Patterns in grass silicification: response to grazing history and defoliation

Summary Morphologically distinct populations of a North American perennial grass, Agropyron smithii, collected from a heavily grazed prairie dog (Cynomys ludovicianus) colony (PDC) and a grazing exclosure (EX), were grown in an environmental chamber to determine whether: (1) leaf silicon (Si) concentrations are greater in plant populations which differentiated under heavy grazing pressure, and (2) leaf silicification is inducible by defoliation. Mean shoot Si concentration of nondefoliated plants was greater in the PDC population (2.2%) than the EX population (1.9%) over the 18 wk experiment, largely as a result of differences in Si concentrations in leaf blades. However, leaf Si concentration was lower in defoliated plants of each population than in nondefoliated plants, indicating that leaf silicification was not an inducible herbivore defense mechanism in A. smithii. The higher leaf Si concentrations from the heavily grazed population may be associated with grazingrelated environmental stresses such as a warmer, drier microclimate or with morphological characteristics related to grazing tolerance or avoidance.     

Plant-herbivore interactions in a North American mixed-grass prairie

Summary Seasonal dynamics of soil nematodes and root biomass were examined from under western wheatgrass (Agropyron smithii) and little bluestem (Andropogon scoparius) from a heavily grazed prairie dog (Cynomys ludovicianus) colony occupied for 5 to 10 years and an adjacent lightly grazed, uncolonized area in Wind Cave National Park, South Dakota, USA. Nematodes were differentiated into classes of plant-parasitic Tylenchida and Dorylaimida and nonparasitic Dorylamida and Rhabditida. Root-feeding nematodes were generally more numerous from A. smithii than from A. scoparius, while nonparasitic populations were not different in soil from beneath the two plant species. Rhabditida, parasitic Dorylaimida and Tylenchida (from A. scoparius only) were more numerous on the prairie dog colony than from the uncolonized site, but nonparasitic Dorylaimida populations did not differ between the two areas. Mean total (live plus dead) root biomass beneath A. scoparius and A. smithii on the prairie dog colony averaged 71% and 81%, respectively, of values from the uncolonized area. Estimated consumption by root-feeding nematodes averaged 12.6% and 5.8% of annual net root production in the upper 10 cm from the prairie dog colony and uncolonized site, respectively. We conclude that, because of microhabitat modification or reductions in plant resistance to nematodes, heavy grazing by aboveground herbivores apparently facilitates grazing by belowground herbivores. Because heavily grazed plants have less roots than lightly grazed or ungrazed plants, the impact of root-feeding nematodes on primary producers is likely to be greatest in heavily grazed grasslands.     

GRAZING HISTORY,DEFOLIATION, AND FREQUENCY-DEPENDENT COMPETITION: EFFECTS ON TWO NORTH AMERICAN GRASSES

Agropyron smithii and Bouteloua gracilis plants from intensively grazed prairie dog colonies and from a grazing exclosure in Wind Cave National Park, South Dakota, were used to compare responses of conspecific populations with different histories of exposure to grazing and to competition for light. In separate experiments for each species, plants grown in monocultures and two-population replacement-series mixtures were used to examine effects of defoliation, frequency-dependent competition, and population on biomass and morphology. Colony and exclosure plants frequently responded differently. Defoliation more often adversely affected exclosure plants than colony plants, while interpopulation competition more often adversely affected colony plants. Defoliation frequently negated the competitive advantage of exclosure plants. Intrapopulation competition appeared to be greater among exclosure than colony plants. Our results indicate that conclusions based on studies of plants in long-term exclosures may not apply to plant populations having long histories of intensive grazing. While there were differences between species, in both, these experiments provide evidence of population differentiation, resulting in morphologically dissimilar populations which responded differently to defoliation and to inter- and intrapopulation competition.     

