Growth,nutrition and gas exchange of Pinus resinosa following artificial defoliation

Summary In three experiments, red pine (Pinus resinosa Ait.) seedlings and trees were subjected to artificial defoliations of varying intensities and subsequent growth, gas exchange and nutritional responses were monitored. In Experiment 1, 2-year-old seedlings received 0, 1 or 2 50% defoliations during a single growing season and were maintained in 1 of 3 low nutrient supply treatments. In Experiment 2, response of 4-year-old seedlings was monitored in the year following 0, 25, 50 or 75% defoliation, while in Experiment 3, response of 11-year-old trees was measured 1 year after being defoliated by 0, 33 or 66%. Regardless of intensity of defoliation, or plant size, clipped plants made qualitatively similar allocational and metabolic adjustments over time. First, leaf diffusive conductance and rates of net photosynthesis were stimulated, especially by light to intermediate defoliation. However, there was no effect of defoliation on foliar nitrogen concentration, and elevated gas exchange rates apparently resulted from altered root-shoot dynamics. Second, allocation of new biomass was preferentially shifted towards foliage at the expense of roots, gradually restoring (but undershooting or overshooting) the ratio of foliage: roots of control plants. During the period when foliage: root balance was being restored, the stimulation of needle gas exchange rates disappeared. Plants defoliated by 25% overcompensated in terms of whole plant growth (were larger at harvest than controls), due to shifts in allocation and enhanced photosynthesis. Defoliated plants also stored a proportionally greater share of their carbohydrate reserves in roots than did control plants, even 1 year after clipping.     

Defoliation effects on isoprene emission from Populus deltoides

Isoprene emission from plants is one of the principal ways in which plant processes alter atmospheric chemistry. Despite the importance of this process, few long-term controls over basal emission rates have been identified. Stress-induced changes in carbon allocation within the entire plant, such as those produced by defoliation, have not been examined as potential mechanisms that may control isoprene production and emission. Eastern cottonwood (Populus deltoides) saplings were partially defoliated and physiological and growth responses were measured from undamaged and damaged leaves 7 days following damage. Defoliation reduced isoprene emission from undamaged and damaged leaves on partially defoliated plants. Photosynthetic rates and leaf carbon and nitrogen pools were unaffected by damage. Photosynthetic rate and isoprene emission were highly correlated in undamaged leaves on undamaged plants and damaged leaves on partially defoliated plants. There was no correlation between photosynthetic rate and isoprene emission in undamaged leaves on partially defoliated plants. Isoprene emission was also highly correlated with the number of source leaves on the apical shoot in damage treatments. Increased carbon export from source leaves in response to defoliation may have depleted the amount of carbon available for isoprene synthesis, decreasing isoprene emission. These results suggest that while isoprene emission is controlled at the leaf level in undamaged plants, emission from leaves on damaged plants is controlled by whole-branch allocation patterns. Received: 12 May 1998 / Accepted: 9 November 1998     

Environmental influences on growth and defence responses of the invasive shrub,Lonicera maackii,to simulated and real herbivory in the juvenile stage

Background and Aims

Tolerance and defence against herbivory are among the many mechanisms attributed to the success of invasive plants in their novel ranges. Because tolerance and defence against herbivory differ with the ontogeny of a plant, the effects of herbivore damage on plant fitness vary with ontogenetic stage and are compounded throughout a plant''s lifetime. Environmental stresses such as light and nutrient limitations can further influence the response of the plant. Much is known about the response of plants in the seedling and reproductive adult stages, but less attention has been given to the pre-reproductive juvenile stage.

Methods

Juvenile plants of the North American invasive Lonicera maackii were exposed to simulated herbivory under high and low light and nitrogen availability and growth, allocation patterns and foliar defensive chemistry were measured. In a second experiment, complete nutrient availability and damage type (generalist caterpillar or simulated) were manipulated.

