Bunch Grass Research Articles

Vineyard δ15N, nitrogen and water status in perennial clover and bunch grass cover crop systems of California's central valley

Perennial cover crops offer a wide range of agronomic and environmental benefits to agroecosystems. Two such cover crops, a perennial legume (strawberry clover) and a perennial native bunch grass mix, were analyzed for water and nitrogen status in a California central valley vineyard. Soil in the rhizomatous clover cover crop system had high water content, total N, soluble N, and vegetative cover as well as low weed density compared to the soil in the bunch grass cover crop system. Similarly, grape leaves in the clover system had higher percent N and δ15N than those in the bunch grass system. High δ15N in the clover system may be due to increased rates of anaerobic denitrification and other fractionating processes associated with N turnover in the wet, N-rich system. The strawberry clover appears to be a more effective cover crop in this vineyard, where the increased N could mitigate the need for chemical fertilizers and reduce nitrate leaching into surrounding watersheds.

Genetic diversity does not affect the invasiveness of fountain grass (Pennisetum setaceum) in Arizona, California and Hawaii

ABSTRACT Pennisetum setaceum (Poaceae) is a perennial bunch grass that invaded the United States during the 20th century and is highly invasive in Hawaii, moderately invasive in Arizona, and not yet invasive in southern California. Pennisetum setaceum is apomictic, a condition that is normally associated with low genetic variation within populations, but even moderate levels of genetic variation among populations could account for differences in invasiveness. To determine whether genetic factors are causing the variable invasion success, we used Inter‐Simple Sequence Repeat markers (ISSRs) to examine genetic variation in populations from the three areas. Screening of 16 primers revealed no genetic variation within any population or between any geographical areas, a pattern consistent with complete apomixis. Variation in invasion success appears unrelated to genetic differences among populations. Differences in the seasonal timing of rainfall among the regions may be the cause of variable invasiveness of fountain grass. Alternatively, differences in timing of introduction or duration of lag phase may have limited invasiveness in Arizona and southern California.

Response Of Okra (A Moschus esculentus (L) Moench) To Some Mulch Materials

Two experiments commenced in the dry and wet seasons, growth of two commonly grown okra cultivars, not mulched and mulched separately with oil palm bunch refuse, grass and brown wood shavings were studied. The two okra cultivars were: Emerald (green, smooth pods) and Clemson Spineless (green, smooth, ribbed pods). In both experiments soil temperatures were similar in mulch and unmulched treatments in the wet season planting while the mulch treatment reduced soil temperature by about 20C in the bunch refuse and up to 4.10C in the wood shavings during the dry season planting. These reductions in soil temperature led to a greater enhancement of crop growth in the mulched treatment of dry season planting, and equally, significant differences for most growth parameters within these mulch treatments. Highest yield of 26.8 pods/plant was obtained from Emerald cultivars and was significantly (P = 0.05) higher than 22.4 pods/plant obtained from Clemson spineless with bunch refuse treatment. The importance of mulching was emphasized by higher soil moisture content which occurred in the mulched treatment during the dry season. In both experiments, bunch refuse was outstanding in enhancing crop growth. During the two seasons of study, mulching with wood shaving suppressed weed growth while grass and especially unmulched soil encouraged weed growth. KEY WORDS: Response of Okra, Mulch Materials, Production, Economic Potentials. Global Journal of Agricultural Sciences Vol.3(1&2) 2004: 53-57

Two New Liolaemus from the Puna Region of Argentina and Chile: Further Resolution of Purported Reproductive Bimodality in Liolaemus alticolor (Iguania: Liolaemidae)

