agropyron cristatum) were grown at various temperatures typical of autumn and springtime when resource are abundant and dominance is determined by rapid growth and acquisition of resources. Bromus tectorum and perennial grasses had similar rates of nitrate acquisition at low temperature, but acquisition by B. tectorum significantly exceeded perennial grasses at higher temperature. Consequently, B. tectorum had the highest acquisition plasticity, showcasing its ability to take advantage of transient warm periods in autumn and spring. Nitrate acquisition by perennial grasses was limited either by root production or rate of acquisition per unit root mass, suggesting a trade-off between nutrient acquisition and allocation of growth to structural tissues. Our results indicate the importance of plasticity in resource acquisition when temperatures are warm such as following autumn emergence by B. tectorum. Highly flexible and opportunistic nitrate acquisition appears to be a mechanism whereby invasive annual grasses exploit soil nitrogen that perennials cannot use.     

Effects of perennial neighbors and nitrogen pulses on growth and nitrogen uptake by Bromus tectorum

An experiment was conducted to determine if growth and biomass responsesof the annual grass Bromus tectorum are affected by themagnitude and timing of nitrogen (N) pulses and if these responses areinfluenced by different perennial neighbor species. Nitrogen(NH₄:NO₃) was applied in three pulse treatments of varyinginterpulse length (3-d, 9-d, or 21-d between N additions). The total amount of Nadded was the same among treatments; hence, both the frequency and magnitude ofN pulses varied (i.e., the longer the interpulse period,the greater the amount of N added for a single pulse).Bromus showed little response to the different N-pulsetreatments. The only characteristic that varied among pulse treatments wasspecific leaf area (SLA), which was significantly greater whenBromus was grown under the 21-d N pulse than when grownunder the 3-d or 9-d N pulses. Bromus height, leaf andtiller numbers, leaf area and aboveground biomass were not affected by theN-pulse treatments nor were tissue-N contents and concentrations. However,Bromus production and tissue-N were significantly differentwhen Bromus was grown with different perennial neighborspecies. Tiller production, aboveground biomass, and seed numbers ofBromus were lowest when the perennial neighbor was thetussock grass Agropyron desertorum, intermediate when theneighbor was the evergreen shrub Artemisia tridentata, andgreatest when the neighbor was the deciduous shrub Chrysothamnusnauseosus. N contents of Bromus leaves were alsolowest when the neighbor was Agropyron. In contrast, root Nuptake capacities were greatest for Agropyron-Bromus rootmixes and lowest for Chrysothamnus-Bromus root mixes. Theseresults suggest that perennial neighbors affect growth, seed production, and Nuptake of Bromus to a greater extent than the timing andmagnitude of N pulses.     

Root growth response to defoliation in two Agropyron bunchgrasses: field observations with an improved root periscope

Summary Root growth responses to defoliation were observed in the field with an improved root periscope technique, which is described. The grazing tolerant, Eurasian bunchgrass, Agropyron desertorum, was compared with the very similar but grazing sensitive, North American bunchgrass, A. spicatum. Root length growth of clipped A. desertorum was about 50% of that of intact plants, while root elongation of clipped A. spicatum continued relatively unabated during ninety days of regrowth following severe defoliation. The reduced root growth in A. desertorum was correlated with the allocation of relatively more resources to aboveground regrowth, thus aiding reestablishment of the root: shoot balance. This balance was apparent in similar root mortality patterns of clipped and control A. desertorum plants in the season following defoliation. In clipped A. spicatum, however, root mortality increased in the winter following the season in which the clipping was done and continued into the subsequent growing season. Reduction of root growth following defoliation appears to be an effective mechanism to aid reestablishment of the photosynthetic canopy and the root: shoot balance. As such it contributes to both herbivory tolerance and maintenance of competitive ability.     

