Field measurements of photosynthesis,water-use efficiency,and growth inAgropyron smithii (C3) andBouteloua gracilis (C4) in the Colorado shortgrass steppe

Summary Field measurements of gas exchange and growth were conducted on a C₃ grass,Agropyron smithii, and a C₄ grass,Bouteloua gracilis, in order to further establish the adaptive significance of the C₄ pathway under natural conditions. Maximum rates of leaf area expansion in tillers and maximum seasonal photosynthesis rates of both species occurred during the cool, early summer month of June. The occurrence of maximum growth and photosynthesis inB. gracilis during this cool period was apparently related to its occupation of warm microenvironments next to the ground surface. As temperatures increased during the midsummer, photosynthesis rates decreased to 47% and 55% of the seasonal maximum inB. gracilis andA. smithii, respectively. Water-use efficiencies in both species were similar or slightly higher forB. gracilis during June, the period of maximum growth. By mid-July, however, leaves of the C₃ grass,A. smithii, exhibited water-use efficiencies approximately half as high asB. gracilis. These differences in water-use efficiency were the result of differences in stomatal conductance, rather than differences in daily CO₂ uptake rates which were similar in both species. The results demonstrate that in certain environments there are no offset periods of growth and maximum photosynthesis during the growing season in these C₃ and C₄ species. The greater amounts of daily water use inA. smithii during the midsummer might contribute to its much greater abundance in lowland sites in the shortgrass steppe. The C₄ grass,B. gracilis, occurs in dry upland sites in addition to the more mesic lowland sites.     

Effects of earthworms and organic litter distribution on plant performance and aphid reproduction

Human management practices and large detritivores such as earthworms incorporate plant litter into the soil, thereby forming a heterogeneous soil environment from which plant roots extract nutrients. In a greenhouse experiment we investigated effects of earthworms and spatial distribution of ¹⁵N-labelled grass litter on plants of different functional groups [Lolium perenne (grass), Plantago lanceolata (forb), Trifolium repens (legume)]. Earthworms enhanced shoot and root growth in L. perenne and P. lanceolata and N uptake from organic litter and soil in all plant species. Litter concentrated in a patch (compared with litter mixed homogeneously into the soil) increased shoot biomass and ¹⁵N uptake from the litter in L. perenne and enhanced root proliferation in P. lanceolata when earthworms were present. Growth of clover (T. repens) was rather independent of the presence of earthworms and organic litter distribution: nevertheless, clover took up more nitrogen in the presence of earthworms and exploited more ¹⁵N from the added litter than the other plant species. The magnitude of the effects of earthworms and organic litter distribution differed between the plant species, indicating different responses of plants with contrasting root morphology. Aphid (Myzus persicae) reproduction was reduced on P. lanceolata in the presence of earthworms. We suggest that earthworm activity may indirectly alter plant chemistry and hence defence mechanisms against herbivores.     

Photosynthetic pathway and ontogeny affect water relations and the impact of CO2 on Bouteloua gracilis (C4) and Pascopyrum smithii (C3)

The eastern Colorado shortgrass steppe is dominated by the C₄ grass, Bouteloua gracilis, but contains a mixture of C₃ grasses as well, including Pascopyrum smithii. Although the ecology of this region has been extensively studied, there is little information on how increasing atmospheric CO₂ will affect it. This growth chamber study investigated gas exchange, water relations, growth, and biomass and carbohydrate partitioning in B. gracilis and P. smithii grown under present ambient and elevated CO₂ concentrations of 350 μl l⁻¹and 700 μl l⁻¹, respectively, and two deficit irrigation regimes. The experiment was conducted in soil-packed columns planted to either species over a 2-month period under summer-like conditions and with no fertilizer additions. Our objective was to better understand how these species and the functional groups they represent will respond in future CO₂-enriched environments. Leaf CO₂ assimilation (A _n), transpiration use efficiency (TUE, or A _n/transpiration), plant growth, and whole-plant water use efficiency (WUE, or plant biomass production/water evapotranspired) of both species were greater at elevated CO₂, although responses were more pronounced for P. smithii. Elevated CO₂ enhanced photosynthesis, TUE, and growth in both species through higher soil water content (SWC) and leaf water potentials (Ψ) and stimulation of photosynthesis. Consumptive water use was greater and TUE less for P. smithii than B. gracilis during early growth when soil water was more available. Declining SWC with time was associated with a steadily increased sequestering of total non-structural carbohydrates (TNCs), storage carbohydrates (primarily fructans for P. smithii) and biomass in belowground organs of P. smithii, but not B. gracilis. The root:shoot ratio of P. smithii also increased at elevated CO₂, while the root:shoot ratio of B. gracilis was unresponsive to CO₂. These partitioning responses may be the consequence of different ontogenetic strategies of a cool-season and warm-season grass entering a warm, dry summer period; the cool-season P. smithii responds by sequestering TNCs belowground in preparation for summer dormancy, while resource partitioning of the warm-season B. gracilis remains unaltered. One consequence of greater partitioning of resources into P. smithii belowground organs in the present study was maintenance of higher Ψ and A _n rates. This, along with differences in photosynthetic pathway, may have accounted for the greater responsiveness of P. smithii to CO₂ enrichment compared to B. gracilis. Received: 21 July 1997 / Accepted: 16 December 1997     

