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 ten year cycle of the willow grouse of Lower Kolyma

Summary The effects of defoliation on growth and nitrogen (N) nutrition were examined in populations of Agropyron smithii (western wheatgrass) collected from a heavily grazed black-tailed prairie dog (Cynomys ludovicianus) colony (ON-colony) and a nearby lightly grazed, uncolonized area (OFF-colony). Defoliated and nondefoliated plants were grown at low soil N availability with similar sized defoliated individuals of A. smithii from a grazing-exclosure population as a common competitor. Sequential harvests were made over 24 days following defoliation. Growth analysis plus biomass and N yield and distribution data were used to identify features which may contribute to plant defoliation tolerance. Defoliation reduced total production 34% across populations. Defoliated plants produced as much new blade tissue, but only 67% as much new root biomass as did nondefoliated controls. Plants from prairie dog colonies accumulated biomass at a faster relative rate than did plants from uncolonized sites, in part, because of a 250% greater mean relative growth rate of blades and more than 200% greater rate of biomass production per unit blade biomass. Total N accumulation was significantly greater in defoliated ON- than OFF-colony individuals. The mean relative accumulation rate of N was increased by defoliation in ON-colony plants, but reduced by defoliation in OFF-colony plants. The mean rate of N accumulation per unit root biomass was more than 300% greater in the ON- than OFF-colony population. Colony plants initially had a greater proportion of biomass and N remaining after defoliation in roots. Initial differences between populations in the distribution of biomass and N were eliminated as colony plants concentrated 24-day accumulation of biomass and N in aboveground structures. The data suggest that the combination of growth, N nutrition, and biomass and N distribution characteristics of the colony population likely confer a high rate of resource capture on heavily grazed prairie dog colonies.     

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.     

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.     

中国东北羊草草原生长季内产量生态模拟及信息参数应用

通过对中国北方羊草草原生物量动态、生物量垂直空间格局及其与环境因子相互关系等主要产量生态数量特征的模拟与内在相关性的研究 ,结果表明 ,草地地上生物量的生长规律呈“单峰”型 ,最大地上生物量出现在 8月 5日 ,其值为1 97.3g· m- 2 干物质 ,而后下降 ;在达到峰值前 ,符合 logistic模型 ,进一步分析模型有关特征值获得了草地有效管理期为返青后的第 73天到第 1 1 9天等十分重要的产量生态信息参数。生长季内地上生物量动态与前一个月的平均气温 ( R=0 .82 87)和积累降雨量 ( R=0 .8932 )均呈极显著正相关 ,这是实施科学水肥管理的重要参数 ;而地上部生物量最大绝对增长速率 ( AGR)出现在 6月 2 0日至 7月 5日 ,平均为 3.35 33g· m- 2 · d- 1干物质 ;而地上部生物量最大相对增长速率( RGR)出现在 5月 2 0日至 6月 5日 ,平均为 0 .0 6 6 2 g· g- 1· d- 1干物质 ;在生长后期绝对增长速率和相对增长速率均出现负值 ,这表明地上部生物量的生长效率在生长初期最高。地上生物量垂直空间格局由下向上呈幂函数变化 ,其模型为 :Bn=a Xb,其中 93%的产量集中在 4 0 cm以下 ,这对不同的家畜的选择利用与刈割利用提供了依据 ;不同种群对草原牧草产量形成的作用是不同的 ,羊草种群对草原牧草产量形成的正向     