Effectiveness of phosphate acquisition by juvenile cold-desert perennials from different patterns of fertile-soil microsites

Summary Phosphate uptake was measured for Artemisia tridentata, Agropyron desertorum and Pseudoroegneria spicata, three common perennial North American Great Basin species. Four patterns of nutrient-rich microsites were used in the experiments (different distances, densities and nutrient concentrations) All species were more efficient at taking up P from microsites nearest the plants than from more distant microsites. Artemisia and Agropyron acquired P more rapidly from the distant microsites when there was a larger number of microsites and, therefore, a greater probability of encounter. Uptake from the nearest microsites did not increase after 26 days, while uptake from distant microsites increased and was equal to uptake from the nearest microsites by the end of the experiment. Phosphate uptake was four to five times higher for Artemisia than for Agropyron on a shoot mass basis and seven to eight times greater than for Pseudoroegneria, which reflects species relative growth rates. Differences in shoot dry mass were significant among species, but little evidence was found for interspecific competition. Root density, root dry mass and P uptake in the upper part of the soil mixture was higher for Artemisia than the other species. Phosphate acquisition seems to be influenced by the distance of microsites and their density and the ability of plants to encounter and proliferate absorbing organs in the microsites.     

Global change effects on Bromus tectorum L. (Poaceae) at its high‐elevation range margin

Global change is likely to affect invasive species distribution, especially at range margins. In the eastern Sierra Nevada, California, USA, the invasive annual grass, Bromus tectorum, is patchily distributed and its impacts have been minimal compared with other areas of the Intermountain West. We used a series of in situ field manipulations to determine how B. tectorum might respond to changing climatic conditions and increased nitrogen deposition at the high‐elevation edge of its invaded range. Over 3 years, we used snow fences to simulate changes in snowpack, irrigation to simulate increased frequency and magnitude of springtime precipitation, and added nitrogen (N) at three levels (0, 5, and 10 g m^?2) to natural patches of B. tectorum growing under the two dominant shrubs, Artemisia tridentata and Purshia tridentata, and in intershrub spaces (INTR). We found that B. tectorum seedling density in April was lower following deeper snowpack possibly due to delayed emergence, yet there was no change in spikelet production or biomass accumulation at the time of harvest. Additional spring rain events increased B. tectorum biomass and spikelet production in INTR plots only. Plants were primarily limited by water in 2009, but colimited by N and water in 2011, possibly due to differences in antecedent moisture conditions at the time of treatments. The threshold at which N had an effect varied with magnitude of water additions. Frequency of rain events was more influential than magnitude in driving B. tectorum growth and fecundity responses. Our results suggest that predicted shifts from snow to rain could facilitate expansion of B. tectorum at high elevation depending on timing of rain events and level of N deposition. We found evidence for P‐limitation at this site and an increase in P‐availability with N additions, suggesting that stoichiometric relationships may also influence B. tectorum spread.     

The impact of ‘available area’ onArtemisia tridentata seedling dynamics

Seedling survival ofArtemisia tridentata ssp.tridentata (basin big sagebrush) was studied during 1984 and 1986 on a semi-arid rangeland in central Utah, USA. Seedlings were mapped in ungrazed pastures and observed on a 10 day interval for survival. Available resource area around individual seedlings was calculated using Theissen polygons. Over 90% of the naturally-occurring seedlings occurred in polygons of 150 mm² or less. Seedlings with available areas greater than 300 mm² were more likely to survive than seedlings with less resource area. Differences in overall seedling survival in different study sites were noted. Survival was similar for all seedlings with less than 50 mm² available area. In 1986, precipitation and seedling survival were considerably lower than in 1984. Even in 1986, however, seedling survival in polygons greater than 300 mm² was always higher than for seedlings in smaller polygons. Survival patterns over each growing season closely followed the precipitation patterns.Mortality of first yearArtemisia seedlings seemed to be density independent at the beginning of the growing season. Patterns of survival for seedlings in different available area classes were similar for the first 20 days after emergence. Survival rates were then consistently higher for seedlings with more than 300 mm² resource area. Changes in survival rates were linked to differences in pastures and the presence of nurse plants.     

