Invasive plants decrease microbial capacity to nitrify and denitrify compared to native California grassland communities

Exotic plant invasions are a major driver of global environmental change that can significantly alter the availability of limiting nutrients such as nitrogen (N). Beginning with European colonization of California, native grasslands were replaced almost entirely by annual exotic grasses, many of which are now so ubiquitous that they are considered part of the regional flora (“naturalized”). A new wave of invasive plants, such as Aegilops triuncialis (Barb goatgrass) and Elymus caput-medusae (Medusahead), continue to spread throughout the state today. To determine whether these new-wave invasive plants alter soil N dynamics, we measured inorganic N pools, nitrification and denitrification potentials, and possible mediating factors such as microbial biomass and soil pH in experimental grasslands comprised of A. triuncialis and E. caput-medusae. We compared these measurements with those from experimental grasslands containing: (1) native annuals and perennials and (2) naturalized exotic annuals. We found that A. triuncialis and E. caput-medusae significantly reduced ion-exchange resin estimates of nitrate (NO₃ ^?) availability as well as nitrification and denitrification potentials compared to native communities. Active microbial biomass was also lower in invaded soils. In contrast, potential measurements of nitrification and denitrification were similar between invaded and naturalized communities. These results suggest that invasion by A. triuncialis and E. caput-medusae may significantly alter the capacity for soil microbial communities to nitrify or denitrify, and by extension alter soil N availability and rates of N transformations during invasion of remnant native-dominated sites.     

Are gas exchange responses to resource limitation and defoliation linked to source:sink relationships?

Productivity of trees can be affected by limitations in resources such as water and nutrients, and herbivory. However, there is little understanding of their interactive effects on carbon uptake and growth. We hypothesized that: (1) in the absence of defoliation, photosynthetic rate and leaf respiration would be governed by limiting resource(s) and their impact on sink limitation; (2) photosynthetic responses to defoliation would be a consequence of changing source:sink relationships and increased availability of limiting resources; and (3) photosynthesis and leaf respiration would be adjusted in response to limiting resources and defoliation so that growth could be maintained. We tested these hypotheses by examining how leaf photosynthetic processes, respiration, carbohydrate concentrations and growth rates of Eucalyptus globulus were influenced by high or low water and nitrogen (N) availability, and/or defoliation. Photosynthesis of saplings grown with low water was primarily sink limited, whereas photosynthetic responses of saplings grown with low N were suggestive of source limitation. Defoliation resulted in source limitation. Net photosynthetic responses to defoliation were linked to the degree of resource availability, with the largest responses measured in treatments where saplings were ultimately source rather than sink limited. There was good evidence of acclimation to stress, enabling higher rates of C uptake than might otherwise have occurred.     

Competition,legacy, and priority and the success of three invasive species

Competitive ability, the ability to generate legacy effects, and the potential to benefit from priority, individually or interactively, are traits that may increase the invasive potential of plants. In this project we examine these three traits in three invasive species (Agropyron cristatum, Bromus tectorum, and Taeniatherum caput-medusae). Specifically in this study, we examine competitive effects of these invasive species, the ability of these invasive species to generate legacy effects (as plant–soil feedback), and the potential of these three species benefit from priority (being sown concurrently, 30 days before, and 30 days after the restoration species) in a greenhouse study using field collected soil. Our results suggest that all three invasive species can benefit from priority and all three have high competitive ability. However, only A. cristatum benefited from legacy effects of plant–soil feedback.     

Plant community diversity and native plant abundance decline with increasing abundance of an exotic annual grass

Exotic plants are generally considered a serious problem in wildlands around the globe. However, some argue that the impacts of exotic plants have been exaggerated and that biodiversity and other important plant community characteristics are commonly improved with invasion. Thus, disagreement exists among ecologists as to the relationship of exotic plants with biodiversity and native plant communities. A better understanding of the relationships between exotic plants and native plant communities is needed to improve funding allocation and legislation regarding exotic plants, and justify and prioritize invasion management. To evaluate these relationships, 65 shrub–bunchgrass plant communities with varying densities of an exotic annual grass, Taeniatherum caput-medusae (L.) Nevski (medusahead), were sampled across 160,000 ha in southeastern Oregon, United States. Environmental factors were generally not correlated with plant community characteristics when exotic annual grass density was included in models. Plant diversity and species richness were negatively correlated with exotic annual grass density. Exotic annual grass density explained 62% of the variation in plant diversity. All native plant functional groups, except annual forbs, exhibited a negative relationship with T. caput-medusae. The results of this study suggest that T. caput-medusae invasions probably have substantial negative impacts on biodiversity and native plant communities. The strength of the relationships between plant community characteristics and T. caput-medusae density suggests that some exotic plants are a major force of change in plant communities and subsequently threaten ecosystem functions and processes. However, experimental studies are needed to fully substantiate that annual grass invasion is the cause of these observed correlations.     

