Overcompensation in response to simulated herbivory in the perennial herb <Emphasis Type="Italic">Sedum maximum</Emphasis>

A positive effect of herbivory on plant reproduction (overcompensation) has been documented mostly in monocarpic plants. Iteroparous perennials can be used to test whether enhanced reproduction in 1 year has negative future consequences as predicted by optimal allocation models. This study was intended to verify this prediction in the iteroparous herb Sedum maximum, applying mechanically simulated herbivory. I monitored 132 labelled S. maximum individuals during 2 years of study. They were randomly assigned to two groups: clipped and control. Infructescence dry mass, total seed dry mass, seed size, germination rate and an increase of root dry mass during the season were assessed in the experimental plants. Since only roots can survive to the next season, root dry mass was considered a reliable measure of allocation to future performance. Clipped plants showed increased fruit and seed dry mass versus the controls, with no other aspect of reproduction affected. Apical bud removal also had a positive effect on increase of root dry mass. The results indicate true overcompensation in response to simulated herbivory with no future costs of increased reproduction. Moreover, increased plant reproduction as a result of herbivory is likely to persist in the following years: clipping increased not only seed production but also root growth. This response is inconsistent with the results of optimal allocation models and the discrepancy is probably due to violation of the resource limitation assumption. Plants adapted to tolerate herbivory seem not to reproduce at the maximum rate when undamaged, but rather withhold resources to be allocated to reproduction after herbivory.     

Flowering phenology and compensation for herbivory in<Emphasis Type="Italic"> Ipomopsis aggregata</Emphasis>

The mechanisms and circumstances that affect a plant's ability to tolerate herbivory are subjects of ongoing interest and investigation. Phenological differences, and the timing of flowering with respect to pollinators and pre-dispersal seed predators, may provide one mechanism underlying variable responses of plants to herbivore damage. The subalpine wildflower, Ipomopsis aggregata, grows across a wide range of elevations and, because phenology varies with elevation, phenological delays associated with elevation may affect the ability of I. aggregata to compensate for or tolerate browsing. Thus, we examined the response of I. aggregata to herbivory across an elevation gradient and addressed the interactions among phenological delays imposed by damage, elevation, pre-dispersal seed predation and pollination, on I. aggregata's compensatory response. Among high and low elevation populations in areas near the Rocky Mountain Biological Laboratory (RMBL) in Gothic, Colorado, we compared the responses of naturally browsed, artificially browsed (clipped), and unbrowsed (control) plants of I. aggregata. We compared responses in the date of initiation of flowering, timing of peak bloom, floral display, nectar production and sugar concentration, oviposition and fruit destruction by the pre-dispersal seed predator Hylemya sp. (Anthomyiidae), fruit production, and aboveground biomass production. Clipping had the greatest effect on reproductive success and clipped plants at high elevation exhibited the lowest tolerance for herbivory. The effects of browsing appear to be mediated by flowering phenology, and both browsing and elevation delayed flowering phenology. Time needed for regrowth delays flowering, and thus affects the overlap with seed predators and pollinators. As a result of delayed flowering, naturally browsed and clipped plants incurred lower rates of seed predation. In the absence of seed predation, plants would exhibit a lower tolerance to herbivory since naturally and artificially browsed plants had fewer fruits destroyed by Hylemya larvae. We provide additional evidence that, for populations near the RMBL, clipping and natural browsing do not have the same effect on I. aggregata plants. This may be due to the selection of larger plants by herbivores. Although under some conditions plants may tolerate browsing, in areas where the growing season is short a phenological delay imposed by damage is likely to significantly reduce plant fitness. Identifying the mechanisms that allow plants to tolerate herbivore damage will help to develop a general framework for understanding the role of tolerance in plant population and community dynamics, as well as plant-herbivore interactions.     

