Plant structural traits and their role in anti-herbivore defence

We consider the role that key structural traits, such as spinescence, pubescence, sclerophylly and raphides, play in protecting plants from herbivore attack. Despite the likelihood that many of these morphological characteristics may have evolved as responses to other environmental stimuli, we show that each provides an important defence against herbivore attack in both terrestrial and aquatic ecosystems. We conclude that leaf-mass–area is a robust index of sclerophylly as a surrogate for more rigorous mechanical properties used in herbivory studies. We also examine herbivore counter-adaptations to plant structural defence and illustrate how herbivore attack can induce the deployment of intensified defensive measures. Although there have been few studies detailing how plant defences vary with age, we show that allocation to structural defences is related to plant ontogeny. Age-related changes in the deployment of structural defences plus a paucity of appropriate studies are two reasons why relationships with other plant fitness characteristics may be obscured, although we describe studies where trade-offs between structural defence and plant growth, reproduction, and chemical defences have been demonstrated. We also show how resource availability influences the expression of structural defences and demonstrate how poorly our understanding of plant structural defence fits into contemporary plant defence theory. Finally, we suggest how a better understanding of plant structural defence, particularly within the context of plant defence syndromes, would not only improve our understanding of plant defence theory, but enable us to predict how plant morphological responses to climate change might influence interactions at the individual (plant growth trade-offs), species (competition), and ecosystem (pollination and herbivory) levels.     

Trade-off among antiherbivore defences in a south american blackberry (Rubus bogotensis)

Summary The idea of trade-offs among antiherbivore defences in plants is examined using data from a South American blackberry (Rubus bogotensis). Two distinct morphs of R. bogotensis, one with glandular trichomes and one without, were compared with respect to leaf toughness, number of prickles and prickle length. The two morphs were sympatric and grew under similar environmental conditions. The morph lacking trichomes had significantly tougher leaves and also tended to have more and longer prickles. Bioassay showed that Ithomiid larvae fed to a lesser extent on tough leaves than on more tender ones. Correlations between antiherbivore defences within each phenotype revealed three significant or almost significant negative relationships. The comparisons support the hypothesis that trade-offs exist among antiherbivore defences.     

Tight Coupling Between Shoot Level Foliar N and P,Leaf Area,and Shoot Growth in Arctic Dwarf Shrubs Under Simulated Climate Change

Nutrient availability limits productivity of arctic ecosystems, and this constraint means that the amount of nitrogen (N) in plant canopies is an exceptionally strong predictor of vegetation productivity. However, climate change is predicted to increase nutrient availability leading to increases in carbon sequestration and shifts in community structure to more productive species. Despite tight coupling of productivity with canopy nutrients at the vegetation scale, it remains unknown how species/shoot level foliar nutrients couple to growth, or how climate change may influence foliar nutrients–productivity relationships to drive changes in ecosystem carbon gain and community structure. We investigated the influence of climate change on arctic plant growth relationships to shoot level foliar N and phosphorus (P) in three dominant subarctic dwarf shrubs using an 18-year warming and nutrient addition experiment. We found a tight coupling between total leaf N and P per shoot, leaf area and shoot extension. Furthermore, a steeper shoot length-leaf N relationship in deciduous species (Vaccinium myrtillus and Vaccinium uliginosum) under warming manipulations suggests a greater capacity for nitrogen to stimulate growth under warmer conditions in these species. This mechanism may help drive the considerable increases in deciduous shrub cover observed already in some arctic regions. Overall, our work provides the first evidence at the shoot level of tight coupling between foliar N and P, leaf area and growth i.e. consistent across species, and provides mechanistic insight into how interspecific differences in alleviation of nutrient limitation will alter community structure and primary productivity in a warmer Arctic.     

