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Keywords:

  • 15N tracer;
  • Artemisia tridentata;
  • Bromus tectorum;
  • cheatgrass;
  • Elymus elymoides;
  • Great Basin;
  • N uptake;
  • resource competition;
  • semiarid;
  • Sitanion hystrix;
  • squirreltail;
  • time domain reflectometry

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    In the Great Basin of the western United States of America, the invasive annual grass Bromus tectorum has extensively replaced native shrub and bunchgrass communities, but the native bunchgrass Elymus elymoides has been reported to suppress Bromus. Curlew Valley, a site in Northern Utah, provides a model community to test the effects of particular species on invasion by examining competitive relationships among Elymus, Bromus and the native shrub Artemisia tridentata.
  • 2
    The site contains Bromus/Elymus, Elymus/Artemisia and monodominant Elymus stands. Transect data indicate that Elymus suppresses Bromus disproportionately relative to its above-ground cover. Artemisia seedlings recruit in Elymus stands but rarely in the presence of Bromus. This relationship might be explained by competition between the two grasses involving a different resource or occurring in a different season to that between each grass and Artemisia.
  • 3
    Time reflectometry data collected in monodominant patches indicated that in spring, soil moisture use by Bromus is rapid, whereas depletion under Elymus and Artemisia is more moderate. Artemisia seedlings may therefore encounter a similar moisture environment in monodominant or mixed perennial stands. However, efficient autumn soil moisture use by Elymus may help suppress Bromus.
  • 4
    In competition plots, target Artemisia grown with Bromus were stunted relative to those grown with Elymus, despite equivalent above-ground biomass of the two grasses. Competition for nitrogen in spring and autumn, assessed with 15N tracer, appears to be secondary to moisture availability in determining competitive outcomes.
  • 5
    Elymus physiology and function appear to play an important role in determining the composition of communities in Curlew Valley, by maintaining zones free of Bromus where Artemisia can recruit.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Invasions by exotic species are an increasingly serious threat to ecosystem integrity and function (Vitousek 1986; D’Antonio & Vitousek 1992). Certain species can dramatically change ecosystem properties such as light availability and water and nutrient balance, potentially influencing the trajectory of vegetation community development (Connell & Slatyer 1977; Pastor & Post 1986; Huston & Smith 1987). The most successful invasive species are probably those that restructure the environment to favour their own long-term persistence, thus ultimately resetting the course of plant community development. In the semiarid environments of the western United States of America (USA), invasions by Euphorbia esula (leafy spurge), Centaurea spp. (yellow star thistle, spotted knapweed and russian knapweed) and an assortment of annual grasses including Taeniatherum caput-medusae ssp. asperum (medusahead wildrye) and Bromus tectorum (L.) (cheatgrass) are widespread and often represent complete replacement of native shrub and bunchgrass communities. Not all communities are equally invasible, but much remains to be learned about the factors that confer resistance to invasion. Various studies have examined the role of community diversity for ecosystem nutrient cycling and invasibility (Tilman 1997; Hooper & Vitousek 1998; Knops et al. 1999; Dukes 2001), in order to determine whether diverse communities use resources more thoroughly (Tilman 1996) and thus reduce resource availability to invaders. However, some work has concluded that the presence or absence of particular species can be as important as overall diversity for controlling nutrient availability. Species whose phenology overlaps that of an invader may monopolize resources when both plants are active (Dukes 2001), but nutrient availability (and presumably invasibility) can also be affected by non-uptake-related effects, such as with litter quality and quantity feedbacks on soil nutrient availability (Wedin & Tilman 1990; Wardle et al. 1997; Hooper & Vitousek 1998). Allelopathic effects are another way that species can interfere with each others’ resource acquisition (Callaway & Aschehoug 2000; Ridenour & Callaway 2001). Many plant communities of the semiarid west are relatively simple and the presence or absence of a particular species may therefore have large consequences for ecosystem function. Whether an exotic species gains a foothold within a community, or whether native species pre-empt resource uptake by exotics, are questions that emphasize the importance of individual species’ physiology and function for shaping community development.

In the Great Basin of the western USA, the exotic annual grass Bromus tectorum is an example of the sweeping influence of invaders, having replaced millions of hectares of native shrub and bunchgrass communities (Whisenant 1990). Transpiration by densely growing Bromus depletes soil moisture in the spring, preventing establishment of native shrub and perennial grass seedlings (Harris 1967; Cline et al. 1977; Monsen 1994) and depressing the moisture status even of mature shrubs (Melgoza et al. 1990). Some work suggests that Bromus may also influence inorganic N availability (Booth 2001; Evans et al. 2001), although the direction of the response may vary depending on other environmental variables such as timing of precipitation (Evans et al. 2001). Bromus germinates after autumn rains or in the early spring and sets seed before senescing when soil water becomes scarce, in late May or early June (Klemmedson & Smith 1964). Standing dead stems often serve to spread fire through otherwise sparsely distributed perennial vegetation (Stewart & Hull 1949; Mayeux et al. 1994). The most abundant shrub of the Intermountain West, Artemisia tridentata (sagebrush), does not resprout following fire (West & Young 2000), thus its displacement by Bromus can persist for decades (Pellant 1990; Whisenant 1990; Knapp 1996). Noted early in the vegetation literature as a ‘disclimax’ (Clements 1936), Bromus has long been recognized as a source of system impoverishment and altered ecosystem dynamics (Billings 1992).