Explaining patterns of primary production from individual level traits

Question: Do species traits explain differences in productivity in grazed and ungrazed plots? Location: Río de la Plata grasslands, Uruguay (31°54′S, 58°15′W). Methods: In a greenhouse experiment, we measured the relative growth rate (RGR) of grasses with contrasting responses to grazing (increasers and decreasers). We performed six harvests at weekly intervals in order to calculate the RGR and assess 12 plant traits. We compared the RGR between increaser and decreaser species after 2 and 5 weeks using t‐tests. Linear and potential regression models were fitted to time versus natural logarithm of total dry biomass relationships. The RGR temporal trajectories of increaser and decreaser species were obtained from the derivatives of the best‐fit functions. Principal component analysis (PCA) was used to sort species according to their traits. Results: The RGR of decreaser grasses was higher than that of increasers at the second week, while at the fifth week the opposite was observed. The RGR of decreasers dropped through time, while the RGR of increaser species was constant. The PCA separated increaser from decreaser species. The attributes related to increaser species were: high specific leaf area, tillering rate, green leaf rate, total leaf number, root weight ratio and leaf weight ratio; while those associated with decreaser species were: high dead biomass, senescence rate, reproductive biomass, leaf elongation rate and total biomass. Conclusions: Traits possessed by decreasers reduce light availability and increase the reproductive investment, explaining the drop in RGR. Specific differences in RGR seem to scale up to the ecosystem level and would explain the pattern in aboveground net primary production observed in the field under contrasting grazing regimes.     

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.     

刈割、施肥和浇水对矮嵩草补偿生长的影响

通过对青海海北高寒矮嵩草(Kobresia humilis)草甸进行为期3年的野外控制试验, 研究了刈割(留茬1 cm、3 cm及不刈割)、施肥(2.5 g·m^-2尿素+ 0.6 g·m^-2磷酸二胺、不施肥)和浇水(20.1 kg·m^-2、不浇水)处理对矮嵩草补偿生长(包括分株密度、株高和分株地上生物量)的影响, 及其比叶面积、叶片净光合速率和相对增长率的变化, 探讨矮嵩草补偿生长的机制。研究结果表明: 刈割后, 矮嵩草的补偿生长高度和比叶面积显著降低; 分株密度有增加的趋势, 但会随刈割强度的增加而下降; 株高和生物量的相对增长率随刈割强度的增加而呈上升趋势; 补偿地上生物量在重度刈割处理下最高。施肥能显著增加矮嵩草的补偿高度、分株密度、补偿地上生物量、株高相对增长率、生物量相对增长率、比叶面积和净光合速率; 与不浇水处理相比, 浇水处理对重度刈割处理下的分株地上生物量、密度相对增长率、比叶面积和净光合速率无影响, 而显著降低了中度刈割处理下的补偿高度和株高相对增长率, 提高了不刈割处理下的分株密度和重度刈割处理下的生物量相对增长率。刈割、施肥和浇水处理的交互作用也显示出刈割与施肥对矮嵩草补偿生长具有拮抗效应, 而刈割与浇水具有协同效应。上述结果说明, 矮嵩草在刈割后可通过增加分株密度和相对增长率等途径来提高补偿能力, 弥补在生长高度上出现的低补偿, 而施肥可显著抵消刈割的不利影响, 提高矮嵩草的补偿能力。     

Variation in grazing tolerance among three tallgrass prairie plant species

Three tallgrass prairie plant species, two common perennial forbs (Artemisia ludoviciana and Aster ericoides [Asteraceae]) and a dominant C(4) perennial grass (Sorghastrum nutans) were studied under field and greenhouse conditions to evaluate interspecific variation in grazing tolerance (compensatory growth capacity). Adaptation to ungulate grazing was also assessed by comparing defoliation responses of plants from populations with a 25-yr history of no grazing or moderate ungulate grazing. Under field conditions, all three species showed significant reductions in shoot relative growth rates (RGR), biomass, and reproduction with defoliation. In the two forbs, clipping resulted in negative shoot RGR and reductions in both number and length of shoot branches per ramet. Sorghastrum nutans maintained positive RGR under defoliation due to a compensatory increase in leaf production. Defoliation reduced rhizome production in A. ericoides and S. nutans, but not in A. ludoviciana. Clipping significantly reduced sexual reproductive allocation in all three species, although S. nutans showed a smaller reduction than the forbs. All three species showed similar responses to defoliation in burned and unburned sites. Under greenhouse conditions, a similar clipping regimen resulted in smaller reductions in growth and reproduction than those observed in the field. For all three species, the grazing tolerance indices calculated under natural field conditions were significantly lower than those estimated from greenhouse-grown plants, and the interspecific patterns of grazing tolerance were different. Aster ericoides exhibited the highest overall defoliation tolerance under greenhouse conditions, followed by S. nutans. Artemisia ludoviciana, the only study species that is typically not grazed by ungulates in the field, showed the lowest grazing tolerance. In the field experiment S. nutans showed the highest grazing tolerance and the two forbs had similar low tolerance indices. These patterns indicate that, despite high compensatory growth potential, limited resource availability and competition in the field significantly reduce the degree of compensation and alter interspecific differences in grazing tolerance among prairie plants. In all three species, defoliation suppressed sexual reproduction more than growth or vegetative reproduction. Significant interactions between plant responses to defoliation and site of origin (historically grazed or ungrazed sites) for some response variables (root/shoot ratios, rhizome bud initiation, and reproductive allocation) indicated some degree of population differentiation and genetic adaptation in response to a relatively short history of ungulate grazing pressure. The results of this study indicate that patterns of grazing tolerance in tallgrass prairie are both genetically based and also environmentally dependent.     