Cumulative effects of wild ungulate and livestock herbivory on riparian willows

We examined the effects of wild ungulates (deer and elk) and domestic sheep browsing on the growth, structure, and reproductive effort of two common willow species, Salix boothii and S. geyeriana, in a montane northeast Oregon riparian zone. With the use of exclosures, large herbivore effects on willows were studied in an area browsed by native mammals only and an adjacent area in which domestic sheep also lightly grazed during summer months. Growth variables were repeatedly measured on individual plants over a 5-year period to understand physiognomic and flowering responses of native willows to different levels of browsing pressure. At the beginning of the study, all willows were intensely browsed but were significantly taller in the area browsed only by native mammals than in the area also grazed by sheep (69 versus 51 cm, respectively). Willows inside exclosures responded with pronounced increases in height, crown area, and basal stem diameters while the stature of browsed plants outside exclosures stayed constant or declined. In the area browsed by both sheep and wild herbivores, the size of browsed plants remained at pre-treatment levels (<60 cm in height) for the duration of the study. There was no significant difference in growth rates of enclosed willows, indicating that current herbivory was the primary cause of growth retardation in the study area. Foliar area was strongly correlated with basal stem numbers for enclosed plants but much less so for browsed plants. Willows inside exclosures had more than twice as much foliar area per stem. Stem diameters were a positive function of crown area: stem-number ratios, suggesting lower photosynthetic potential was correlated with diminished radial growth among browsed plants. No flowering was observed until 2 years after exclusion when plants inside all exclosures and browsed willows in the wild ungulate area responded with a large pulse in flowering. Browsed plants in the sheep + wild ungulate area did not flower. The number of catkins produced per plant was significantly associated with willow height and plants <70 cm in height did not flower, thus suggesting a size threshold for reproduction in these species. Our results suggest that even relatively light levels of domestic livestock grazing, when coupled with intense wild ungulate browsing, can strongly affect plant structure and limit reproduction of riparian willows.     

Trade-off between plant growth and defense? A comparison of sagebrush populations

We used ecotypic variation in big sagebrush (Artemisia tridentata) to examine potential trade-offs between inherent growth rate and tolerance or resistance to herbivory. Seeds were obtained from seven geographic populations, and 1,120 seedlings were established in a common garden. In one set of plots, plants were subjected to five treatments: control, regular insecticide spray, moderate browsing, severe browsing, or moderate browsing plus insecticide. Plants in a second set of plots were all untreated, and were used to estimate ambient growth, flower production, and susceptibility to herbivorous insects. In the first growing season, population differences in relative growth rate produced approximately seven-fold variation in mean biomass. Two populations of basin big sagebrush (A. tridentata tridentata) and one population of mountain big sagebrush (A. tridentata vaseyana) grew fastest; those of Wyoming big sagebrush (A. tridentata wyomingensis) showed the slowest growth. Bi-weekly application of insecticide for two growing seasons had no effect on the growth of either browsed or unbrowsed plants. All populations showed compensatory growth (but not overcompensation) in response to browsing, but the degree of compensation was unrelated to inherent growth rate. Similarly, there was no consistent relationship between plant growth rate and flower production in the second growing season. Some insects colonized fast-growing populations more frequently than slow-growing ones, but patterns of insect colonization were species-specific. At the level of geographic populations and subspecies, we found little evidence of a built-in trade-off between inherent growth rate and the ability to tolerate or resist herbivory. Because population ranks for growth rate changed substantially between seasons, attempts to correlate growth and defense characters need to account for differences in the growth trajectories of perennial plants.     

Plant responses and herbivory following simulated browsing and stem cutting of Combretum apiculatum

Abstract. Time and mode of herbivory on savanna trees and their subsequent responses are dependent on, among other things, earlier herbivory and fire. We used clipping (simulated browsing) and stem cutting (simulated heavy browsing and to some extent simulated fire) to evaluate such interactions. Study organisms were a deciduous, broad‐leaved tree species, Combretum apiculatum (Combretaceae), browsing large herbivores and leaf‐eating insects. The treatments were done in the late dry season before bud break. Late in the following wet season, we recorded plant responses to treatment and browsing. The treated trees, especially the cut ones, responded by producing larger and fewer annual shoots. Compared to control trees, there was a slight increase in shoot biomass of clipped trees and a strong reduction of cut ones. Leaf area increased in clipped trees, but decreased in cut ones. A marked increase in the number of browsed trees was recorded amongst treated trees. Number of bites, consumption and utilization also increased with severity of treatment. In contrast, insect herbivory was reduced on both clipped and cut trees. The observed patterns are discussed in relation to current ideas on plant ‐ herbivore interactions.     

