The effect of clipping frequency on the competitive interaction between two perennial grass species

Summary The effect of clipping frequency on competition between Lolium perenne and Agrostis tenuis was investigated. The yield of clippings of both species increased and then declined during the 12-week period of the experiment, but the clip yield of Lolium was always significantly greater than that of Agrostis. Lolium was clearly the better competitor in unclipped controls. The proportion of the biomass contributed to the mixture by Agrostis increased as the interval between clips decreased. Tiller production was unaffected by increased clipping frequency in Lolium but was increased in Agrostis. Total yield was much more drastically reduced by frequent clipping in Lolium than in Agrostis, where yield was practically unaffected by wide variations in clipping frequency.These results are in agreement with the field distributions of the two species. They also suggest that differences in height and response to clipping are likely to confound any attempt to monitor the progress of competition experiments by measuring the yield of clippings.     

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

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

A multiple species approach to biomass production from native herbaceous perennial feedstocks

Due to the rapid rate of worldwide consumption of nonrenewable fossil fuels, production of biofuels from cellulosic sources is receiving increased research emphasis. Here, we review the feasibility to produce lignocellulosic biomass on marginal lands that are not well-suited for conventional crop production. Large areas of these marginal lands are located in the central prairies of North America once dominated by tallgrass species. In this article, we review the existing literature, current work, and potential of two native species of the tallgrass prairie, prairie cordgrass (Spartina pectinata), and little bluestem (Schizachyrium scoparium) as candidates for commercial production of biofuel. Based on the existing literature, we discuss the need to accelerate research in the areas of agronomy, breeding, genetics, and potential pathogens. Cropping systems based on maintaining biodiversity across landscapes are essential for a sustainable production and to mitigate impact of pathogens and pests.     

The frequency, distribution and reproductive biology of dioecious species in the native flora of Britain and Ireland

The proportion of dioecious species in the flora of the British Isles has been underestimated in the past, partly because the total number of native species was overestimated and partly because some dioecious species were omitted for various reasons. We estimate that, in a total native angiosperm flora of 1377 species and species-equivalents, 4.3% (59 species) are dioecious, a substantially higher percentage than the previous estimates of 2, 3.1 and 3%. If gymnosperms are included, 4.4% (61 of 1380 species of seed plant) are dioecious. A review of published work shows that few of these dioecious species have been well-studied, and that for some little or no information is available about their sex ratios and reproductive biology.     

Genotypic variation in the response of two perennial grass species to elevated carbon dioxide

Shoot and reproductive biomass of genotypes of Bromus erectus and Dactylis glomerata grown in competition at ambient and elevated CO₂ were examined for 2 consecutive years in order to test whether genetic variation in those traits exists and whether it is maintained over time. At the species level, a positive CO₂ response of shoot biomass of both species was only found in the first year of treatment. At the genotype level, no significant CO₂2genotype interaction was found at any single harvest either for vegetative or reproductive biomass of either species. Analysis over time, however, indicated that there is a potential for evolutionary adaptation only for D. glomerata: (1) repeated measures ANOVA detected a marginally significant CO₂2genotype2time interaction for shoot biomass, because the range of the genotypes CO₂ response increased over time; (2) genotypes that displayed the highest response during the first year under elevated CO₂ also showed the highest response the second year. Null (B. erectus) or weak (D. glomerata) selective potentials of elevated CO₂ were detected in this experiment, but short time series could underestimate this potential with perennial species.     

Response to ammonium and nitrate by a mycorrhizal annual invasive grass and native shrub in southern California

The goal of this study was to determine the interaction of mycorrhizae and two N sources, ammonium (NH(4)(+)) and nitrate (NO(3)(-)), on the growth of a coastal sage scrub (CSS) species, Artemisia californica, and an exotic annual grass, Bromus madritensis ssp. rubens. Anthropogenic nitrogen deposition may be influencing the decline of CSS and replacement by exotic grasses, but the extent to which mycorrhizae are involved in shrubland decline is unknown. NO(3)(-) is the dominant form of deposition in southern California, although the native, uneutrophied soils have a greater concentration of NH(4)(+). Seeds of each species were germinated in pots of sterile soil, inoculated with native soil containing mycorrhizal spores and infective root fragments, and fertilized with 50 μg/g of either NO(3)(-) or NH(4)(+). NH(4)(+) enhanced the growth of both mycorrhizal species, while NO(3)(-) did not. Control plants of B. madritensis under low N had a significant response to mycorrhizae, but A. californica did not. Nitrate increased the growth of nonmycorrhizal A. californica as much as the mycorrhizal NH(4)(+)-treated plants. There is no evidence in this study to suggest that the decline of A. californica or increase in B. madritensis is due to a mycorrhizal response to NO(3)(-). Other life history traits of the two species must be used to explain the invasive behavior of the annual grass. Mycorrhizae may be more important in controlling plant growth in native uneutrophied soils dominated by NH(4)(+) rather than NO(3)(-).     

