Plant-herbivore interactions: Examination of potential effects of bison saliva on regrowth of Bouteloua gracilis (H.B.K.) lag

Summary Laboratory experiments were performed to determine whether regrowth of blue grama was affected by potential growth-promoting substances in saliva of North American bison. We observed no statistically significant effects of foliar application of whole bison saliva on net photosynthesis (P_N), root respiration (R_R), allocation patterns of photosynthetically fixed ¹⁴C, or regrowth rates over a 10-day period following clipping to various heights. In a 10-week experiment, there were no significant effects of saliva on leaf, crown or root growth or tiller production in plants clipped to heights of 6, 4 or 2 cm above crowns. Similarly, nitrogen-stressed plants failed to show significant changes in growth rates or tillering in response to saliva over a 3-week period. Clipped blue grama plants did exhibit significant compensatory growth responses, including higher P_N rates from 3–10 days following clipping and allocation of a higher proportion of current photosynthate to synthesis of new leaf tissue with increasing severity of defoliation. Nevertheless, unclipped plants invariably outproduced clipped plants following defoliation.     

Compensatory growth responses to clipping defoliation in Leymus chinensis (Poaceae) under nutrient addition and water deficiency conditions

Compensatory growth responses of Leymus chinensis, a dominant species in Inner Mongolia steppe, to clipping defoliation were evaluated in a pot-cultivated experiment under different nutrient (N and P) and water availability conditions. Leymus chinensis exhibited over-compensatory growth at the light and moderate clipping intensities (20% and 40% aerial mass removed) with a greater accumulated aboveground biomass, higher relative growth rate (RGR), more rhizomatic tillers and a stimulation of compensatory photosynthesis to the remnant leaves as compared with those of the unclipped plants. Intense clipping (80% aerial mass removed), which removed most of the aboveground tissues, greatly reduced the growth of aboveground biomass in comparison with that of the unclipped plants. Nitrogen addition only slightly improved the biomass production and RGR in light and moderately clipped plants, and it did not allow plants in the intense clipping condition to over-compensate. Phosphorus addition had no obvious influences on the growth and physiological responses to clipping defoliation. These results indicated that nutrient addition could not compensate for the negative effects of severe clipping on the defoliated grass. On the other hand, there were no distinct positive responses under water deficiency condition for L. chinensis at all clipping intensities with a significant reduction of aboveground and belowground biomass, lower RGR, fewer rhizomatic tillers, and a lower net photosynthetic rate than other wet treatments. Additionally, the chlorophyll contents of remnant leaves gradually increased with the increase of clipping intensities in each treatment. In conclusion, although L. chinensis could compensate for tissues removal by some morphological and physiological responses, intense clipping and drought can result in a significant decrease of biomass and growth rate, even under enriched nutrition conditions.     

Effect of light and simulated herbivory on growth of endangered northeastern bulrush,shape Scirpus ancistrochaetus Schuyler

The independent and interactive effects of light and simulated herbivory on growth and biomass allocation in the endangered emergent sedge, Scirpus ancistrochaetus Schuyler, were examined in a 15-week common garden experiment. Light level was manipulated through the use of shade cloth to attain shading levels of 0%, 30%, 63% and 90%, while herbivory was simulated by reducing plant height by 50% six weeks into the experiment. Multivariate Analysis of Variance (MANOVA) of final total (shoot + root) plant mass, leaf width and root to shoot ratio (R:S) indicated overall significant effects of both light and the interaction of light and clipping. Means of these variables decreased with decreasing light level. Furthermore, means of these variables were generally higher for clipped plants than unclipped plants in 0% shade, while they tended to be lower in clipped plants in all other levels of shade. Maximum leaf height was also significantly affected by light. Leaf height was similar in all treatments for the first 11 weeks. For the last four weeks of the experiment, however, leaf height was lowest in plants grown in 0% shade, highest in plants grown in 63 and 90% shade, and intermediate in plants grown in 30% shade. Clipped plants tended to be taller than unclipped plants at lower levels of shade, while the opposite was true at higher levels of shade, although effects were marginally significant. Both light and the interactive effect of light and clipping were important determinants of growth of S. ancistrochaetus in this experiment and therefore may be important influences on its distribution within and among pond habitats.     