One way to reduce the impact of Bromus may be to use plant competitors to reduce its resource availability. Bromus is probably most vulnerable at germination, when seedling growth is inhibited by low soil moisture (Upadhyaya et al. 1986; Anderson 1996) and reductions in inorganic N availability (Young et al. 1995). The native perennial bunchgrass Elymus elymoides (Raf.) (formerly Sitanion hystrix) is reported to reduce cover both of Bromus (Hironaka & Tisdale 1963; Stevens & Anderson 1997) and Taeniatherum caput-medusae, probably by altering moisture availability (Hironaka & Tisdale 1963). Elymus appears to be fire tolerant (Wright & Klemmedson 1965; Rickard & Vaughan 1988; Hosten & West 1994), in some cases increasing its biomass after burning (Trent et al. 1994; Jones 1998). Like Bromus, Elymus is capable of autumn germination (Hironaka & Sinelar 1973; Allen et al. 1994), and shows relatively high rates of root elongation throughout winter (Hironaka & Tisdale 1963; Hironaka & Sinelar 1973). Tussocks are extremely responsive to autumn precipitation (Coyne 1969), proliferating new foliage at about the same time as Bromus germinates in the autumn. Elymus may be useful for site restoration if it can suppress Bromus but also serve as a link to a community that includes other species of perennial grasses and shrubs (Hironaka & Sinelar 1973; Jones 1998).

The similar phenology of Elymus and Bromus suggests that the reported success of Elymus in suppressing the annual (Hironaka & Tisdale 1963; Stevens & Anderson 1997) may be due to resource competition. However, if Elymus acquires resources with the same intensity and timing as Bromus, it would be unlikely to facilitate the re-invasion of other native perennials such as Artemisia. A non-hierarchical competitive relationship, in which Elymus suppresses Bromus, but not other perennials, could come about if competition between Elymus and Bromus is mediated by a different resource, or occurs in another season than competition between Elymus and other perennials.

Curlew Valley, a shrub-steppe site in Northern Utah, is well-suited as a model community to investigate links between plant resource use and vegetation dynamics. It currently contains interspersed patches of Bromus/Elymus (‘mixed grass’) stands, Artemisia/Elymus (‘grass/shrub’) stands, and monodominant Elymus stands. The site is grazed by cattle from late November until late April, but cattle-use surveys indicate that the animals use the different vegetation types equivalently (Booth, unpublished data). Vegetation surveys (Rice & Westoby 1978), site photos and aerial photos suggest that both Bromus and Elymus have substantially increased since the 1960s, particularly at the expense of the subshrub Atriplex nutallii, which was formerly present in monodominant stands (Mitchell & West 1966; Rice & Westoby 1978) and now only occurs as isolated individuals. Artemisia cover has also increased since the 1960s. While it is not clear what factors were responsible for the initial distribution of Elymus and Bromus following the displacement of Atriplex nutallii, current species distribution suggests that Elymus can coexist with both Artemisia and Bromus, while excluding the annual grass altogether from certain areas.

This study had two primary objectives. The first was to examine the case for Elymus both suppressing Bromus and facilitating Artemisia recruitment at Curlew Valley. Vegetation communities were characterized by transect sampling, which quantified Bromus cover relative to Elymus cover within mixed grass stands, and assessed whether Artemisia seedlings occurred with equivalent frequency in mixed grass and monodominant Elymus stands. Because invasion of Bromus stands by Elymus is a prerequisite for increase in perennial grass cover, an experiment was conducted at Curlew Valley to compare performance of Elymus tussocks transplanted into mixed grass and monodominant Elymus stands.

Resource competition is often hypothesized to drive plant community dynamics, but the covariance of resource availability in natural settings often renders the underlying mechanism elusive. Additionally, the relative importance of resource competition may diminish once plants are past the vulnerable seedling stage. The second objective of the study was therefore to disentangle the effects of competition for water from that for nitrogen and determine their relative influence on plant community composition at Curlew Valley, with particular reference to how Artemisia seedlings ‘perceive’ their environment. How may Elymus suppress Bromus, which is a strong competitor with Artemisia, yet itself coexist with or even facilitate the shrub? Is Elymus/Bromus competition mediated by a different resource than Elymus/Artemisia competition, or does it occur at a different time? The timing and intensity of soil moisture use by the two grasses was compared with that of mature Artemisia, to characterize the competitive environment faced by germinating Artemisia seedlings in stands of the three species. With regard to water use, is the perennial grass more functionally similar to the shrub or the annual grass?

To investigate patterns of N use by the three species, a competition experiment was set up in constructed plots where Artemisia seedlings were grown alone, or in a background of Bromus or Elymus, and seasonal uptake of 15N tracer was monitored. The experiment addressed two main questions. First, what is the competitive effect of Bromus and Elymus on Artemisia seedling biomass and N status? Secondly, are autumn and spring similarly important periods of N uptake for all three species? Autumn, when precipitation returns but temperatures are still warm, is a potentially important period for plant water and nitrogen uptake. Aggressive N acquisition by Elymus during this time might suppress Bromus but have little effect on Artemisia, if autumn resource uptake is relatively less important to the shrub than the grasses.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

site descriptions

A mixed vegetation community at Curlew Valley, in Northern Utah, provided the opportunity to examine community dynamics in mixed stands of Artemisia, Elymus and Bromus. The site is located just north of the Great Salt Lake at 41°52′ N, 113°5′ W at an elevation of 1350 m, and receives about 310 mm precipitation year−1. Soils in the lower part of the valley are fine silty Xerollic Calciorthids of the Thiokol series, and are of lacustrine origin (Skujins & West 1973). The site is characterized by large areas of Bromus/Elymus (‘mixed-grass’) stands, Artemisia/Elymus (‘shrub-grass’) stands, and monodominant areas of all three species. Monodominant Elymus patches are extensive and are characterized by bare or cryptogamic crust-covered soils between grass tussocks. Monodominant Bromus patches are small areas lacking Elymus that are embedded within mixed-grass stands. Likewise, monodominant Artemisia patches are not extensive, and occur within shrub-grass stands. Some Bromus is found in the understorey of mature and senescent shrubs in shrub-grass stands, but there are no widespread areas where all three species are found together. Previous work at the site has established that soils associated with each species are characterized by distinct temporal patterns of inorganic N availability. In late summer, inorganic N accumulates in Bromus-dominated soils to concentrations of 5–25 mg kg−1 soil, mostly as inline image, whereas extractable inorganic N in Artemisia- and Elymus-dominated soils is typically lower (1–10 mg kg−1 soil), and is less inline image dominated (Booth 2001).

