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

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

Role of nutrient level and defoliation on symbiotic function: experimental evidence by tracing 14C/15N exchange in mycorrhizal birch seedlings

High nutrient availability and defoliation generally reduce ectomycorrhizal colonization levels in trees, but it is not known how this affects the functional aspects of mycorrhizal symbiosis. It was therefore investigated whether (1) defoliation or increasing substrate N availability reduce C allocation from the plant to the fungus and N allocation from the fungus to the plant (symbiotic resource exchange), (2) symbiotic resource exchange depends on relative N and P availability, and (3) fungal N translocation to plant and plant C allocation to fungus are interdependent. Birch (Betula pendula) seedlings were grown in symbiosis with the ectomycorrhizal fungus Paxillus involutus at five times excess N, or at five times excess N and P for 6 weeks. One-half of the plants were defoliated and the plant shoots were allowed to photosynthesize ¹⁴CO₂ while the fungal compartment was exposed to ¹⁴NH₄. After 3 days, the ¹⁴C of plant origin in fungal tissues and ¹⁵N of fungal origin in plant tissues were quantified. Nutrient availability had no observable effect on symbiotic resource exchange in non-defoliated systems. Defoliation reduced symbiotic N acquisition by plants at all levels of nutrient availability, with the reduction being most marked at higher N availability, indicating an increased tendency in the symbiotic system to discontinue resource exchange after defoliation at higher fertility levels. The concentration of ¹⁴C in extramatrical mycelium correlated significantly with the concentration of ¹⁵N in birch shoots. The results support the assumption that N delivery to the host by the mycorrhizal fungus is dependent on C flow from the plant to the fungus, and that exchanges between the partners are reciprocal. No significant reductions in root ¹⁴C content as a response to defoliation were observed, indicating that defoliation specifically reduced allocation to fungus, but not markedly to roots.     

Growth,morphology and leaf characteristics after simulated herbivory in Chinese subtropical evergreen saplings

Growth, morphology and leaf characteristics were assessed in late spring following simulated autumnal defoliation in second-year saplings of three Chinese subtropical evergreen tree species.Castanopsis fargesii showed strong compensatory growth in terms of plant biomass after removal of both 50 and 75% of leaf biomass and slight compensatory growth after 90% defoliation. DefoliatedC. fargesii saplings had more leaves per unit shoot length than non-defoliated saplings. New leaves on defoliated plants were smaller and had higher per area nitrogen content than new leaves on non-defoliated plants.Pinus massoniana andElaeocarpus japonicus showed strong and no compensatory growth, respectively, after 50% defoliation. The strong compensatory growth inP. massoniana andC. fargesii may partly explain why these species predominate in the early and late successional phases of evergreen broad-leaved forests     

Clonal integration and effects of simulated herbivory in old-field perennials

Summary We compared the growth, phenology and leaf demography of partly defoliated, connected shoots with that of partly defoliated, severed shoots in four old-field perennials (Solidago canadensis, S. altissima, S. gigantea, Aster lanceolatus) with differing genet architectures (rhizome systems), in a common garden and in the field. Our main hypothesis was that defoliation would have fewer negative effects on shoot performance if shoots were connected than if their rhizomes were severed. Since degree of clonal integration is related to differences in genet architecture, our second hypothesis was that the effects of defoliation would be less pronounced in more integrated than in less integrated clones. Removing about 50% of the total leaf area from shoots had different effects depending on plant species, shoot density, and in particular whether rhizome connections between shoots were left intact or severed. In agreement with our prediction, experimentally isolated shoots in the field or in high density clumps in the garden suffered the most from defoliation, while shoots with intact connections or in low density clumps suffered the least. Our second prediction was neither confirmed nor falsified in the present study. Solidago altissima showed overcompensation in response to simulated herbivory in the common garden, i.e. defoliated shoots grew faster and were larger at harvest than their non-defoliated neighbours.     

