Compensatory growth and competitive ability of an invasive weed are enhanced by soil fungi and native neighbours

Compensatory responses to herbivory by invasive weeds may foil attempts to arrest their spread with biological controls. We conducted an experiment to study the effects of defoliation and soil fungi on interactions between Centaurea melitensis , an invasive annual from Eurasia, and Nassella pulchra , a native Californian bunchgrass. Defoliation of C. melitensis reduced its final biomass in all species–fungicide treatments, except when C. melitensis was grown with both Nassella and non-treated soil fungi at the same time. In this treatment, the biomass of clipped C. melitensis plants was equal to that of unclipped plants, indicating that soil fungi and Nassella promoted a compensatory response in the weed. Overall, the biomass of C. melitensis was 44% lower when soil fungi were reduced. However, in soil not treated with fungicide, the total biomass of C. melitensis increased in the presence of Nassella , but decreased when it was grown alone. When stressed by defoliation, C. melitensis may benefit from a form of mycorrhizae-mediated parasitism through a common mycorrhizal network, or Nassella may alter the fungal community in a way that enhances the positive direct effects of soil fungi on Centaurea .     

Effects of simulated herbivory in three old field Compositae with different inflorescence architectures

The effects of simulated herbivory (early or late defoliation and cutting of the flowering shoot) on the growth and reproduction of three species of monocarpic composite forbs (Crepis pulchra, Picris hieracioides and C. foetida) with different inflorescence architectures were studied in experimental plots. For the three species studied, early defoliation had no significant effect on subsequent growth. In contrast, late defoliation, occurring at the start of the season of drought, had a negative effect on growth and reproduction in the two Crepis species, particularly C. foetida, but had less effect on P. hieracioides. Sexual biomass was more clearly affected by late defoliation than was vegetative biomass, although the effects differed markedly among species possibly as a result of differences in phenology. Clipping the flowering shoot removed about 3 times less biomass than late defoliation and had little effect on vegetative biomass. It had much greater effects on the sexual biomass in P. hieracioides and C. pulchra, and resulted in the production of many shoots sprouting from the rosette, allowing the treated plants to regain a vegetative biomass close to that of control plants. Clipping did however lead to the production of shorter shoots and a reduction in the number of capitula formed. In C. foetida, much branching occurred even when the main shoot was not cut; the architecture of individual plants was therefore only slightly changed by clipping the apical bud and the sexual biomass of this species was not affected by ablation of the flowering shoot. Overcompensation was found in only two families of C. pulchra for vegetative biomass. No over-compensation was found for sexual biomass, despite an increase in the number of flowering shoots in C. pulchra and P. hieracioides following clipping. However situations close to compensation for the vegetative biomass in the three species and in P. hieracioides for the sexual biomass were recorded. The response of the three study species to simulated herbivory were related to their architecture and to the time of defoliation.     

Centaurea solstitialis Invasion Success Is Influenced by Nassella pulchra Size

Replacement of perennial grasses with non‐native annual grasses in California's Central Valley grasslands and foothills has increased deep soil water availability. Yellow starthistle (Centaurea solstitialis), a deep‐rooted invasive thistle, can use this water to invade annual grasslands. Native perennial bunchgrasses, such as Purple needlegrass (Nassella pulchra), also use deep soil water, so there is an overlap in resource use between N. pulchra and C. solstitialis. Restoration of N. pulchra to annual grasslands could result in strong competitive interactions between N. pulchra and C. solstitialis, which may reduce survival, growth, and reproduction of the invader. The strength of this competitive interaction can increase as N. pulchra plants mature, increase in size, and develop more extensive root systems. We studied how the size of N. pulchra affected the success of C. solstitialis invasion over 2 years. We allowed C. solstitialis seed to fall naturally into plots containing N. pulchra plants. For each plot, we measured the number of C. solstitialis seedlings and mature plants, as well as C. solstitialis biomass and seedhead production. In both years of the study, C. solstitialis number, biomass, and seedhead production declined significantly as N. pulchra size increased. However, even C. solstitialis grown with the largest N. pulchra plants produced some seed, especially during the higher rainfall year. We conclude that restoration plantings with larger, established N. pulchra plants will be more resistant to invasion by C. solstitialis than young N. pulchra plantings, but site management must continue as long as a C. solstitialis seed source is present.     