Non-native competitive perennial grass impedes the spread of an invasive annual grass

Invasive plants are degrading wildlands around the globe by displacing native species, reducing biodiversity, and altering ecological functions. The current approach of applying herbicides to invasive plants in wildlands has not been effective at curtailing their expansion and, in certain circumstances, may do more harm than good. Preventing the spread of invasive species has been identified as an important strategy to protect wildlands. However, few prevention strategies have actually been tested. We hypothesized that establishing competitive vegetation next to infestations would increase the biotic resistance of the plant community to invasion and decrease the invasive species propagule pressure beyond the competitive vegetation. To evaluate this, we established twelve competitive vegetation barriers in front of invasive annual grass, Taeniatherum caput-medusae (L.) Nevski, infestations. The non-native perennial grass Agropyron desertorum (Fisch. ex Link) Schult. was seeded into plant communities adjacent to the infestations to create the competitive vegetation barriers. Soil nutrient concentrations and the spread of T. caput-medusae were compared between where A. desertorum was seeded and not seeded (control treatment) 3 years after treatment. Less T. caput-medusae and lower soil ammonium and potassium concentrations in the competitive vegetation barrier than control treatment (P ≤ 0.01) suggest that establishing competitive vegetation increased the biotic resistance of the plant communities to invasion. Taeniatherum caput-medusae cover and density in the plant communities protected by the competitive vegetation barrier (locales across the barriers from the infestations) were ~42- and 47-fold less, respectively, than unprotected plant communities (P < 0.01). This suggests that invasive plant propagule pressure was decreased in the plant communities protected by competitive vegetation barriers. The establishment of competitive vegetation around infestations may be an effective strategy to prevent or at least reduce the spread of invasive plant species.     

Resource allocation in an annual herb: Effects of light,mycorrhizal fungi,and defoliation

Concurrent interactions and the availability of resources (e.g., light) affect the cost/benefit balance during mutualistic and antagonistic interactions, as well as plant resource allocation patterns. Mycorrhizal interactions and herbivory concur in most plants, where mycorrhizae can enhance the uptake of soil nutrients by plants as well as consuming a large fraction of the plant's carbon, and defoliation usually reduces light interception and photosynthesis, thereby causing direct losses to the hosts of mycorrhizal fungi. Both types of interactions affect the carbon budget of their host plants and thus we predict that the relative costs of herbivory and mycorrhizal colonization will increase when photosynthesis is reduced, for instance in light limited environments. We conducted a greenhouse experiment using Datura stramonium to investigate the effects of defoliation and mycorrhizal inoculation on the resource allocation patterns in two different light environments. Defoliated plants overcompensated in terms of leaf mass in both light environments, but total seed mass per fruit was negatively affected by defoliation in both light environments. Mycorrhizal inoculation had a positive effect on vegetative growth and the leaf nitrogen content, but defoliation negates the benefit of mycorrhizal interactions in terms of the leaf nitrogen content. In general, D. stramonium compensated for the relative costs of concurrent mycorrhizal interactions and defoliation; plants that lacked both interactions exhibited the same performance as plants with both types of interactions.     

Combined Effects of Partial Defoliation and Nutrient Availability on Cloned Betula pendula Saplings: II. CHANGES IN NET PHOTOSYNTHESIS AND RELATED BIOCHEMICAL PROPERTIES