Density- and growth stage-dependent responses to defoliation in two rhizomatous grasses

Summary Responses to defoliation were studied in two tallgrass prairie perennials (Andropogon gerardii and Panicum virgatum) established from seed at three densities. P. virgatum was also grown from transplanted rhizomes of established clones. Plants of both species displayed a continuum of responses to defoliation, from large reductions in biomass, tillering and seed production to significant increases in one or more performance measures. In crowded populations, defoliation shifted plants into subordinate positions within the competitive hierarchy. Plants competing intraspecifically and those that were initially small suffered more from defoliation than either plants grown at low density or those that were larger than their neighbors. At the highest plant density, the effects of defoliation or initial plant size were overshadowed by the effects of crowding. When defoliated and grown at similar densities, P. virgatum and A. gerardii grown from seed showed large reductions in biomass, seed production, and new rhizome production, but established P. virgatum ramets grown from rhizomes showed increases in these performance measures. Thus, herbivory may be particularly detrimental to P. virgatum during juvenile stages before perennating organs have developed. Overcompensation of P. virgatum clones in response to defoliation only occurred if all ramets within the clone were defoliated. In clones containing both defoliated and undamaged ramets, there were no differences in their performance, suggesting that genets are capable of integrating the effects of differential defoliation among shoots. Defoliated P. virgatum clones allocated a smaller fraction of their total biomass to new rhizomes, indicating that the short-term regrowth response following defoliation may incur a longer-term cost associated with gradual reduction in biomass of the perennating organs and reduced genet success.     

Nitrogen cycling in poplar stands defoliated by insects

Large-scale outbreaks of defoliating insects are common in temperate forests. These outbreaks are thought to be responsible for substantial cycling of nitrogen (N), and its loss from the system. Gypsy moth (Lymantria dispar) populations within poplar plots were manipulated over 2 years so that the ecosystem-wide consequences of catastrophic defoliation on N cycling could be examined. The quantities of N in leaf litter-fall, ammonia volatilization and soil N pools were estimated across the two seasons. Defoliated leaf biomass was estimated from experimentally derived approximate digestibility factors and added to the mass of senesced leaf to determine total annual leaf production. Throughout the growing season the defoliation treatment peaked at about 40% in year 1 and 100% in year 2. Rapid regrowth after defoliation meant that only 45% of the annual leaf biomass was consumed in the defoliation treatment in year 2, while control plots suffered about 20% consumption each year. In each year, defoliated plots produced 20% more leaf biomass and N than the controls, a phenomenon attributed to compensatory photosynthesis. No substantial losses of N via ammonia volatilization, nitrous oxide emission or nitrate leaching were observed. Neither was there any sustained or substantial gain in the soils microbial biomass or inorganic N pools. These observations suggest that the defoliated poplars were able to compete with soil microbes and N loss mechanisms for soil N as it became available, thereby ameliorating the effects of defoliation on soil nitrogen cycling. We conclude from this study that the N mineralized from defoliation residues was conserved in this plantation ecosystem.     

Plant morphology and grazing history:

Grazing-related, intraspecific, morphological variation was studied in four North American grasses (Bouteloua gracilis, Agropyron smithii, Schizachyrium scoparium, and Andropogon gerardii) from eight locales in Wind Cave National Park, South Dakota: three locales currently occupied and heavily grazed by prairie dogs (Cynomys ludovicianus), colonized (since settlement) for 2–100 years, where native ungulates concentrate grazing activities; an extinct colony locale from which prairie dogs were removed 30 years previously, moderately to lightly grazed by ungulates; two noncolony locales, moderately to lightly grazed by ungulates; and two locales from within a 50-year-old grazing exclosure, with no known history of grazing by prairie dogs nor any recent grazing by ungulates. Data were collected both in situ and in common environments.Active-colony plants were more frequently and more heavily grazed than those at other grazed locales. In situ, plants from heavily grazed populations were smaller and more prostrate than those from populations with little or no grazing (including the extinct colony) and interpopulation variation corresponded to current grazer use. After several growing seasons in common environments, there were still significant interpopulation differences; however, variation often corresponded with grazing history. Although differences between active-colony and noncolony plants were somewhat reduced (indicating some phenotypic plasticity), active-colony plants were still smaller and more prostrate. However, extinct-colony plants more closely resembled active-colony plants than noncolony plants. Morphological variation among these populations is the result of more than simple grazer use; historical factors and the dynamic nature of the grazing regimes are also contributing factors.Abbreviations A. gerardii Andropogon gerardii - A. smithii Agropyron smithii - BFC Bison Flats colony locale - BFN noncolony locale at Bison Flats - B. gracilis Bouteloua gracilis - EXT Upper Highland extinct-colony locale - GDN common garden - GH greenhouse - NEW new satellite colony locale - PVC Pringle Valley colony locale - PVN noncolony locale in Pringle Valley - S. scoparium Schizachyrium scoparium - WCNP Wind Cave National Park, South Dakota, USA - XFN exclosure locale just inside exclosure fence from BFN - XHQ exclosure locale near headquarters buildings     

Analytical studies on the responses of sunflower (Helianthus annuus) to various defoliation treatments

The effects of defoliation treatments on plant growth in sunflower (Helianthus annuus) were studied in the field. Four defoliation treatments, 0 (control), 37.4, 56.1 and 93.4% of the total leaf dry weight, were applied to plants that had small third leaves. Decreased leaf weight/whole plant weight (F/W) ratios in defoliated plants rapidly recovered to almost the same ratio as that observed in the control within 12 to 16 days after defoliation according to the degree of defoliation. The mechanism involved in the recovery of the F/W ratio in defoliated plants mainly consisted of three parameters: enhancement of (1) carbon distribution ratios in the leaves, (2) photosynthetic activity in the remaining leaves, and (3) retranslocation of carbon from the stem and/or roots to leaves. Inhibitive effects of defoliation on relative growth rate and net assimilation rate were seen at an early stage, but subsequently both rates became larger in defoliated plants than in controls. Defoliated plants tended to show rapid development and expansion of new leaves, and to show increased specific leaf area and protein synthesis in individual leaves. The sugar content of leaves in defoliated plants was higher than that in controls, while the content in both stem and roots was lower. These responses seem to be advantageous for development of the photosynthetic system. Heights of defoliated plants were clearly depressed according to the degree of defoliation, and this was attributed largely to differences in the elongation rates of the internodes resulting from defoliation.     

Interactive effects of defoliation and an AM fungus on plants and soil organisms in experimental legume–grass communities