The interaction of defoliation and nutrient uptake in Sporobolus kentrophyllus,a short-grass species from the serengeti plains

Summary Sporobolus kentrophyllus, a grazing-tolerant C₄ grass from the southeastern Serengeti Plains, was grown in solution culture to examine the effects of clipping on the uptake, preference and subsequent transport of varying nitrogen forms. Clipping reduced offtake mass, crown mass ane root mass, resulting in a 58% decline in plant mass. Proportional biomass allocation to roots decreased with clipping, while tillering rates increased. Clipping also increased the nitrogen concentrations of all tissues, and plant nitrogen uptake (nitrogen accumulated throughout the experiment per gram root). The ¹⁵N concentrations (% atom excess) of all tissues were higher in clipped compared with unclipped plants, and the average ¹⁵N uptake rate of clipped plants was twice that of unclipped plants. The relative ¹⁵N allocation to aboveground mass, a measure of canopy sink strength, was higher in clipped plants. Plants fed ¹⁵N-ammonium or ¹⁵N-nitrate during the ¹⁵N pulse experiment had greater ¹⁵N tissue concentrations compared with urea-fed plants, and ¹⁵N uptake rates were higher in ammonium-fed and nitrate-fed plants, compared with urea-fed plants. The relative magnitudes of these differences were higher when plants were clipped. Clipped plants had higher uptake rates for potassium, phosphorus and sodium, while differences between clipping treatments for calcium, iron, and magnesium were indistinguishable. Rapid uptake rates for species on the southeastern Serengeti plains, particularly during grazing periods, have important implications for nutrient cycling in this system.     

Production and nitrogen responses of the African dwarf shrub Indigofera spinosa to defoliation and water limitation

Summary The dwarf shrub Indigofera spinosa Forsk. (Papilionacea), a native forage species of arid Northwest Kenya, was propogated from seed, grown in a controlled environment, and subjected to three treatments of defoliation and watering frequencies in a factorial experimental design. Biomass production and nitrogen accumulation in tissue components were measured to determine defoliation responses in a water-limited environment. We hypothesized that plants would maintain biomass and nitrogen flows despite removal of aboveground meristems and tissues by defoliation. Principal experimental results included a slight reduction (11%; P=0.08) of total biomass production by clipping ca. 1/3 or 2/3 of new leaves and stems and all apical meristems every month. Total aboveground production was not affected by clipping, while final root biomass was reduced 17% by the 2/3 clipping. The least water stressed plants were affected most negatively by defoliation, and the unclipped plants responded more negatively to greater water limitation. Plants achieved partial biomass compensation through alterations in shoot activity and continued allocation of photosynthate to roots. A smaller fraction of leaf production was directed to litter in clipped plants although clipping only removed the youngest tissues, suggesting that clipping increased leaf longevity. In turn, each leaf probably contributed a greater total quantity of photosynthate. Photosynthetic rates were also likely to have been increased by clipping water-stressed plants. In contrast to biomass, plants overcompensated for nitrogen lost to defoliation. Total nitrogen uptake by individual plants was stimulated by defoliation, as there was more total nitrogen in leaves and stems. Increased nitrogen uptake was achieved by clipping stimulation of total uptake per unit of root rather than of total root mass.     