A comparison of Bromus and Elymus competitive effects on target Artemisia plants was carried out at the Utah State University Green Canyon Ecological Research Station, in Logan, Utah. The site is located at 41°45′ N, 111°48′ W, at 1460 m elevation, and receives an average of 468 mm annual precipitation. Soils are Typic Haploxeralls formed on alluvial material.

transect sampling

Transect sampling was conducted at Curlew Valley in June 1996, May 1997 and late May 1998. In 1996, seven transects, each consisting of seven 0.5 m2 sampling areas spaced at 10-m intervals, were installed in two regions of the site where monodominant Elymus stands intergrade with mixed-grass stands. Transects were laid perpendicular to the vegetation boundary. Data on percentage cover of Artemisia, Elymus and Bromus were collected using a sampling frame with a 10-cm2 grid. In the 1997 sampling, two 150-m transects were laid out at random across another part of the study site that is characterized by numerous small patches of Bromus, Artemisia and Elymus. Sampling was conducted in 106 plots at random intervals between 1 and 5 m using a 0.5-m2 sampling frame with a 10-cm2 grid. Percentage cover of Elymus, Bromus and Artemisia was noted, as well as the number of Artemisia seedlings in each plot. Plots were classed as having Elymus and Bromus present or absent, with a species considered present if it constituted 4% or greater cover in the plot. Artemisia seedling numbers were then analysed using a two-by-two chi-square contingency table, with presence and absence of Bromus and Elymus as factors, to determine if seedling recruitment occurred with the same frequency in the presence of the two grasses.

Data collected in 1996 and 1997 were analysed for the relationship between Bromus and Elymus cover in mixed-grass stands. Analysis was confined to the subset of plots where Artemisia cover was less than 10% and at least one of the grasses was present. All cover recorded in frames, including that of bare ground, summed to 100%. The regression slope of Bromus cover on Elymus cover was determined using quantile regression analysis at the 90th quantile (Thompson et al. 1996; Cade et al. 1999; Koenker 2000), using an algorithm in EasyReg International (Bierens 2002). A slope of −1 would imply that a unit increase in Elymus cover corresponds with a unit decrease in Bromus cover. The data sets from 1996 and 1997 were analysed separately.

In the 1998 transect sampling, three 50-m belt transects were laid perpendicular to the boundary between a grass-dominated area and a shrub-grass area. Visual estimation of vegetation cover was carried out in continuous 2 × 4 m blocks. Mean Bromus and Elymus cover was compared in plots containing 10% or greater Artemisia cover, using a t-test.

competition between artemisia and each of the grasses

In November 1996, eight plots, each containing six 1-m2 subplots, were established at the Green Canyon Ecological Research Station. In mid-April 1997, seedlings of Artemisia tridentata subspp. wyomingensis were purchased from Plants of the Wild (Tekoa, WA, USA). Two subplots of each main plot were planted with a solitary Artemisia seedling, two with an Artemisia seedling surrounded by three Elymus tussocks transplanted from Curlew Valley, and two with an Artemisia seedling and 2 g of Bromus seeds. Inter-tussock distances for Elymus (17 cm) and Bromus seed densities approximated those observed in monodominant species stands at Curlew Valley. All plants were located within a 50-cm circle within each subplot and were watered as necessary to ensure establishment. Bromus plants senesced in early June 1997, leaving seeds that germinated in the plots in the autumn. In the first week of October 1997, these seedlings were thinned or more seeds were added as necessary to achieve consistency among the plots, and watering was temporarily reinitiated until new grass seedlings were established.

To compare growth among Artemisia seedling treatments, ongoing non-destructive measurements were conducted three times during the summer of 1997 on seedling height, circumference and elongation of three marked twigs per plant. Data were analysed as a repeated measures anova. In mid-October 1997, subsamples of Artemisia, Elymus and Bromus were collected from half the subplots for determination of background tissue 15N concentration. Each of these subplots then received 0.96 kg ha−1 99% labelled (15NH4)2SO4 added in 2 L water. The NH+4 added represented about 0.2 µg N g soil−1, equivalent to a 23% increase in NH+4 concentrations over the concentrations determined in Green Canyon soils the previous November, when plots were installed (Booth, unpublished data). Two weeks after the nitrogen addition, above-ground biomass of Artemisia, Bromus and Elymus was harvested (autumn harvest). A second addition of labelled N was carried out in mid-May 1998 on the remaining plots, using the same protocol, except that (15NH4)2SO4 was added in 1 L of water because soils were already moist. The above-ground portion of the plants was again harvested 2 weeks after N addition (spring harvest).

Above-ground plant biomass was oven-dried at 65 °C, weighed, and a separate weight obtained for green biomass (i.e. excluding Elymus crowns and Artemisia woody stems). All green tissue from each plant was coarsely chopped, and a subsample was ground on a Wiley mill using a 600-µm mesh screen. Samples were analysed for percentage N and percentage 15N using continuous-flow, direct combustion mass spectrometry on an ANCA 2020 system (Europa Scientific, Cincinnati, Ohio, USA). Natural enrichment of tissue samples collected prior to the autumn addition of 15N was subtracted from post-treatment values from autumn and spring.