Aboveground dry-matter allocation in ungrazed and grazed stands of Indigofera spinosa in the arid zone of Turkana, Kenya

The dwarf shrub Indigofera spinosa , indigenous to arid and semi-arid rangelands of northeastern Africa, is an important food source for livestock. Proper management of the shrub requires improved understanding of the effects of grazing and climatic variability on aboveground dry-matter allocation. Between 1986 and 1990, we compared the temporal variability of aboveground dry-matter allocation to different plant biomass compartments. We also compared dry-matter transfers between components; total live biomass to litter, standing dead to litter and live biomass to standing dead between continuously grazed and an ungrazed treatments. Partitioning of combined total dry-matter production among different structural organs (called allocation ratio) is influenced by phenological changes, episodic rainfall and her-bivory. Dry-matter production in the grazed treatment responded more markedly to episodic rainfall events more than in the ungrazed treatment. Exclusion of grazers failed to improve the relative growth rate (RGR) of shrub biomass, while grazing improved it. RGR declined in the ungrazed treatment following the accumulation of standing dead dry-matter, while m the grazed treatment it declined following the shedding of leaves. The shrub allocated more to total live biomass than to standing dead. Greater reduction of total live allocation ratio in the grazed than in the ungrazed treatment occurred during a dry year. The ungrazed treatment had higher standing dead allocation ratio than did the grazed treatment. Plants transferred more dry-matter from total live biomass compartment to litter, than from standing dead to or from total live biomass to standing dead independent of treatment. The rates of transfer were higher in the ungrazed than in the grazed treatment. The results suggest that I spinosa has evolved to respond to climatic variability and grazmgbyallocating dry allocating dry-matter differently between various compartments.     

黄土丘陵半干旱区引种禾草柳枝稷的生物量与水分利用效率

加强优良引种禾草植物的生态适应性研究对促进我国黄土高原半干旱区草地建设和草业科学发展具有十分重要的意义。比较研究了引种禾草柳枝稷(Panicumvirgatum)在黄土丘陵半干旱区不同立地条件下地上生物量的大小和季节累积差异及其水分利用特征。2001~2002年川地柳枝稷草地地上生物量达13000~16000kg/hm2,山地梯田和坡地为2300~2650kg/hm2。不同立地条件柳枝稷返青后的生物量累积过程呈二次或三次多项式。柳枝稷的绝对生长速率(AGR)在整个生长季内呈双峰曲线变化,川地柳枝稷草地的现存量和总量AGR最大值分别为158.93和169.83kg/(hm2·d),梯田分别为27.31和38.25kg/(hm2·d),坡地为37.0和36.69kg/(hm2·d)。坡地柳枝稷生物量较大值和AGR最大值出现时间最早。不同立地柳枝稷生物量相对生长速率(RGR)在整个生长季内呈双峰曲线变化,均以返青后的20d内最大,平均AGR以川地最大,坡地和梯田相近,但坡地两峰值高于梯田。不同立地柳枝稷草地土壤水分主要利用层次为0~2m,月平均含水量顺序为梯田>坡地>川地。川地柳枝稷叶片和整体生物量水分利用效率均最高,梯田整体生物量水分利用效率大于坡地,但二者叶片水分利用效率相近。川地和山地地表下5cm生育期平均地温均为17.60℃,但4~5月份川地地温高出山地1.2~2.8℃,川地4~10月份平均气温较山地高1.5℃,这些差异影响不同立地条件柳枝稷草地水分利用和生长进程。     