Do life history traits predict responses to defoliation in co‐occurring prairie grasses?

Abstract. This study examines whether competition between the unpalatable grass Hilaria mutica and three co‐occurring, palatable grasses in a Texan mixed prairie is altered by non‐selective or selective defoliation. In this four‐year study, plants were grown in monoculture or in combination with the unpalatable Hilaria in a replacement design. Under no defoliation, the unpalatable Hilaria had a lower growth potential than Bouteloua curtipendula and Nassella leucotricha that were of equal stature, and produced only as much as the shorter grass, Buchloe dactyloides. Bouteloua had the highest growth potential under no‐defoliation and was defoliation tolerant, except when defoliated at ground level. Nassella was more productive than the unpalatable Hilaria, since the ability to grow earlier in the year enabled it to compete successfully with Hilaria. These results indicate that with adequate deferment Bouteloua and Nassella should compete successfully with Hilaria and Buchloe should be able to maintain itself in the presence of Hilaria. Under non‐selective defoliation, Hilaria was able to compete successfully only with Buchloe. Hilaria was sensitive to defoliation, despite being rhizomatous, and competed less successfully with Buchloe after non‐selective defoliation than it did when not defoliated. This indicates that the management practice of burning and stocking heavily with livestock until Hilaria is avoided, resulting in non‐selective defoliation, will not cause Hilaria to be more competitive with the more palatable Bouteloua, Buchloe or Nassella. Hilaria was able to compete most successfully under selective defoliation when it was not defoliated. Under selective defoliation, by avoiding herbivory, Hilaria is able to compete strongly with at least Buchloe and Nassella. The reaction of Nassella and Buchloe to selective defoliation indicates that they may have been displaced by Hilaria in the past. In contrast, under the short‐term and non‐limiting growth conditions of this study, Bouteloua competed successfully with Hilaria even under selective defoliation. These results do not rule out the possibility that, through selective defoliation, Hilaria may have displaced other grasses including Bouteloua in the past.     

Clipping defoliation and nitrogen addition shift competition between a C3 grass (Leymus chinensis) and a C4 grass (Hemarthria altissima)

• Human‐induced disturbances, including grazing and clipping, that cause defoliation are common in natural grasslands. Plant functional type differences in the ability to compensate for this tissue loss may influence interspecific competition.
• To explore the effects of different intensities of clipping and nitrogen (N) addition on compensatory growth and interspecific competition, we measured accumulated aboveground biomass (AGB), belowground biomass (BGB), tiller number, non‐structural carbohydrates concentrations and leaf gas exchange parameters in two locally co‐occurring species (the C₃ grass Leymus chinensis and the C₄ grass Hemarthria altissima) growing in monoculture and in mixture.
• For both grasses, the clipping treatment had significant impacts on the accumulated AGB, and the 40% clipping treatment had the largest effect. BGB gradually decreased with increasing defoliation intensity. Severe defoliation caused a significant increase in tiller number. Stored carbohydrates in the belowground biomass were mobilised and transported aboveground for the growth of new leaves to compensate for clipping‐induced injury. The net CO₂ assimilation rate (A) of the remaining leaves increased with clipping intensity and peaked under clipping intensities of 20% or 40%. Nitrogen addition, at a rate of 10 g·N·m^?2·year^?1, enhanced A of the remaining leaves and non‐structural carbohydrate concentrations, which benefited plant compensatory growth, especially for the C₃ grass. Under the mixed planting conditions, the clipping and N addition treatments lowered the competitive advantage of the C₄ grass.
• The results suggest that a combination of defoliation and N deposition have the potential to benefit the coexistence of C₃ and C₄ grasses.