川西林线交错带岷江冷杉幼苗异龄叶形态对长期模拟增温的响应

全球气候变暖对高纬度、高海拔地区的植物形态产生强烈影响。川西林线交错带是青藏高原东部高寒生态系统的重要组成部分,对全球变化极度敏感。以川西林线交错带岷江冷杉(Abies faxoniana)幼苗异龄叶为对象,采用原位开顶式生长室(Open-top chamber,OTCs)模拟增温,研究了长期模拟增温下岷江冷杉幼苗异龄叶片叶长、叶厚等叶形态的响应,采用表型可塑性指数和变异系数对叶形态的可塑性进行分析。结果表明:增温限制岷江冷杉幼苗叶片的增长、增宽和叶面积、体积的扩大,使叶长、叶宽、叶面积、叶体积分别较对照减小12.77%、11.86%、17.76%、11.49%;增温促进叶片厚度的增加,较对照增加7.27%;除叶长宽比外,增温对其余叶形态均产生显著影响(P0.05)。叶形态对模拟增温的响应具有显著的年龄差异(P0.05)。温度、叶龄的交互效应对叶长和叶面积影响显著(P0.05),对叶宽和叶厚影响不显著(P0.05)。两种表型可塑性分析结果表明,除1 a叶叶长外,增温不同程度增大各叶形态可塑性。长期增温使冷杉幼苗叶片有旱生倾向且形态值更发散。研究提供了岷江冷杉幼苗叶片对长期增温的差异性响应证据,为评估青藏高原东缘优势植物对响应气候变暖提供了基础数据和理论参考。     

Evolutionary responses to environmental change: trophic interactions affect adaptation and persistence

According to recent reviews, the question of how trophic interactions may affect evolutionary responses to climate change remains unanswered. In this modelling study, we explore the evolutionary dynamics of thermal and plant–herbivore interaction traits in a warming environment. We find the herbivore usually reduces adaptation speed and persistence time of the plant by reducing biomass. However, if the plant interaction trait and thermal trait are correlated, herbivores can create different coevolutionary attractors. One attractor has a warmer plant thermal optimum, and the other a colder one compared with the environment. A warmer plant thermal strategy is given a head start under warming, the only case where herbivores can increase plant persistence under warming. Persistence time of the plant under warming is maximal at small or large thermal niche width. This study shows that considering trophic interactions is necessary and feasible for understanding how ecosystems respond to climate change.     

CO2‐mediated changes of plant traits and their effects on herbivores are determined by leaf age

1. Concentration of atmospheric CO₂ is predicted to double during the 21st century. However, quantitative effects of increased CO₂ levels on natural herbivore–plant interactions are still little understood. 2. In this study, we assess whether increased CO₂ quantitatively affects multiple defensive and nutritive traits in different leaf stages of cyanogenic wildtype lima bean plants (Phaseolus lunatus), and whether plant responses influence performance and choice behaviour of a natural insect herbivore, the Mexican bean beetle (Epilachna varivestis). 3. We cultivated lima bean plants in climate chambers at ambient, 500, 700, and 1000 ppm CO₂ and analysed cyanogenic precursor concentration (nitrogen‐based defence), total phenolics (carbon‐based defence), leaf mass per area (LMA; physical defence), and soluble proteins (nutritive parameter) of three defined leaf age groups. 4. In young leaves, cyanide concentration was the only parameter that quantitatively decreased in response to CO₂ treatments. In intermediate and mature leaves, cyanide and protein concentrations decreased while total phenolics and LMA increased. 5. Depending on leaf stage, CO₂‐mediated changes in leaf traits significantly affected larval performance and choice behaviour of adult beetles. We observed a complete shift from highest herbivore damage in mature leaves under natural CO₂ to highest damage of young leaves under elevated CO_2. Our study shows that leaf stage is an essential factor when considering CO₂‐mediated changes of plant defences against herbivores. Since in the long run preferred consumption of young leaves can strongly affect plant fitness, variable effects of elevated CO₂ on different leaf stages should receive highlighted attention in future research.     

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.     