We provide descriptions for two new cryptic species belonging to the Liolaemus alticolor group from northern Argentina and northeastern Chile. The new species were previously considered conspecific with either Liolaemus walkeri in northeastern Chile or L. alticolor in northwestern Argentina and adjacent Chile. However, the new species differ from these taxa, and all other members of the alticolor group, in a number of characteristics. Liolaemus chaltin n. sp. differs from L. alticolor from the type locality (Tiahuanaco, Bolivia) in the following ways: this new species has a larger body size; a fragmented vertebral stripe; and a pigmented subocular (white in L. alticolor), and is one of just three members of the alticolor group that is oviparous. Liolaemus puna n. sp. differs from all other members of the alticolor group in that male L. puna lack paravertebral markings and dorsolateral and vertebral stripes. Females, however, are similar to other members of the alticolor group but can be distinguished from them by several meristic characters. Liolaemus puna is widely distributed throughout the high-elevation (3680–4400 m) Puna regions (a flat or gently sloping steppe dominated by perennial bunch grasses and small shrubs) in northwestern Argentina and northeastern Chile. Liolaemus chaltin is known only from the Puna of central Jujuy Province, Argentina (3400–3750 m). Based on examinations of the type series of L. alticolor and L. walkeri, we determined that virtually all northern Chilean populations of Liolaemus previously considered to belong to either of these two species should be assigned to L. puna. Thus, the range of L. alticolor is restricted to Bolivia and southern Perú, and the range of L. walkeri is restricted to central and southern Andean Perú. Liolaemus chaltin is oviparous, and L. puna is viviparous, and because both are morphologically similar to L. alticolor, some investigators have suggested that some populations of L. alticolor may be reproductive bimodal. Our studies, however, indicate that these populations represent sympatric populations of the cryptic species described herein. A diagnostic key is provided for the currently recognized members of the alticolor group.

Spatio-temporal variation in the demography of a bunch grass in a patchy semiarid environment

Plants face different environmental pressures in different patches of vegetation mosaics, so their demography cannot be completely understood if it is not studied in each patch-type. Banded patterns of vegetation surrounded by bare areas occur in semiarid landscapes. At one level, two phases of the mosaic are the banded vegetation-patches (vegetation arcs) and the bare areas, but at another level two phases can be distinguished inside the vegetation arc. One phase (frontal zone) is always in the upslope boundary of the arc, has only herbs and it has been suggested that it functions as a colonization area, while the other one (central zone) is at the middle of the arcs and has both shrubs and herbs. The demography of a tussock grass (Hilaria mutica) growing in the two phases of the vegetation arcs was studied under the hypothesis that it will show the demographic parameters of a ruderal species in the frontal zone and those of a more competitive species in the central zone. Temporal variability was assessed through annual, average, periodic and stochastic matrices. λ-values are higher in the frontal than in the central zone, and lower in dry years than in years with moderate precipitation. The influence of the demographic processes on λ-values shows spatial and temporal variation. In dry years, λ-values are more sensible to stasis (permanence in the same size class) and retrogression (transition to a smaller size class) in both zones, whereas in years of moderate precipitation the influence of fecundity and growth increases in the frontal zone and the influence of stasis and retrogression continue to be the most important in the central zone. Variations in the demographic parameters observed in the frontal zone are evidences of a life history plasticity finely tuned with environmental variation, and these results support the hypothesis that frontal zones function as colonization areas.

Summer and winter drought in a cold desert ecosystem (Colorado Plateau) part I: effects on soil water and plant water uptake

We investigated the effects of winter and summer drought on plants of the Colorado Plateau in western North America. This winter-cold, summer-hot desert region receives both winter and summer precipitation. Droughts were imposed for two consecutive years using rainout shelters. Here, we examine drought effects on the hydrologic interactions between plants and soil. We chose three perennial species for this study, representing different rooting patterns and responsiveness to precipitation pulses: Oryzopsis hymenoides, a perennial bunch grass with shallow roots; Gutierrezia sarothrae, a subshrub with dimorphic roots; and Ceratoides lanata, a predominantly deep-rooted woody shrub. Drought effects on plant water status were qualitatively similar among species, despite morphological differences. Summer drought affected the water status of all species more negatively than winter drought. Isotopic analysis of stem water revealed that all three species took up deeper soil water under drought conditions and shallow soil water after a large rainfall event in summer. Thus all three species appeared to use the same water sources most of the time. However, after a particularly dry summer, only the deepest-rooted species continued to take up soil water, while the more shallow-rooted species were either dead or dormant. Our study suggests therefore that increased occurrence of summer drought could favor the most deep-rooted species in ecosystem.