The combined effects of partial defoliation and nutrient availabilityon net photosynthesis and related biochemical variables werestudied in cloned Betula pendula Roth saplings. The saplingswere randomly assigned to different nutrient levels (5, 1·5and 0·5 mol N m^–3) in aerated nutrient cultureand to the following defoliation treatments: (1) control (nodamage), (2) damage of the developing main stem leaves (halfof the leaf lamina removed), and (3)removal of the developingmain stem leaves (entire leaf lamina removed). The leaf immediatelybelow the damaged area in the treated plants, and the correspondingleaf in the control plants, were selected for study. Net photosynthesismeasurements and biochemical determinations were made 2, 8 and14 d after assigning the treatments. At intermediate and lownutrient levels the final net photosynthetic capacity was significantlyhigher in the saplings with the topmost leaves removed thanin the undamaged control saplings, indicating that the expressionof compensatory photosynthesis after partial defoliation isnot inhibited by nutrient deficiency. The photosynthetic enhancementwas closely associated with the increased initial activity ofribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). However,the increased activity of Rubisco was not exclusively the resultof a higher amount of Rubisco. The expression of compensatoryphotosynthesis after partial defoliation in our study cannotunequivocally be attributed to an increased flow of nitrogento the remaining leaves. Key words: Partial defoliation, nutrient availability, net photosynthesis, nitrogen, Rubisco     

Aboveground biomass of naturally regenerated and replanted semi-tropical shrublands derived from aerial imagery

Rapid assessment of plant size and population densities is important for estimating biomass over large areas, but it has often been limited by methods requiring intensive labor and resources. In this study, we demonstrate how shrub biomass can be estimated from fine-grained aerial photographs for a large area (23,000 ha) located in the Lower Rio Grande Valley, Texas, USA. Over the past 30 years, refuge land management has included the replanting of native shrubs to promote the restoration of wildlife habitat and carbon sequestration. To assess shrub regrowth, we developed a method to estimate individual shrub canopy areas from digital aerial imagery that was used to calculate biomass from allometric equations. The accuracy of the automated delineation of individual canopies was 79 % when compared to that of hand-digitized shrub canopies. When applied to photographs across the refuge, we found higher shrub densities for older naturally regenerated sites (174 individuals ha^?1) compared to those of younger replanted sites (156 individuals ha^?1). In contrast, naturally regenerated sites had less biomass (3.43 Mg ha^?1) than replanted sites (4.78 Mg ha^?1) indicating that shrubland restored for habitat conservation has the potential to sequester more carbon in a shorter period. There was an inverse relationship between aridity and aboveground shrub biomass for replanted sites in the drier west (p < 0.05). We found a difference in predicted biomass among shrub species in replanted sites that was also associated with climate (p < 0.05). We conclude that the canopy of individual shrubs detected from remote sensing can be used to estimate and monitor vegetation biomass over large areas across environmental gradients.     

Salicylic acid-mediated reductions in yield in Nicotiana attenuata challenged by aphid herbivory

Aphid herbivory decreases primary production in natural ecosystems and reduces crop yields. The mechanism for how aphids reduce yield is poorly understood as some studies suggest aphid feeding directly impedes photosynthesis, whereas other studies suggest a change in allocation of resources from growth to defense compounds reduces yield. To determine the mechanisms underlying reduced plant growth by aphids, Nicotiana attenuata plants, native tobacco, were infested with Myzus persicae ssp. nicotianae, tobacco-adapted green peach aphids, at low and high densities, and plant performance including fitness was assessed. To test the direct defense capacity of salicylic acid (SA) on aphid performance, we fed aphids an artificial diet with varying levels of SA and measured their survivorship and fecundity. There was no detectable effect of aphid herbivory on net photosynthesis, yet herbivory reduced plant growth, final biomass (43 % at high aphid density), and seed set (18 % at high aphid density) at both low and high aphid infestation levels. High-density aphid attack during the rosette and flowering stage caused an increase in SA levels, but caused only a transient decrease in jasmonic acid concentration at low aphid density. SA concentrations similar to those found in infested flowering plants decreased aphid fecundity, suggesting that SA was an effective chemical defense response against aphids. These results suggest that as aphid densities increased the proximal cause of reduced growth and yield was not reduced photosynthesis, but instead resources may have been mobilized for defense via the SA pathway, decreasing the availability of resources for building plant biomass.     