     

Shoot herbivory on the invasive plant, Centaurea maculosa, does not reduce its competitive effects on conspecifics and natives

Herbivory can have negative, positive, or no effect on plants. However, insect biological control assumes that herbivory will negatively affect the weed and release natives from competition. Centaurea maculosa, an invader in North America, is tolerant to herbivory, and under some conditions, herbivory may increase its competitive effects on natives. Therefore, we investigated two hypotheses: 1) herbivory stimulates compensatory growth by C. maculosa, which increases its competitive effects, and 2) herbivory stimulates the allelopathic effect of C. maculosa. In the greenhouse, Trichoplusia ni shoot herbivory reduced C. maculosa biomass when shoot damage exceeded 40% of the total original leaf area. Conspecific neighbors had no effect on C. maculosa biomass, and the presence of the natives Festuca idahoensis and F. scabrella had a positive effect on C. maculosa. Neighbors did not alter the effects of shoot herbivory. More importantly, even intense shoot herbivory on C. maculosa did not benefit neighboring plants. In a field experiment, clipping 50% of C. maculosa aboveground biomass in the early summer and again in the late summer reduced final biomass by 40% at the end of the season; however, this clipping did not affect total biomass production or reproductive output. Festuca idahoensis neighbors did not increase the effects of clipping, and aboveground damage to C. maculosa did not release F. idahoensis from competition. In the greenhouse we used activated carbon to adsorb allelochemicals, which reduced the competitive effects of C. maculosa on F. idahoensis but not on F. scabrella or other C. maculosa. However, we found no increase in the allelopathic effects of C. maculosa after shoot herbivory. In summary, our results correspond with others indicating that exceptionally high intensities of herbivory are required to suppress C. maculosa growth and reproduction; however, even intense herbivory on C. maculosa does not insure that native bunchgrasses will benefit.     

Compensatory growth response of the legume, Medicago sativa, to defoliation and denodulation

A laboratory study was conducted to determine the effects of defoliation and denodulation on compensatory growth of Medicago sativa (L.). Plants grown hydroponically in clear plastic growth pouches were subjected to 0 and 50% nodule pruning, and 0, 25, 50, and 75% defoliation by clipping trifoliate leaves. An additional experiment was conducted to determine if clipping leaves simulated herbivory by Hypera postica (Gyllenhal) larvae. Previously, we determined that nodule pruning accurately simulated herbivory by Sitona hispidulus (L.) larvae (Quinn & Hall, 1992). Results indicated that denodulation stimulated nodule growth and caused exact compensation in standing and total number of nodules per plant within 15 days and in standing nodule biomass within 22 days of treatment. Denodulation caused a significant reduction (13%) in final shoot biomass, but did not affect significantly final root biomass. Percentage of change in number of trifoliate leaves per plant increased with the level of defoliation. Within 22 days of treatment, total number of trifoliate leaves per plant was similar to controls. However, final standing shoot biomasses were significantly less that controls, indicating undercompensatory growth. Shoot biomasses of the 25-, 50-, and 75%-defoliated plants were 18, 20, and 36% lower than controls, respectively. Nodule biomass per plant was reduced by 24 and 32% in 50- and 75%-defoliated plants, respectively, but was not affected significantly by 25% defoliation. Root biomass was affected by all levels of defoliation. Clipping trifoliate leaves accurately simulated defoliation by H. postica larvae. Our results indicated that partial defoliation affected shoot, root, and nodule biomass of M. sativa, but that partial denodulation only affected shoot biomass.     

Contribution of flexible allocation priorities to herbivory tolerance in C4 perennial grasses: an evaluation with 13C labeling