Key Results

Juvenile plants receiving 50 % defoliation had lower total biomass and a higher root^:^shoot ratio than controls for all treatment combinations except low nitrogen/low light. Low light and defoliation increased root^:^shoot ratio. Light, fertilization and defoliation had little impact on foliar defensive chemistry. In the second experiment, there was a reduction in total biomass when caterpillar damage was applied. The root^:^shoot ratio increased under low soil fertility and was not affected by defoliation. Stem-diameter growth rates and specific leaf area did not vary by damage type or fertilization. Foliar protein increased through time, and more strongly in defoliated plants than in controls, while peroxidase activity and total flavonoids decreased with time. Overall, resource limitations were more influential than damage in the growth of juvenile L. maackii plants.

Conclusions

The findings illustrate that even when resources are limited, the tolerance and defence against herbivory of a woody invasive plant in the juvenile stage may contribute to the establishment and persistence of some species in a variety of habitats.     

Analytical studies on the responses of sunflower (Helianthus annuus) to various defoliation treatments

The effects of defoliation treatments on plant growth in sunflower (Helianthus annuus) were studied in the field. Four defoliation treatments, 0 (control), 37.4, 56.1 and 93.4% of the total leaf dry weight, were applied to plants that had small third leaves. Decreased leaf weight/whole plant weight (F/W) ratios in defoliated plants rapidly recovered to almost the same ratio as that observed in the control within 12 to 16 days after defoliation according to the degree of defoliation. The mechanism involved in the recovery of the F/W ratio in defoliated plants mainly consisted of three parameters: enhancement of (1) carbon distribution ratios in the leaves, (2) photosynthetic activity in the remaining leaves, and (3) retranslocation of carbon from the stem and/or roots to leaves. Inhibitive effects of defoliation on relative growth rate and net assimilation rate were seen at an early stage, but subsequently both rates became larger in defoliated plants than in controls. Defoliated plants tended to show rapid development and expansion of new leaves, and to show increased specific leaf area and protein synthesis in individual leaves. The sugar content of leaves in defoliated plants was higher than that in controls, while the content in both stem and roots was lower. These responses seem to be advantageous for development of the photosynthetic system. Heights of defoliated plants were clearly depressed according to the degree of defoliation, and this was attributed largely to differences in the elongation rates of the internodes resulting from defoliation.     

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.     

Invasive and native populations of common ragweed exhibit strong tolerance to foliar damage

Tolerance and resistance are defence strategies evolved by plants to cope with damage due to herbivores. The introduction of exotic species to a new biogeographical range may alter the plant–herbivore interactions and induce selection pressures for new plant defence strategies with a modified resource allocation. To detect evolution in tolerance to herbivory in common ragweed, we compared 3 native (North America) and 3 introduced (France) populations, grown in a common garden environment. We explored the effect of leaf herbivory on plant vegetative and reproductive traits. Plants were defoliated by hand, simulating different degrees of insect grazing by removing 0%, 50% or 90% of each leaf blade. Total and shoot dry biomasses were not affected by increasing defoliation, whereas root dry biomass and root:shoot ratio decreased significantly for native and introduced populations. Furthermore, defoliation treatments did not affect any of the plant reproductive traits measured. Hence, common ragweed displayed an efficient reallocation of resources in shoot biomass at the expense of roots following defoliation, which allows the species to tolerate herbivory without obvious costs for fitness. We did not detect any difference in herbivory tolerance between introduced and native populations, but significant differences were found in reproduction with invasive populations producing more seeds than native populations. As a result, tolerance to herbivory has been maintained in the introduced plant populations. We discuss some implications of these preliminary results for biological control strategies dedicated to common ragweed.     