How to induce defense responses in wild plant populations? Using bilberry (Vaccinium myrtillus) as example

Inducible plant defense is a beneficial strategy for plants, which imply that plants should allocate resources from growth and reproduction to defense when herbivores attack. Plant ecologist has often studied defense responses in wild populations by biomass clipping experiments, whereas laboratory and greenhouse experiments in addition apply chemical elicitors to induce defense responses. To investigate whether field ecologists could benefit from methods used in laboratory and greenhouse studies, we established a randomized block‐design in a pine‐bilberry forest in Western Norway. We tested whether we could activate defense responses in bilberry (Vaccinium myrtillus) by nine different treatments using clipping (leaf tissue or branch removal) with or without chemical treatment by methyljasmonate (MeJA). We subsequently measured consequences of induced defenses through vegetative growth and insect herbivory during one growing season. Our results showed that only MeJA‐treated plants showed consistent defense responses through suppressed vegetative growth and reduced herbivory by leaf‐chewing insects, suggesting an allocation of resources from growth to defense. Leaf tissue removal reduced insect herbivory equal to the effect of the MeJa treatments, but had no negative impact on growth. Branch removal did not reduce insect herbivory or vegetative growth. MeJa treatment and clipping combined did not give an additional defense response. In this study, we investigated how to induce defense responses in wild plant populations under natural field conditions. Our results show that using the chemical elicitor MeJA, with or without biomass clipping, may be a better method to induce defense response in field experiments than clipping of leaves or branches that often has been used in ecological field studies.     

Interactions between herbivory and resource availability on grazing tolerance of Leymus chinensis

Herbivory and resource interact to influence plant regrowth following grazing, but few detailed investigations on grazing tolerance at population levels are available. We conducted two pot experiments along a simulated grazing gradient (0%, 25%, 50% and 75% of shoot removal) at three water or nutrient levels to determine the interaction of resource and herbivory on Leymus chinensis, a perennial, dominant species in the eastern Eurasian steppes. Interactions between water availability and clipping intensity on the relative height growth rate (RHGR) and bud number were significant. Significant interactions between nutrient and clipping on RHGR, total biomass and specific leaf area (SLA) were also found. Total biomass and bud number, showing a unimodal curve along the clipping gradient in resource-rich environments, were highest at light clipping level, suggesting that this species has the plastic compensatory responses from under- to overcompensation. Interactions between herbivory and water or nutrient were opposite to each other. The “cooperative” interactions between water and herbivory magnified the difference in grazing tolerance of L. chinensis between high and low water treatments. The “antagonistic” interactions between nutrient and herbivory, on the other hand, were reflected in the lower tolerance to heavy clipping in the high nutrient than low nutrient treatments. Results partly support the limiting resource model (LRM). A modified and simplified graphic model of the LRM was proposed based on our results. The new LRM clearly demonstrated that “cooperative” interactions between varying water levels and clipping intensities aggravate the detrimental impacts of herbivores on plant growth and reproduction, whereas “antagonistic” interactions between nutrient and grazing alleviate the negative effects of herbivores. Biomass compensation and density compensation were identified as main mechanisms of herbivory tolerance in this clonal species.     

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.     