Above-ground green biomass (g plot−1), tissue N concentration (g N g green tissue−1 × 100), 15N concentration (µg 15N g green tissue−1), and total green biomass 15N (mg 15N plot−1) were calculated for each species and compared between Bromus and Elymus using season and species as factors. Spring values for Bromus are weighted averages of both mature autumn-germinated plants and spring-germinated seedlings, except where values for spring-germinated plants are presented separately. Artemisia target plants were analysed separately from the grasses. Above-ground green biomass, tissue N concentration, tissue 15N concentration and total green biomass 15N were compared using planting combination (alone, with Bromus, and with Elymus) and season as factors, using anova (systat, version 2). To improve normality of the data, biomass, total biomass N and 15N data were log or square-root transformed, and percentage N and tissue 15N concentration were arcsine transformed. Tukey pairwise comparison tests were used for multiple comparisons among treatments.

elymus transplant experiment

In October of 1997, Elymus tussocks from monodominant stands at Curlew Valley were excavated and re-planted either in monodominant Elymus stands or in mixed-grass stands (n = 6). Pairs of locations were chosen in each stand type to include at least three Elymus tussocks that surrounded each transplant at a similar mean distance of 25–50 cm. All tussocks received 25 mL water at the time of planting to aid establishment. To assess soil inorganic N availability in monodominant and mixed-grass soils at the time of planting, soil cores (0–10 cm) were taken at each planting location, and an additional six soil cores were collected in monodominant Bromus stands. All cores were extracted in the field in 2-M KCl. After processing, extractants were analysed for inline image and inline image using a flow-injection colourimetric analyser (Lachat Instruments, Mequon, Wisconsin, USA). Eight days after planting, all above-ground biomass was clipped from transplanted Elymus tussocks and dried at 65 °C. Length of individual leaves was measured, then all leaves were weighed together, ground on a Wiley mill using a 600-µm mesh screen, and analysed for N content using continuous-flow, direct combustion mass spectrometry. Total N content of above-ground biomass was calculated as proportion tissue N multiplied by green biomass. Data on leaf length, leaf number and total green biomass were square-root transformed to improve normality and were compared between planting locations using a paired samples t-test.

determination of volumetric soil moisture in bromus, elymus AND artemisia stands using tdr

Time domain reflectometry (TDR) was used to characterize soil moisture depletion curves in Artemisia, Elymus and Bromus stands at Curlew Valley. Measurements were conducted in monodominant Elymus stands, in Bromus-dominated areas within mixed grass stands, and in Artemisia-dominated patches in grass/shrub stands. Three probes (18 cm) were inserted vertically at the corners of eight 1-m2 plots per vegetation type. One probe measured soil moisture from 5 to 23 cm, one from 30 to 48 cm, and one from 50 to 68 cm. Probe readings were taken periodically with a cable tester (Campbell Scientific, Logan, Utah, USA), using a car battery as a power source. Time-domain reflectometry measures the bulk dielectric constant of the soil, so readings were calibrated against an empirically derived curve of gravimetric moisture content. Values were converted to a volumetric basis using a bulk density value of 1.26 g cm−3. In 1997, measurements were initiated on 25 March and conducted at approximately 2- to 3-week intervals until 5 September. In 1998, measurements were initiated on 19 March and continued until 23 June. Soil water content, depletion rates and response to precipitation events were compared among the vegetation stands at the three probe depths. For data from a single year, soil moisture content on particular dates and depletion/recharge rates between dates were analysed using a repeated measures anova with species and depth as factors. Depletion/recharge rates were calculated as the difference in soil moisture between dates divided by the number of days in the interval. In cases when measurement dates were similar in 1997 and 1998 (25/26 March, 28 April, 5 May, 19/23 May, 26/23 June) statistical analysis was carried out as a repeated measures anova on year, using species and depth as factors. The depletion rate between 25/26 March and 5 May was also analysed as a repeated measures anova on year. Data collected in the mid- and late summer of 1997 (14 July, 30 July, 14 August, 5 September) had no counterpart in 1998, and were analysed separately as a repeated measures anova on date, with species and depth as factors.

determination of moisture-holding capacity in soils of the three vegetation types

In spring of 2000, PVC tubes (10 cm in length and 8 cm in diameter) were installed near each TDR station to a depth of 7 cm, and 225 mL water was added to each tube. After 48 h, a TDR probe was inserted within each tube to take a reading spanning 2–20 cm. Five replicate tubes in each vegetation type were then cored from 2 to 7 cm and three from 2 to 20 cm. Soils were analysed for gravimetric soil moisture, and analysis of soil moisture-holding capacity (field capacity) was conducted using vegetation type and core depth as factors. Soil moisture did not differ among vegetation types or between the two depths at which cores were collected. Mean gravimetric soil moisture was 19.1% (SE 0.2).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

transect sampling

The 1997 census showed only one Artemisia seedling in the 37 sampling locations where both Elymus and Bromus occurred, whereas there was a total of 31 seedlings in the 46 locations containing Elymus but no Bromus. The 11 locations containing just Bromus had no seedlings and the 12 locations containing neither grass contained 10 seedlings. The chi-square analysis thus showed a significant negative association between even minimal Bromus cover (4% and greater) and presence of Artemisia seedlings, yet seedlings were found in Elymus-dominated areas more often than expected (d.f. = 1, χ2 = 31.84, P < 0.001). Results of the 0.9 quantile regression on vegetation cover data collected in mixed-grass stands in 1996 and 1997 showed a significant decrease of Bromus with increase in Elymus (Fig. 1a; slope of −5.05, P < 0.001 in 1996; slope of −4.04 and P < 0.005 in 1997, compared with the slope of −1 expected for direct replacement). Elymus cover of 15–20% virtually eliminated Bromus, whereas Elymus tussocks persisted in locations containing much higher percentages of Bromus.

image

Figure 1. Relationship between Bromus and Elymus cover for transect sampling conducted at Curlew Valley in June 1996 (a) and May 1997 (b). Lines delineate 0.9 quantile.