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     

Variation in defence strategies in two species of the genus Beilschmiedia under differing soil nutrient and rainfall conditions

The relationships between various leaf functional traits that are important in plant growth (e.g., specific leaf area) have been investigated in recent studies; however, research in this context on plants that are highly protected by chemical defences, particularly resource-demanding nitrogen-based defence, is lacking. We collected leaves from cyanogenic (N-defended) Beilschmiedia collina B. Hyland and acyanogenic (C-defended) Beilschmiedia tooram (F. M. Bailey) B. Hyland at high- and low-soil nutrient sites in two consecutive years that varied significantly in rainfall. We then measured the relationships between chemical defence and morphological and functional leaf traits under the different environmental conditions. We found that the two species differed significantly in their resource allocation to defence as well as leaf morphology and function. The N defended species had a higher leaf nitrogen concentration, whereas the C-defended species had higher amounts of C-based chemical defences (i.e., total phenolics and condensed tannins). The C-defended species also tended to have higher force to fracture and increased leaf toughness. In B. collina, cyanogenic glycoside concentration was higher with higher rainfall, but not with higher soil nutrients. Total phenolic concentration was higher at the high soil nutrient site in B. tooram, but lower in B. collina; however, with higher rainfall an increase was found in B. tooram, while phenolics decreased in B. collina. Condensed tannin concentration decreased in both species with rainfall and nutrient availability. We conclude that chemical defence is correlated with leaf functional traits and that variation in environmental resources affects this correlation.     

Regrowth dynamics of <Emphasis Type="Italic">Calamagrostis epigejos</Emphasis> after defoliation as affected by nitrogen availability

Young plants of a rhizomatous grass Calamagrostis epigejos (L.) Roth were grown from seed in nutrient solutions containing nitrogen in concentrations 0.1, 1.0, and 10 mM. After six weeks of cultivation the plants were defoliated and changes in growth parameters and in content of storage compounds were measured in the course of regrowth under highly reduced nitrogen availability. Plants grown at higher nitrogen supply before defoliation had higher amount of all types of nitrogen storage compounds (nitrates, free amino acids, soluble proteins), which was beneficial for their regrowth rate, in spite of lower content of storage saccharides. Amino acids and soluble proteins from roots and stubble bases were the most important sources of storage compounds for regrowth of the shoot. Faster growth of plants with higher N content was mediated by greater leaf area expansion and greater number of leaves. In plants with lower contents of N compounds number of green leaves decreased after defoliation significantly and senescing leaves presumably served as N source for other growing organs. Results suggest that internal N reserves can support regrowth of plants after defoliation even under fluctuating external N availability. Faster regrowth of C. epigejos with more reserves was mediated mainly by changes in plant morphogenesis.     

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.     

Nitrogen uptake and root morphological responses of defoliated Lolium perenne (L.) to a heterogeneous nitrogen supply

Lolium perenne L. (c.v. Magella) plants were grown under three nutrient treatments for six weeks and then defoliated to test the hypothesis that for their regrowth they could acquire N equally well irrespective of N distribution. Two different N levels were applied; uniform level 1 N (U1), uniform level 2 (U2) and heterogeneous level 2 (H2). A system where the nutrient patch could be applied without barriers to root growth was adopted. A single defoliation to 4 cm height resulted in a reduction in tillering, biomass increment and N uptake at 3 weeks after defoliation. Root growth was reduced by defoliation under all N treatments. Defoliation was found to reduce the proportion of N in the shoots which was derived from root uptake from 7 to 14 days. At 21 days this effect was significant for the plants with a heterogeneously distributed supply only. By the end of the regrowth period, the undefoliated plants from H2 had a shoot biomass and N content equal to that of plants receiving the same total N but supplied homogeneously (U2). However, defoliation reduced the ability of the plant to acquire N from the patch. No preferential root growth was measured into the N-rich patch, but an increased root diameter within the patch was found. Root diameter was reduced by defoliation, coinciding with a reduction in concentration of N in the root tissue. As a result of the increased sink strength of the growing leaves after defoliation, the roots may become a source of carbon and also nitrogen. These responses to an N-rich patch under defoliation could alter a plant's competitive balance in a mixed sward. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mechanisms and traits associated with compensation for defoliation in <Emphasis Type="Italic">Ruellia nudiflora</Emphasis>