The effects of grazers on the performance of individuals and populations of scarlet gilia,Ipomopsis aggregata

Summary I. aggregata exhibits considerable powers of regrowth following removal of its primary shoot by herbivores, but we found no evidence of overcompensation (i.e. of significantly higher plant performance where plants were exposed to ungulate herbivory) in a comparison between individuals on grazed and ungrazed sides of exclosure fences, in a comparison between artificially clipped and control plants in one population in the Okanagan National Forest, or in comparisons between grazed and ungrazed plants in 14 natural populations. We tested whether ungulate grazing affects the population size of Ipomopsis aggregata by comparing populations inside and outside deer exclosures at 7 sites in the Western United States. We found consistent, highly significant differences in plant population density on the grazed and ungrazed sides of these exlosure fences. Plant density was a modal 25-fold higher on the protected side of the fence, suggesting that exposure to ungulate grazing increases plant death rates at some stage in the life cycle. Our results show that the presence of ungulate grazers leads to a substantial decrease in plant density despite the fact that grazing on young bolting shoots has very little influence on fruit production. Since this decrease in population density is not correlated with a decrease in the fecundity of individuals, it must instead be due to other direct and indirect effects of ungulate grazers.     

Simulated ungulate herbivory affects differently two herbivorous arthropod guilds in bilberry

Dense ungulate populations, both domestic and wild, accompanied by high grazing intensities, are known to have effects on the abundance and diversity of the community of arthropods hosted by plants. However, research combining ungulate herbivory with nutrient addition effects is scarce. By means of a nitrogen (N) addition and clipping experiment, we investigated the effects of both factors on the diversity, abundance and trophic structure of the arthropod community hosted by the bilberry, Vaccinium myrtillus, focusing on the response of herbivorous arthropods by considering leaf-chewers and sap-feeders separately. In addition, we assessed altitudinal variation on the arthropod community and also specifically on herbivores. Neither of the variables estimated for the arthropod community (total abundance, richness and biomass) were affected by simulated ungulate herbivory. However, clipping reduced the abundance of sap-feeders although had no significant effect on leaf-chewer insects. We therefore suggest that ungulates can compete with sap-feeders by reducing suitable food resources, or via changing plant nutritional state. Moreover, the lack of response on leaf-chewers could correspond with the absence of chemical defence in bilberry after repeated damage. Despite this effect on sap-feeder abundance, clipping did not cause a cascade effect on the trophic structure of the arthropod community associated with bilberry, at least in the short term. In turn, N fertilization at levels close to atmospheric N deposition did not cause any significant effect on the invertebrate community. Finally, the number of arthropod morphospecies and biomass decreased with altitude.     

Heavy browsing affects the hydraulic capacity of Ceanothus rigidus (Rhamnaceae)

Defoliation by herbivores can reduce carbon assimilation, change plant water relations, and even shift the biotic structure of plant communities. In this study, we took advantage of a long-term deer exclosure experiment to examine the consequences of persistent deer herbivory on plant water relations and the xylem structure–function relationships in Ceanothus rigidus, a maritime chaparral shrub in coastal California. Browsed plants had thicker stems with many intertwined short distal twigs, and significantly higher sapwood-to-leaf area ratios than their non-browsed counterparts. Leaf area-specific hydraulic conductivity was similar in both browsed and non-browsed plants, but xylem area-specific conductivity was significantly lower in the browsed plants. Vessel diameters were equivalent in both plant groups, but the number of vessels on a transverse area basis was nearly 40 % lower in the browsed plants, accounting for their lower transport efficiency. Mid-day in situ water potentials and losses of hydraulic conductivity due to embolism were similar in both groups of plants but stomatal conductance was higher in the browsed shrubs in the early part of the growing season. We discuss our findings in the context of whole-plant ecophysiology, and explore the consequences of herbivory on hormonal signals, wood anatomy, and xylem function.     

Tolerance of a perennial herb, <Emphasis Type="Italic">Pimpinella saxifraga</Emphasis>, to simulated flower herbivory and grazing: immediate repair of injury or postponed reproduction?