The relation of biomass production with leaf traits varied under different land-use and precipitation conditions in an Inner Mongolia steppe

The analyses of plant leaf traits that strongly influence aboveground net primary production (ANPP) are indispensable for understanding the process of plant biomass formation. However, there are few studies that have attempted to relate patterns of ANPP under contrasting management practices to plant leaf-level traits. To assess how leaf traits affect plant biomass accumulation under different land-use practices, we examined leaf traits and biomass production in three differently managed sites in the Inner Mongolia steppe: a site fenced since 1979 (UG), a winter grazing site (WG), and a heavily grazed site (HG). Low soil water content, leaf area index, and potential growth ability of species at site HG led to low crop growth rate (CGR), net assimilation rate (NAR), and relative growth rate (RGR); resulting in lower ANPP as compared to sites WG and UG. Irrespective of land-use management, prolonged drought significantly decreases ANPP even though it systematically increases mean CGR and RGR. However, leaf N content and leaf weight ratio are the crucial components necessary to determine the RGR at site WG. This suggests that low leaf N and availability of soil N due to haymaking may be responsible for neither over-compensatory nor compensatory growth in this site. The low ANPP in dry years is not due to the low mean CGR and RGR but rather to the short effective growing days (referring to the days the vegetation actually grows), suggesting that production-adjusted grazing regimes may be the most suitable measures for precision land management and avoiding grassland degradation.     

Linear relationships between aboveground biomass and plant cover in low open herbaceous vegetation

Abstract. Four herbaceous plant species of a sand dune area and several herb species of an open early‐successional patch were investigated for the occurrence of a simple relationship between aboveground biomass and plant cover. Without exception linear regressions of aboveground biomass on plant cover were found with slope factors depending on the growth form of the species. These results suggest that (early) growth of herbaceous plants in low and (temporarily) open vegetation is not affected by possible constraints caused by a decreasing ratio of plant cover to aboveground biomass. The obtained linear relationships could be used for rapid non‐destructive determination of aboveground biomass by image‐analysis of cover data.     

Relative growth rates and the grazing optimization hypothesis

Summary A mathematical analysis of the changes in plant relative growth rates necessary to increase aboveground production following grazing was conducted. The equation derived gives an isoline where production of a grazed and ungrazed plant will be the same. The equation has four variables (mean shoot relative growth rate, change in relative growth rate after grazing, grazing intensity, and recovery time) and may be analyzed graphically in a number of ways.Under certain conditions, small increases in shoot relative growth rate following grazing will lead to increased aboveground production. Under other conditions, very large increases in relative growth rate after grazing can occur without production being increased over that of ungrazed plants. Plants growing at nearly their maximum potential relative growth rate have little opportunity to respond positively to grazing and potentially can sustain less grazing than plants with growth rates far below maximum. Plants with high relative growth rates at the time of grazing require large increases in growth rate while slow growing plants require only small increases. High grazing intensities are least likely to increase production and high grazing frequencies require greater responses than infrequent grazing events.     

Growth rate and biomass partitioning of wildtype and low-gibberellin tomato (Solanum lycopersicum) plants growing at a high and low nitrogen supply