A full understanding of the ecology and evolution of plant tolerance to damage requires the measurement of a diversity of traits (including multiple fitness-correlates) and underlying mechanisms. Here, we address the compensatory response to defoliation in the perennial herb Ruellia nudiflora, measure biomass allocation patterns and relate them to compensation, and address multiple mechanisms and traits that determine compensatory ability. We used maternal full-sib lines of R. nudiflora and conducted a defoliation experiment in which half the plants of each line were subjected to removal of 40% of leaf area (the other half remained undamaged). Fitness-correlated traits, physiological traits, and leaf longevity were measured during a 2-month period after defoliation. Using another set of plants, we conducted a second defoliation experiment and measured the concentration of non-structural carbohydrates to test for root-to-shoot carbon mobilization as a compensatory mechanism. R. nudiflora showed full compensation in terms of fruit output, and compensatory ability was positively correlated with investment in root biomass in the absence of damage. In addition, defoliated plants produced shorter-lived leaves and had a greater concentration of starch in roots, suggesting that reduced leaf longevity and accumulation of below-ground carbon reserves act as compensatory mechanisms. By measuring multiple fitness-correlates and induced traits, we provide a comprehensive evaluation of R. nudiflora compensatory responses to herbivory.     

Effects of defoliation on the saplings of a gap-colonizing neotropical tree

This paper describes a defoliation experiment on saplings of the gap-colonizing pioneer tree Heliocarpus appendiculatus in the tropical rain forest of Los Tuxtlas (Southeast Mexico). Four levels of defoliation (0, 25, 50, and 75% of the leaf area removed) were applied to naturally established plants in a medium-sized forest gap. Records were made of growth (height, diameter, and leaf production) and of whole-plant and leaf survival. Statistical comparisons for all variables showed that plants that had not been defoliated grew better than defoliated ones, but there were no statistically significant differences between the defoliation levels. The risk of death was significantly lower for control plants than for defoliated plants at any level of damage. Leaf survivorship was highly irregular. The most consistent pattern was that the leaves of intact plants always showed higher survivorship, while the most heavily defoliated ones always had the poorest survival. The survivorship pattern of leaves at intermediate defoliation levels was irregular. The results illustrate the lack of a monotonic response to a wide range of defoliation levels, and suggest the potential effect of herbivores as reducers of vegetative growth and survival in pioneer tropical species, and as limiting agents of plant establishment in regenerating forest gaps.     

Regrowth of spring canola (Brassica napus) after defoliation

Aims

Regrowth of dual-purpose canola after grazing is important for commercial success and the aim of this research was to investigate the effects of defoliation on the development, growth, photosynthesis and allocation of carbohydrates.

Methods

We conducted two pot experiments in which defoliation was conducted at multiple intensities with scissors. Experiment 1 determined changes in flowering date due to defoliation while Experiment 2 investigated the effects of defoliation on growth, photosynthesis and allocation of carbohydrates in canola.

Results

Time to the appearance of the first flower was delayed by up to 9 days after the removal of all leaves at the start of stem elongation (GS30), and up to 19 days if the elongating bud was also removed. Stem growth rate decreased by 56–86 % due to defoliation and tap roots did not increase in mass when plants were completely defoliated. Leaf area continued to expand at the same rate as in un-defoliated plants. The new leaf area established per gram of regrowth biomass over 20 days was 158 cm².g^-1 for the complete defoliation treatments compared with 27 cm².g^?1 for the half-defoliated treatment and 13 cm².g^?1 for the un-defoliated treatment. Despite a reduction in total biomass of up to 60 %, the proportion of dry matter partitioned to the leaves was 18 % for all treatments within 20 days after defoliation. Total non-structural carbohydrate levels were reduced rapidly in the stem by day two (predominately sucrose) and the tap root by day four (predominately starch) after defoliation and did not recover to match un-defoliated plant levels within 20 days. Residual leaves on defoliated plants maintained photosynthetic rate compared with the same leaf cohorts on un-defoliated plants in which photosynthetic rate decreased to 39 % by day 12.

Conclusions

The rapid recovery of leaf area in defoliated canola was facilitated by the sustained high photosynthetic rate in remaining leaves, rapid mobilisation of stored sugars (stem) and starch (root), and a cessation of root and stem growth.     