Summer and winter drought in a cold desert ecosystem (Colorado Plateau) part II: effects on plant carbon assimilation and growth

We investigated the effects of winter and summer drought on a shrub/grass community of the Colorado Plateau in western North America, a winter-cold, summer-hot desert that receives both winter and summer precipitation. Summer, winter and yearlong drought treatments were imposed for 2 consecutive years using rainout shelters. We chose three perennial species for this study, representing different rooting patterns and responsiveness to precipitation pulses: Oryzopsis hymenoides, a perennial bunch grass with shallow roots; Gutierrezia sarothrae, a subshrub with dimorphic roots; and Ceratoides lanata, a predominantly deep-rooted woody shrub. Growth for all three species was far more sensitive to winter than to summer drought. The primary reason was that plants did not grow in summer and also did not appear to use summer-assimilated carbon to support growth in the following spring. We hypothesize that the relative scarcity and uncertainty of summer rain on the Colorado Plateau prevents most species from evolving adaptations that would improve their use of summer rain. Together with the results of the companion paper, which focused on plant water relations, we conclude that variation in fall to spring precipitation would have strong effects on primary productivity, and could cause reversible fluctuations in community composition, while increased variation in summer precipitation, through causing high rates of mortality among shallow-rooted species in dry years, has the potential to cause lasting and perhaps irreversible community change, especially if coinciding with the invasion of western landscapes by cheatgrass, tumble weed and other grazing tolerant exotics.

Pairing season habitat selection by Montezuma quail in southeastern Arizona

Montezuma quail (Cyrtonyx montezumae Vigors) are closely associated with oak woodlands (Quercus spp.). Livestock grazing and cover availability are considered important factors affecting Montezuma quail distribution and density. While habitat conditions during pairing season (April–June) are thought to be important to Montezuma quail survival and reproduction, information on habitat selection during that time is limited. We investigated habitat selection by Montezuma quail in grazed and ungrazed areas within the Huachuca and Santa Rita mountain foothills in southeastern Arizona. We used pointing dogs to locate quail during the pairing seasons of 1998 and 1999, and measured habitat characteristics at 60 flush sites and 60 associated random plots (within 100 m of flush sites). We recorded information on landform, substrate, vegetation, and cover. Montezuma quail selected (P < 0.10) areas with higher grass canopy cover and more trees than randomly available. Short (≤ 50 cm tall) visual obstruction (cover), usually associated with bunch grass, was greater (P < 0.10) at use sites than at random plots. Land management practices that reduce grass and tree cover may affect Montezuma quail habitat quality and availability in southeastern Arizona. Based on habitat selection patterns of Montezuma quail, we recommend that oak woodland habitats should contain a minimum tree canopy of 26%, and 51–75% grass canopy cover at the 20-cm height to provide optimum cover availability.