The growth-promoting effects of gibberellins (GAs) on plants are well documented, but a complete growth analysis at the whole plant level on plants with an altered GA biosynthesis has never been reported. In the present work, the relative growth rate (RGR), biomass partitioning and morphological parameters of wildtype (Wt) tomato ( Solanum lycopersicum L. cv. Moneymaker) plants were compared with those of isogenic ( gib ) mutants with a reduced biosynthesis of gibberellins. GA deficiency reduced RGR and specific leaf area (SLA, leaf area per unit leaf mass) and increased the net assimilation rate (NAR, the rate of biomass increment per unit leaf area). Despite the free access to nitrogen in the rooting medium, the low-GA mutants had a much higher root mass ratio (RMR, the root mass per unit plant biomass) than the Wt, suggesting that the mutants were disturbed in their growth response to nitrate supply. The experiment was repeated at a low exponential nitrate supply, which forced all plants to grow at the same low RGR. The persistence of the differences in RMR at low N-supply indicated that the high RMR of the mutants was a direct effect of low GA, which was independent of nitrate supply. Because the low N-supply increased the RMRs of all genotypes to the same extent, the response of RMR to N-supply does not seem to depend on GA. Although many of the traits of the slow growing GA mutants were very similar to those of inherently slow growing plant species from unproductive habitats, gibberellins are unlikely to be a main determinant of a plant's potential RGR.     

盐渍化胁迫下油葵对土壤原油污染的适应性及其改良措施

为了探究植物在盐渍化胁迫下对原油污染的适应性及改良措施,本研究以油葵作为研究对象,进行了原油-氯化钠-脱硫石膏盆栽正交试验和煤渣-沸石-脱硫石膏-锯沫盆栽正交试验.结果表明: 在盐渍化条件下,随着原油浓度的增大,油葵幼苗株高相对生长率(RGR)、地上生物量RGR、根氮磷比均显著减小,超氧化物歧化酶(SOD)和过氧化氢酶(CAT)活性呈先增加后显著降低的趋势;随着锯沫体积分数的加大,油葵株高RGR和地上生物量RGR均显著增加,SOD活性逐渐降低,说明锯沫在改良盐渍化原油污染土壤方面比煤渣、沸石和脱硫石膏效果显著.在盐渍化条件下,原油污染能够降低油葵幼苗的生长率,锯沫对改良原油污染有较好的效果.     

Plant and soil microbial responses to defoliation in temperate semi-natural grassland

There is much interest in understanding the nature of feedback mechanisms between plants and soil organisms in grazed ecosystems. In this study, we examine the effects of different intensities of defoliation on the growth of three dominant grass species, and observe how these plant responses relate to the biomass and activity of the microbial community in the root zone. Our data show that grassland plants with varying tolerances to grazing have markedly different growth responses to defoliation, and that these responses vary with the intensity of cutting. Defoliation of grasses which are tolerant to grazing, namely Festuca rubra and Cynosurus cristatus, leads to a reduction in root mass and an increase in the allocation of resources to shoots. In contrast, defoliation of a grass with low tolerance to grazing, Anthoxanthum odoratum, had little effect on root mass, but increased the relative allocation of resources below-ground. In all plant species, defoliation led to an increase in soil microbial biomass and C use efficiency in the root zone. This response was greatest in the root zone of A. odoratum and is likely to be related to changes in root exudation pattern following defoliation. The significance of these changes in relation to soil nutrient dynamics and plant nutrient uptake during regrowth require further exploration. This revised version was published online in June 2006 with corrections to the Cover Date.     

Grazing alters species relative abundance by affecting plant functional traits in a Tibetan subalpine meadow

Domestic livestock grazing has caused dramatic changes in plant community composition across the globe. However, the response of plant species abundance in communities subject to grazing has not often been investigated through a functional lens, especially for belowground traits. Grazing directly impacts aboveground plant tissues, but the relationships between above‐ and belowground traits, and their influence on species abundance are also not well known. We collected plant trait and species relative abundance data in the grazed and nongrazed meadow plant communities in a species‐rich subalpine ecosystem of the Qinghai–Tibet Plateau. We measured three aboveground traits (leaf photosynthesis rate, specific leaf area, and maximum height) and five belowground traits (root average diameter, root biomass, specific root length, root tissue density, and specific root area). We tested for shifts in the relationship between species relative abundance and among all measured traits under grazing compared with the nongrazed meadow. We also compared the power of above‐ and belowground traits to predict species relative abundance. We observed a significant shift from a resource conservation strategy to a resource acquisition strategy. Moreover, this resource conservation versus resource acquisition trade‐off can also determine species relative abundance in the grazed and nongrazed plant communities. Specifically, abundant species in the nongrazed meadow had aboveground and belowground traits that are associated with high resource conservation, whereas aboveground and belowground traits that are correlated with high resource acquisition determined species relative abundance in the grazed meadow. However, belowground traits were found to explain more variances in species relative abundance than aboveground traits in the nongrazed meadow, while aboveground and belowground traits had comparable predictive power in the grazed meadow. We show that species relative abundance in both the grazed and the nongrazed meadows can be predicted by both aboveground traits and belowground traits associated with a resource acquisition versus conservation trade‐off. More importantly, we show that belowground traits have higher predictive power of species relative abundance than aboveground traits in the nongrazed meadow, whereas in the grazed meadows, above‐ and belowground traits had comparable high predictive power.     