The ability of plants to rapidly replace photosynthetic tissues following defoliation represents a resistance strategy referred to as herbivory tolerance. Rapid reprioritization of carbon allocation to regrowing shoots at the expense of roots following defoliation is a widely documented tolerance mechanism. An experiment was conducted in a controlled environment to test the hypothesis that herbivory-sensitive perennial grasses display less flexibility in reprioritizing carbon allocation in response to defoliation than do grasses possessing greater herbivory tolerance. An equivalent proportion of shoot biomass (60% dry weight) was removed from two C₄ perennial grasses recognized as herbivory-sensitive, Andropogon gerardii and Schizachyrium scoparium, and two C₄ perennial grasses recognized as herbivory-tolerant, Aristida purpurea and Bouteloua rigidiseta. Both defoliated and undefoliated plants were exposed to ¹³CO₂ for 30 min, five plants per species were harvested at 6, 72 and 168 h following labeling, and biomass was analyzed by isotope ratio mass spectrometry. The tallgrass, A. geraiddii, exhibited inflexible allocation priorities while the shortgrass, B. rigidiseta, exhibited flexible allocation priorities in response to defoliation which corresponded with their initial designations as herbivory-sensitive and herbivory-tolerant species, respectively. A. gerardii had the greatest percentage and concentration of ¹³C within roots and lowest percentage of ¹³C within regrowth of the four species evaluated. In contrast, B. rigidiseta had a greater percentage of ¹³C within regrowth than did A. gerardii, the greatest percentage of ¹³C within new leaves of defoliated plants, and the lowest concentration of ¹³C within roots follwing defoliation. Although both midgrasses, S. scoparium and A. purpurea, demonstrated flexible allocation priorities in response to defoliation, they were counter to those stated in the initial hypothesis. The concentration of ¹³C within new leaves of S. scoparium increased in response to a single defoliation while the percentage and concentration of ¹³C within roots was reduced. A. purpurea was the only species in which the percentate of ¹³C within new leaves decreased while the percentage of ¹³C within roots increased following defoliation. The most plausible alternative hypothesis to explain the inconsistency between the demonstrated responsiveness of allocation priorities to defoliation and the recognized herbivory resistance of S. scoparium and A. purpurea is that the relative ability of these species to avoid herbivory may make an equal or greater contribution to their overall herbivory resistance than does herbivory tolerance. Selective herbivory may contribute to S. scoparium's designation as a herbivorysensitive species even though it possesses flexible allocation priorities in response to defoliation. Alternatively, the recognized herbivory resistance of A. purpurea may be a consequence of infrequent and/or lenient herbivory associated with the expression of avoidance mechanisms, rather than the expression of tolerance mechanisms. A greater understanding of the relative contribution of tolerance and avoidance strategies of herbivory resistance are required to accurately interpret how herbivory influences plant function, competitive interactions, and species abundance in grazed communities.     

The Role of Light and Soil Moisture in Plant Community Resistance to Invasion by Yellow Starthistle (Centaurea solstitialis)

To resist establishment by an invasive plant, a community may require one or more species functionally similar to the invader in their resource acquisition pattern. In this study, communities consisting of native winter annual forbs, non‐native annual grasses, native perennials, or a combination of the two native communities were established with and without Centaurea solstitialis to determine the effect of soil moisture and light availability on plant community invasion resistance. The annual plant communities were unable to resist invasion by C. solstitialis. In the native winter annual forb community, senescence in late spring increased light penetration (>75%) to the soil surface, allowing seeded C. solstitialis to quickly establish and dominate the plots. In addition, native annual forbs utilized only shallow soil moisture, whereas C. solstitialis used shallow and deep soil moisture. In communities containing native perennials, only Elymus glaucus established well and eventually dominated the plots. During the first 2 years of establishment, water use pattern of perennial communities was similar to native annual forbs and resistance to invasion was associated with reduced light availability during the critical stages of C. solstitialis establishment. In later years, however, water use pattern of perennial grass communities was similar or greater than C. solstitialis‐dominated plots. These results show that Central Valley grasslands that include E. glaucus resist C. solstitialis invasion by a combination of light suppression and soil water competition. Spatiotemporal resource utilization patterns, and not just functional similarity, should be considered when developing restoration strategies to resist invasion by many non‐native species.     