Rhizosphere interactions, carbon allocation, and nitrogen acquisition of two perennial North American grasses in response to defoliation and elevated atmospheric CO2

Carbon allocation and N acquisition by plants following defoliation may be linked through plant-microbe interactions in the rhizosphere. Plant C allocation patterns and rhizosphere interactions can also be affected by rising atmospheric CO(2) concentrations, which in turn could influence plant and microbial responses to defoliation. We studied two widespread perennial grasses native to rangelands of western North America to test whether (1) defoliation-induced enhancement of rhizodeposition would stimulate rhizosphere N availability and plant N uptake, and (2) defoliation-induced enhancement of rhizodeposition, and associated effects on soil N availability, would increase under elevated CO(2). Both species were grown at ambient (400 μL L(-1)) and elevated (780 μL L(-1)) atmospheric [CO(2)] under water-limiting conditions. Plant, soil and microbial responses were measured 1 and 8 days after a defoliation treatment. Contrary to our hypotheses, we found that defoliation and elevated CO(2) both reduced carbon inputs to the rhizosphere of Bouteloua gracilis (C(4)) and Pascopyrum smithii (C(3)). However, both species also increased N allocation to shoots of defoliated versus non-defoliated plants 8 days after treatment. This response was greatest for P. smithii, and was associated with negative defoliation effects on root biomass and N content and reduced allocation of post-defoliation assimilate to roots. In contrast, B. gracilis increased allocation of post-defoliation assimilate to roots, and did not exhibit defoliation-induced reductions in root biomass or N content. Our findings highlight key differences between these species in how post-defoliation C allocation to roots versus shoots is linked to shoot N yield, but indicate that defoliation-induced enhancement of shoot N concentration and N yield is not mediated by increased C allocation to the rhizosphere.     

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

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

Defoliation increases carbon limitation in ectomycorrhizal symbiosis of <Emphasis Type="Italic">Betula pubescens</Emphasis>

Boreal forest trees are highly dependent on root-colonizing mycorrhizal fungi. Since the maintenance of mycorrhizal symbiosis implies a significant carbon cost for the host plant, the loss of photosynthetic leaf area due to herbivory is expected to reduce the host investment in mycorrhizae. We tested this hypothesis in a common garden experiment by exposing ectomycorrhizal white birch (Betula pubescens Ehrh.) seedlings to simulated insect defoliation of 50 or 100% intensity during either the previous or the current summer or repeatedly during both seasons before harvest. The shoot and root growth of the seedlings were distinctly reduced by both 100% defoliation and repeated 50% defoliation, and they were more strongly affected by previous-year than current-year defoliation. The root to shoot ratio significantly decreased after 100% defoliation, indicating reduced proportional allocation to the roots. Ergosterol concentration (i.e. fungal biomass) in the fine roots decreased by 100% defoliation conducted either in the year of harvest or in both years. No such decrease occurred following the 100% defoliation conducted in the previous year, indicating the importance of current photosynthates for fungal symbionts. The trend was similar in the colonization percentage of thick-mantled mycorrhizae in the roots, the most marked decline occurring in the repeatedly defoliated seedlings. The present results thus support the prediction that the plant investment in ectomycorrhizae may decline as a response to foliage loss. Moreover, the colonization percentage of thick-mantled mycorrhizae correlated positively with the ratio of leaf to heterotrophic plant biomass in the defoliated birch seedlings, but not in the control ones. This tends to indicate a stronger carbon limitation of ectomycorrhizal colonization in defoliated seedlings.     

Extreme defoliation reduces tree growth but not C and N storage in a winter-deciduous species

Background and Aims There is a growing concern about how forests will respond to increased herbivory associated with climate change. Carbon (C) and nitrogen (N) limitation are hypothesized to cause decreasing growth after defoliation, and eventually mortality. This study examines the effects of a natural and massive defoliation by an insect on mature trees’ C and N storage, which have rarely been studied together, particularly in winter-deciduous species.Methods Survival, growth rate, carbon [C, as non-structural carbohydrate (NSC) concentration] and nitrogen (N) storage, defences (tannins and total polyphenols), and re-foliation traits were examined in naturally defoliated and non-defoliated adult trees of the winter-deciduous temperate species Nothofagus pumilio 1 and 2 years after a massive and complete defoliation caused by the caterpillar of Ormiscodes amphimone (Saturniidae) during summer 2009 in Patagonia.Key Results Defoliated trees did not die but grew significantly less than non-defoliated trees for at least 2 years after defoliation. One year after defoliation, defoliated trees had similar NSC and N concentrations in woody tissues, higher polyphenol concentrations and lower re-foliation than non-defoliated trees. In the second year, however, NSC concentrations in branches were significantly higher in defoliated trees while differences in polyphenols and re-foliation disappeared and decreased, respectively.Conclusions The significant reduction in growth following defoliation was not caused by insufficient C or N availability, as frequently assumed; instead, it was probably due to growth limitations due to factors other than C or N, or to preventative C allocation to storage. This study shows an integrative approach to evaluating plant growth limitations in response to disturbance, by examining major resources other than C (e.g. N), and other C sinks besides storage and growth (e.g. defences and re-foliation).     