Spatial patterns of light gaps in mesic grasslands

The spatial pattern of light gaps in mesic grasslands in central Texas with contrasting disturbance histories was assessed using patch-based landscape metrics determined from a threshold level (25% of full sunlight), as light intensities below this threshold substantially decrease survival of honey mesquite (Prosopis glandulosa var. glandulosa Torr.) seedlings. The spatial pattern of light gaps, with the exception of edge density, were significantly different between annually-disturbed and non-disturbed grasslands on all sample dates (2 April, 30 April, 29 May, and 26 June 1998). Differences in patch metrics did not occur between non-disturbed grasslands despite contrasting vegetation composition [perennial forbs and perennial bunch (tussock) grasses]. Patch-based landscape metrics of light gaps did vary temporally in both annually-disturbed and non-disturbed grasslands. The structure and spatial configuration of light gaps were distinctly different between annually-disturbed and non-disturbed grasslands: a low density of large patches characterized light gaps in annually-disturbed grassland, whereas non-disturbed grasslands had a high density of small patches. Our findings demonstrate that the current disturbance regime is the principal environmental driver influencing species dominance and composition, and indirectly vegetation structure, which collectively contribute to the observed dynamics of light gap patches in these mesic grasslands. Incorporating spatially explicit consideration of light gap structure and dynamics into experimental studies addressing invasion of weedy plant species such as honey mesquite may be an effective approach to address mechanisms and the ecological significance of disturbance operating as a driver facilitating woody plant invasions in mesic grasslands.

Grazing Systems Contribute to Atmospheric Quality

Significant progress has been made by a variety of U.S. industries to reduce or compensate for effluent carbon dioxide, CO2, degeneration of the local atmosphere. Among these beneficial efforts is the practice of contracting with and financially reimbursing local farmers who grow appropriate forage crops, such as legumes and grasses, which will respond to various practices that prolong the green forage growing period. The principle involved in this arrangement is to increase and prolong the local production of atmospheric oxygen through the process of photosynthesis of green, growing plants. The object is to help compensate for carbon dioxide emitted from the local industry. For decades, professionals in range and pasture management have promoted livestock grazing systems designed to enhance and maintain the ecological status, forage quality and production of rangeland and pasture forage crops. Irrigated pastures and subirrigated native meadows are commonly maintained in high productivity by rotating the grazing herd among several pastures during the greenforage season. This practice promotes greenforage production and helps maximize the process of photosynthesis, which in turn, utilizes carbon dioxide, CO2, from the atmosphere, storing the carbon in the plant and releasing the oxygen into the atmosphere. This is an extremely significant contribution to atmosphere quality. In response to rangelands and grazed forests, regionally adapted grazing systems have effectively improved ecological status of the plant communities: (1) (2). This, in turn, benefited the stability and hydrologic quality of the watershed, forage quality and quantity for livestock, habitat for wild animals and birds, and aesthetic values for recreationists. The ecological, biological and economic benefits of conservative utilization and rotational grazing systems are well documented. The contribution this kind of resource management makes to atmospheric quality through increased amount and prolongation of greenherbage photosynthesis is not well known or if known, has not been publicized. Obviously, quantification is impossible and actually not needed. However, the magnitude and significance of conservation resource management, in terms of its contribution to atmospheric quality, needs to be widely publicized. It is quite likely that research data is available that cites the amount of oxygen and carbon produced during photosynthesis of a variety of agronomic crops currently used by industries which contract with local farmers for this purpose. It seems reasonable that such data could also be made available for native meadow vegetation and each of the various rangeland types i.e., shortgrass prairie, bunch grass prairie, desert shrub, to form a basis for generalizing the contribution to atmospheric quality being made by ranchers and farmers who manage their grazing animals so as to maximize and prolong green growth on their grazing lands. The general public, especially in urban areas, need to be informed of this huge contribution to what is commonly known as conservation grazing management.

Late Oligocene bunch grassland and early Miocene sod grassland paleosols from central Oregon, USA