Endozoochorous dispersal, seedling emergence and fruiting success in disturbed and undisturbed successional stages of sheep-grazed inland sand ecosystems

This study examines the seedling emergence and fruiting success of plant species endozoochorously dispersed by sheep in a field experiment in a Koelerion glaucae area. We designed a factorial experiment (faeces/soil disturbance/successional stage/year) that mimics sheep-mediated microsites and analysed all occurring vascular plant species from September 2002–December 2004. The faeces samples were collected in the same ecosystem and additionally analysed in a common-garden experiment (seedling emergence potential). According to the latter, 28 vascular plant taxa were identified (on average 124 seedlings per 100 g air-dry sheep faeces). In the field, 15 species emerged directly out of faeces, only five of which were able to set seeds (the threatened species Medicago minima, Phleum arenarium, Silene conica, Vicia lathyroides and the non-threatened Vulpia myuros). Graminoid competitors were not able to set seedlings in the field but did so in the common-garden experiment (mainly Carex hirta). The total numbers of seedlings emerging out of faeces (5% of the potential) and reaching fruit ripeness (0.4%) were very low.In the stage of seedling emergence, ten of 42 species were significantly influenced by faeces, 11 species by disturbance and two species by an interaction of the two treatment factors (Bromus tectorum, Saxifraga tridactylites). In the fruiting stage, two species profited by faeces (Bromus tectorum, Vulpia myuros) and two species by disturbance (Erophila verna, Corynephorus canescens). Overall, the density and diversity of fruiting individuals were significantly influenced by both treatment factors. The faeces factor showed a strong dependence on year (increasing effect on both dependent variables in the first year, decreasing effect in the second year). The disturbance factor is dependent particularly on successional stage (increasing effect especially in the later successional stage).Our study revealed the paradox that threatened species with low nutrient demands are the most successful direct colonisers of sheep-faeces deposits. For them to reach the fruiting stage is rare but is of importance if new habitats are to be made accessible.     

Morphological Analysis of Tillering in Agropyron spicatum and Agropyron desertorum

The caespitose grasses Agropyron spicatum and Agropyron desertorumexhibit a striking difference in tillering response followingexperimental clipping treatment, with plants of A. desertorumproducing up to 18 times more tillers. The two species are similarin many aspects of their phenology and physiology. Previousexamination of current photosynthate production and levels ofstored carbohydrates indicate only slight differences betweenthe species. The possible role of three anatomical/morphologicalconstraints in controlling tillering was examined. No evidencefor such constraints was found. A basal cluster of buds is presenton the parent tillers. The mean bud number per tiller was similarfor both species and the range (3–9) was identical. Nearlyall of the bud apical meristems appeared anatomically viablethroughout the growing season and vascular development occurredto within 250 to 490 µm of the various bud apices of bothspecies. Both normal fall tillers and summer tillers producedunder clipping treatment originated from the largest, most distalbuds of the basal cluster of buds. However, precocious, morphologicallydistinctive, second-order tillers occasionally grew out fromthe smaller, most basal buds of some elongating fall tillers. Agropyron spicatum, Agropyron desertorum, bluebunch wheatgrass, crested wheatgrass, bud, tiller, tillering ability, meristematic potential, vascular development, regrowth     

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

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

The population biology of Bromus tectorum in forests: effect of disturbance,grazing, and litter on seedling establishment and reproduction