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A paired samples t-test on 1998 transect data from sample locations in shrub-grass stands containing at least 8%Artemisia showed higher mean Elymus cover (3.94%, SE ± 0.38) than Bromus cover (0.31%, SE ± 0.15; n = 31, P < 0.001).

competition between artemisia and each of the grass species

Biomass of Bromus and Elymus grown with Artemisia in the competition plots increased from autumn to spring (seasonal effect, Fig. 2a; F1,28 = 29.59; P < 0.001), but did not differ between the two grasses in either season. On an area basis, mature Bromus biomass in the spring was within the range determined by sampling at Curlew Valley in May 1997 (Booth, unpublished data). Elymus tissue N concentration in the autumn was higher than that of Bromus (species effect, F1,28 = 16.33; P < 0.001). Tissue N concentration declined from autumn to spring in both grasses (seasonal effect, F1,28 = 284.22; P < 0.001). In the spring, Elymus tissue N concentration was higher than that of mature autumn-germinated Bromus, but was similar to that of spring-germinated Bromus seedlings (species effect, Fig. 2b; F2,21 = 8.11; P = 0.002).

image

Figure 2. Effect of season on green biomass, tissue N concentration, tissue 15N concentration, and total green biomass 15N of Bromus and Elymus from the competition experiment. Autumn harvest consisted of newly germinated Bromus and Elymus tussocks, spring harvest also included spring-germinated Bromus. Separate bars are shown for spring-germinated Bromus for comparison. Data shown are untransformed means and standard errors. Letters refer to Tukey pairwise comparisons. Upper letter refers to difference within a species, between seasons, and the lower to differences between species within a season; all differences significant at P < 0.05.

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Green tissue 15N concentration of Bromus declined from autumn to spring (seasonal effect, F1,28 = 83.63; P < 0.001), whereas values for Elymus did not differ significantly between seasons (Fig. 2c). In the spring it was higher for spring-germinated Bromus seedlings than Elymus or autumn-germinated Bromus (species effect, F2,21 = 11.01; P < 0.001). Total biomass 15N was higher in Bromus than Elymus in the autumn (species effect, F1,28 = 7.25; P < 0.001), but mature plants of the two species showed similar values in the spring (Fig. 2d).

Artemisia seedling height, circumference and measurements of individual twigs were conducted on 14 May, 22 July and 14 October 1997. All measures showed similar patterns and for simplicity, only height values are reported. By 22 July (12 weeks after planting) Artemisia growing alone and with Elymus did not differ in height (29.7 cm, SE ± 1.1 and 27.5 cm, SE ± 1.5, respectively) but were significantly larger than those growing with Bromus (17.1 cm, SE ± 1.3) (species effect, F2,45 = 26.92; P < 0.001). These differences persisted and by October, mean height of shrubs growing alone and with Elymus was 35.2 cm (SE ± 1.5), whereas that of Artemisia with Bromus was 21.6 cm (SE ± 1.9). Above-ground green biomass in autumn and spring was greatest in Artemisia growing alone, followed by Artemisia with Elymus, and least in Artemisia with Bromus (species effect, F2,42 = 43.34; P < 0.05; Fig. 3a). Values increased from autumn to spring in all three planting combinations (seasonal effect, F1,42 = 24.26; P < 0.001). Total Artemisia above-ground biomass (i.e. including woody biomass) showed the same relationships.

image

Figure 3. Effect of season and grass species on green biomass, tissue N concentration, tissue 15N concentration, and total 15N in green biomass of Artemisia grown in the competition experiment. Statistical conventions as in Fig. 2.

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Artemisia15N concentration and total 15N uptake were low and did not differ among treatments in the autumn. Tissue 15N concentration (Fig. 3c; season by species interaction, F2,41 = 4.07; P = 0.02) and total 15N uptake (Fig. 3d; season by species interaction, F2,41 = 18.56; P < 0.001) increased from autumn to spring in shrubs growing alone and with Elymus, but those with Bromus did not show a similar increase.

Overall tissue N was positively correlated with biomass in shrubs growing alone (n = 16, R2 = 0.38, P = 0.01; Fig. 4), though the relationship was not significant by season (in autumn, n= 8, R2 = 0.39, P = 0.09; in spring, n= 8, R2 = 0.45, P = 0.07). In shrubs grown with Elymus, however, the relationship was negative (in autumn, n= 8, R2 = 0.58, P = 0.03; in spring, n= 8, R2 = 0.92, P = 0.001). There was no consistent pattern for Artemisia grown with Bromus.

image

Figure 4. Effect of season and grass species on the relationship between tissue N concentration and biomass for Artemisia grown in the competition experiment. Closed symbols represent autumn-harvested plants; open symbols represent spring-harvested plants.