Responses in leaf functional traits and resource allocation of a dominant alpine sedge (Kobresia pygmaea) to climate warming in the Qinghai-Tibetan Plateau permafrost region

Assessing the influence of warming on leaf traits, carbon, and nutrient concentrations above and below ground to understand how the dominant sedge Kobresia pygmaea (C. B. Clarke) C. B. Clarke may respond and adapt to extant and future climate in the alpine meadow of the Qinghai-Tibetan Plateau. A warming experiment was conducted in the permafrost region of the Qinghai-Tibetan Plateau from 2008 to 2009. Two 2-year warming treatments (T1, annual warming of 2.1°C; T2, annual warming of 4.4°C) were used, and responses of leaf traits and above- and belowground carbon, nitrogen, and phosphorus concentrations of K. pygmaea were examined. The results show that both moderate (T1) and more extensive (T2) warming decreased leaf mass, leaf thickness, and vascular bundle size, and increased the mass-based photosynthetic rate (A_mass) and photosynthetic nitrogen use efficiency (PNUE). A moderate warming significantly decreased leaf carbon (C), nitrogen (N), and phosphorus (P), and root C and N concentrations of K. pygmaea. These decreases were even more pronounced under the more extensive warming. The decreases in leaf N and P were significantly larger than the decrease in leaf C concentration. Root P concentration increased more under the extensive than the moderate warming. The observed increase in leaf C:N ratio in the warming treatment indicates that enhanced temperature may increase the long-term nitrogen use efficiency of K. pygmaea leaves. This again suggests that K. pygmaea might adapt well to future climate warming, and that nitrogen might be a more important factor for K. pygmaea dominated alpine meadows under future climate warming.     

Effects of light on direct and indirect defences against herbivores of young plants of Mallotus japonicus demonstrate a trade-off between two indirect defence traits

Background and Aims

Although most studies on plant defence strategies have focused on a particular defence trait, some plant species develop multiple defence traits. To clarify the effects of light on the development of multiple defence traits, the production of direct and indirect defence traits of young plants of Mallotus japonicus were examined experimentally under different light conditions.

Methods

The young plants were cultivated under three light conditions in the experimental field for 3 months from May to July. Numbers of ants and pearl bodies on leaves in July were examined. After cultivation, the plants were collected and the developments of trichomes and pellucid dots, and extrafloral nectaries (EFNs) on the leaves were examined. On plants without nectar-collecting insects, the size of EFNs and the volume of extrafloral nectar secreted from the EFNs were examined.

Key results

Densities of trichomes and pellucid dots did not differ significantly among the plants under the different light conditions, suggesting that the chemical and physical defences function under both high and low light availability. The number of EFNs on the leaves did not differ significantly among the plants under the different light conditions, but there appeared to be a trade-off between the size of EFNs and the number of pearl bodies; the largest EFNs and the smallest number of pearl bodies were found under high light availability. EFN size was significantly correlated with the volume of extrafloral nectar secreted for 24 h. The number of ants on the plants was smaller under low light availability than under high and moderate light availability.

Conclusions

These results suggest that direct defence traits function regardless of light conditions, but light conditions affected the development of indirect defence traits.     

Leaf‐trait plasticity and species vulnerability to climate change in a Mongolian steppe

Climate change is expected to modify plant assemblages in ways that will have major consequences for ecosystem functions. How climate change will affect community composition will depend on how individual species respond, which is likely related to interspecific differences in functional traits. The extraordinary plasticity of some plant traits is typically neglected in assessing how climate change will affect different species. In the Mongolian steppe, we examined whether leaf functional traits under ambient conditions and whether plasticity in these traits under altered climate could explain climate‐induced biomass responses in 12 co‐occurring plant species. We experimentally created three probable climate change scenarios and used a model selection procedure to determine the set of baseline traits or plasticity values that best explained biomass response. Under all climate change scenarios, plasticity for at least one leaf trait correlated with change in species performance, while functional leaf‐trait values in ambient conditions did not. We demonstrate that trait plasticity could play a critical role in vulnerability of species to a rapidly changing environment. Plasticity should be considered when examining how climate change will affect plant performance, species' niche spaces, and ecological processes that depend on plant community composition.     