Functional compatibility in cucumber mycorrhizas in terms of plant growth performance and foliar nutrient composition

Functional compatibility in cucumber mycorrhizas in terms of plant and fungal growth, and foliar nutrient composition from all possible combinations of six cucumber varieties and three species of arbuscular mycorrhizal (AM) fungi was evaluated. Measurements of foliar nutrient composition included N, P, K, Mg, Ca, Na, Fe, Zn, Mn and Cu. Growth of AM fungi was measured in terms of root colonisation, as examined with microscopy and the AM fungus biomarker fatty acid 16:1ω5 from both phospholipids and neutral lipids. Different responses of plant growth and foliar nutrient profiles were observed for the different AM symbioses examined. The AM fungus Claroideoglomus claroideum caused growth depression in association with four out of six cucumber varieties; Rhizophagus irregularis caused growth promotion in one of six cucumber varieties; whereas Funneliformis mosseae had no effect on the growth performance of any of the cucumber varieties examined. All three AM fungi markedly altered host plant shoot nutrient composition, with the strongest contrast observed between cucumber–R. irregularis symbioses and non‐mycorrhizal cucumber plants, independent of cucumber variety. On the other hand, AM fungal growth in roots differed between the three AM fungi, but was unaffected by host genotype. Strong build‐up of storage lipids was observed for R. irregularis, which was more moderate in the two other AM fungi. In conclusion, strong differential responses of cucumber varieties to inoculation with different AM fungi in terms of growth and shoot nutrient composition revealed high functional diversity in AM symbioses in cucumber plants.     

Linking above-ground and below-ground effects in autotrophic microcosms: effects of shading and defoliation on plant and soil properties

Although factors affecting plant growth and plant carbon/nutrient balance – e.g., light availability and defoliation by herbivores – may also propagate changes in below‐ground food webs, few studies have aimed at linking the above‐ground and below‐ground effects. We established a 29‐week laboratory experiment (～one growing season) using autotrophic microcosms to study the effects of light and defoliation on plant growth, plant carbon/nutrient balance, soil inorganic N content, and microbial activity and biomass in soil. Each microcosm contained three substrate layers – mineral soil, humus and plant litter – and one Nothofagus solandri var. cliffortioides seedling. The experiment constituted of the presence or absence of two treatments in a full factorial design: shading (50% decrease in light) and artificial defoliation (approximately 50% decrease in leaf area in the beginning of the growing season). At the end of the experiment a range of above‐ground and below‐ground properties were measured. The shading treatment reduced root and shoot mass, root/shoot ratio and leaf production of the seedlings, while the defoliation treatment significantly decreased leaf mass only. Leaf C and N content were not affected by either treatment. Shading increased NO ₃–N concentration and decreased microbial biomass in humus, while defoliation did not significantly affect inorganic N or microbes in humus. The results show that plant responses to above‐ground treatments have effects which propagate below ground, and that rather straightforward mechanisms may link above‐ground and below‐ground effects. The shading treatment, which reduced overall seedling growth and thus below‐ground N use and C allocation, also led to changes in humus N content and microbial biomass, whereas defoliation, which did not affect overall growth, did not influence these below‐ground properties. The study also shows the carbon/nutrient balance of N. solandri var. cliffortioides seedlings to be highly invariant to both shading and defoliation.     