Lack of compensatory growth under phosphorus deficiency in grazing-adapted grasses from the Serengeti Plains

Summary Two shortgrass species (Sporobolus ioclados and Eustachys paspaloides) and two midgrass species (E. paspaloides and Pennisetum mezianum) from the Serengeti grasslands of Tanzania were grown under conditions of extreme phosphorus (P) deficiency. Production of each of these species is maintained or enhanced by defoliation under adequate nutrient supply (McNaughton et al. 1983). However, under the P-deficient conditions of our experiment, defoliation caused a reduction in biomass of all plant parts of each species. Green leaf biomass was reduced most strongly by defoliation, and crowns were least affected. Yield of biomass and nutrients to grazers (green leaves+clipped material) was enhanced by weekly defoliation in the shortgrass grazing-adapted species, whereas yield to producers (live biomass and nutrients retained by the plant) and yield to decomposers (litter) were strongly reduced by defoliation in all species. Phosphate absorption capacity (V _max) measured on excised roots was enhanced by defoliation in the grazing-adapted Sporobolus, but, due to low affinity (high K _m) of roots of defoliated plants for phosphate, absorption rate was not greatly altered at low solution concentrations. Phosphate absorption capacity was reduced or unaffected by defoliation in other species. We conclude that under conditions of P deficiency, plants are unable to acquire the nutrients necessary to replenish large nutrient losses to grazers. In low-nutrient environments, compensatory growth (stimulation of production by grazing) is not a viable strategy. Therefore, in these environments plants respond evolutionarily to herbivores by developing chemical or morphological defenses.     

Increased Defoliation Frequency Depletes Remobilization of Nitrogen for Leaf Growth in Grasses

Plants ofLolium perenneandFestuca rubrawere grown in sand culturereceiving all nutrients as a complete nutrient solution containing1.5 mMNH₄NO₃, and subjected to one of three defoliation treatments:undefoliated, defoliated on one occasion, or defoliated weekly.¹⁵Nlabelling was used to determine the rate of N uptake, allowingthe amount of N remobilized from storage for the growth of thetwo youngest leaves (subsequently referred to as ‘newleaves’) growing over a 14 d period after defoliationto be calculated. The total plant N uptake by both species wasreduced, compared with undefoliated plants, by both a singleand repeated defoliation, although neither caused complete inhibitionof uptake. Regularly defoliatedL. perennehad a greater reductionin root mass, concomitant with a greater increase in N uptakeper g root than did regularly defoliatedF. rubra. In both species,the amount of N derived from uptake recovered in the new leaveswas unaffected by the frequency of defoliation. BothL. perenneandF.rubramobilized nitrogen to the new leaves after a single defoliation,mobilization being sufficient to supply 50 and 41%, respectively,of the total nitrogen requirement. In regularly defoliated plants,no significant nitrogen was mobilized to the new leaves inL.perenne, and only a small amount was mobilized inF. rubra. Plantsachieved greater leaf regrowth when only defoliated once. Weconclude that increasing the frequency of defoliation of bothL.perenneandF. rubrahad little effect on the uptake of nitrogenby roots which was subsequently supplied to new leaves, butdepleted their capacity for nitrogen remobilization, resultingin a reduction in the rate of growth of new leaves. Lolium perenne; Festuca rubra; defoliation frequency; mobilization; root uptake; nitrogen     

Seasonal growth of tree lupins (Lupinus arboreus) and the effect of defoliation on leaf production