Established native perennial grasses out-compete an invasive annual grass regardless of soil water and nutrient availability

Competition and resource availability influence invasions into native perennial grasslands by non-native annual grasses such as Bromus tectorum. In two greenhouse experiments we examined the influence of competition, water availability, and elevated nitrogen (N) and phosphorus (P) availability on growth and reproduction of the invasive annual grass B. tectorum and two native perennial grasses (Elymus elymoides, Pascopyrum smithii). Bromus tectorum aboveground biomass and seed production were significantly reduced when grown with one or more established native perennial grasses. Conversely, average seed weight and germination were significantly lower in the B. tectorum monoculture than in competition native perennial grasses. Intraspecific competition reduced per-plant production of both established native grasses, whereas interspecific competition from B. tectorum increased production. Established native perennial grasses were highly competitive against B. tectorum, regardless of water, N, or P availability. Bromus tectorum reproductive potential (viable seed production) was not significantly influenced by any experimental manipulation, except for competition with P. smithii. In all cases, B. tectorum per-plant production of viable seeds exceeded parental replacement. Our results show that established plants of Elymus elymoides and Pascopyrum smithii compete successfully against B. tectorum over a wide range of soil resource availability.     

Optimal individual growth and reproduction in perennial species with indeterminate growth

Summary A model predicting optimal timing of growth and reproduction in perennial species with indeterminate growth living in a seasonal environment, is presented. According to the model, the optimal fraction of growing season devoted to growth decreases with increasing individual age and size, which leads to S-shaped growth curves. Winter mortality seems to be a crucial factor affecting the timing of growth and reproduction, under the same function describing the dependence of growth rate and reproductive rate on body size. When winter mortality is heavy, it is often optimal to start reproducing in the first year, and to devote a large proportion of the subsequent years to reproduction, thus leading to small adult body sizes.The model has been applied to two species of mollusc and one species of fish. The model predictions fit well to the field data for these three species.     

Fire frequency and species associations in perennial grasslands of south-west Ethiopia

Fires play an important role in shaping species composition and associations in East African grasslands. Grassland plains of Omo National Park (ONP), Ethiopia, which are dominated by perennial grass species, exist in a fire-prone environment. Our objective was to determine if the current pattern of plant species composition in ONP's grassland plains was correlated with the historical pattern of fire frequency. Species composition was determined at 160 plots along 30 west-trending transects, approximately 2 km apart. Fire frequency for each plot was estimated using eleven Landsat satellite images that spanned a 23-year period. The Mantel and partial Mantel tests were used to test for correlation between species composition and fire frequency.
Plots in the northern grassland plain appear to burn every other year, while plots in the southern grassland plain burn once every 4–5 years. However, no significant correlation was found between patterns in species composition and fire frequency. Likewise, a selective analysis by functional group (i.e. grass, shrubs) revealed no relationship with fire frequency. It appears that fire does play a role in dictating species composition in ONP, but only in the sense that species that can tolerate the current fire regime persist. Species distribution, however, appears to be under the influence of other factors.     

Plant-induced changes in soil nutrient dynamics by native and invasive grass species

Alteration of soil nutrient dynamics has recently garnered more attention as both a cause and an effect of plant invasion. This project examines how nutrient dynamics are affected by native (Elymus elymoides, Pseudoroegneria spicata, and Vulpia microstachys) and invasive (Aegilops triuncialis, Agropyron cristatum, Bromus tectorum, and Taeniatherum caput-medusae) grass species. This research questions whether natives and invasives differ in their effects on nutrient dynamics. A greenhouse study was conducted using two field-collected soils. Effects on nutrient dynamics were compared using an integrated index that evaluates the total nutrients in soil and in plant tissue compared to an unplanted control. With this index, we evaluated whether soil nutrients increased or decreased as a result of plant growth, controlling for plant uptake. We found no consistent support for our hypothesis that invasive grass species as a group influence nutrient dynamics differently than native grass species as a group. Our results indicate species-specific effects on nutrient dynamics. Alteration of nutrient dynamics is not a trait shared by all of the invasive grass species in our study. However, alteration of nutrient dynamics may be a mechanism by which some individual species increase their invasive potential.     