Mycorrhiza and seedling establishment in a subarctic meadow: Effects of fertilization and defoliation

Abstract. Question: How does changing resource availability induced by fertilization and defoliation affect seedling establishment and mycorrhizal symbiosis in a subarctic meadow? Location: 610 m a.s.l., Kilpisjärvi (69°03’N, 20°50’E), Finland. Methods: A short‐term full‐factorial experiment was established, with fertilization and defoliation of natural established vegetation as treatments. Seeds of two perennial herbs Solidago virgaurea and Gnaphalium norvegicum were sown in natural vegetation and their germination and growth followed. At the final harvest we measured the response in terms of arbuscular mycorrhizal (AM) colonization, biomass and nitrogen concentration of the seedlings and the established vegetation. Results: Germination rate was negatively affected by defoliation in the unfertilized plots. The shoot biomass of S. virgaurea seedlings was reduced by the defoliation and fertilization treatments, but not affected by their interaction. In G. norvegicum, the germination rate and the seedling shoot biomass were negatively correlated with moss biomass in the plots. In the established plants the arbuscular colonization rate was low and defoliation and fertilization treatments either increased or did not affect the colonization by AM fungi. In the seedlings, the colonization rate by AM fungi was high, but it was not affected by treatments. Both seedlings and established plants were colonized by dark septate fungi. Conclusions: Reduction of plant biomass by herbivores can have different effects on seedling growth in areas of high and low soil nutrient availability. The weak response of AM colonization to defoliation and fertilization suggests that AM symbiosis is not affected by altering plant resource availability under the conditions employed in this study.     

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

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

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.     

Comparative responses of the Savanna grasses Cenchrus ciliaris and Themeda triandra to defoliation

Summary Two perennial tussock grasses of savannas were compared in a glasshouse study to determine why they differed in their ability to withstand frequent, heavy grazing; Cenchrus ciliaris is tolerant and Themeda triandra is intolerant of heavy grazing. Frequent defoliation at weekly intervals for six weeks reduced shoot biomass production over a subsequent 42 day regrowth period compared with previously undefoliated plants (infrequent) in T. triandra, but not in C. ciliaris. Leaf area of T. triandra expanded rapidly following defoliation but high initial relative growth rates of shoots were not sustained after 14 days of regrowth because of reducing light utilising efficiency of leaves. Frequently defoliated plants were slower in rate of leaf area expansion and this was associated with reduced photosynthetic capacity of newly formed leaves, lower allocation of photosynthate to leaves but not lower tiller numbers. T. triandra appears well adapted to a regime where defoliation is sufficiently infrequent to allow carbon to be fixed to replace that used in initial leaf area expansion. In contrast, C. ciliaris is better adapted to frequent defoliation than is T. triandra, because horizontally orientated nodal tillers are produced below the defoliation level. This morphological adaptation resulted in a 10-fold higher leaf area remaining after defoliation compared with similarly defoliated T. triandra, which together with the maintenance of moderate levels of light utilising efficiency, contributed to the higher leaf area and shoot weight throughout the regrowth period.     

Effects of defoliation intensity on soil food-web properties in an experimental grassland community

We established a greenhouse experiment based on replicated mini‐ecosystems to evaluate the effects of defoliation intensity on soil food‐web properties in grasslands. Plant communities, composed of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and plantain (Plantago lanceolata) with well‐established root and shoot systems, were subjected to five defoliation intensity treatments: no trimming (defoliation intensity 0, or DI 0), and trimming of all plant material to 35 cm (DI 1), 25 cm (DI 2), 15 cm (DI 3) and 10 cm (DI 4) above soil surface every second week for 14 weeks. Intensification of defoliation reduced shoot production and standing shoot and root mass of plant communities but increased their root to shoot ratio. Soil microbial activity and biomass decreased with intensification of defoliation. Concentrations of NO₃–N in soil steadily increased with intensifying defoliation, whereas NH₄–N concentrations did not vary between treatments. Numbers of microbi‐detritivorous enchytraeids, bacterial‐feeding rotifers and bacterial‐feeding nematodes steadily increased with intensifying defoliation, while the abundance of fungal‐feeding nematodes was significantly enhanced only in DI 3 and DI 4 relative to DI 0. The abundance of herbivorous nematodes per unit soil mass was lower in DI 3 and DI 4 than in DI 0, DI 1 and DI 2, but when calculated per unit root mass, their abundance tended to increase with defoliation intensity. The abundance of omnivorous and predatory nematodes appeared to be highest in the most intensely defoliated systems. The ratio of abundance of fungal‐feeding nematodes to that of bacterial‐feeding nematodes was not significantly affected by defoliation intensity. The results infer that defoliation intensity may significantly alter the structure of soil food webs in grasslands, and that defoliation per se is able to induce patterns observed in grazing studies in the field. The results did not support hypotheses that defoliation per se would cause a shift between the bacterial‐based and fungal‐based energy channels in the decomposer food web, or that herbivore and detritivore densities in soil would be highest under intermediate defoliation. Furthermore, our data for microbes and microbial feeders implies that the effects of defoliation intensity on soil food‐web structure may depend on the duration of defoliation and are therefore likely to be dynamic rather than constant in nature.     