The growth of tree lupins was investigated in two experiments. In the first, two ages of plant, 4-wk-old seedlings and 1-year-old plants, were transplanted into a ryegrass sward in an upland environment. Growth, in terms of leaf production, branching and stem elongation, was measured over two successive growing seasons. Plant dry matter and nutrient contents were determined at the beginning and end of each growing season. In the first summer, the rate of production of new leaves on the main stem of seedling plants averaged 1.8 leaves per wk and main stem length increased from 5 to 67 cm. On older plants, where floral apices had been initiated on main and primary stems, there was a 3–10 fold increase in secondary branch length. In the second season, there was no effect of plant age on rates of leaf appearance or stem extension; dry matter production was higher than in the first season. In the second experiment, the effect of removal of 0%, 50% or 100% of fully expanded leaves on the subsequent growth of 23-wk-old plants was investigated. During the 7-wk growth period, defoliation promoted the rate of production of mature leaves, and area and dry weight of new laminae were slightly higher in defoliated plants. Defoliation did not affect the concentrations of N, P or K in the new laminae, but P and K concentrations in petioles of defoliated plants were significantly higher than those in intact plants. The results from the experiments are discussed in relation to the potential use of tree lupins as nurse species and biomass crops in hill and upland environments of the UK.     

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 of the annual herb Abutilon theophrasti: mechanisms underlying reproductive compensation

A number of studies have shown that under some conditions plants may fully or partially compensate for leaf tissue loss; however, the mechanisms underlying compensatory responses are not well understood. Previous work demonstrated that the annual herb Abutilon theophrasti fully compensated for 75% defoliation, but only when grown in the absence of stem competition. We examined potential mechanisms of compensatory response and how they are influenced by resource limitation (i.e., competition for light). Full compensation for these annual plants was defined as equal final reproductive output in defoliated and control plants. In the current study we observed substantial compensation in defoliated plants growing at low density, despite losing 75% of leaf area prior to the onset of flowering. Plant responses associated with compensation included (1) increased reproductive efficiency, which may in turn may have resulted from increased canopy light penetration and transient increases in leaf-level photosynthetic rates; (2) greater allocation to reproduction (RA); (3) changes in biomass allocation from roots to shoots; (4) lower leaf longevity, and (5) increased percent fruit set. Although some of these responses were also observed in defoliated plants grown at high density, the inability of high-density plants to compensate appeared to result from competition for light; these plants delayed reproduction and continued to produce new leaves late in the growing season after low-density, defoliated plants had shifted allocation of resources to reproduction. Received: 20 June 1996 / Accepted: 12 February 1997     

川西北高寒草地3种主要植物的生长及物质分配对温度升高的响应

在野外自然条件下采用开顶式生长室模拟增温的方法, 研究了增温对川西北高寒草地3种主要植物(单子叶草本植物垂穗披碱草(Elymus nutans)和双子叶草本植物尼泊尔酸模(Rumex acetosa)和鹅绒委陵菜(Potentilla anserina))的生长及物质分配的影响。研究结果表明, 增温对3种植物的生长产生了显著影响, 垂穗披碱草和尼泊尔酸模的比叶面积和生物量积累在增温后显著增加, 而鹅绒委陵菜在增温后显著减少。在各组分中, 增温处理使尼泊尔酸模的叶生物量显著增加, 根生物量却显著下降, 而鹅绒委陵菜叶和茎的生物量在增温后显著减少, 根生物量却显著增加。增温对尼泊尔酸模各组分的养分含量产生了显著影响, 其中, 根部碳含量在增温后显著增加, 而氮含量在增温后显著减少。增温对尼泊尔酸模和鹅绒委陵菜的生物量在各组分中的分配产生了显著影响, 增温显著增加了尼泊尔酸模的叶重比(LMR)、根重比(RMR)和地下生物量/地上生物量(R/S), 而茎重比(SMR)在增温后却显著降低; 增温显著增加了鹅绒委陵菜的RMR和R/S, 而SMR和LMR在增温后却显著降低。增温对尼泊尔酸模和鹅绒委陵菜各组分中的碳、氮分配产生了显著影响, 增温显著增加了碳、氮在尼泊尔酸模叶片的分配比例, 并且使尼泊尔酸模根部的碳分配比例也显著增加, 而茎部的碳、氮分配比例却显著减少; 增温显著减少了碳在鹅绒委陵菜叶片的分配比例, 而根部的碳、氮分配比例却显著增加。