Effects of different herbivores on an actinorhizal species in Northwest Patagonia

Above-ground herbivory has a direct impact on plant life cycles, particularly at more sensitive stages, due to reduction of vegetative biomass. However, this effect may not be negative if it results in net biomass compensation. As sapling stage could be the best stage for native species to be outplanted, understanding the impact of aboveground herbivory on tree saplings is necessary for restoration purposes. We studied the effect of herbivory on saplings of Ochetophila trinervis (Rhamnaceae), a native woody species from North-west Patagonia, which forms an actinorhizal symbiosis with the N₂-fixing actinobacteria Frankia. This tree species has the potential to be used for recovering degraded lands. Nevertheless, there is a perplexing contradiction between the high seed output of O. trinervis and the scarcity of saplings in the field. For 4 months, 1-year-old O. trinervis saplings were exposed to aboveground herbivory by generating different protection degrees (unprotected, protected against some kind of walking herbivores—protected saplings; and protected against all kind of walking herbivores—excluded saplings). The impact of herbivores over sapling survival was minimal (92?±?3%, mean?±?SE) and it was similar among saplings exposed to different protection degrees. The highest frequency of foliar damage in excluded saplings suggests the attack of flying herbivores. The increased emergence of new sprouts and root length growth in saplings highly damaged by herbivores (about three fold and two fold higher than in excluded saplings, respectively), evidenced the capacity of O. trinervis to develop a compensatory growth. The results contradict the assumption that herbivory explains the low density of saplings despite high seed production. Given the high-sapling survival and biomass compensation of O. trinervis after herbivory, we suggest that this species might be appropriate for restoration of degraded areas in the region.     

Temperature and functional traits influence differences in nitrogen uptake capacity between native and invasive grasses

Performance differences between native and exotic invasive plants are often considered static, but invasive grasses may achieve growth advantages in western North America shrublands and steppe under only optimal growing conditions. We examine differences in N uptake and several morphological variables that influence uptake at temperatures between 5 and 25 °C. We contrast two native perennial grasses in western North America: Elymus elymoides and Pseudoroegneria spicata; two invasive annual grasses: Bromus tectorum and Taeniatherum caput-medusae; and one highly selected non-native perennial grass: Agropyron cristatum. The influence of temperature on N uptake is poorly characterized, yet these invasive annual grasses are known to germinate in warm soils in the autumn, and both experience cool soils during the short growing season following snowmelt in the spring. To further explore the influence of temperature on the correlation between morphological variables and N uptake, our data are applied to a previously published path model and one proposed here. Differences in N uptake between native and invasive grasses were small at the lowest temperature, but were large at the highest temperature. At lower temperatures, uptake of N by annuals and perennials was correlated with leaf N and mass. At higher temperatures, uptake by annuals was correlated only with these leaf traits, but uptake by perennials was correlated with these leaf traits as well as root N and mass. Consequently, our results imply that annual grasses face fewer morphological constraints on N uptake than perennial grasses, and annual grasses may gain further advantage in warmer temperature conditions or during more frequent warm periods.     

Recovery after defoliation in <Emphasis Type="Italic">Eucalyptus globulus</Emphasis> saplings: respiration and growth

Key message

Recovery after partial defoliation and/or debudding treatments was found to be more closely related to the release of latent buds rather than temporal changes in leaf-level respiration and carbon uptake.

Abstract

Despite the importance of respiration in the overall carbon balance of plants, recovery after defoliation and debudding has been largely related to changes in carbon uptake; the significance of respiration has received much less attention. Growth, biomass and leaf-level carbon balance (both photosynthesis and dark respiration at night) responses of young Eucalyptus globulus potted-saplings to debudding (B), partial defoliation (D) and combined B&D treatments were assessed over a 12-week recovery period. Light-saturated photosynthetic rates (A ₁₅₀₀) were asynchronous with night respiration rates (R _dark) throughout the course of the experiment; 5 weeks after defoliation, significant increases in A ₁₅₀₀ were accompanied by concomitant increases in R _dark in the B&D and B and D treatments. By week 8, while A ₁₅₀₀ returned to control values, R _dark had increased, particularly in the B&D treatment. Saplings in the B and D treatments showed full recovery with growth, biomass and leaf area being similar to control saplings by week 12. In contrast, saplings in the B&D treatment appeared unable to compensate for the combined removal of all buds and 35 % leaf area as evidenced by slowed height increments and reductions in total biomass of >30 %. Simple modelling of whole-plant net CO₂ uptake showed that saplings in the B&D treatment fixed 20 % less CO₂ than the other treatments at week 12, suggesting that recovery following this treatment and the D treatment was dependent on changes in total leaf area development and whole-tree assimilation rather than differences in assimilation or respiration per unit foliage area. Increased biomass allocation to bud in weeks 5 and 8 suggested that the pattern of refoliation after defoliation and debudding was related to changes in tree architecture from the release of latent buds.

     

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.     