Influences of Arbuscular Mycorrhizal Fungus Glomus mosseae on Growth and Nutrition of Lentil Irrigated with Arsenic Contaminated Water

Arsenic (As) contamination of irrigation water represents a major constraint to Bangladesh agriculture. While arbuscular mycorrhizal (AM) fungi have their most significant effect on P uptake, they have also been shown to alleviate metal toxicity to the host plant. This study examined the effects of As and inoculation with an AM fungus, Glomus mosseae, on lentil (Lens culinaris L. cv. Titore). Plants were grown with and without AM inoculum for 9 weeks in a sand and terra green mixture 50:50 v/v and watered with five levels of As (0, 1, 2, 5, 10 mg As L⁻¹ arsenate). Inoculum of Rhizobium leguminosarum b.v. Viceae strain 3841 was applied to all plants. Plants were fed with modified Hoagland solution (1/10 N of a full-strength solution and without P). Plant height, leaf number, pod number, plant biomass and shoot and root P concentration/offtake increased significantly due to mycorrhizal infection. Plant height, leaf/ pod number, plant biomass, root length, shoot P concentration/offtake, root P offtake and mycorrhizal infection decreased significantly with increasing As concentration. However, mycorrhizal inoculation reduced As concentration in roots and shoots. This study shows that growing lentil with compatible AM inoculum can minimise As toxicity and increase growth and P uptake.     

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.     

Relative importance of an arbuscular mycorrhizal fungus (Rhizophagus intraradices) and root hairs in plant drought tolerance

Both arbuscular mycorrhizal (AM) fungi and root hairs play important roles in plant uptake of water and mineral nutrients. To reveal the relative importance of mycorrhiza and root hairs in plant water relations, a bald root barley (brb) mutant and its wild type (wt) were grown with or without inoculation of the AM fungus Rhizophagus intraradices under well-watered or drought conditions, and plant physiological traits relevant to drought stress resistance were recorded. The experimental results indicated that the AM fungus could almost compensate for the absence of root hairs under drought-stressed conditions. Moreover, phosphorus (P) concentration, leaf water potential, photosynthetic rate, transpiration rate, stomatal conductance, and water use efficiency were significantly increased by R. intraradices but not by root hairs, except for shoot P concentration and photosynthetic rate under the drought condition. Root hairs even significantly decreased root P concentration under drought stresses. These results confirm that AM fungi can enhance plant drought tolerance by improvement of P uptake and plant water relations, which subsequently promote plant photosynthetic performance and growth, while root hairs presumably contribute to the improvement of plant growth and photosynthetic capacity through an increase in shoot P concentration.     

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

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

Shifts in plant nutrient use strategies under secondary forest succession

In evergreen broad-leaved forests (EBLFs) in Tiantong National Forest Park, Eastern China, we studied the soil chemistry and plant leaf nutrient concentration along a chronosequence of secondary forest succession. Soil total N, P and leaf N, P concentration of the most abundant plant species increased with forest succession. We further examined leaf lifespan, leaf nutrient characteristics and root–shoot attributes of Pinus massoniana Lamb, the early-successional species, Schima superba Gardn. et Champ, the mid-successional species, and Castanopsis fargesii Franch, the late-successional species. These species showed both intraspecific and interspecific variability along succession. Leaf N concentration of the three dominant species increased while N resorption tended to decrease with succession; leaf P and P resorption didn’t show a consistent trend along forest succession. Compared with the other two species, C. fargesii had the shortest leaf lifespan, largest decay rate and the highest taproot diameter to shoot base diameter ratio while P. massoniana had the highest root–shoot biomass ratio and taproot length to shoot height ratio. Overall, P. massoniana used ‘conservative consumption’ nutrient use strategy in the infertile soil conditions while C. fargesii took up nutrients in the way of ‘resource spending’ when nutrient supply increased. The attributes of S. superba were intermediate between the other two species, which may contribute to its coexistence with other species in a wide range of soil conditions.