Fossil soils, burrows and mammals of the upper John Day Formation in central Oregon are evidence of bunch grasses and open, semiarid vegetation as old as late Oligocene (earliest Arikareean, 30 Ma). Root traces in these paleosols include both stout, tapering tubes, like roots of trees, as well as sinuous filamentous tubes, similar to roots of grasses. Paleosol structure is fine subangular blocky, with patchy distribution of grass-like roots, as in wooded grassland and sagebrush steppe with bunch grasses. Cursoriality in horses ( Mesohippus, Miohippus) and hypsodonty in rhinos ( Diceratherium) is also evidence for open grassy vegetation. Trace fossils of Pallichnus (dung beetle boli) and Edaphichnium (earthworm chimneys) are characteristic of wooded grassland paleosols, whereas Taenidium (cicada burrows) dominates desert shrubland paleosols, as has also been found in Quaternary paleosols and soils of eastern Washington. In both Oligocene and Quaternary paleosol sequences, arid shrubland and semiarid grassland paleosols alternate on Milankovitch frequencies (23, 41, 100 ka). The oldest known paleosols in Oregon with crumb structure and abundant fine fossil root traces characteristic of sod grasslands are dated by mammalian biostratigraphy as Hemingfordian (early Miocene, ca. 19 Ma). Wooded grassland habitats are indicated by scattered chalcedony-calcite rhizoconcretions from large woody plants, and by fossil chalicotheres ( Moropus), camels ( Gentilicamelus, “ Paratylopus”) and horses ( Parahippus). Silty texture and silcrete horizons are evidence of semiarid to arid paleoclimate, and are in striking contrast to highly calcareous, and clayey underlying paleosols of the John Day Formation. These silcrete paleosols may represent the Miocene onset of summer-dry (Mediterranean) seasonality, as opposed to a summer-wet (monsoonal) pattern of seasonality found in this region during the Oligocene. Oregon's early rangelands can be compared with those in the North American Great Plains. Granular-structured calcareous paleosols of the Brule Formation of South Dakota are evidence of dry, bunch grasslands as old as 33 Ma (early Orellan, early Oligocene), and crumb-structured paleosols of the Anderson Ranch Formation of Nebraska are evidence of sod grasslands as old as 19 Ma (late Arikareean, early Miocene). Although grasses were a conspicuous part of dry rangelands well back into the Oligocene, early and middle Miocene sod grasslands in North America were restricted to regions estimated to have had less than 400 mm mean annual precipitation.

Pairing season habitat selection by Montezuma quail in southeastern Arizona

Montezuma quail (Cyrtonyx montezumae Vigors) are closely associated with oak woodlands (Quercus spp.). Livestock grazing and cover availability are considered important factors affecting Montezuma quail distribution and density. While habitat conditions during pairing season (April-June) are thought to be important to Montezuma quail survival and reproduction, information on habitat selection during that time is limited. We investigated habitat selection by Montezuma quail in grazed and ungrazed areas within the Huachuca and Santa Rita mountain foothills in southeastern Arizona. We used pointing dogs to locate quail during the pairing seasons of 1998 and 1999, and measured habitat characteristics at 60 flush sites and 60 associated random plots (within 100 m of flush sites). We recorded information on landform, substrate, vegetation, and cover. Montezuma quail selected (P &lt; 0.10) areas with higher grass canopy cover and more trees than randomly available. Short (≤ 50 cm tall) visual obstruction (cover), usually associated with bunch grass, was greater (P &lt; 0.10) at use sites than at random plots. Land management practices that reduce grass and tree cover may affect Montezuma quail habitat quality and availability in southeastern Arizona. Based on habitat selection patterns of Montezuma quail, we recommend that oak woodland habitats should contain a minimum tree canopy of 26%, and 51-75% grass canopy cover at the 20-cm height to provide optimum cover availability.