We established a 13‐week greenhouse experiment based on replicated microcosms to test whether the effects of defoliation on grassland plants and soil organisms depend on plant species composition and the presence of arbuscular mycorrhizal (AM) fungi. The experiment constituted of three treatment factors – plant species composition, inoculation of an AM fungus and defoliation – in a fully factorial design. Plant species composition had three levels: (1) Trifolium repens monoculture (T), (2) Phleum pratense monoculture (P) and (3) mixture of T. repens and P. pratense (T+P), while the AM inoculation and the defoliation treatment had two levels: (1) no inoculation of AM fungi and (2) inoculation of the AM fungus Glomus claroideum BEG31, and (1) no trimming, and (2) trimming of all plant material to 6 cm above the soil surface three times during the experiment, respectively. At the final harvest, AM colonization rate of plant roots differed between the plant species compositions, being on average 45% in T, 33% in T+P and 4% in P. Defoliation did not affect the colonization rate in T but raised the rate from 1% to 7% in P and from 20% to 45% in T+P. Shoot production and standing shoot and root biomass were 48%, 85% and 68% lower, respectively, in defoliated than in non‐defoliated systems, while the AM fungus did not affect shoot production and root mass but reduced harvested shoot mass by 8% in non‐defoliated systems. Of the plant quality attributes, defoliation enhanced the N concentration of harvested shoot biomass by 129% and 96% in P and T+P, respectively, but had no effect in T, while the C concentration of shoot biomass was on average 2.7% lower in defoliated than in non‐defoliated systems. Moreover, defoliation reduced shoot C yield (the combined C content of defoliated and harvested shoot biomass) on average by 47% across all plant species compositions and shoot N yield by 37% in T only. In contrast to defoliation, the AM fungus did not affect shoot N and C concentrations or shoot N yield, but induced 10% lower C yield in non‐defoliated systems and 17% higher C yield in defoliated T. In roots, defoliation led to 56% and 21% higher N concentration in P and T+P, respectively, and 28% higher C concentration in P, while the mycorrhizal fungus lowered root N concentration by 9.7% in defoliated systems and had no effect on root C concentrations. In the soil, the nematode community was dominated by bacterivores and the other trophic groups were found in a few microcosms only. Bacterivores were 45% more abundant in defoliated than in non‐defoliated systems, but were not affected by plant species composition or the AM fungus. Soil inorganic N concentration was significantly increased by defoliation in T+P, while the mycorrhizal fungus reduced NH₄–N concentration by 40% in T. The results show that defoliation had widespread effects in our experimental systems, and while the effects on plant growth were invariably negative and those on bacterivorous nematodes invariably positive, most effects on plant C and N content and soil inorganic N concentration varied depending on the plant species present. In contrast, the effects of defoliation did not depend on the presence of the AM fungus, which suggests that while the relative abundance of legumes and grasses is likely to have a significant role in the response of legume–grass communities to defoliation, the role of AM fungi may be less important. In line with this, the AM fungus had only a few significant effects on plant and soil attributes in our systems and each of them was modified by defoliation and/or plant species composition. This suggests that the effects of AM fungi in legume–grass communities may largely depend on the plant species present and whether the plants are grazed or not.     

Growth,nutrition and gas exchange of Pinus resinosa following artificial defoliation

Summary In three experiments, red pine (Pinus resinosa Ait.) seedlings and trees were subjected to artificial defoliations of varying intensities and subsequent growth, gas exchange and nutritional responses were monitored. In Experiment 1, 2-year-old seedlings received 0, 1 or 2 50% defoliations during a single growing season and were maintained in 1 of 3 low nutrient supply treatments. In Experiment 2, response of 4-year-old seedlings was monitored in the year following 0, 25, 50 or 75% defoliation, while in Experiment 3, response of 11-year-old trees was measured 1 year after being defoliated by 0, 33 or 66%. Regardless of intensity of defoliation, or plant size, clipped plants made qualitatively similar allocational and metabolic adjustments over time. First, leaf diffusive conductance and rates of net photosynthesis were stimulated, especially by light to intermediate defoliation. However, there was no effect of defoliation on foliar nitrogen concentration, and elevated gas exchange rates apparently resulted from altered root-shoot dynamics. Second, allocation of new biomass was preferentially shifted towards foliage at the expense of roots, gradually restoring (but undershooting or overshooting) the ratio of foliage: roots of control plants. During the period when foliage: root balance was being restored, the stimulation of needle gas exchange rates disappeared. Plants defoliated by 25% overcompensated in terms of whole plant growth (were larger at harvest than controls), due to shifts in allocation and enhanced photosynthesis. Defoliated plants also stored a proportionally greater share of their carbohydrate reserves in roots than did control plants, even 1 year after clipping.     