Potential trajectories of old‐growth Neotropical forest functional composition under climate change

Quantifying relationships between plant functional traits and abiotic gradients is valuable for evaluating potential responses of forest communities to climate change. However, the trajectories of change expected to occur in tropical forest functional characteristics as a function of future climate variation are largely unknown. We modeled community level trait values of Costa Rican rain forests as a function of current and future climate, and quantified potential changes in functional composition. We calculated per‐plot community weighted mean (CWM) trait values for leaf area (LA), specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen (N) and phosphorus (P) content, and wood basic specific gravity (WSG), for tree and palm species in 127 0.25 ha plots. We modeled the response of CWM traits to current temperature and precipitation gradients using generalized additive modeling. We then predicted and mapped CWM traits values under current and future climate, and quantified potential changes under a global warming scenario (RCP8.5, year 2050). We calculated the area within the multi trait functional space occupied by forest plots under both current and future climate, and determined potential changes in functional space occupied by forest plots. Overall, precipitation predicted CWM traits better than temperature. Models indicated increases in CWM SLA, N and P, and a decrease in CWM LDMC under climate change. Lowland forest communities converged on a single direction of change towards more acquisitive CWM trait values, indicating a change in forest functional composition resulting from a changed climate. Functional space occupied by forest plots was reduced by 50% under the future climate. Functional composition changes may have further effects on forests ecosystem services. Assessing functional trait spatial‐gradients can help bridge the gap between species‐based biogeography and biogeochemical approaches to strengthen biodiversity and ecosystem services conservation efforts.     

Effects of warming climate on early-season carbon allocation and height growth of defoliated mountain birches

Tree carbohydrate reserves are usually compromised following insect outbreak, which results in a delay in leaf emergence and a reduction in growth, especially in cold environments. However, in recent times, severe defoliation of subarctic mountain birches (Betula pubescens ssp. czerepanovii) by the winter moth (Operophtera brumata) has not induced such responses. This may be the result of a warming climate stimulating plant primary metabolism. We examined if increasing thermal sum (sum of daily mean temperatures above +5 °C, d.d.) and complete foliage loss affected the concentrations of carbohydrates in sap, juvenile leaves, and fine roots of mountain birches in northern Finland and Norway. The sampling was conducted at the beginning of the growing season, two years after the insect outbreak. We also investigated the morphologic properties of mature leaves and the shoot growth of the trees. Our results showed that the carbohydrate concentrations in leaves and roots (averages 67.8 and 12.5 mg g^?1 DW, respectively) decreased in defoliated trees with increasing thermal sum (>400 d.d.), whereas the response in intact trees was the opposite. The carbohydrates in the sap were unaffected by defoliation or thermal sum accumulation. The leaf area of mature leaves and the height growth of long shoots were greater in trees at warmer sites, irrespective of defoliation. However, defoliation increased the leaf weight per area (SLW: specific leaf weight). We conclude that under warmer growing conditions, low early-season leaf and fine root carbohydrate concentrations of previously defoliated trees cannot be used as indicators of aboveground growth.     

Correlations between leaf toughness and phenolics among species in contrasting environments of Australia and New Caledonia

Background and Aims

Plants are likely to invest in multiple defences, given the variety of sources of biotic and abiotic damage to which they are exposed. However, little is known about syndromes of defence across plant species and how these differ in contrasting environments. Here an investigation is made into the association between carbon-based chemical and mechanical defences, predicting that species that invest heavily in mechanical defence of leaves will invest less in chemical defence.

Methods

A combination of published and unpublished data is used to test whether species with tougher leaves have lower concentrations of phenolics, using 125 species from four regions of Australia and the Pacific island of New Caledonia, in evergreen vegetation ranging from temperate shrubland and woodland to tropical shrubland and rainforest. Foliar toughness was measured as work-to-shear and specific work-to-shear (work-to-shear per unit leaf thickness). Phenolics were measured as ‘total phenolics’ and by protein precipitation (an estimate of tannin activity) per leaf dry mass.

Key Results

Contrary to prediction, phenolic concentrations were not negatively correlated with either measure of leaf toughness when examined across all species, within regions or within any plant community. Instead, measures of toughness (particularly work-to-shear) and phenolics were often positively correlated in shrubland and rainforest (but not dry forest) in New Caledonia, with a similar trend suggested for shrubland in south-western Australia. The common feature of these sites was low concentrations of soil nutrients, with evidence of P limitation.