Effects of simulated herbivory and resource availability on the invasive plant, Alternanthera philoxeroides in different habitats

In biological control programs, the insect natural enemy’s ability to suppress the plant invader may be affected by abiotic factors, such as resource availability, that can influence plant growth and reproduction. Understanding plant tolerance to herbivory under different environmental conditions will help to improve biocontrol efficacy. The invasive alligator weed (Alternanthera philoxeroides) has been successfully controlled by natural enemies in many aquatic habitats but not in terrestrial environments worldwide. This study examined the effects of different levels of simulated leaf herbivory on the growth of alligator weed at two levels of fertilization and three levels of soil moisture (aquatic, semi-aquatic, and terrestrial habitats). Increasing levels of simulated (manual) defoliation generally caused decreases in total biomass in all habitats. However, the plant appeared to respond differently to high levels of herbivory in the three habitats. Terrestrial plants showed the highest below–above ground mass ratio (R/S), indicating the plant is more tolerant to herbivory in terrestrial habitats than in aquatic habitats. The unfertilized treatment exhibited greater tolerance than the fertilized treatment in the terrestrial habitat at the first stage of this experiment (day 15), but fertilizer appears not to have influenced tolerance at the middle and last stages of the experiment. No such difference was found in semi-aquatic and aquatic habitats. These findings suggest that plant tolerance is affected by habitats and soil nutrients and this relationship could influence the biological control outcome. Plant compensatory response to herbivory under different environmental conditions should, therefore, be carefully considered when planning to use biological control in management programs against invasive plants.     

Effects of fire and defoliation on the life history of native and invader C4 grasses in a Neotropical savanna

African grasses, introduced into Neotropical savannas to improve forage quality, have spread successfully and displaced native plants. To understand their competitive relationships, we compared biomass production and allocation, plant architecture and phenology, net photosynthesis (Pn), water relations, and nutrient content under fire and simulated herbivory between two C₄ grasses, the native Trachypogon plumosus and the introduced Hyparrhenia rufa from a seasonal savanna in Venezuela. All variables were strongly influenced by the rainfall regime. Hyparrhenia produced bigger plants (in mass and size) with a large proportion of mass (>75%) allocated to leaves and culms. Its biomass production was more affected by fire than by defoliation. In contrast, Trachypogon was more affected by defoliation than by fire which promoted a flush of leaf growth even in the dry season. Fire caused up to 85% mortality in Hyparrhenia but none in Trachypogon where it increased inflorescence production. However, fire promoted abundant seed germination and fast seedling growth in Hyparrhenia, enabling it to colonize new areas. During the growing season Trachypogon had higher Pn and lower leaf water potential (Ψ) than Hyparrhenia but differences among treatments were not significant for either grass. Pn of Trachypogon ceased at a lower Ψ (−3.0 MPa) than in Hyparrhenia (−2.0 MPa), indicating its higher tolerance to water stress. During the dry season, Trachypogon leaves remained alive and retained low Pn. Leaf nutrient content was higher during the rainy season in both species. Differences in Pn could not explain the higher seasonal biomass production of Hyparrhenia. However, its water stress evasion strategy, larger biomass allocated to leaves, abundant germination and fast seedling growth appeared to be responsible for the success of Hyparrhenia as an invader of Neotropical savannas. Received: 17 August 1998 / Accepted: 3 March 1999     

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.     

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.     

Effect of herbivory on the growth and photosynthesis of replanted Calligonum caput-medusae saplings in an infertile arid desert

Replanted Calligonum caput-medusae saplings in the Tarim River watershed face short-term and frequent herbivory by goats, which can result in either growth inhibition or stimulation. The effects of herbivory on shrub saplings are unclear. We simulated herbivory with clipping to test two hypotheses. We hypothesized that (1) moderate herbivory may positively affect replanted shrub saplings due to overcompensatory growth and compensatory photosynthesis and that (2) high amounts of defoliation may change water availability and impair photosynthesis and growth of saplings. We applied four defoliation treatments (0, 30, 50, and 70 %) to 2-year-old C. caput-medusae saplings to test the effects of herbivory. Moderately defoliated (~30 %) saplings grew faster and had higher photosynthetic performance than controls; however, defoliation of 50 % or more reduced growth due to undercompensatory photosynthesis and reduced water availability. Non-photochemical quenching by thermal dissipation provided photoprotection when absorbed light energy used in PSII photochemistry was inhibited, reducing excess excitation energy and allowing saplings with high amounts of defoliation to maintain adequate photosystem functioning. This suggests that moderate herbivory of replanted shrubs used as forage in arid ecological restoration projects is feasible, but that uncontrolled grazing should be forbidden.     