Early physiological responses of Abies alba and Rubus fruticosus to ungulate herbivory

For the last 50 years, Chamois (Rupicapra rupicapra) population exponentially increased in the French Alps. This herbivore faces a food shortage in the winter; its diet mainly includes woody species (Abies alba Mill.) and brambles (Rubus fruticosus). The physiological response following simulated herbivory of these two plants was assessed. More precisely, the effects of leaf clipping and ungulate saliva application on the epidermal flavonoids and chlorophyll contents were considered as an induced structural response. The chlorophyll fluorescence (Fv/Fm) was also measured to provide information on the photosynthesis status as an induced functional response. The non-invasive techniques used in this experiment highlighted an induced response, emerging during the days following the simulated herbivory. R. fruticosus was drastically affected, photosynthesis was disturbed (decreased Fv/Fm) and mortality began on day 5 reaching 100 % less than 1 week later. Physiological parameters were also affected in A. alba (slight decrease of Fv/Fm and lower content of flavonoids), but this species recovered and survived no matter what the stressing treatment was. These results did not point out an efficient repulsive-induced response of A. alba and R. fruticosus to herbivory. The existence of constitutive defences such as prickles (R. fruticosus) or low-digestible tannins (A. alba) is no more efficient to escape from ungulates consumption. Nevertheless, in spite of the increase of the ungulates exponential demography, these two plants do not face rarefaction problem in the French Alps. Thus, survival of A. alba stands is probably linked to the recovery ability of saplings, when R. fruticosus stands maintain themselves thanks to the strong resprouting capacity of this species.     

The effect of age on height growth in even-sized saplings of Fagus sylvatica L.

Key message

After controlling for the effects of size and light, partial regression revealed that height growth of common beech saplings was negatively affected by sapling age.

Abstract

Common beech (Fagus sylvatica L.) saplings were studied along gradients of light availability (4–82 % of full sunlight), initial size (9–290 cm), and age (2–25 years) to examine the interactive effect of these variables on saplings’ annual height growth. Although age was non-significant as a main effect in a linear model, sapling age had a significant interaction with the other variables. After controlling for the effects of size and light, partial regression revealed that height growth was negatively affected by sapling age. Observed growth decline in older common-sized saplings may be explained not as effect of age per se, but as indirect age-related effect probably induced through plastic response of saplings to past growth conditions.     

Experimental defoliation affects male but not female reproductive performance of the tropical monoecious plant Croton suberosus (Euphorbiaceae)

Background and Aims

Monoecious plants have the capacity to allocate resources separately to male and female functions more easily than hermaphrodites. This can be advantageous against environmental stresses such as leaf herbivory. However, studies showing effects of herbivory on male and female functions and on the interaction with the plant''s pollinators are limited, particularly in tropical plants. Here, the effects of experimental defoliation were examined in the monoecious shrub Croton suberosus (Euphorbiaceae), a wasp-pollinated species from a Mexican tropical dry forest.

Methods

Three defoliation treatments were applied: 0 % (control), 25 % (low) or 75 % (high) of plant leaf area removed. Vegetative (production of new leaves) and reproductive (pistillate and staminate flower production, pollen viability, nectar production, fruit set, and seed set) performance variables, and the abundance and activity of floral visitors were examined.

Key Results

Defoliated plants overcompensated for tissue loss by producing more new leaves than control plants. Production of staminate flowers gradually decreased with increasing defoliation and the floral sex ratio (staminate : pistillate flowers) was drastically reduced in high-defoliation plants. In contrast, female reproductive performance (pistillate flower production, fruit set and seed set) and pollinator visitation and abundance were not impacted by defoliation.

Conclusions

The asymmetrical effects of defoliation on male and female traits of C. suberosus may be due to the temporal and spatial flexibility in the allocation of resources deployed by monoecious plants. We posit that this helps to maintain the plant''s pollination success in the face of leaf herbivory stress.     

Long-term effects of defoliation on quaking aspen in relation to genotype and nutrient availability: plant growth,phytochemistry and insect performance

This research tested the long-term effects of defoliation on aspen chemistry and growth in relation to genotype and nutrient availability. We grew saplings of four aspen genotypes in a common garden under two conditions of nutrient availability, and subsequently subjected them to two levels of artificial defoliation. Artificial defoliation suppressed plant growth, and saplings of the four genotypes did not show evidence of genetic variation in tolerance to defoliation. Phenolic glycoside concentrations did not respond to defoliation, but were influenced by genotype and nutrient availability. Condensed tannins responded to defoliation and varied among genotypes. Although defoliation affected condensed tannins, plant quality was not altered in a manner important for gypsy moth performance. Regression analyses suggested that phenolic glycoside concentrations accounted for most of the variation in insect performance. The lack of a strong response important for herbivores was surprising given the severity of the defoliation treatment (nearly 100% of leaf area was removed). In this study, plant genotype was of primary importance, nutrient availability was of secondary importance and long-term induced responses were unimportant as determinants of insect performance.     