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Total above-ground green biomass N within a plot (Fig. 5a) did not differ between plots with Artemisia growing alone and Artemisia growing with Elymus, but was significantly lower in plots with Artemisia and Bromus in both autumn and spring (species effect, F2,41 = 9.66; P < 0.001). Total biomass N was higher in spring than autumn (seasonal effect, F1,41 = 61.87; P < 0.001). In the autumn, total green biomass 15N was higher in plots with Artemisia and Bromus than in the other treatments (Fig. 5b; season by species interaction, F2,41 = 12.57, P < 0.001), but did not differ significantly between the three treatments in the spring.

image

Figure 5. Effect of season and grass species on N and 15N contained in total above-ground green biomass within a plot, separated by species. Data shown are untransformed means and standard errors. Left-hand in a pair refers to autumn harvested plots, right-hand bar to spring harvested plots. Error bars and Tukey pairwise comparisons refer to plot totals. Upper letter corresponds to difference within a growing combination, between seasons; lower to differences among growing combinations within a season; all differences significant at P < 0.05.

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elymus transplant experiment

Total inorganic N concentrations in pure Elymus stands were 3.2 µg N g−1 soil (SE ± 0.38) when tussocks were transplanted, significantly lower than mean inorganic N concentrations of 7.13 µg g−1 soil (SE ± 0.84) in Bromus patches and 7.20 µg g−1 soil (SE ± 1.1) in Bromus/Elymus stands (location effect, F2,15 = 7.59; P = 0.007). Nitrate comprised 43% (SE ± 5%) of total inorganic N in Elymus stands, but values of 70% (SE ± 3%) in Bromus patches and 64% (SE ± 4%) in Bromus/Elymus stands were similar (overall location effect, F2.15 = 11.79; P= 0.001). Soil moisture was significantly higher under Bromus (at 8%) and Bromus/Elymus (7.8%) than under monodominant Elymus (6.5%; location effect, F2,15 = 16.5; P = 0.0002). Elymus tussocks transplanted to Bromus/Elymus stands showed increases in leaf length above those transplanted to pure Elymus stands (6.7 cm, SE ± 0.8 vs. 3.6 cm, SE ± 0.7; location effect, F1,9 = 8.04, P = 0.02). Tissue N concentration was also higher in transplants to Bromus/Elymus stands (3.31%, SE ± 0.1 vs. 2.9%, SE ± 0.1; location effect, F1,9 = 8.56, P = 0.02). However, biomass was slightly lower for transplants in Bromus/Elymus than in Elymus stands though the effect was not significant (0.11 g, SE ± 0.03 vs. 0.18, SE ± 0.03; location effect, F1,9 = 3.97, P = 0.07).

determination of volumetric soil moisture in bromus, elymus and artemisia stands using tdr

Patterns of spring and early summer moisture depletion and recharge were broadly similar in 1997 and 1998. Both years had an initial drawdown period, a recharge phase following precipitation, then another period of depletion (Fig. 6). Averaged over all depths, soil moisture on 25 March 1997 and 26 March 1998 was highest in Artemisia, followed by Bromus, then Elymus (species effect, F2,108 = 12.06; P < 0.001), and was higher overall in 1997 than 1998 (year effect, F1,108 = 53.55; P < 0.001). Soil moisture at 50–68 cm on 25/26 March was lower in Elymus stands than in Artemisia or Bromus stands (species-by-depth interaction, F4,108 = 3.25; P = 0.01).

image

Figure 6. Mean volumetric soil moisture as determined by TDR in stands of Bromus (solid triangles), Elymus (squares) and Artemisia (circles) for dates in 1997 and 1998. Error bars are standard error of the mean; n = 8.

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Between 25/26 March and 5 May of both years, soil moisture declined most quickly beneath Bromus (−0.20% day−1) followed by Artemisia (−0.15% day−1) then Elymus (−0.11% day−1) with depletion rates all differing significantly (species effect, F2,107 = 19.63; P < 0.001). Depletion rates in the 5–23 cm and 30–48 cm layers were two to three times greater than in the 50–68 cm layer (depth effect, F2,107 = 49.61; P < 0.001). By 5 May of both years, soil moisture at all measured depths was lowest in Bromus, followed by Elymus, then Artemisia (species effect, F2,125 = 19.28; P < 0.001).

Between 19 May and 16 June 1997, the 5–23 cm layer was recharged by precipitation, but the overall slope of recharge was negative for the lower depths. Overall, recharge under Bromus was higher than under Elymus or Artemisia (species effect, F2,61 = 10.16; P < 0.001). The following year, the significant spring recharge event occurred between 5 May and 23 May, when soil moisture recharge was measured at all three depths. Recharge averaged over all depths tended to be higher under Elymus than the other species, although the effect was not significant (F2,63 = 2.74; P = 0.07).

By the end of June 1997, soil moisture averaged over all depths in Bromus stands was 10.7%, but was 9.1% at the same time in the following year. In contrast, mean soil moisture in Elymus stands was more depleted at the end of June in 1997 (9.4%) than 1998 (12.1%). This was also the case in Artemisia stands (9.4% in 1997 and 11.0% in 1998), resulting in a significant species-by-year interaction for the June 26/23 sampling date (F2,121 = 6.62; P < 0.001). By 5 September, the last date in 1997 on which measurements were taken, there was no significant difference in soil moisture at any depth among the three species.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Distinct patterns of water and N uptake by Bromus, Elymus and Artemisia appear to mediate interactions among the three species, contributing to spatial distribution of plant communities at Curlew Valley. Community composition is consistent with an asymmetric relationship in which Elymus permits Artemisia recruitment, but appears to suppress Bromus, as indicated by the near total suppression of Bromus when Elymus cover approaches 20%. The seedling stage is the most vulnerable period for reestablishment of native perennials within invaded communities. In assessing the competitive environment encountered by Artemisia seedlings, however, the nature, timing and intensity of resource use by competitors must all be taken into account. Results of TDR monitoring at Curlew Valley and the competition experiment indicate that intense water and N uptake by Bromus in the spring overlaps that by associated Artemisia seedlings, resulting in reduced growth and rates of resource uptake by the latter. In contrast, while timing of water use by Elymus is similar to that of Artemisia, its intensity in Elymus stands is similar to that in Artemisia stands. The prevalence of shrub seedlings in Elymus-dominated areas and their near absence in areas of even low Bromus cover suggests the importance of Elymus for development of a diverse plant community at Curlew Valley.