Responses of an African tall-grass (Hyparrhenia filipendula stapf.) to defoliation and limitations of water and nitrogen

Summary Hyparrhenia filipendula stapf., a tall (1–1.5 m) perennial grass common in dry-subhumid African savannas, was collected from Serengeti National Park in Tanzania, propagated vegetatively, and grown in controlled environments simulating conditions in nature. Plants were subjected to a factorial experiment with combinations of watering frequency, nitrogen supply, clipping height (10 and 15 cm) and clipping frequency (7 and 14 d). Biomass yield and allocation to various tissue types, morphometric traits, and growth processes were measured.Watering frequency affected leaf elongation rate while nitrogen affected tiller number. Clipped yield was strongly correlated with leaf elongation rate but not tiller number, therefore it was primarily controlled by the activities of intercalary rather than apical meristems. There was a negative exponential relationship between tillering and clipped yield per tiller. Plants that received both high nitrogen and high water closely followed a-3/2 power law in this tradeoff. The fraction of total net photosynthate allocated to roots was not significantly related to any environmental treatment. Root and crown growths were not affected by defoliation treatment; leaf blade and stem growths were inhibited; and sheaths were inhibited only under low water.Despite a tall stature, H. filipendula tolerated herbivory by increased photosynthetic rate (Wallace et al. 1984), through continued production of young tissues by intercalary meristems balanced against tiller number, and by a statistically constant proportional allocation to roots. Clipped yield increased only when both nitrogen and water were abundant, and then, proportional clipped yield did not surpass an upper asymptotic limit.     

Interactive effects of clipping and nutrient availability on the compensatory growth of a grass species

Resource availability is an important factor affecting the capacity of compensatory growth after grazing. We performed a greenhouse experiment with Poa bulbosa, a small perennial grass of the Mediterranean and Central Asian grasslands, to test the importance of nutrient availability for compensatory growth after clipping. We also compared the results with predictions of the limited resource model (LRM). Plants were grown at low and high fertilization levels and subjected to a clipping treatment. Contrary to the LMR, we found that in Poa plants compensatory growth occurred under the high fertilization level, while it did not occur under the low level. The LMR predicts a higher tolerance for grazing in the stressful environment. Our plants showed a significant decrease in their relative growth rates (RGR) after clipping. Although the plants allocated a 32–188% greater fraction of the mass to lamina growth after clipping, this greater allocation to the leaves did not fully compensate for the initial reduction in leaf area ratio (LAR). A sensitivity analysis showed for the clipped plants under the high fertilization treatment, that changes in leaf allocation (f _lam) enabled the plants to compensate for a part of the potential loss caused by defoliation. Probably, the increased biomass allocation comes largely from the bulbs. We conclude that the inconsistency of the LRM with our results originates in the lack of compensatory mechanisms in the model. To better understand how environmental conditions affect tolerance to herbivory, the effects of compensatory growth should be taken into account.     

Growth responses of a woody species to clipping and goat saliva

Studies on the role of mammalian herbivore saliva in plant–animal interactions have mostly focused on graminoid species and bovine saliva. A trial was performed in Botswana with clipping treatments to simulate browsing of shoots and the application of goat saliva on the woody species Combretum apiculatum Sonder (Combretaceae). Treatments were performed during early growing season while shoot growth was rapid, and responses of trees were recorded later in the same season. Clipped shoots with saliva had significantly enhanced shoot growth (tripled in length) and leaf production (2.7 times more leaves) compared to clipped shoots without saliva. However, unclipped shoots still grew more than clipped shoots, with or without saliva treatment.     

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.     

Biomass and mineral element responses of a Serengeti short-grass species to nitrogen supply and defoliation: compensation requires a critical [N]