Biomass allocation and leaf chemical defence in defoliated seedlings of Quercus serrata with respect to carbon-nitrogen balance

BACKGROUND AND AIMS: Both nutrient availability and defoliation affect the carbon-nutrient balance in plants, which in turn influences biomass allocation (e.g. shoot-to-root ratio) and leaf chemical composition (concentration of nitrogen and secondary compounds). In this study it is questioned whether defoliation alters biomass allocation and chemical defence in a similar fashion to the response to nutrient deficiency. METHODS: Current-year seedlings of Quercus serrata were grown with or without removal of all leaves at three levels of nutrient availability. KEY RESULTS: Plant nitrogen concentration (PNC), a measure of the carbon-nutrient balance in the plant, significantly decreased immediately after defoliation because leaves had higher nitrogen concentrations than stems and roots. However, PNC recovered to levels similar to or higher than that of control plants in 3 or 6 weeks after the defoliation. Nitrogen concentration of leaves produced after defoliation was significantly higher than leaf nitrogen concentration of control leaves. Leaf mass per plant mass (leaf mass ratio, LMR) was positively correlated with PNC but the relationship was significantly different between defoliated and control plants. When compared at the same PNC, defoliated plants had a lower LMR. However, the ratio of the leaf to root tissues that were newly produced after defoliation as a function of PNC did not differ between defoliated and control plants. Defoliated plants had a significantly lower concentration of total phenolics and condensed tannins. Across defoliated and control plants, the leaf tannin concentration was negatively correlated with the leaf nitrogen concentration, suggesting that the amount of carbon-based defensive compounds was controlled by the carbon-nutrient balance at the leaf level. CONCLUSIONS: Defoliation alters biomass allocation and chemical defence through the carbon-nutrient balance at the plant and at the leaf level, respectively.     

Defoliation and below-ground herbivory in the grass Muhlenbergiaquadridentata : Effects on plant performance and on the root-feeder Phyllophaga sp. (Coleoptera, Melolonthidae)

In this study we evaluated (1) the combined effects of simulated defoliation and below-ground herbivory (BGH) on the biomass and nitrogen content of tillers and roots of the bunchgrass Muhlenbergia quadridentata and (2) the effect of defoliation on the survival of third-instar root-feeder larvae of Phyllophaga sp. The experiment was performed in a pine forest area at an altitude of 3200 m above sea level. The grass and the root-feeder species were native and dominant in the understory and in the macroarthropod root-feeder communities, respectively. Plants were established in pots in the field and were subjected to the following treatments in a factorial design: simulated defoliation (three levels) and BGH (with or without root-feeder larvae) with ten replicates per treatment. Plants were defoliated three times at 2-month intervals. The interaction between defoliation and root herbivory was significant for all components of plant biomass. In every case, light defoliation with BGH decreased live above-ground, root and total plant biomass, and the number of live tillers by more than 50% with respect to the same defoliation level without root-feeders. Plants apparently did not compensate for the carbon drain by root-feeders when a high proportion of older leaves were not removed by defoliation. Plants under heavy defoliation were not affected by the presence of root-feeders and showed a greater live/dead above-ground biomass ratio than lightly defoliated and control plants. Defoliation and BGH did not change tiller and root N concentrations but root herbivores did decrease live-tiller N content in lightly defoliated plants. Root-feeders but not defoliation decreased the root/shoot ratio by 40% and the live/dead above-ground biomass ratio by 45% through increased tiller mortality. Survivorship and final biomass of Phyllophaga sp. larvae were not affected by defoliation treatments during the 6-month study period. Received: 17 May 1996 / Accepted: 1 November 1996     

The Effect of Defoliation on the Nitrogen Economy of White Clover: Regrowth and the Remobilization of Plant Organic Nitrogen