Summary The effect of tree canopy, understory, herbivores, and litter depth on seedling establishment, survival, and reproduction of the alien grass, Bromus tectorum (cheatgrass), was examined in a series of experiments in four forest habitat types in western North America. Higher recruitment, survival, and reproduction on clearcuts, which would be expected if the overstory alone is limiting the distribution of cheatgrass in forests, were not observed. Removing the understory in an otherwise undisturbed Pinus ponderosa forest did, however, increase the emergence of B. tectorum, but plants in these experimentally-created openings were more vulnerable to grazing by small mammals. In contrast, removing the sparse understory in an Abies forest neither enhanced recruitment nor increased the incidence of grazing of B. tectorum seedlings. Regardless of the forest habitat, most grazed plants died before maturity; even fewer grazed plants produced seeds. Litter depth influenced both recruitment and biomass production: both the rate of germination and the size of resultant seedlings were lower on thick litter (6 cm) compared to results on thin litter (1.5 cm). In the more open Pinus ponderosa and Pseudotsuga menziesii forests, cheatgrass colonization may often occur in openings in the understory alone. Colonization in the more shady A. grandis and Thuja plicata forests is unlikely, however, unless the opening extends through both the understory and the overstory. As a result, cheatgrass is unlikely to increase in any of these forests unless the scale and incidence of disturbance increases substantially.     

Nitrogen deposition enhances Bromus tectorum invasion: biogeographic differences in growth and competitive ability between China and North America

Increased resource supply commonly facilitates invasion by exotic plants, raising concerns over atmospheric nitrogen (N) deposition; fast‐growing annual invaders may have exceptional abilities to outperform native perennials in response to N pulses. However, it remains unclear whether this advantage is due to growth differences or to shifts in competitive outcomes, and whether annual invaders are favored by N deposition in their introduced range over native range. We conducted an experiment to compare the growth and competitive ability of Bromus tectorum and its native perennial grasses either at three different N regimes or between China and North America. The soil used in this experiment was from mountain grasslands as a neutral growth medium. The total biomass of three natives from China and North America did not increase along the N deposition gradient. Nitrogen addition enhanced the growth of North American B. tectorum instead of Chinese B. tectorum. Nitrogen addition increased the competitive ability of B. tectorum, but had no effect on that of natives. North American B. tectorum was bigger and had greater competitive ability and root weight ratio than Chinese B. tectorum. In contrast, North American natives were less competitive than Chinese natives. There was a significantly positive correlation between the growth of B. tectorum grown alone and its competitive ability. These findings suggest that N deposition may enhance the B. tectorum invasion through disproportionally increasing the growth and maintaining inherent competitive advantages of North American B. tectorum, further increasing threats to introduced ranges. There were differences in the growth and competitive ability of B. tectorum and natives between China and North America, which explains why B. tectorum is a minor component at home and becomes a successful invader abroad.     

Morphological plasticity following species-specific recognition and competition in two perennial grasses

Morphological characteristics and biomass allocation of two perennial grasses, Pseudoroegneria spicata (Pursh) A. Löve ssp. spicata (bluebunch wheatgrass) and Agropyron desertorum (Fisch. ex Link) Schult. (crested wheatgrass), were compared under different competition and nutrient treatments. The competitive responses of two plants grown in containers under field conditions were assessed in monocultures and mixtures in two experiments using different scales of nutrient application. In the Small-Scale Experiment, a localized fertilization was applied in the rooting zone between two plants; in the Large-Scale Experiment the entire container was supplied with nutrients. Agropyron responded more vigorously to fertilization than did Pseudoroegneria, but based on the relative performance of Agropyron in monoculture and mixture, it was not superior to Pseudoroegneria in resource competition. Pseudoroegneria was apparently able to recognize neighboring plants as either conspecifics or individuals of the other species. The responses included changes in shoot architecture, root morphology, and allocation between roots and shoots. Agropyron generally did not exhibit such morphological flexibility. In field plot plantings of 4-yr-old tussocks similar shoot differences were seen in Pseudoroegneria. There was, however, no indication of superior resource competition for Agropyron. Thus, any early advantage of Agropyron in vigorous growth of young plants in response to nutrients was apparently lost by the time the plants had reached this stage of development. Morphological and allocation flexibility of Pseudoroegneria may have compensated for slower, less vigorous growth. If species-specific recognition and morphological plasticity are common in nature, this complicates our attempts to understand mechanisms of competition.