Spatial patterns of light gaps in mesic grasslands

The spatial pattern of light gaps in mesic grasslands in central Texas with contrasting disturbance histories was assessed using patch-based landscape metrics determined from a threshold level (25% of full sunlight), as light intensities below this threshold substantially decrease survival of honey mesquite (Prosopis glandulosa var. glandulosa Torr.) seedlings. The spatial pattern of light gaps, with the exception of edge density, were significantly different between annually-disturbed and non-disturbed grasslands on all sample dates (2 April, 30 April, 29 May, and 26 June 1998). Differences in patch metrics did not occur between non-disturbed grasslands despite contrasting vegetation composition [perennial forbs and perennial bunch (tussock) grasses]. Patch-based landscape metrics of light gaps did vary temporally in both annually-disturbed and non-disturbed grasslands. The structure and spatial configuration of light gaps were distinctly different between annually-disturbed and non-disturbed grasslands: a low density of large patches characterized light gaps in annually-disturbed grassland, whereas non-disturbed grasslands had a high density of small patches. Our findings demonstrate that the current disturbance regime is the principal environmental driver influencing species dominance and composition, and indirectly vegetation structure, which collectively contribute to the observed dynamics of light gap patches in these mesic grasslands. Incorporating spatially explicit consideration of light gap structure and dynamics into experimental studies addressing invasion of weedy plant species such as honey mesquite may be an effective approach to address mechanisms and the ecological significance of disturbance operating as a driver facilitating woody plant invasions in mesic grasslands.

Facilitative interactions on coastal dunes in response to seasonal weather fluctuations and benefactor size

:We analyzed spatial plant-plant associations (during three seasons: nortes, dry, and rainy) and performed a field experiment on seedling survival, in order to test whether the occurrence of facilitation varied in response to seasonal weather fluctuations and to benefactor size. In a mobile dune system (central Gulf of Mexico) we focused on the endemic early colonizer shrub Chamaecrista chamaecristoides and two late colonizer bunch grasses. Total shrub cover and density were highest during the rainy period. Density of adult Trachypogon plumosus, but not that of adult Schizachyrium scoparium, was higher beneath the canopy of the shrub. No seasonal variations in spatial associations were observed. Density of grass seedlings decreased during the nortes beneath and beyond the canopy of C. chamaecristoides. Fitness parameters of both grasses improved beneath the benefactor plants, while sand accretion decreased. The number of grass individuals benefiting from facilitation increased with shrub size except for the largest shrubs, which had a reduced density of grasses. Temperature on the sand surface was lowest beneath the larger shrubs. Our results highlight the importance of different life stages at which facilitation may occur together with a varying effect of benefactor size in the overall role of facilitation in structuring communities.

Species-specific patterns of hydraulic lift in co-occurring adult trees and grasses in a sandhill community.

Plants can significantly affect ecosystem water balance by hydraulic redistribution (HR) from dry to wet soil layers via roots (also called hydraulic lift, HL, when the redistribution is from deep to shallow soil). However, the information on how co-occurring species in natural habitats differ in HL ability is insufficient. In a field study, we compared HL ability of four tree species (including three congeneric oak species) and two C4 bunch grass species that co-occur in subxeric habitats of fall-line sandhills in southeastern USA. Soil water potentials (psi(s)) were recorded hourly for 3 years both in large chambers that isolated roots for each species and outside the chambers. Outside of root chambers, soil drying occurred periodically in the top 25 cm and corresponded with lack of precipitation during the summer growing season. Soil moisture was continuously available at a 1 m depth. HL activity was observed in three of the tree species, with greater frequency for Pinus palustris than for Quercus laevis and Q. incana. The fourth tree species Q. margaretta did not exhibit HL activity even though it experienced a similar psi(s) gradient. For the C4 bunch grasses, Aristida stricta exhibited a small amount of HL activity, but Schizachyrium scoparium did not. The capacity for HL activity may be linked to the species ecological distribution. The four species that exhibited HL activity in this subxeric habitat are also dominant in adjacent xeric sandhill habitats, whereas the species that did not exhibit HL are scarcely found in the xeric areas. This is consistent with other studies that found greater fine root survival in dry soil for the four xeric species exhibiting HL activity. The differential ability of these species to redistribute water from the deep soil to the rapidly drying shallow soil likely has a strong effect on the water balance of sandhill plant communities, and is likely linked to their differential distribution across edaphic gradients.