Perennial, polycarpic herbs can respond to herbivory either by (1) regrowth in the same season in order to compensate for lost reproductive structures or by (2) postponing reproduction until the following growing season. We tested these response patterns with the perennial umbellifer Pimpinella saxifraga by simulating flower herbivory and shoot grazing both in the field and in a common garden experiment. In the field, both simulated flower herbivory and grazing effectively suppressed current reproduction, whereas no statistically significant effects of previous-year treatments on growth or reproduction were found in the following year. In the common garden, in the first year the species fully compensated for simulated flower herbivory in vegetative parameters but seed set was reduced by 26%. After 2 years of flower removal, the plants overcompensated in shoot and root biomass by 47 and 46%, respectively, and compensated fully in reproductive performance. Simulated grazing resulted in 21% lower shoot biomass in the first season, but the root biomass was not affected. In the second season the root biomass increased by 43% as compared to the control plants. However, regrowth following simulated grazing resulted in a significant delay in flowering with the consequence that the seed yield of fertile plants was reduced by 55% as compared to the control plants. These results suggest that in resource-rich garden conditions P. saxifraga may immediately repair injuries caused by flower herbivory, but repairs more extensive shoot injury less successfully. Delayed phenology decreases the benefits of immediate repair. In resource-poor conditions, the benefits of regrowth can be negligible. Accordingly, in our field population, the plants postponed their reproduction until the following year in response to simulated grazing and frequently in response to flower removal. When the plants gain very little from regrowth, the costs of reproduction would select for postponed reproduction in response to injury.     

The effects of herbivory on paternal fitness in scarlet gilia: better moms also make better pops

Abstract Whether plants can benefit from the direct effects of herbivory has been contentious among ecologists and evolutionary biologists. Although previous studies have provided experimental evidence of enhanced maternal fitness following herbivory in a natural system, an accurate depiction of plant-herbivore interactions must include the effects of herbivory on male as well as female fitness. Here we show that ungulate herbivory on scarlet gilia results in an increase in paternal as well as maternal fitness. This study represents the first evidence of overcompensation in a natural system where both paternal and maternal components of fitness have been assessed.     

Does pollination limit tolerance to browsing in <Emphasis Type="Italic">Ipomopsis aggregata</Emphasis>?

Ungulate browsing of flowering stalks of the semelparous herb Ipomopsis aggregata leads to regrowth of lateral inflorescences, a response that has been reported to yield overcompensation in some cases (browsed plants with higher reproductive success than unbrowsed), but undercompensation in others. Little is known about the mechanisms that cause such variable tolerance to herbivory. We explored one possible mechanism—variation in effects of browsing on pollination—by clipping I. aggregata inflorescences to mimic browsing, observing subsequent visits by pollinators and nectar-robbers, and adding pollen by hand to flowers of some clipped and unclipped plants. Clipping reduced floral display size and increased inflorescence branching, but neither hummingbirds, the primary pollinators, nor nectar-robbing bumblebees showed any preference for unclipped versus clipped plants. Clipping delayed flowering; this shift in phenology caused clipped plants to miss the peak of hummingbird activity and to have lower per-flower visitation rates than unclipped controls in one year, but to have greater overlap with birds and higher visitation rates in the subsequent year. In three sites and 2 years, clipped plants exposed to natural pollination suffered extreme undercompensation, producing on average only 16% as many seeds as unclipped controls. This was not directly attributable to clipping effects on pollination, however, because clipped plants were unable to increase fecundity when provided with supplemental pollen by hand. Taken altogether, our results suggest that compensation was constrained less by indirect effects of browsing on pollination than by its direct impacts on resource availability and hence on the ability of plants to regrow lost inflorescence tissue and to fill seeds. Exploring the physiological and developmental processes involved in regrowth of inflorescences and provisioning of seeds is a promising future direction for research designed to understand variation in browsing tolerance.     