Large mammalian herbivores in grassland ecosystems influence plant growth dynamics in many ways, including the removal of plant biomass and the return of nutrients to the soil. A 10-week growth chamber experiment examined the responses of Sporobolus kentrophyllus from the heavily grazed short-grass plains of Serengeti National Park, Tanzania, to simulated grazing and varying nitrogen nutrition. Plants were subjected to two clipping treatments (clipped and unclipped) and five nitrogen levels (weekly applications at levels equivalent to 0, 1, 5, 10, and 40 g N m⁻²), the highest being equivalent to a urine hit. Tiller and stolon production were measured weekly. Total biomass at harvest was partitioned by plant organ and analyzed for nitrogen and mineral element composition. Tiller and stolon production reached a peak at 3–5 weeks in unclipped plants, then declined drastically, but tiller number increased continually in clipped plants; this differential effect was enhanced at higher N levels. Total plant production increased substantially with N supply, was dominated by aboveground production, and was similar in clipped and unclipped plants, except at high nitrogen levels where clipped plants produced more. Much of the standing biomass of unclipped plants was standing dead and stem; most of the standing biomass of clipped plants was live leaf with clipped plants having significantly more leaf than unclipped plants. However, leaf nitrogen was stimulated by clipping only in plants receiving levels of N application above 1 g N m⁻² which corresponded to a tissue concentration of 2.5% N. Leaf N concentration was lower in unclipped plants and increased with level of N. Aboveground N and mineral concentrations were consistently greater than belowground levels and while clipping commonly promoted aboveground concentrations, it generally diminished those belowground. In general, clipped plants exhibited increased leaf elemental concentrations of K, P, and Mg. Concentrations of B, Ca, K, Mg, and Zn increased with the level of N. No evidence was found that the much greater growth associated with higher N levels diminished the concentration of any other nutrient and that clipping coupled with N fertilization increased the total mineral content available in leaf tissue. The results suggest that plants can (1) compensate for leaf removal, but only when N is above a critical point (tissue [N] 2.8%) and (2) grazing coupled with N fertilization can increase the quality and quantity of tissue available for herbivore removal. Received: 25 August 1997 / Accepted: 14 April 1998     

The effect of shading on photosynthesis,growth, and regrowth following defoliation for Bromus tectorum

Summary The effect of full sunlight, 60%, or 90% attenuated light on photosynthetic rate, growth, leaf morphology, dry weight allocation patterns, phenology, and tolerance to clipping was examined in the glasshouse for steppe populations of the introduced grass, Bromus tectorum. The net photosynthetic response to light for plants grown in shade was comparable to responses for plants grown in full sunlight. Plants grown in full sunlight produced more biomass, tillers and leaves, and allocated a larger proportion of their total production to roots than plants grown in shade. The accumulation of root and shoot biomass over the first two months of seedling growth was primarily responsible for the larger size at harvest of plants grown in full sunlight. Plants grown under 60% and 90% shade flowered an average of 2 and 6 weeks later, respectively, than plants grown in full sunlight. Regrowth after clipping was greater for plants grown in full sunlight compared to those grown in shade. Even a one-time clipping delayed flowering and seed maturation; the older the individual when leaf area was removed, the greater the delay in its phenology. Repeated removal of leaf area was more frequently fatal for plants in shade than in full sunlight. For plants originally grown in full sunlight, regrowth in the dark was greater than for shaded plants and was more closely correlated to non-flowering tiller number than to plant size. This correlation suggests that etiolated regrowth is more likely regulated by the number of functional meristems than by differences in the size of carbohydrate pools. Thus, shading reduces the rate of growth, number of tillers, and ability to replace leaf area lost to herbivory for B. tectorum. These responses, in turn, intensify the effect of competition and defoliation for this grass in forests. B. tectorum is largely restricted to forest gaps at least in part because of its inability to acclimate photosynthetically, the influence of shade on resource allocation, and the role of herbivory in exacerbating these effects.     

Effect of stress and time for recovery on the amount of compensatory growth after grazing

Summary We tested the hypothesis that the amount of compensatory growth after defoliation is affected by the level of stress at which plants grow when defoliated and by the length of time for recovery. Growth response to defoliation went from partial compensation when plants were growing at high relative growth rates (RGR) to overcompensation when plants were more stressed and growing at low RGR. Defoliation released plants from the limitation imposed by the accumulation of old and dead tissue and this release overrode the negative effect of biomass loss. Compensatory growth resulted from a higher RGR aboveground that was not associated with a reduction in RGR belowground. Time available for recovery had a major impact on the outcome of defoliation. With a short time for recovery, RGR was decreased by defoliation because an immediate increase in net assimilation rate was overridden by a reduction in the ratio of leaf area to plant weight. After defoliation, this ratio increased quickly due to a larger allocation to leaf growth and lower leaf specific weights, resulting in higher RGR. We conclude that the compensatory response to grazing depends on the type and level of stress limiting growth. Allocation and physiological responses to stress may positively or negatively affect the response to grazing and, simultaneously, grazing may alleviate or aggravate the effects of different types of stress.     