The response of mycorrhizal colonization to elevated CO2 and climate change in Pascopyrum smithii and Bouteloua gracilis

Large intact soil cores of nearly pure stands of Pascopyrum smithii (western wheatgrass, C₃) and Bouteloua gracilis (blue grama, C₄) were extracted from the Central Plains Experimental Range in northeastern Colorado, USA and transferred to controlled environment chambers. Cores were exposed to a variety of water, temperature and CO₂ regimes for a total of four annual growth cycles. Root subsamples were harvested after the completion of the second and fourth growth cycles at a time corresponding to late winter, and were examined microscopically for the presence of mycorrhizae. After two growth cycles in the growth chambers, 54% of the root length was colonized in P. smithii, compared to 35% in blue grama. Field control plants had significantly lower colonization. Elevation of CO₂ increased mycorrhizal colonization in B. gracilis by 46% but had no effect in P. smithii. Temperatures 4° C higher than normal decreased colonization in P. smithii by 15%. Increased annual precipitation decreased colonization in both species. Simulated climate change conditions of elevated CO₂, elevated temperature and lowered precipitation decreased colonization in P. smithii but had less effect on B. gracilis. After four growth cycles in P. smithii, trends of treatments remained similar, but overall colonization rate decreased.     

Do endogeic earthworms change plant competition? A microcosm study

Plants compete for limited resources. Although nutrient availability for plants is affected by resource distribution and soil organisms, surprisingly few studies investigate their combined effects on plant growth and competition. Effects of endogeic earthworms (Aporrectodea jassyensis), root-knot nematodes (Meloidogyne incognita) and the spatial distribution of ¹⁵N labelled grass litter on the competition between a grass (Lolium perenne), a forb (Plantago lanceolata) and a legume (Trifolium repens) were investigated in the greenhouse. Earthworms promoted N uptake and growth of L. perenne. Contrastingly, shoot biomass and N uptake of T. repens decreased in the presence of earthworms. P. lanceolata was not affected by the earthworms. We suggest that earthworms enhanced the competitive ability of L. perenne against T. repens. Nematodes increased the proportion of litter N in each of the plant species. Litter distribution (homogeneous vs. patch) did not affect the biomass of any plant species. However, P. lanceolata took up more ¹⁵N, when the litter was homogeneously mixed into the soil. The results suggest that endogeic earthworms may affect plant competition by promoting individual plant species. More studies including decomposers are necessary to understand their role in determining plant community structure.     

Responses of a C3 and a C4 perennial grass to elevated CO2 and temperature under different water regimes

An experiment was carried out to determine the effects of elevated CO₂, elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated by Bouteloua gracilis, a C₄ perennial grass, or Pascopyrum smithii, a C₃ perennial grass, were placed in growth chambers with 350 or 700 μL L^?1 atmospheric CO₂, and under either normal or elevated temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high-temperature regime was 4 °C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under elevated CO₂, with no significant difference between the C₃ and C₄ grass. The effect of elevated CO₂ on biomass was greatest at the intermediate water level. The positive effect of elevated CO₂ on shoot biomass was greater at normal temperatures in B. gracilis, and greater at elevated temperatures in P. smithii. Neither root-to-shoot ratio nor production of seed heads was affected by elevated CO₂. Plant tissue N and soil inorganic N concentrations were lower under elevated Co₂, but no more so in the C₃ than the C₄ plant. Elevated CO₂ appeared to increase plant N limitation, but there was no strong evidence for an increase in N limitation or a decrease in the size of the CO₂ effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated CO₂, in spite of greater N limitation.     