Single plants of white clover (Trifolium repens) were establishedfrom stolon cuttings rooted in acid-washed silver sand. Allplants were inoculated with Rhizobium trifolii, and receivednutrient solution containing 0·5 mg ¹⁵N as either ammoniumor nitrate weekly for 12 weeks (i.e. 6 mg ¹⁵N in total). Plantswere then leniently defoliated or left intact, and the labelledN supply was replaced with unlabelled N. Lenient defoliationremoved fully expanded leaves only, leaving immature leaveswhich accounted for 50–55% of the total; growing pointnumbers were not reduced. Nodules, leaves and growing pointswere counted over the following 21 d period, and d. wts, N contents,and ¹⁵N enrichments of individual plant organs were determined. Defoliated plants had fewer nodules, but numbers of growingpoints were unaffected by defoliation. The rates of both leafemergence and expansion were accelerated in defoliated plants;in consequence the number of young leaves remained less thanin intact plants until day 21. Total dry matter (DM) and N accumulationwere less in defoliated plants, and a greater proportion oftotal plant DM was invested in roots. About 97 % of plant totalN was derived from fixed atmospheric N, but there was incompletemixing of fixed and mineral N within the plant. Relatively moremineral N was incorporated into roots, whereas there was relativelymore fixed N in nodules. There was isotopic evidence that Nwas remobilized from root and stolon tissue for leaf regrowthafter defoliation; approximately 2 % of plant N turned overdaily in the 7-d period after defoliation, and this contributedabout 50% of the N increment in leaf tissue. White clover, Trifolium repens L. cv. SI84, lenient defoliation, N economy, regrowth, N remobilization     

Rates of vegetation change associated with prairie dog (Cynomys ludovicianus) grazing in North American mixed-grass prairie

A prairie dog (Cynomys ludovicianus) colony with a known history of habitation was studied to quantify the effects of herbivory on plant species composition, dominance, stature and diversity in a North American mixedgrass prairie. Gradient analysis was used to quantify the relationship between plant community structure, prairie dog density, burrow density and habitation history and to document community-level responses of plants subjected to heavy grazing pressure. The results quantify the type, rate and extent of change which plant populations and communities may undergo in response to the differential grazing of plants variously tolerant of defoliation.Detrended correspondence analysis indicated that 69% of the between-sample floristic variance on the site was attributable to prairie dog habitation. Perennial grasses were rapidly displaced from the site within 3 yr of colonization and were replaced by annual forbs. The net result was an increase in species richness and diversity on the prairie dog colony. Within the colony, however, the number of species was more a function of stand size than colonization history.Significant decreases in canopy stature after 2 yr of habitation resulted from replacement of mid-height grass species by shortgrass species and forbs. In addition, there was a shift from tall growth forms of off-colony species to dwarf growth forms of the same species on the colony. Decreases in litter and increases in bare soil cover were substantial during the first 2 yr of habitation but changed little thereafter.Abbreviations WCC Wind Cave Canyon - DCA Detrended Correspondence Analysis     

Responses of clonal architecture to experimental defoliation: a comparative study between ten grassland species

Clonal architecture may enable plants to effectively respond to environmental constraints but its role in plant tolerance to defoliation remains poorly documented. In several non-clonal species, modifications of plant architecture have been reported as a mechanism of plant tolerance to defoliation, yet this has been little studied in clonal plants. In a glasshouse experiment, five rhizomatous and five stoloniferous species of grazed pastures were subjected to three frequencies of defoliation in order to test two hypotheses. (1) We expected plant clonal response to defoliation to be either a more compact architecture (low clonal propagation, but high branching), or a more dispersed one (long-distance propagation and low branching). Such plastic adjustments of clonal architecture were assumed to be involved in tolerance to defoliation i.e. to promote genet performance in terms of biomass and number of ramets. (2) The response of clonal architecture to defoliation was expected to be dependent on the species and to be more plastic in stoloniferous than in rhizomatous species. Most genets of each species were tolerant to defoliation as they survived and developed in every treatment. Architectural modifications in response to defoliation did not match our predictions. Clonal growth was either maintained or reduced under defoliation. Relative growth rate (RGR) decreased in eight species, whereas defoliated genets of seven species produced as many ramets as control genets. Biomass allocation to ramet shoots remained stable for all but one species. In defoliated genets, the number and mean length of connections, and mean inter-ramet distance were equal to or lower than those in control genets. Four groups of species were distinguished according to their architectural response to defoliation and did not depend on the type of connections. We hypothesised that dense clonal architectures with low plasticity may be the most advantageous response in defoliated conditions such as in grazed pastures. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ecophysiology of Zigadenus nuttallii,a toxic spring ephemeral in a warm season grassland: effect of defoliation and fire