Carbon and nitrogen content of congeneric annual and perennial grass species: relationships with growth

Seven annual-perennial pairs of grass species (six congeneric and one pair taken at random) were grown under productive conditions in the laboratory in order to investigate which plant characters were responsible for the higher relative growth rate (RGR) of annuals as compared to perennials under these conditions. The nitrogen and carbon concentrations of shoot organs and of the whole plant were higher in annuals than in perennials. This was also the case for the specific absorption rate for nitrate and nitrogen productivity (on whole plant and leaf basis). The range of RGR displayed by the 14 species was large enough (0.15–0.33d⁻¹) to examine the general relationships between RGR and the various parameters measured in the present study. RGR was positively related to plant, leaf blade and sheath nitrogen concentrations, but there was no relationship between RGR and any of the carbon concentrations. RGR also strongly correlated with specific absorption rate for nitrate and with nitrogen productivity. A new factorization of this latter parameter led to the definition of the ‘leaf nitrogen productivity’ (N_LP), which is likely to depend on photosynthetic nitrogen use efficiency. RGR was shown to be strongly correlated with N_LP, but not with the second term of the factorization, namely the proportion of plant nitrogen allocated to the leaves.     

Effect of species, root branching order and season on the root traits of 13 perennial grass species

Aims

Inter-specific comparisons of plant traits may vary depending on intra-specific variation. Here we examine the impact of root branching order and season on key functional root traits for grass species. We also compare root traits among co-existing grass species as a step towards defining root trait syndromes.

Methods

Monocultures of 13 grass species, grown under field conditions and subjected to intensive management, were used to record root trait values for coarse roots (1st order, >0.3?mm), fine roots (2nd and 3rd orders, <0.2?mm) and mixed root samples over three growing seasons.

Results

Branching order and species had a significant effect on root trait values, whereas season showed a marginal effect. The diameter of coarse roots was more variable than that of fine roots and, as expected, coarse roots had higher tissue density and lower specific root length values than fine roots. Principal component analysis run on eight root traits provided evidence for two trait syndromes related to resource acquisition and conservation strategies across grass species.

Conclusions

Our data show that root branching order is the main determinant of root trait variation among species. This highlights the necessity to include the proportion of fine vs coarse roots when measuring traits of mixed root samples.     

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

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

Root morphological plasticity and nutrient acquisition of perennial grass species from habitats of different nutrient availability

We studied the root foraging ability and its consequences for the nutrient acquisition of five grass species that differ in relative growth rate and that occur in habitats that differ widely in nutrient availability. Foraging responses were quantified, based on the performance of the plants in homogeneous and heterogeneous soil environments of the same overall nutrient availability. Although all species tended to produce a significantly higher root length density in a nutrient-rich patch, this response was significant only for the faster-growing species. The increased root length density resulted from small, though not significant, changes in root biomass and specific root length. The effectiveness of root proliferation was determined by quantifying the total amount of nutrients (N and P) accumulated by the plants over the course of the experiment. Plants acquired more N in a heterogeneous environment than in a homogeneous environment, although the total nutrient availability was the same. The ability to acquire nutrients (N or P) in the heterogeneous environment was not related to the ability of species to increase root length density in response to local nutrient enrichment. In contrast to other studies, our results suggest that the role of morphological plasticity of roots in acquiring patchily distributed resources is limited. Possible reasons for this discrepancy are discussed. Received: 11 September 1997 / Accepted: 28 February 1998     

Physiological responses to short-term water and light stress in native and invasive plant species in southern California

As climate variability increases in low-resource environments, the ability of native and invasive species to tolerate stress and respond to large, ephemeral resource pulses will strongly influence plant fitness and, consequently, competitive outcomes. We examined how native and invasive species occurring in arid coastal sage scrub communities in southern California responded to water and high-light stress. We also examined how plants responded to irrigation following short-term water stress. While species responded differently to water and light treatments, no general pattern emerged between native and invasive species. Photosynthetic function of Ricinus communis (invasive) and Salvia mellifera (native) was most robust to water stress and most responsive to irrigation following water stress. Leaf transpiration data suggested that Ricinus and Salvia maintained photosynthetic function by high water use efficiency rather than higher water status via large root biomass. Brassica nigra (invasive) and Encelia californica (native) were more resistant to photoinhibition in response to high-light stress than Ricinus, Salvia, Artemesia californica (native) or Nicotiana glauca (invasive). Our data suggest that native and invasive species in these arid systems display a range of physiological responses to stress and that strategies for invasive species control or native ecosystem restoration based on plant responses to stress may require species-specific approaches.