THE EFFECT OF COMPLETE REMOVAL OF HIPPOPOTAMUS ON GRASSLAND IN THE QUEEN ELIZABETH NATIONAL PARK, UGANDA

Coincident with the shooting out of hippopotamus in May 1958 20 permanent 30.46 m line intercept transects were established on the denuded grassland of the Mweya Peninsula, Queen Elizabeth National Park, western Uganda, to assess botanical changes in basal cover, species composition and frequency. Between May 1958 and May 1959, grass basal cover (GBC) decreased from 14.7% to 5.3% and bare ground increased from 78.3% to 83.6%. After four years total ground cover was 10.9% GBC, 27.2% litter, 1.3% dicotyledons and 60.6% bare ground. Changes occurred in grass species composition. The perennial carpet grass, Chrysochloa orientalis, declined from 37.7% GBC (transect frequency 100%) to 6.6% (87%) by May 1962. Bunch grasses showed marked recovery: Sporobolus pyramidalis increased from 8.0% GBC (60%) to 27.6% (100% frequency). Cynodon dactylon, initially prominent in two lakeside transects only, declined from 9.2% GBC to 2.1%, then became re-established in inland localities and recovered to 11.7% GBC. The variable rainfall (714 mm annual average) and its influence on plant growth is discussed and the advantage of mechanically increasing effective rainfall mentioned. Authorities for plant names are given in Table 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.     

Plant population and community responses to removal of dominant species in the shortgrass steppe

Question: What are the plant population‐ and community‐level effects of removal of dominant plant species in the shortgrass steppe? Location: The Shortgrass Steppe Long‐Term Ecological Research site in northern Colorado, USA. Methods: We annually measured plant cover and density by species for 10 years after a one‐time aboveground removal of the dominant perennial grass, Bouteloua gracilis. Removal and control plots (3 m × 3 m) were within grazed and ungrazed locations to assess the influence of grazing on recovery dynamics. Our analyses examined plant species, functional type, and community responses to removal, paying special attention to the dynamics of subdominant and rare species. Results: Basal cover of B. gracilis increased by an average of 1% per year, but there was significantly less plant cover in treatment compared to control plots for 5 years following removal. In contrast to the lower cover in treatment plots, the plant density (number of plants m^?2) of certain subdominant perennial grasses, herbaceous perennial and annual forbs, a dwarf shrub, and cactus increased after removal of the dominant species, with no major change in species richness (number of species per 1 m × 1 m) or diversity. Subdominant species were more similar between years than rare species, but dominant removal resulted in significantly lower similarity of the subdominant species in the short term and increased the similarity of rare species in the long term. Conclusions: Removal of B. gracilis, the dominant perennial grass in the shortgrass steppe, increased the absolute density of subdominant plants, but caused little compensation of plant cover by other plants in the community and changes in species diversity.     

Extirpation or Coexistence? Management of a Persistent Introduced Grass in a Prairie Restoration

Abstract Introduced perennial grasses are one of the greatest constraints to prairie restoration. Herbicides suppress but do not eliminate introduced grasses, so we explored the interaction of herbicide with two additional controls: heavy clipping (to simulate grazing) and competition from native species. A 50‐year‐old stand of the introduced perennial grass Agropyron cristatum (crested wheatgrass) in the northern Great Plains was seeded with native grasses and treated with herbicide annually for 7 years in a factorial experiment. Clipping was applied as a subplot treatment in the final 3 years. Both herbicide and clipping significantly reduced the cover of A. cristatum, but clipping produced an immediate and consistent decrease, whereas herbicide control varied among years. The cover of A. cristatum decreased significantly with increasing cover of a seeded native grass, Bouteloua gracilis (blue grama), suggesting that both top‐down (i.e., grazing) and bottom‐up (i.e., resource competition) strategies can contribute to A. cristatum control. No treatment had any effect on the seed bank of A. cristatum. Even in the most effective control treatments, A. cristatum persisted at low amounts (approximately 5% cover) throughout the experiment. The cover of B. gracilis increased significantly with seed addition and herbicide, and, after 7 years, was similar to that in undisturbed prairie. The total cover of native species increased significantly with clipping and herbicide, and species richness was significantly higher in plots receiving herbicide. Clipping season had no effect on any variable. In summary, no method extirpated A. cristatum, but clipping reduced its cover by 90% and doubled the cover of native species. Extirpation might not be a realistic goal, but relatively simple management allowed coexistence of native species.     