Defoliation of a grass is mediated by the positive effect of dung deposition,moss removal and enhanced soil nutrient contents: results from a reindeer grazing simulation experiment

Herbivory is one of the key drivers shaping plant community dynamics. Herbivores can strongly influence plant productivity directly through defoliation and the return of nutrients in the form of dung and urine, but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. However, the relative importance of these processes is poorly understood. We, therefore, established a common garden experiment to study plant responses to defoliation, dung addition, moss cover, and the soil legacy of reindeer grazing. We used an arctic tundra grazed by reindeer as our study system, and Festuca ovina, a common grazing‐tolerant grass species as the model species. The soil legacy of reindeer grazing had the strongest effect on plants, and resulted in higher growth in soils originating from previously heavily‐grazed sites. Defoliation also had a strong effect and reduced shoot and root growth and nutrient uptake. Plants did not fully compensate for the tissue lost due to defoliation, even when nutrient availability was high. In contrast, defoliation enhanced plant nitrogen concentrations. Dung addition increased plant production, nitrogen concentrations and nutrient uptake, although the effect was fairly small. Mosses also had a positive effect on aboveground plant production as long as the plants were not defoliated. The presence of a thick moss layer reduced plant growth following defoliation. This study demonstrates that grasses, even though they suffer from defoliation, can tolerate high densities of herbivores when all aspects of herbivores on ecosystems are taken into account. Our results further show that the positive effect of herbivores on plant growth via changes in soil properties is essential for plants to cope with a high grazing pressure. The strong effect of the soil legacy of reindeer grazing reveals that herbivores can have long‐lasting effects on plant productivity and ecosystem functioning after grazing has ceased.     

Differential responses to defoliation frequency in little bluestem (<Emphasis Type="Italic">Schizachyrium scoparium</Emphasis>) in tallgrass prairie: implications for herbivory tolerance and avoidance

Plant responses to herbivory are complex. In grasses, relative growth rate (RGR), seed, and vegetative reproduction, resource allocation, and architecture vary differentially and often nonlinearly with grazing intensity. High grazing tolerance may be achieved through compensatory photosynthesis and leaf growth, or through demographic mechanisms such as activation of a belowground dormant bud bank. This study assessed the relationship between grazing frequency and responses of Schizachyrium scoparium (little bluestem) in a tallgrass prairie, and examined the roles of tiller growth, reproduction, and bud (meristem) populations in its persistence under grazing. Genets were subjected to varying simulated grazing frequencies for a period of 2 years. Strong differential responses were observed among plant traits. RGR, biomass, and flowering showed strong nonlinear reductions in response to increasing clipping frequency, with no evidence of threshold effects. However, meristem density was unaffected, and plants maintained a large bud bank across all clipping treatments. Tiller natality decreased initially, but increased with >4 clippings, suggesting that declines in tiller RGR are partially offset by increasing tiller natality, and that variation in genet size is driven more by demography than by variation in individual tiller growth. Increased grazing frequency also resulted in differential activation of buds at different positions (emerging within vs. outside the subtending leaf sheath), explaining the shift to a more prostrate growth form observed in many caespitose grasses under persistent grazing. Thus, although this grass species lacks the capacity for compensatory foliage re-growth, the maintenance of a large dormant bud bank and the differential activation of buds in different positions contribute to its grazing tolerance and avoidance, respectively, and its long-term persistence in grazed grasslands.