Nitrogen and Carbon Cycling in a Model Longleaf Pine Community as Affected by Elevated Atmospheric CO2

Increasing global atmospheric CO2 concentration has led to concerns regarding its potential effects on terrestrial ecosystem and the long-term storage of C and N in soil. This study examined responses to elevated CO2 in a typical regenerating longleaf pine-wiregrass community. The model community consisted of five plant species: (1) an evergreen conifer (Pinus palustris), (2) a bunch grass (Aristida stricta), (3) a broadleaf tree (Quercus margaretta), (4) a perennial herbaceous legume (Crotalaria rotundifolia), and (5) a herbaceous perennial (Asclepias tuberosa) grown at two CO2 concentrations (ambient and twice ambient). The CO2-enriched plots had greater aboveground biomass than ambient plots, mainly due to increased pine biomass. After 3 years, samples of the soil (Blanton loamy sand: loamy, siliceous, semiactive, thermic Grossarenic Paleudult) were collected from 0- to 5-, 5- to 10-, and 10- to 20-cm depth increments. Microbial respiration, potential C and N mineralization, and C turnover were measured during a 120-day incubation of the soil samples. Elevated CO2 decreased soil C respiration and C turnover, but increased N mineralization. Results indicate that soil C sequestration is likely for soils in this longleaf pine ecosystem.

Optimum Planting Procedures for Eastern Gamagrass

Eastern gamagrass (Tripsacum dactyloides L.) is a warm‐season, perennial bunch grass with great potential for use in grazing, cut forage, and conservation systems. Current recommendations for planting eastern gamagrass during the winter (November to February) are suitable for many areas but are inadequate in locales were the ground is frozen during most of this period. The objectives of this study were to determine the best combinations of planting date, planting depth, and seed stratification for eastern gamagrass in the northern range of its adaptation. Unstratified and stratified gamagrass seed was planted in mid‐August, late October–early November, mid‐April, mid‐May, and mid‐June at 2.5‐ and 5.0‐cm planting depths over two growing seasons. Planting in the late summer and fall resulted in better stand establishment than planting in the spring and early summer. The net seedling survival from plantings in April and May was 39 and 48% less than from summer and fall plantings of unstratified seed in 1999–2000 and 2000–2001, respectively. Seed stratification was beneficial to April plantings in only one year of the study and did not improve May or June plantings. At most planting dates, depth had little influence on final stand. Full stands contained at least one plant per 25 cm of row. Plantings producing stands above this level were the August, November, and June plantings in both years and the April planting of stratified seed in the first study year. While the June plantings resulted in adequate final stand numbers, stand establishment was delayed for more than 10 mo because most of the plants did not emerge until the following spring.

Cost of resistance and tolerance under competition: the defense-stress benefit hypothesis

Defense costs provide a major explanation for why plants in nature have not evolved to be better defended against pathogens and herbivores; however, evidence for defense costs is often lacking. Plants defend by deploying resistance traits that reduce damage, and tolerance traits that reduce the fitness effects of damage. We first tested the defense-stress cost (DSC) hypothesis that costs of defenses increase and become important under competitive stress. In a greenhouse experiment, uniparental maternal families of the host plant Arabis perennans were grown in the presence and absence of the bunch grass Bouteloua gracilis and the herbivore Plutella xylostella. Costs of resistance and tolerance manifest as reduced growth in the absence of herbivory were significant when A. perennans grew alone, but not in the competitive environment, in contrast to the DSC hypothesis. We then tested the defense-stress benefit (DSB) hypothesis that plant defenses may benefit plants in competitive situations thereby reducing net costs. For example, chemical resistance agents and tolerance may also have functions in competitive interactions. To test the DSB hypothesis, we compared differentially competitive populations for defense costs, assuming that poorer competitors from less dense habitats were less likely to have evolved defenses that also function in competition. Without competitive benefits of defenses, poorer competitors were expected to have higher net costs of defenses under competition in accordance with DSB. Populations of A. perennans and A. drummondii that differed dramatically in competitiveness were compared for costs, and as the DSB hypothesis predicts, only the poor competitor population showed costs of resistance under competition. However, cost of tolerance under competition did not differ among populations, suggesting that the poor competitors might have evolved a general stress tolerance. Although the DSC hypothesis may explain cases where defense costs increase under stress, the DSB hypothesis may explain some cases where costs decrease under competitive stress.