The Effect of Clipping on Net Photosynthesis and Dark Respiration Rates of Plants from an Upland Grassland, with Reference to Carbon Partitioning in Festuca ovina

Four co-existing species (Deschampsia flexuosa, Festuca ovina,Juncus squarrosus and Nardus stricta) were subjected to clippingand the net photosynthetic and dark respiration rates were followedafter this treatment for 50 d. Concurrently carbon partitioningin F. ovina plants clipped initially and again at 50 and 100d was examined. An expansion of new leaf lamina was observed with F. ovina,which had a greater net photsynthetic rate per unit leaf areathan unclipped lamina. The remaining leaf lamina (stubble) afterclipping also showed net photosynthetic and dark respirationrates greater than unclipped lamina; these responses were uniqueto F. ovina plants. N. stricta was the only other species toattain a pre-clipping photosynthetic rate within 6 d. Clipped F. ovina plants showed a change in carbon allocationpattern, with a reduction in carbon allocated to roots. ¹⁴Caccumulated in roots and stubble was shown to have a role inregrowth, as was current assimilate via the production of newleaf lamina. Plants initially clipped before exposure to ¹⁴C,redistributed less ¹⁴C to new shoot growth and, therefore, lostless when subsequently clipped. Further redistribution of ¹⁴Ccame from leaf stubble tissue and not at the expense of roots.The variation between species in clipping response are discussedin terms of the implications for coexistence. Carbon partitioning, clipping, gas exchange, grasses     

Ecological conditions that determine when grazing stimulates grass production

Summary We report the results of a pot experiment that examined the effects of three ecologically important factors controlling plant growth rates in savanna grasslands: defoliation, soil nitrogen and soil water availability. The experiment was conducted in the Amboseli region in east Africa, and was designed to simulate natural conditions as far as possible, using local soils and a grass species that is heavily grazed by abundant large herbivores. Productivity by different plant components was reduced, stimulated or unchanged by defoliation, depending on specific watering and fertilization treatments. Total above-ground production was stimulated by defoliation and was maximized at moderate clipping intensities, but this was statistically significant only when plants were watered infrequently (every 8 days), and most important, periods between clipping events were extended (at least 24 days). Under these conditions, plant growth rates were limited by water availability at the time of clipping, and soil water conserved in clipped, compared to unclipped plants. Within a given fertilization treatment, whole-plant production was never stimulated by defoliation because root growth was unaffected or inhibited by clipping. However, when fertilization was coupled to defoliation, as they are in the field, whole-plant production by fertilized and moderately clipped plants exceeded production by infertilized, unclipped plants. Under this interpretation, maximum whole-plant production coincided with optimum conditions for herbivores (maximum nitrogen concentration in grass leaves) when watering was frequent, and plants were moderately defoliated. However, these conditions were not the same as those that maximized relative above-ground stimulation of growth (infrequent watering and clipping).The results indicate that above-ground grass production can be stimulated by grazing, and when that is likely to occur. However, the results emphasize that plant production responses to defoliation can vary widely, contigent upon a complex interaction of ecological factors.     

Attack frequency and the tolerance to herbivory of Neotropical savanna trees

Tolerance is the ability of a plant to regrow or reproduce following damage. While experimental studies typically measure tolerance in response to the intensity of herbivory (i.e., the amount of leaf tissue removed in one attack), the impact of how many times plants are attacked during a growing season (i.e., the frequency of damage) is virtually unexplored. Using experimental defoliations that mimicked patterns of attack by leaf-cutter ants (Atta spp.), we examined how the frequency of herbivory influenced plant tolerance traits in six tree species in Brazil’s Cerrado. For 2 years we quantified how monthly and quarterly damage influenced individual survivorship, relative growth rate, plant architecture, flowering, and foliar chemistry. We found that the content of leaf nitrogen (N) increased among clipped individuals of most species, suggesting that Atta influences the allocation of resources in damaged plants. Furthermore, our clipping treatments affected tree architecture in ways thought to promote tolerance. However, none of our focal species exhibited a compensatory increase in growth (increment in trunk diameter) in response to herbivory as relative growth rates were significantly lower in clipped than in unclipped individuals. In addition, the probability of survival was much lower for clipped plants, and lower for plants clipped monthly than those clipped quarterly. For plants that did survive, simulated herbivory dramatically reduced the probability of flowering. Our results were similar across a phylogenetically distinct suite of species, suggesting a potential extendability of these findings to other plant species in this system.     