The effect of simulated herbivory on growth and nutrient status of focal and neighbouring early successional woody plant species

Defoliation through herbivory is well known to affect target plants and their associated belowground properties, but the response of plants and their soil environment to defoliation of their neighbours is less well understood. We performed a controlled shade‐house experiment involving three plant species that colonize New Zealand floodplains during primary succession, i.e. a palatable N₂‐fixing shrub (Carmichaelia odorata), a palatable deciduous small tree (Fuchsia excorticata) and a less palatable evergreen tree (Weinmannia racemosa). All species were grown in large pots for 40 months both singly and in two species pairs, and either one or both of the species grown in pairs were clipped to simulate herbivory. Responses of growth and foliar nutrient status to clipping varied strongly among species, with Carmichaelia having the largest response and Fuchsia having the smallest. Carmichaelia also enhanced soil microbial biomass and activity, and foliar N concentrations of Weinmannia. However, this did not translate to a net positive effect; instead Carmichaelia competitively reduced growth and foliar P concentrations of both other species. Most effects of Carmichaelia on the soil microflora, and growth and nutrient status of its neighbours, disappeared when Carmichaelia was clipped. Further, the effect of clipping Carmichaelia had a stronger impact on growth, soil activity and nutrient status of the other two species than did the clipping of those species. These results contradict expectations that N₂‐fixing plants should promote growth of other species in pioneer communities or that defoliation of N₂‐fixers exacerbate positive effects; in our study, defoliation of Carmichaelia merely mitigated the negative effects that it had on other species. They also suggest that interplay of competition and differential herbivory among coexisting plants has important implications for soil microflora and processes, relative nutrient acquisition and stoichiometry of coexisting plant species, and potentially plant community development.     

Effects of endomycorrhizal infection,artificial herbivory,and parental cross on growth of Lotus corniculatus L.

Summary We examined how combinations of parentage, fungicide application, and artificial herbivory influence growth and shoot phosphorus content in pre-reproductive Lotus corniculatus, using young offspring arising from three parental crosses, two of which had one parent in common. Soil with vesicular-arbuscular mycorrhizal (VAM) fungi was treated with either water or benomyl, an anti-VAM fungicide, and added to trays containing groups of four full siblings. There were two experiments; in the first no plants were clipped while in the second two of the four plants were clipped to simulate herbivory. In both experiments plants of the two related crosses accumulated more biomass and total shoot P than did plants of the third cross. Plants inoculated with watertreated soil had greater shoot mass and P concentration than did fungicide-treated replicates but the extent of increase in P concentration varied among crosses. In Experiment 2, clipping reduced root mass and resulted in higher shoot P concentration. In this experiment there was a significant interaction of fungicide application and clipping: both unclipped and clipped plants grew better in soil not treated with fungicide, but the increase in shoot mass, total mass, and total P was greater in unclipped plants. Significant interaction of fungicide treatment and clipping is most likely due to reduced availability of carbon to the roots of clipped plants, resulting in poorer symbiotic functioning.     

Pea leaf weevil herbivory on pea seedlings: effects on growth response and yield

Field experiments were conducted to determine growth and yield responses ofPisum sativum L. to defoliation by adultSitona lineatus (L.). Seedlings grown under conventional (moldboard plowed) and conservation (chisel plowed) tillage treatments were infested for a 1-week period with 0, 1 and 8 weevils per plant at two times: at 75% field emergence and 1 week later. After the early infestation, defoliation for the control, low and high weevil densities was about 0,15 and 50%, respectively, while defoliation after the late infestation was about 0, 10 and 35%. An undercompensatory growth response was observed in one experiment after seedlings were subjected to moderate levels of early defoliation. Exact compensation was observed in two experiments after early infestations of low and highSitona densities.Sitona defoliation reduced the number of pods per plant and pod length in two experiments. However, seed biomass was never significantly reduced. Averaged over all experiments, reduction in seed biomass due to highSitona densities was 10 and 5% for early and late infestations, respectively. Tillage treatments did not affectPisum compensatory growth response, although yield components were sometimes greater in conservation tillage than in conventional tillage, possibly due to slightly greater soil moisture in the conservation tillage plots.     