Elevated atmospheric CO2 affects structure of a model regenerating longleaf pine community

AbstractDifferences in plant morphology, physiology, life form, and symbiotic relationships can generate differences in species responses to CO2‐enrichment, which can alter competitive interactions, thus affecting community structure and function. Here, we present data from a two‐year study, examining the species and community responses to elevated [CO2] of a model regenerating longleaf pine community. The model community was constructed from an assemblage of early successional forest species representing major functional guilds within a typical longleaf pine–wiregrass community: (1) a C3 evergreen conifer (Pinus palustris); (2) a C4 bunch grass (Aristida stricta); (3) a C3 broadleaf tree (Quercus margaretta); (4) a C3 perennial herbaceous legume (Crotalaria rotundifolia); and (5) a C3 herbaceous perennial (Asclepias tuberosa). After 2 years, CO2‐enriched plots had 109% greater above‐ground biomass than ambient plots, mainly due to a 117% increase in pine biomass. Community structure was altered by CO2 enrichment; Crotalaria and Asclepias had higher mortality and less biomass in high‐CO2 plots, suggesting that not all species will perform well as global [CO2] rises. Our data suggest that longleaf pine communities as a whole will perform well in a future higher CO2 world, but some species may fall prey to altered competitive interactions for light and soil moisture.

Root dynamics in an artificially constructed regenerating longleaf pine ecosystem are affected by atmospheric CO 2 enrichment

Differential responses to elevated atmospheric CO 2 concentration exhibited by different plant functional types may alter competition for above- and belowground resources in a higher CO 2 world. Because C allocation to roots is often favored over C allocation to shoots in plants grown with CO 2 enrichment, belowground function of forest ecosystems may change significantly. We established an outdoor facility to examine the effects of elevated CO 2 on root dynamics in artificially constructed communities of five early successional forest species: (1) a C 3 evergreen conifer (longleaf pine, Pinus palustris Mill.); (2) a C 4 monocotyledonous bunch grass (wiregrass, Aristida stricta Michx.); (3) a C 3 broadleaf tree (sand post oak, Quercus margaretta); (4) a C 3 perennial herbaceous legume (rattlebox, Crotalaria rotundifolia Walt. ex Gemel); and (5) an herbaceous C 3 dicotyledonous perennial (butterfly weed, Asclepias tuberosa L.). These species are common associates in early successional longleaf pine savannahs throughout the southeastern USA and represent species that differ in life-form, growth habit, physiology, and symbiotic relationships. A combination of minirhizotrons and soil coring was used to examine temporal and spatial rooting dynamics from October 1998 to October 1999. CO 2-enriched plots exhibited 35% higher standing root crop length, 37% greater root length production per day, and 47% greater root length mortality per day. These variables, however, were enhanced by CO 2 enrichment only at the 10–30 cm depth. Relative root turnover (flux/standing crop) was unchanged by elevated CO 2. Sixteen months after planting, root biomass of pine was 62% higher in elevated compared to ambient CO 2 plots. Conversely, the combined biomass of rattlebox, wiregrass, and butterfly weed was 28% greater in ambient compared to high CO 2 plots. There was no difference in root biomass of oaks after 16 months of exposure to elevated CO 2. Using root and shoot biomass as a metric, longleaf pine realized the greatest and most consistent benefit from exposure to elevated CO 2. This finding suggests that the ability of longleaf pine to compete with sand post oak, a common deciduous tree competitor, and wiregrass, the dominant understory herbaceous species, in regenerating ecosystems may be significantly enhanced by rising atmospheric CO 2 concentrations.