Interactive regulation of grass yield and chemical properties by defoliation,a salivary chemical,and inorganic nutrition

Summary Clones of 2 C₄ grass species, Sprobolus ioclados and S. pyramidalis, were obtained from more and less heavily grazed grasslands, respectively, in Tanzania's Serengeti National Park. Plants were grown in a factorial experiment to determine the effects of severe defoliation, nutrient limitation, and a salivary chemical (thiamine) on plant growth, nitrogen content, and non-structural carbohydrate content. The experimental design included: (1) species; (2) clipping, with plants either unclipped or clipped weekly to a height of 5 cm; (3) 0.2 ml of distilled water of 0.2 ml of 10 ppb aqueous thiamine sprayed on plants from an atomizer after clipping and identical treatments at the same time to unclipped plants; (4) phosphorus (P) at 0.2 or 1 mM; (5) nitrogen (N) at 3 or 15 mM. Clipping was the major variable affecting plant growth. Total and litter yields were reduced to half and residual plant yield was reduced to 30% of the values for unclipped plants. Clipping interacted strongly with other variables since they commonly had minor effects on clipped plants and major effects on unclipped plants. Exceptions to this generalization were generally due to better performance by S. ioclados under clipping. Compared to lower treatment levels, higher treatment levels promoted total yield of unclipped plants by 52% for N, 43% for thiamine, and 33% for P. In general, thiamine had greater effects than P but lesser effects than N. Thiamine promoted yield and modified the chemical balance of plants by promoting carbohydrate (CHO) concentrations and reducing N concentrations. N and P deficiencies promoted CHO accumulation. Clipping promoted the N of leaves and crowns and reduced the N levels in roots. Leaf blade water and N contents were positively correlated with very little scatter. The slope of the line was different for S. ioclados and S. pyramidalis. Leaf blade water and CHO contents were negatively related but there was more scatter and the species could not be distinguished. The species from more heavily grazed grasslands was conspicuously more sensitive to thiamine application. The results indicated that leaf treatment with thiamine, the only likely source of which in natural grasslands is saliva deposited by feeding herbivores, can have major effects on plant yield and metabolic balances at very low application levels. But under defoliation levels as severe as those imposed in this experiment, which reduced above ground plant biomass to a fourth of the level produced by unclipped plants, growth was so strongly limited by defoliation that neither thiamine nor inorganic nutrients affected plant yield residual from clipping. Therefore, whether chemicals such as thiamine that may be introduced onto grass foliage by grazing ungulates and other herbivores will influence the growth of grazed plants will depend upon the grazing intensity associated with the transfer.     

Urea as a promotive coupler of plant-herbivore interactions

Summary Growth responses of Kyllinga nervosa Steud., a sedge from the Serengeti short-grass plains, were examined in a factorial experiment which included clipped and unclipped plants, and nitrogen supplied as either urea or ammonium nitrate. Results were expressed in relation to three transfer processes: flow to grazers, flow to producers and flow to reproduction. Clipping increased biomass and nitrogen flow to grazers by significantly increasing nitrogen uptake, aboveground nitrogen flow, and the weights of and proportional allocation to green leaf production. This was at the expense of flow to vegetative and sexual reproduction, since the weights and proportional investments in roots, crowns and reproductive structures were reduced. Urea nutrition increased flow to grazers and plant reproduction through increases in green leaf weight, flower weight, allocation to green leaves, flowers and stems, and aboveground: belowground biomass ratios. Stimulation of aboveground productivity by urea was a consequence of increased tillering rates.Interactive responses of clipping and nitrogen source regulated plant growth, thus controlling flow to each transfer process. Combined effects of clipping and urea resulted in compensatory production of both green leaves and flowers, and maximized biomass and nitrogen flow to grazers. Both urea and clipping tightened herbivore-producer recycling by significantly reducing litter nitrogen and carbon masses. In contrast, when plants were unclipped and grown on NH₄NO₃, biomass allocation and weights of roots and crowns were increased at the expense of aboveground tissues, thus increasing flow to primary producers. Plant growth responses to experimental treatment combinations simulating nutritional status of grazed and ungrazed field plants indicate that urea represents a potential importance beyond it nitrogen contribution by introducing a positive feedback to herbivores.     