role of n uptake in mediating competitive interactions among the three species

Like many early successional species (Bazazz 1986), Bromus is fast-growing and shows high rates of nutrient uptake. In the competition experiment, it was the most effective of the three species in acquiring 15N added in both autumn and spring (Fig. 4b). However, a high resource requirement may also be the Achilles heel of the invasive grass. Germinating Bromus is sensitive to inorganic N limitation (Young et al. 1997), showing diminished tissue N and growth rates under low inorganic N availability (Uresk et al. 1979). If Elymus acted as a significant sink for N in autumn, it might thus suppress annual grass growth, and if autumn N uptake were relatively unimportant to Artemisia, such activity by Elymus would not greatly impact Artemisia seedlings. In the autumn harvest of the competition experiment, Artemisia did indeed show minimal 15N uptake, even when grown alone with no grass competitor. However, autumn 15N uptake by Elymus was not particularly aggressive, relative to that by Bromus. While Elymus tissue N concentration (providing an integration of long-term N status of the plant) was higher in autumn than that of Bromus (Fig. 2b), Elymus tissue 15N concentration (a short-term measure of immediate N-uptake capacity) was lower (Fig. 2c), suggesting that N acquired in autumn constitutes a relatively small portion of perennial grass total N. The perennial habit of Elymus allows it to store large carbohydrate reserves in roots and crowns, which it mobilizes for foliage production after autumn rains (Coyne 1969). A similar mobilization of N reserves may reduce reliance on immediately available soil inorganic N. In contrast, autumn 15N acquisition by newly germinated Bromus over the 2-week experimental period was substantial, despite minimal seedling biomass (Fig. 2).

Despite low autumn 15N uptake by Elymus in the competition experiment, Elymus tussocks transplanted into Bromus/Elymus stands in Curlew Valley in the autumn apparently did benefit from higher soil moisture and soil inline image concentrations that were threefold greater than those in monodominant Elymus stands. Transplants to mixed-grass stands had higher tissue N and longer leaves than those in monodominant Elymus stands, consistent with findings that Elymus leaf length is stimulated by N fertilization (Trent et al. 1994). However, lower transplant biomass in mixed-grass stands than Elymus stands suggests that despite initially higher soil moisture, tussocks became water-stressed due to soil moisture use by Bromus seedlings over the course of the experiment.

In the spring, Elymus biomass and tissue 15N concentration were equivalent to Bromus, suggesting that the tussock grass may be a strong competitor for N in mixed grass stands. Nutrient incorporation into perennial biomass should reduce N availability to Bromus over the long-term, and may have consequences for ecosystem nutrient budgets. In south-western Spain, more nutrient flushing occurred in annual grass stands than perennial grass stands after autumn rains (Joffre 1990), because annuals were not sufficiently phenologically advanced to retain resources effectively. However, at Curlew Valley, increasing perennial cover appears to have a synergistic effect on nutrient availability beyond the simple effects of uptake. The high soil inline image concentrations observed in mixed-grass stands during the transplant experiment were typical of Bromus-dominated soils in the late summer (Booth 2001), and were not found in monodominant Elymus stands, suggesting that soil nutrient regimes are influenced by the quality and quantity of plant contributions to the soil organic matter pool (Wedin & Pastor 1993; Booth 2001). As Bromus cover is reduced, so is its contribution to the labile soil organic matter pool that apparently contributes to increased inorganic N availability.

Like Elymus, Artemisia was a much stronger competitor for inorganic N in spring than in autumn. In contrast to the grasses, which showed dilution of tissue N concentration with increasing biomass in the spring, spring-harvested Artemisia showed both increased biomass and a trend towards increased tissue N concentration and tissue 15N concentration relative to autumn values. Shrubs grown with Bromus did not show a significant increase in tissue 15N concentration from spring to autumn, probably due in part to aggressive N uptake by associated Bromus, which remained a strong competitor for 15N even when mature and approaching senescence. However, low 15N uptake by shrubs growing with Bromus in the competition plots could also be attributable to high microbial immobilization of inorganic N (Hooper & Vitousek 1998), or physiological limitations to N uptake. Artemisia growing with Bromus were water-limited, which may have reduced uptake of 15N added in the 2-week addition, even if it was available. In contrast, the tissue 15N concentration of Artemisia growing with Elymus was similar to that of Artemisia growing alone, suggesting that short-term N uptake by shrubs was not significantly impacted. Nonetheless, lower biomass in shrubs grown with Elymus indicates some degree of water limitation, with the inverse relationship of N concentration and biomass suggesting slightly decreased N availability to those plants least limited by water. The same pattern was observed by Eissenstat & Caldwell (1988), who found that N concentration tended to be negatively correlated with biomass in field-grown Artemisia growing with the perennial grass Agropyron desertorum, and concluded that shrubs with higher tissue N concentrations were more water-stressed. Overall, the results of the competition experiment suggest that given the ubiquity of water limitation in a semiarid environment, N limitation is likely to be a secondary issue. Where resources were the least limiting, i.e. in shrubs grown alone, increased biomass conveyed greater control over soil resources, as indicated by the positive relationship of tissue N concentration and biomass. As a consequence, total N contained in green biomass did not differ in plots containing just Artemisia and plots with both Artemisia and Elymus (Fig. 4a). This flexibility, allowing perennials to sequester soil resources over the long term, is probably important for exclusion of opportunistic invader species that are rendered less aggressive when nutrients are limiting.