Summary Zigadenus nuttallii, a highly toxic spring ephemeral in tallgrass prairie, was studied in 1985 to ascertain: 1) several ecophysiological characteristics of the species, 2) seasonal patterns of biomass accumulation, and 3) its response to defoliation and fire. The maximum photosynthetic rate of Z. nuttallii measured in unburned prairie was 13.2 moles CO₂ m^-2 s^-1 which occurred at 24–28° C and an incident quantum flux of 0.8–1.0 mmoles m^-2 s^-1. Maximum stomatal conductance measured was 5.4 mm s^-1. Early in the season, belowground storage organs (bulbs) decreased in mass and supplied much of the energy for growth of leaves, even though CO₂ uptake was possible. Buld mass did not increase until about 6 weeks after shoot emergence implying that, at this time, leaves had become a source rather than a sink for carbohydrates. The result of a single, severe defoliation event was a decrease in biomass of bulbs, leaves and reproductive structures in Z. nuttallii. Intrinsic compensatory mechanisms were not detected. In contrast, fire, which also defoliated plants, did not result in any biomass decrease at the end of the season. Improved post-fire microclimate and increased nutrient supply (extrinsic factors) may have contributed to higher photosynthetic rates and led to biomass compensation in burned prairie. These data support arguments that intrinsic compensatory mechanisms have evolved in response to chronic herbivory.     

Plant responses to selective grazing by bison: interactions between light,herbivory and water stress

Two abundant tallgrass prairie forb species, Ambrosia psilostachya and Vernonia baldwinii, are commonly found intact in patches where the grasses have been selectively grazed by bison. Microclimatic patterns and physiological responses of these forbs were measured in grazed and ungrazed patches. These experiments demonstrated that bison herbivory indirectly enhanced water availability and productivity of forbs growing in grazed patches. This was due primarily to the reduction in transpiring grass leaf area in grazed patches and an increase in light availability. In grazed patches, incident light at forb mid-canopy height was 53% greater than ungrazed sites at midseason and soil temperatures were always warmer (e.g., 10°C at 5 cm), perhaps enabling forbs to initiate growth earlier in the spring. Enhanced leaf xylem pressure potential and stomatal conductance in plants in grazed areas were most evident when water availability was low (i.e., late in the growing season and over short-term dry periods characteristic of the tallgrass prairie environment). Relative to individuals in ungrazed areas, end-of-season biomass of A. psilostachya was 40% greater and reproductive biomass and head number of V. baldwinii was 45% and 40% greater, respectively, in plants in grazed patches. A favorable growing environment maintained in grazed patches during periods of water limitation enhances carbon gain in forbs leading to increased biomass and potential fitness.     

Effects of bison grazing on Andropogon gerardii and Panicum virgatum in burned and unburned tallgrass paririe

Summary Responses to clipping and bison grazing in different environmental contexts were examined in two perennial grass species, Andropogon gerardii and Panicum virgatum, on the Konza Prairie in northeastern Kansas. Grazed tillers had lower relative growth rates (RGR) than clipped tillers following defoliation but this difference was transient and final biomass was not affected by mode of defoliation. Grazed tillers of both species had higher RGR throughout the season than ungrazed tillers, resulting in exact compensation for tissue lost to defoliation. However, A. gerardii tillers which had been grazed repeatedly the previous year (1988) had reduced relative growth rates, tiller biomass and tiller survival in 1989. This suggests that the short-term increase in aboveground relative growth rates after defoliation had a cost to future plant growth and tiller survival.In general, the two species had similar responses to defoliation but their responses were altered differentially by fire. The increase in RGR following defoliation of A. gerardii was relatively greater on unburned than burned prairie, and was influenced by topographic position. P. virgatum responses to defoliation were similar in burned and unburned prairie. Thus grazing, fire, and topographical position all interact to influence tiller growth dynamics and these two species respond differently to the fire and grazing interaction. In addition, fire may interact with grazing pattern to influence a plants' grazing history and thus its long-term performance.     