MYCORRHIZAL AND CLIPPING EFFECTS ON ANDROPOGON GERARDII PHOTOSYNTHESIS

Mature Andropogon gerardii (Vitman) plants were placed in a 2 × 2 factorial treatment regime with clipping soil fumigation as main treatment effects. Plants were grown in a greenhouse using a Latin square design; pots were watered daily, and fertilized weekly with a low phosphorus nutrient solution. Photosynthesis and conductance were measured prior to and after a clipping which removed about 23% of the total leaf length of clipped plants. Fumigation reduced mycorrhizal colonization levels from 34.3 to 7.4%. Conductance was not affected by either treatment. Fumigated, clipped plants had significantly lower photosynthetic rates than their nonfumigated counterparts. However, there were no significant differences between fumigated and nonfumigated unclipped plants. The initial slope of the photosynthetic light response curve was significantly greater for the nonfumigated plants. Greater mycorrhizal development in nonfumigated plants appeared to be beneficial only when the host plants were under defoliation stress.     

Leaf compensatory growth as a tolerance strategy to resist herbivory in Pancratium sickenbergeri

We examined the effects of leaf herbivory by the dorcas gazelle, Gazella dorcas, on the compensatory growth of the geophyte Pancratium sickenbergeri (Amaryllidaceae) in the Negev desert, Israel. In three populations exposed to different levels of herbivory, we removed different amounts of photosynthetic leaf area from plants in five clipping treatments: 0, 25, 50%-dispersed over all leaves, 50%-entire area of half the leaves, 100%. The population with the lowest level of herbivory showed the lowest relative regrowth rate after clipping. In the population with a constantly high level of herbivory, plants in intermediate-clipping treatments overcompensated in leaf area after clipping. For all the populations, clipped plants produce more new leaves than unclipped plants. In the population with the highest level of herbivory, clipping treatments did not have a significant effect on the number of fruits per plant. In addition, we did not find a trade-off between investments in growth and reproduction in this population. Our results indicated that, in the desert lily, herbivores may select for plant mechanisms that compensate after damage as a tolerant strategy to maintain fitness.     

Net photosynthesis,root respiration,and regrowth of Bouteloua gracilis following simulated grazing

Summary Net photosynthesis (P_N), root respiration (R_R), and regrowth of Bouteloua gracilis (H.B.K.) Lag. were examined in the laboratory over a 10-day period following clipping to a 4-cm height to simulate grazing by large herbivores. Net photosynthesis rates of tissue remaining immediately following defoliation were only about 40% as great as preclipping rates. Three days after clipping, P_N rates of defoliated plants had increased to values about 21% greater (per unit leaf area) than those of unclipped controls and remained at that level through Day 10. No statistically significant changes in R_R occurred following defoliation. Biomass of unclipped plants nearly doubled during the 10-day study period, while that of defoliated plants increased 67%. Over half the new growth of defoliated plants was allocated to new leaf blades and only 18% to new roots, while only 33% of the new growth of control plants was allocated to new leaf blades but 29% went to new roots. As a consequence of increased P_N rates and increased carbon allocation to synthesis of additional photosynthetic tissue following defoliation, net CO₂ uptake per plant increased from 9% to 80% of that of the controls from Day 0 through Day 10.