Grazing tolerance and mycorrhizal colonization: Effects of resource manipulation and plant size in biennial Gentianella campestris

As herbivory usually leads to loss of photosynthesizing biomass, its consequences for plants are often negative. However, in favorable conditions, effects of herbivory on plants may be neutral or even beneficial. According to the compensatory continuum hypothesis plants can tolerate herbivory best in resource-rich conditions. Besides herbivory, also primarily positive biotic interactions like mycorrhizal symbiosis, bear carbon costs. Tritrophic plant–fungus–herbivore interaction further complicates plant's cost-benefit balance, because herbivory of the host plant is expected to cause decline in mycorrhizal colonization under high availability of soil nutrients when benefits of symbiosis decline in relation to costs. To gain insight into above interactions we tested the effects of plant size and resource manipulation (simulated herbivory and fertilization) on both above-ground performance and on root fungal colonization of the biennial Gentianella campestris.Clipping caused allocation shift from height growth to branches in all groups except in large and fertilized plants. For large plants nutrient addition may have come too late, as the number of meristems was most likely determined already before the fertilization. Clipping decreased the amount of DSE (dark septate endophytic) fungi which generally are not considered to be mycorrhizal. The effect of clipping on total fungal colonization and colonization by arbuscular mycorrhizal (AM) fungal coils were found to depend on host size and resource level. Dissimilar mycorrhizal response to simulated herbivory in small vs. large plants could be due to more intensive light competition in case of small plants. Carbon limited small plants may not be able to maintain high mycorrhizal colonization, whereas large clipped plants allocate extra resources to roots and mycorrhizal fungi at the expense of above-ground parts. Our results suggest that herbivory may increase carbon limitation that leads re-growing shoots and fungal symbionts to function as competing sinks for the limited carbon reserves.     

Effects of herbivory and its timing across populations of Trillium grandiflorum (Liliaceae)

The goal of this study was to identify the degree to which the frequency and timing of herbivory by white-tailed deer (Odocoileus virginianus) and subsequent plant response varied across 12 populations of the perennial herb Trillium grandiflorum. Effects of natural and experimental herbivory on the stage and size of reproductive plants were measured. Both the frequency and timing of herbivory varied across T. grandiflorum populations. Reproductive plants were more likely to regress to nonreproductive stages in the next growing season when (1) reproductive plants were consumed by deer (vs. intact reproductive plants); (2) reproductive plants were consumed early in the growing season (vs. reproductive plants consumed late in the growing season); (3) reproductive plants were smaller in size. Clipped plants that remained reproductive were smaller in the following season than unclipped controls. Plant size was positively correlated with the number of ovules, suggesting that reductions in the growth rate of reproductive plants diminish their future reproductive success. Populations with high levels of natural herbivory had a greater proportion of reproductive plants that regressed to nonreproductive stages, probably because reproductive plants in these populations were smaller in size. However, the plant response to herbivory was similar across populations.     

Ungulate herbivory reduces abundance and fluctuations of herbivorous insects in a boreal old-growth forest

Ecological theory predicts the strongest ecosystem effects of herbivory when dominant and ecologically important species are consumed. Bilberry, Vaccinium myrtillus, is such a key plant species, attractive to many other species in the boreal forests, for example ungulate and invertebrate herbivores. Large herbivores may remove substantial biomass and alter plant quality and therefore affect abundance and populations of invertebrate animals sharing the same food plant. We combined experimental exclusion of ungulates with a browsing intensity gradient to investigate the 15-year effect of ungulate (Cervus elaphus and Ovis aries) browsing on bilberry plant size and on bilberry-feeding herbivorous larvae (Lepidoptera and Symphyta), in a Norwegian old growth boreal forest ecosystem. Bilberry ramets in exclosure plots had nearly nine times higher dry mass and three times higher abundance of invertebrates feeding on them than in ungulate-access plots. Sweep-netting data verified these findings as larval numbers were twice as high in exclosure plots. The pattern in the large herbivore effects on bilberry size and abundance of herbivorous larvae were identical along the browsing gradient. Differences in larval abundance between treatments, as indicated by leaf-chewing, increased during the 15-year study period, and the community fluctuations were larger when ungulate herbivores were excluded. The browsing effect was moderated by plant quality as larval densities were lowest on both heavily-browsed and non-browsed plants, and highest on ramets that had 50–74% of annual shoots browsed. Our study supports previous findings in that bilberry is relatively disturbance tolerant and may recover quickly, but that ungulates may compete with herbivorous larvae for food biomass. Additionally, our results strongly indicates that population insect community peaks and fluctuations are dampened by ungulate consumption. Our findings add to the understanding on how ungulates may structure forest ecosystems directly and indirectly.     