Plant species and nutritional-mediated control over rhizodeposition and root decomposition

This study focuses on the influence of nitrogen (N) availability and species on rhizodeposition and on decomposition of rhizodeposits, roots and soil organic matter. Four perennial grass species were studied that are characteristic of grassland habitats that differ in nutrient availability. These perennial grass species, Holcus lanatus L., Festuca rubra L., Anthoxanthum odoratum L. and Festuca ovina L., were homogeneously labeled with ¹⁴CO₂. Plants were grown on soil without N addition and with N addition (14 g N m^–2). After 8 weeks, plants were harvested and root production and the remaining amount of rhizodeposits in the soil were measured. ¹⁴C-labelled roots were incubated in fresh soil. Decomposition was measured of 1) the labeled rhizodeposits in the soil in which the plants had been growing and 2) the labeled dead roots incubated in fresh soil, by trapping the evolved ¹⁴CO₂, over 69 days.In general, decomposability of both roots and rhizodeposits increased when nitrogen availability increased. Moreover, the species differed in their response to N. Higher N supply increased total rhizodeposition of H. lanatus and the decomposability of rhizodeposited carbon compounds of this high fertility species was greater than of the low fertility species F. ovina, but lower than of A. odoratum. The presented study gives no evidence for a relation between root decomposition rate and the nutrient availability of the habitat of the four species. Overall, we suggest on the basis of the results that species can affect nutrient cycling by differences in rates of rhizodeposition and litter production. This offers a mechanism whereby species can influence species replacement during succession.     

Grazing by Kangaroos Limits the Establishment of the Grass Trees Xanthorrhoea gracilis and X. preissii in Restored Bauxite Mines in Eucalypt Forest of Southwestern Australia

This study examined factors affecting germination, survival, and growth of the grass trees Xanthorrhoea gracilis and X. preissii on newly rehabilitated bauxite mine pits in the jarrah forest of southwestern Australia. Grazing by kangaroos (Macropus fuliginosus) was the major factor in reducing survival and growth of both species during the first 2 years. Provision of artificial grazing protection increased survival and growth (plant mass) of both species by 3‐fold. Grazing by native vertebrates has not previously been identified as affecting mine restoration in Western Australia. Initial germination rates from sown seeds of X. preissii at eight replicate sites ranged from 25 to 64% with a mean of 42%. Corresponding figures for X. gracilis were 5–42% with a mean of 17%. Germination of X. gracilis was greater on heavier, moister soils, but X. preissii germinated better on sandier soils. High levels of initial germination did not ensure high survival. Plants of both species grew bigger and survived better on the lighter, sandy soils. Xanthorrhoea seedlings located in the depressions created by the ripping process grew larger than seedlings on the slopes of the riplines. The presence of plants of other species did not have a significant effect on survival. However, these plants facilitated the growth of both species when artificial grazing protection was unavailable. Plants of other species reduced the growth rates of Xanthorrhoea seedlings where artificial grazing protection was provided. Artificial shade by itself had no significant effect on growth of either species. In rehabilitated bauxite mines in the jarrah forest, the provision of grazing protection is recommended to ensure successful establishment and early survival of Xanthorrhoea spp.     

Interference betweenSalsola kali L. seedlings: Implications for plant succession

Studies were conducted to determine the cause of the decline of the early successional species,Salsola kali L., in the years following its colonization of disturbed soils in arid regions of the western United States. DriedS. kali plant material significantly stimulated the growth ofS. kali but had no effect on the growth or levels of mycorrhizal infection ofAgropyron smithii, a later successional grass. In contrast, root leachates fromS. kali caused a depression in the growth ofS. kali, but had no effect on the growth or mycorrhizal infection ofA. smithii. In the reciprical experiment, root leachates fromA. smithii had no effect on mycorrhizal fungi,S. kali, orA. smithii. This study contradicts earlier studies of the allelopathic potential ofS. kali litter and supports the importance of direct interference betweenS. kali seedling as the cause of the die-off ofS. kali during secondary succession.     