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.     

Relative herbivory tolerance and competitive ability in two dominant : subordinate pairs of perennial grasses in a native grassland

We evaluated herbivory tolerance and competitive ability within twodominant : subordinate pairs of C₄, perennial grasses at each of twosites to determine the contribution of these processes to herbivore-inducedspecies replacement. Herbivory tolerance was assessed by cumulative regrowthfrom defoliated plants of each species and competitive ability was evaluated byrelative uptake of a ¹⁵N isotope placed into the soil between pairedspecies in the field. Herbivory tolerance was similar for the dominant andsubordinate species in both plant pairs and defoliation intensity had a greaterinfluence on herbivory tolerance than did defoliation pattern. Both specieswithin the Sorghastrum nutans : Schizachyriumscoparium pairs exhibited comparable nitrogen acquisition from a¹⁵N enriched pulse with or without defoliation. In contrast,S. scoparium acquired more ¹⁵N than did itssubordinate neighbor, Bothriochloa laguroides when thisspecies pair was undefoliated. Uniform defoliation of this species pair at adefoliation intensity removing 70% of the shoot mass accentuated this responsefurther demonstrating the greater competitive ability of the dominant comparedto the subordinate species. Although the 90% defoliation intensity reducednitrogen acquisition by the dominant relative to the subordinate species,B. laguroides, it did not reduce nitrogen acquisition bythe dominant below that of the subordinate neighbor. The occurrence of similarherbivory tolerance among dominant and subordinate species indicates thatselective herbivory suppressed the greater competitive ability, rather than thegreater herbivory tolerance, of the dominant grasses in this experimentaldesign. These data suggest that interspecific competitive ability may be ofequal or greater importance than herbivory tolerance in mediatingherbivore-induced species replacement in mesic grasslands and savannas.     

Compensation and resistance to herbivory in seagrasses: induced responses to simulated consumption by fish

Herbivory can induce changes in plant traits that may involve both tolerance mechanisms that compensate for biomass loss and resistance traits that reduce herbivore preference. Seagrasses are marine vascular plants that possess many attributes that may favour tolerance and compensatory growth, and they are also defended with mechanisms of resistance such as toughness and secondary metabolites. We quantified phenotypic changes induced by herbivore damage on the temperate seagrass Posidonia oceanica in order to identify specific compensatory and resistance mechanisms in this plant, and to assess any potential trade-offs between these two strategies of defence. We simulated three natural levels of fish herbivory by repeatedly clipping seagrass leaves during the summer period of maximum herbivory. Compensatory responses were determined by measuring shoot-specific growth, photosynthetic rate, and the concentration of nitrogen and carbon resources in leaves and rhizomes. Induced resistance was determined by measuring the concentration of phenolic secondary metabolites and by assessing the long-term effects of continued clipping on herbivore feeding preferences using bioassays. Plants showed a significant ability to compensate for low and moderate losses of leaf biomass by increasing aboveground growth of damaged shoots, but this was not supported by an increase in photosynthetic capacity. Low levels of herbivory induced compensatory growth without any measurable effects on stored resources. In contrast, nitrogen reserves in the rhizomes played a crucial role in the plant’s ability to compensate and survive herbivore damage under moderate and high levels of herbivory, respectively. We found no evidence of inducibility of long-term resistance traits in response to herbivory. The concentration of phenolics decreased with increasing compensatory growth despite all treatments having similar carbon leaf content, suggesting reallocation of these compounds towards primary functions such as cell-wall construction.