Effects of Herbivory and Light Conditions on Induced Defense in Quercus crispula

Quercus crispula saplings, and the relation between defense and tolerance involving photosynthesis and growth. We conducted field experiments with herbivorous insects under two light conditions. As an induced defense, the leaf mass per area increased with herbivory regardless of the amount of light available, and the concentration of condensed tannin was significantly higher when light was plentiful. On the other hand, as induced tolerance, the photosynthetic rate decreased with herbivory under conditions of ample light. In conclusion, we found that both the availability of light and herbivory affected defense and tolerance in Q. crispula. In addition, we suggest that the interaction between defense and tolerance was noticeable when much light was available because of the photosynthetic reduction caused by herbivory. Received 21 February 2001/ Accepted in revised form 13 August 2001     

Effects of sap-feeding insect herbivores on growth and reproduction of woody plants: a meta-analysis of experimental studies

The majority of generalisations concerning plant responses to herbivory are based on studies of natural or simulated defoliation. However, effects caused by insects feeding on plant sap are likely to differ from the effects of folivory. We assessed the general patterns and sources of variation in the effects of sap feeding on growth, photosynthesis, and reproduction of woody plants through a meta-analysis of 272 effect sizes calculated from 52 papers. Sap-feeders significantly reduced growth (−29%), reproduction (−17%), and photosynthesis (−27%); seedlings suffered more than saplings and mature trees. Deciduous and evergreen woody plants did not differ in their abilities to tolerate damage imposed by sap-feeders. Different plant parts, in particular below- and above-ground organs, responded similarly to damage, indicating that sap-feeders did not change the resource allocation in plants. The strongest effects were caused by mesophyll and phloem feeders, and the weakest by xylem feeders. Generalist sap-feeders reduced plant performance to a greater extent than did specialists. Methodology substantially influenced the outcomes of the primary studies; experiments conducted in greenhouses yielded stronger negative effects than field experiments; shorter (<12 months) experiments showed bigger growth reduction in response to sap feeding than longer experiments; natural levels of herbivory caused weaker effects than infestation of experimental plants by sap-feeders. Studies conducted at higher temperatures yielded stronger detrimental effects of sap-feeders on their hosts. We conclude that sap-feeders impose a more severe overall negative impact on plant performance than do defoliators, mostly due to the lower abilities of woody plants to compensate for sap-feeders’ damage in terms of both growth and photosynthesis.     

Estimation of dieback process caused by herbivory in an endangered root-sprouting shrub species,Paliurus ramosissimus (Lour.) Poir., using a shoot-dynamics matrix model

Mortality, a critical parameter for population dynamics, is difficult to measure in long-lived trees or clonal herbaceous species because of the extremely low frequency of deaths. A model based on shoot recruitment would be helpful to estimate the population fate of species without a sufficient number of observed deaths. Existing matrix models are applicable to the dynamics of physiologically independent shoots, but not to physiologically dependent ones. We developed a shoot-dynamics model for plants with physiologically-dependent shoots, and used the model to estimate the effects of herbivory and conservation measures on the dynamics of a long-lived, shoot-sprouting shrub species, Paliurus ramosissimus (Rhamnaceae). Two populations of the endangered shrub have been severely damaged by herbivory by sika deer. The damage was mainly to new sprouting shoots. No deaths of individual plants were observed during an 8-year survey, and we could not estimate mortality. Thus, prediction of population dynamics based on births and deaths of individuals was impossible. Because P. ramosissimus is a shoot-sprouting species, we instead estimated the decline of individuals using a shoot-dynamics model. Using this model, we estimated the time to an 80 % decrease in shoot number per individual in the two populations at 37.8 and 37.2 years. These lengths suggest an immediate need for conservation measures to prevent herbivory even though no death of any individual was observed in the field survey. The estimated recovery times from the herbivory damage were 28.7 and 29.2 years if herbivory of new shoots is completely prevented by conservation measure.