Consequences of growth at two carbon dioxide concentrations and two temperatures for leaf gas exchange in Pascopyrum smithii (C3) and Bouteloua gracilis (C4)*

Continually rising atmospheric CO₂ concentrations and possible climatic change may cause significant changes in plant communities. This study was undertaken to investigate gas exchange in two important grass species of the short-grass steppe, Pascopyrum smithii (western wheat-grass), C₃, and Bouteloua gracilis (blue grama), C4, grown at different CO₂ concentrations and temperatures. Intact soil cores containing each species were extracted from grasslands in north-eastern Colorado, USA, placed in growth chambers, and grown at combinations of two CO₂ concentrations (350 and 700 μmol mol⁻¹) and two temperature regimes (field average and elevated by 4°C). Leaf gas exchange was measured during the second, third and fourth growth seasons. All plants exhibited higher leaf CO₂ assimilation rates (A) with increasing measurement CO₂ concentration, with greater responses being observed in the cool-season C₃ species P. smithii. Changes in the shape of intercellular CO₂ response curves of A for both species indicated photosynthetic acclimation to the different growth environments. The photosynthetic capacity of P. smithii leaves tended to be reduced in plants grown at high CO₂ concentrations, although A for plants grown and measured at 700μmol mol⁻¹ CO₂ was 41% greater than that in plants grown and measured at 350 μmol mol⁻¹ CO₂. Low leaf N concentration may have contributed to photosynthetic acclimation to CO₂. A severe reduction in photosynthetic capacity was exhibited in P. smithii plants grown long-term at elevated temperatures. As a result, the potential response of photosynthesis to CO₂ enrichment was reduced in P. smithii plants grown long-term at the higher temperature.     

Direct and indirect effects of exotic annual grasses on species composition of a South Australian grassland

Abstract Invasion by Mediterranean annual grasses, such as Avena L. spp. and Bronms L. spp, is one of the major threats to temperate perennial grassland. This study investigated the effects of annual grasses and their litter on the species composition of a grassland near Burra, South Australia. The placement of annual grass litter on soil samples in the glasshouse decreased the establishment or growth of several exotic annual dicots. In the field the addition of annual grass litter slightly decreased the frequency of Danthonia Lam. & DC. tussocks. Furthermore, litter strongly reduced the species richness from 13 species in plots with no litter to nine species in plots with the highest litter level, mainly by decreasing the frequency of common exotic dicots. Native dicot frequency similarly appeared to be decreased by litter addition. In addition to the negative effects of their litter, annual grasses also directly competed with perennial grasses. The magnitude of the competitive effect varied systematically along a slope, suggesting that other factors such as soil properties may control competitive inter actions. The biomass of annual grasses also tended to increase with the addition of their own litter. This combination of positive and negative feedback mechanisms suggests that brief periods favourable for annual grasses, either through management changes or environmental conditions, can lead to persistent changes in the species composition of the system.     

<Emphasis Type="Italic">Melilotus officinalis</Emphasis> (yellow sweetclover) causes large changes in community and ecosystem processes in both the presence and absence of a cover crop

Non-native species are hypothesized to decrease native species establishment and cover crops are hypothesized to decrease non-native species abundance. Although many studies have compared invaded to non-invaded habitats, relatively few studies have experimentally added non-native species to directly examine their effects. In a greenhouse mesocosm experiment, we tested the effects of non-native forbs (Melilotus officinalis, Verbascum thapsus, and Lespedeza cuneata), a proposed C₃ grass cover crop (Pascopyrum smithii), and a commonly seeded non-native C₃ grass (Bromus inermis) on the establishment of target native C₄ prairie grass species. All treatments contained the same seed density of target C₄ species and were begun on bare soil collected from the field. The legume M. officinalis strongly decreased the abundance of all other species, species diversity, and light and soil moisture levels. Surprisingly, M. officinalis took up relatively large amounts of labeled nitrogen (¹⁵N) from the soil early in its development, but M. officinalis fixed nitrogen, thus increasing nitrogen in biomass nearly fivefold by the end of the study. We found few effects of either C₃ grass species on non-native forbs or C₄ target species, but seeded P. smithii did increase species diversity. Non-native plants therefore impeded native C₄ grass establishment through long-lasting effects of target species seedbank depletion (death of most target seedlings) and altered nutrient availability. The effects of M. officinalis were not reduced by the presence of a cover crop.     

Shrub persistence and increased grass mortality in response to drought in dryland systems