A test of the compensatory continuum: fertilization increases and below-ground competition decreases the grazing tolerance of tall wormseed mustard (Erysimum strictum)

Contrary to the general expectation, the compensatory continuum hypothesis proposes that grazing may not always affect plant performance adversely. Instead, the effects may vary from negative (undercompensation) to positive (overcompensation), depending on the local availability of resources and the intensity of competition experienced by individual plants. We tested this hypothesis in a common garden experiment by growing tall wormseed mustard, Erysimum strictum, under a factorial design involving simulated grazing (0, 10, or 50% of the main stem clipped), supplemental fertilization and below-ground competition. The results supported the hypothesis. On an average, fertilization increased and competition decreased plant performance. Overcompensation was only observed among the fertilized plants growing free of competition. Simulated grazing increased seed yield 1.6 (10% clipping) and 1.4 times (50% clipping) as compared to unclipped plants when the plants were grown with fertilization and without competition. In contrast, clipping did not significantly increase seed yield in the plants grown without fertilization and/or with competition. The breakage of apical dominance provides a proximate mechanism of these regrowth responses. This is consistent with the fact that most plants (85%) had an unbranched shoot architecture in our study population. However, it is not clear why E. strictum has a relatively unbranched architecture in natural populations. We briefly discuss the alternative ecological factors – competition for light, adaptation to herbivory and optimal timing of flowering as a bet-hedging strategy in monocarpic plants – which might maintain unbranched architecture in this species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Biomass allocation and growth responses of Scots pine saplings to simulated herbivory depend on plant age and light availability

This study experimentally analyses the response to simulated herbivory of juvenile Scots pine of two different ages in contrasting abiotic scenarios, focusing on the potential dual role of browsing ungulates: negative, by removing aerial biomass, and positive, by stimulating compensation capacity and providing nutrients by depositing their excrement. Compensation against herbivory was investigated by experimentally clipping a set of Scots pine (Pinus sylvestris L. nevadensis) juveniles, grown under different levels of light and nutrient availability. The responses analysed were survival, trunk-diameter growth, leader-shoot growth, increment in number of meristems, RGR, biomass of needles, shoots, root and whole plant, and root-to-shoot ratio. Clipping consistently resulted in a worse survival and performance of pines with respect to unclipped ones. From the factors analysed, light availability was responsible mainly for the variations in plant performance, while the addition of nutrients was much less important. Age was also important, with older pines showing in general better performance after clipping. Overall, clipping invariably had a negative effect on Scots pine, since none of the combinations of abiotic factors used resulted in overcompensation. However, the intensity on this negative effect proved quite variable, from almost an exact compensation in clipped older pines under full sunlight availability to very poor performance and high death probability in younger pines in shade. Scots pine cannot overcompensate after clipping, but, depending of the environmental conditions, the negative result of clipping varies from severe undercompensation to almost exact compensation. Also, small differences in sapling age can promote significant differences in sapling response to clipping and light environment.     

苗期刈割伤害对春小麦影响的盆栽实验研究

盆栽实验研究了春小麦在苗期（三叶期）受到不同强度刈割伤害（模拟动物的采食）后的补偿作用。结果表明,在保持田间持水量６０％和９０％两种水分梯度下,春小麦苗期的刈割伤害均可刺激生长,出现生物产量和经济产量的超越补偿,而且受到重度刈割伤害春小麦（Ｅ、Ｆ）的补偿作用大于受到轻度刈割伤害春小麦（Ｂ、Ｄ）的补偿作用。这一超越补偿作用是以耗费较多的水分为基础的。