role of soil moisture uptake in mediating competition among the three species

Because soils of all three vegetation types at Curlew Valley had similar moisture-holding capacity, TDR data should reflect soil moisture use by plants. Time domain reflectometry data indicate that rates of soil moisture depletion were higher under Bromus than under the perennials at all depths, in both years. These results were similar to those from a study conducted in eastern Washington, where Bromus used more of the soil water at depths of 45–80 cm than did perennial vegetation (Kremer & Running 1996). The opportunistic use of soil moisture by the annual is highlighted by results from Harris (1967), who found that sparse (5% cover) and dense (95% cover) stands of Bromus showed negligible differences in soil moisture content. Bromus is flexible in its phenology, and behaves as a facultative biennial when moisture conditions permit (Harris 1967; Link et al. 1990). While the spring of 1998 was drier than that of 1997, subsequent deep penetration of precipitation may have delayed Bromus senescence, ultimately allowing the annual to deplete soil moisture more thoroughly than in 1997. Soil moisture depletion conveys no penalty to the annual, which endures the driest part of the summer in seed form, but is detrimental to associated perennials, which appear to persist best where soil moisture is retained after Bromus senescence (Tausch et al. 1994). Data from Curlew Valley indicate that Artemisia seedlings recruiting in Bromus-dominated areas encounter moisture stress earlier in the season than under Artemisia or in Elymus stands. High rates of soil moisture use by Bromus probably also impact Elymus where it occurs in mixed-grass stands. However, the nearly complete senescence of Elymus foliage allows this grass to endure the driest part of the summer, and in the autumn, an extant root system already deployed at depth may give the perennial a head start on resource acquisition over the annual, which must germinate from seed.

Overall, water use patterns by Elymus and Artemisia showed a high degree of congruence, particularly for dates after 5 May of both years. Both perennials appeared to be more drought-sensitive than the annual grass, showing attenuated soil moisture depletion rates in the relatively dry spring of 1998. In contrast to Bromus, soils under Elymus and Artemisia retained more moisture at the end of June 1998 than they did in 1997, although soils overall were drier in the early spring of 1998. The major difference between patterns of soil moisture use by Elymus and Artemisia was lower soil moisture under the tussock grass on dates prior to May, probably due to transpiration following foliage growth in the fall. Apparently such cool season water use by Elymus has little effect on associated Artemisia, as biomass of shrubs grown with Elymus in the competition experiment was similar to that of shrubs grown alone. However, early season water depletion by Elymus may impact associated Bromus where the two grasses occur in mixed stands. Bromus leaf growth primarily occurs in the spring, and the annual has usually completely senesced by late May (Harris 1967), thus early spring moisture availability is critical for both growth and seed production (Richardson et al. 1989). Early spring soil moisture use by the tussock grass may in part explain the strong reduction in Bromus cover as Elymus cover increases.

The climatic regime of the Great Basin has proved to be ideal for the exotic annual grass Bromus tectorum. Autumn rains trigger germination, and root growth continues at low temperatures, so that the annual is ready to take advantage in spring of what is essentially a high-resource environment, if only for a short time. Bromus resets the plant community to a disturbance-driven regime, in the sense that rapid accumulation of biomass is followed by total turnover of vegetation on a yearly basis, causing greater resource flux than occurs under native vegetation. When the system contains living biomass, soil moisture and soil nutrients are low due to high vegetative demand; when the vegetation has senesced, soil moisture and nutrients accumulate. In contrast, ecosystems containing native vegetation show more moderate changes in resource availability. Perennial biomass takes soil nutrients out of circulation, nutrients are recycled with plants, and soil moisture uptake is attenuated in response to climatic conditions.

The composition of the vegetation at Curlew Valley suggests that Elymus suppresses Bromus, thereby indirectly facilitating Artemisia, which only recruits in zones that are free of the annual grass. Elymus appears to share characteristics with both Artemisia and Bromus that support its transitional role. Like Bromus, Elymus is revitalized by autumn precipitation, but as a perennial its root system is already deployed at depth, allowing it to exploit water throughout the soil profile early in the season. In mixed-grass stands, Elymus may thus displace the annual due to the combined effects of more efficient water use, physical usurpation of space, and nutrient sequestration that decreases inorganic N availability. As a perennial, Elymus must be responsive to soil moisture limitation, and Artemisia recruiting into Elymus-dominated areas thus experience resource competition not unlike that found in mature shrub stands. Bromus has been considered as one of the most stable alternative states in semiarid systems of the western United States (Clements 1936; Stewart & Hull 1949; Allen-Diaz & Bartolome 1998). If Elymus is indeed breaking this link and facilitating the re-invasion of Artemisia into previously inaccessible sites, it may serve as a pivot point for plant community development.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Many people contributed to this research. The authors wish to especially thank Charlie Ashurst, for assistance with all things technical. Wally McFarlane conducted much of the data collection at Curlew Valley. Romaine Neilson, Tarek Milleron and others helped in the field, and discussions with Ron Ryel were helpful with data analysis. Thanks as always to Susan Durham for assistance with statistical analysis. Finally, many thanks to Dani Or, for TDR probe calibration and assistance with data interpretation. This work was supported with NASA Graduate Fellowship Grant NGT5-50011 and NSF Grant DEB-9807097.

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  6. Discussion
  7. Acknowledgements
  8. References
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