Plant biomass partitioning and chemical defense: Response to defoliation and nitrate limitation

Summary Plant growth and allocation to root, shoot and carbon-based leaf chemical defense were measured in response to defoliation and nitrate limitation inHeterotheca subaxillaris. Field and greenhouse experiments demonstrated that, following defoliation, increased allocation to the shoot results in an equal root/shoot ratio between moderately defoliated (9% shoot mass removed) and non-defoliated plants. High defoliation (28% shoot mass or >25% leaf area removed) resulted in greater proportional shoot growth, reducing the root/shot ratio relative to moderate or non-defoliated plants. However, this latter effect was dependent on nutritional status. Despite the change in distribution of biomass, defoliation and nitrate limitation slowed the growth and development ofH. subaxillaris. Chronic defoliation decreased the growth of nitrate-rich plants more than that of nitrate-limited plants. The concentration of leaf mono- and sesqui-terpenes increased with nitrate-limitation and increasing defoliation. Nutrient stress resulting from reduced allocation to root growth with defoliation may explain the greater allocation to carbon-based leaf defenses, as well as the defoliation-related greater growth reduction of nitrate-rich plants.     

Impact of defoliation intensities on plant biomass,nutrient uptake and arbuscular mycorrhizal symbiosis in Lotus tenuis growing in a saline‐sodic soil

The impact of different defoliation intensities on the ability of Lotus tenuis plants to regrowth, mobilise nutrients and to associate with native AM fungi and Rhizobium in a saline‐sodic soil was investigated. After 70 days, plants were subjected to 0, 25, 50, 75 and 100% defoliation and shoot regrowth was assessed at the end of subsequent 35 days. Compared to non‐defoliated plants, low or moderate defoliation up to 75% did not affect shoot regrowth. However, 100% treatment affected shoot regrowth and the clipped plants were not able to compensate the growth attained by non‐defoliated plants. Root growth was more affected by defoliation than shoot growth. P and N concentrations in shoots and roots increased with increasing defoliation while Na⁺ concentration in shoots of non‐defoliated and moderately defoliated plants was similar. Non‐defoliated and moderately defoliated plants prevented increases of Na⁺ concentration in shoots through both reducing Na⁺ uptake and Na⁺ transport to shoots by accumulating Na⁺ in roots. At high defoliation, the salinity tolerance mechanism is altered and Na⁺ concentration in shoots was higher than in roots. Reduction in the photosynthetic capacity induced by defoliation neither changed the root length colonised by AM fungi nor arbuscular colonisation but decreased the vesicular colonisation. Spore density did not change, but hyphal density and Rhizobium nodules increased with defoliation. The strategy of the AM symbiont consists in investing most of the C resources to preferentially retain arbuscular colonisation as well as inoculum density in the soil.     

Morphologic and allozymic variation between long-term grazed and non-grazed populations of the bunchgrass Schizachyrium scoparium var. frequens

Summary Plant populations of Schizachyrium scoparium var. frequens with a history of long-term grazing by domestic herbivores were characterized by shorter and narrower leaf blades and tillers of lower weight than plants from populations with a history of no grazing. Following four biweekly defoliation events plants from the grazed populations additionally displayed lower specific leaf weights, lower amounts of biomass removed per tiller upon defoliation and a greater number of new and regrowing tillers. Production values per plant remained similar between the two populations because of a greater number of tillers per plant in the grazed population.A large amount of allozymic polymorphism was observed in both the grazed and non-grazed populations. Results of the electrophoretic analyses indicated that a minimum of 66% of the plants sampled represented separate genotypes. No distinction could be made between grazed and non-grazed populations in terms of allozymic partitioning. The morphological variation observed between these two populations may represent a shift in the relative dominance of a series of genotypes variously adapted to herbivory.