Droughts in the southwest United States have led to major forest and grassland die‐off events in recent decades, suggesting plant community and ecosystem shifts are imminent as native perennial grass populations are replaced by shrub‐ and invasive plant‐dominated systems. These patterns are similar to those observed in arid and semiarid systems around the globe, but our ability to predict which species will experience increased drought‐induced mortality in response to climate change remains limited. We investigated meteorological drought‐induced mortality of nine dominant plant species in the Colorado Plateau Desert by experimentally imposing a year‐round 35% precipitation reduction for eight continuous years. We distributed experimental plots across numerous plant, soil, and parent material types, resulting in 40 distinct sites across a 4,500 km² region of the Colorado Plateau Desert. For all 8 years, we tracked c. 400 individual plants and evaluated mortality responses to treatments within and across species, and through time. We also examined the influence of abiotic and biotic site factors in driving mortality responses. Overall, high mortality trends were driven by dominant grass species, including Achnatherum hymenoides, Pleuraphis jamesii, and Sporobolus cryptandrus. Responses varied widely from year to year and dominant shrub species were generally resistant to meteorological drought, likely due to their ability to access deeper soil water. Importantly, mortality increased in the presence of invasive species regardless of treatment, and native plant die‐off occurred even under ambient conditions, suggesting that recent climate changes are already negatively impacting dominant species in these systems. Results from this long‐term drought experiment suggest major shifts in community composition and, as a result, ecosystem function. Patterns also show that, across multiple soil and plant community types, native perennial grass species may be replaced by shrubs and invasive annuals in the Colorado Plateau Desert.     

DISPLACEMENT OF ANDROPOGON SCOPARIUS ON THE NEW JERSEY PIEDMONT BY THE SUCCESSIONAL SHRUB MYRICA PENSYLVANICA

Andropogon scoparius, a perennial grass found in old fields on the New Jersey Piedmont, can be invaded and displaced by a nitrogen-fixing shrub, Myrica pensylvanica. The progression of Andropogon displacement was followed over a season, and possible contributing mechanisms (shading, allelopathy, physical effects of Myrica litter) were tested through field and greenhouse experiments. In the field, Andropogon seedling growth was inhibited beneath Myrica clumps, and mature plant living crown area was reduced. In the greenhouse, Andropogon seedling growth was reduced significantly in pots with mature Myrica. Both seedling and mature growth of Andropogon were reduced under shaded conditions. No inhibitory physical effects of Myrica litter on Andropogon seed germination or mature plant growth were found; however, there may ba allelopathic effects of Myrica litter leachate on Andropogon seedlings growing under shade or within grass litter, and on mature Andropogon in shaded conditions. Andropogon displacement appears to be the result of a complex interaction of Myrica shade, allelochemic, and competitive effects, rather than being attributable to any single mechanism.     

Impact of grasshoppers and an invasive grass on establishment and initial growth of restoration plant species

Exotic plant invasion can have dramatic impacts on native plants making restoration of native vegetation at invaded sites challenging. Though invasives may be superior competitors, it is possible their dominance could be enhanced by insect herbivores if native plants are preferred food sources. Insect herbivory can regulate plant populations, but little is known of its effects in restoration settings. There is a need to better understand relationships between insect herbivores and invasive plants with regard to their combined potential for impacting native plant establishment and restoration success. The objective of this study was to assess impacts of grasshopper herbivory and the invasive grass Bromus tectorum (cheatgrass) on mortality and growth of 17 native plant species used in restoration of critical sagebrush steppe ecosystems. Field and greenhouse experiments were conducted using moderate densities of a common, generalist pest grasshopper (Melanoplus bivittatus). Grasshoppers had stronger and more consistent impacts on native restoration plants in field and greenhouse studies than cheatgrass. After 6 weeks in the greenhouse, grasshoppers were associated with 36% mortality over all native restoration species compared to 2% when grasshoppers were absent. Herbivory was also associated with an approximately 50% decrease in native plant biomass. However, effects varied among species. Artemisia tridentata, Chrysothamnus viscidiflorus, and Coreopsis tinctoria were among the most negatively impacted, while Oenothera pallida, Pascopyrum smithii, and Leymus cinerus were unaffected. These findings suggest restoration species could be selected to more effectively establish and persist within cheatgrass infestations, particularly when grasshopper populations are forecasted to be high.     

Degree of plant root digestion by the larva of the beetle, Costelytra zealandica

The degree of plant root digestion by grass grubs (Costelytra zealandica) has been evaluated by scanning electron microscope examination of plant residues in faeces. Faecal pellets were examined from grubs fed on roots of pure plant species (perennial ryegrass, Lolium perenne, and white clover, Trifolium repens) under soil-free conditions, as well as from naturally occurring grubs on mixed pasture. There was considerable variation in the degree of digestion observed with different plant species. Clover root was more extensively digested with formation of partially digested xylem vessels, whereas ryegrass root appeared to undergo very little digestive breakdown. These results indicate that plant structural carbohydrates undergo limited digestion by the grass grub. Thus grass grubs may require a high turnover of dietary root material, which would explain the highly destructive effect of the larvae on pasture plants.