Conclusions

Positive correlations between toughness and phenolics in vegetation on infertile soils suggest that additive investment in carbon-based mechanical and chemical defences is advantageous and cost-effective in these nutrient-deficient environments where carbohydrate may be in surplus.Key words: Antiherbivore defence, leaf toughness, mechanical defence, chemical defence, phenolics, trade-offs     

Anatomical defences against bark beetles relate to degree of historical exposure between species and are allocated independently of chemical defences within trees

Conifers possess chemical and anatomical defences against tree‐killing bark beetles that feed in their phloem. Resins accumulating at attack sites can delay and entomb beetles while toxins reach lethal levels. Trees with high concentrations of metabolites active against bark beetle‐microbial complexes, and more extensive resin ducts, achieve greater survival. It is unknown if and how conifers integrate chemical and anatomical components of defence or how these capabilities vary with historical exposure. We compared linkages between phloem chemistry and tree ring anatomy of two mountain pine beetle hosts. Lodgepole pine, a mid‐elevation species, has had extensive, continual contact with this herbivore, whereas high‐elevation whitebark pines have historically had intermittent exposure that is increasing with warming climate. Lodgepole pine had more and larger resin ducts. In both species, anatomical defences were positively related to tree growth and nutrients. Within‐tree constitutive and induced concentrations of compounds bioactive against bark beetles and symbionts were largely unrelated to resin duct abundance and size. Fewer anatomical defences in the semi‐naïve compared with the continually exposed host concurs with directional differences in chemical defences. Partially uncoupling chemical and morphological antiherbivore traits may enable trees to confront beetles with more diverse defence permutations that interact to resist attack.     

Springs and wire plants: anachronistic defences against Madagascar's extinct elephant birds

The extinction of large vertebrates in the last few millennia has left a legacy of evolutionary anachronisms. Among these are plant structural defences that persist long after the extinction of the browsers. A peculiar, and controversial, example is a suite of traits common in divaricate (wide-angled branching) plants from New Zealand. Divaricate architecture has been interpreted as an adaptive response to cold climates or an anachronistic defence against the extinct moas. Madagascar, a larger tropical island, also had a fauna of large flightless birds, the elephant birds. If these extinct ratites selected for similar plant defences, we expected to find convergent features between New Zealand and Malagasy plants, despite their very different climates. We searched the southern thickets of Madagascar for plants with putative anti-ratite defences and scored candidate species for a number of traits common to many New Zealand divaricates. We found many Malagasy species in 25 families and 36 genera shared the same suite of traits, the 'wire plant' syndrome, as divaricates in New Zealand that resist ratite browsing. Neither ecologically, nor phylogenetically, matched species from South Africa shared these traits. Malagasy wire plants differ from many New Zealand divaricates in lacking the distinctive concentration of leaves in the interior of shrubs. We suggest that New Zealand divaricates have a unique amalgam of traits that acted as defences and also confer tolerance to cold. We conclude that many woody species in the thickets of southern Madagascar share, with New Zealand, anachronistic structural defences against large extinct bird browsers.     

Ontogenetic changes in anti-herbivore defensive traits in leaves of four Mediterranean co-occurring <Emphasis Type="Italic">Quercus</Emphasis> species

Investment in anti-herbivore defence in tree species has been one of the priority research topics in plant terrestrial ecology during the last decades. However, despite considerable experimental effort, interspecific differences in the ontogenetic trends in the investment in defence are still a matter of debate, as to date experimental evidence is contradictory. In the present work, insect herbivory levels were measured in seedlings and mature trees of four co-occurring Mediterranean Quercus species with differing leaf life spans, as well as several leaf characteristics that can determine herbivore preference. The measured leaf traits included nitrogen (N), fibre (cellulose, hemicellulose and lignin), total phenolic contents, leaf mass per unit area (LMA) and leaf thickness. The leaves of seedlings had a lower LMA and leaf thickness and lower concentrations of N and cellulose, but higher concentrations of lignin and phenols than those of mature trees. However, the loss of leaf area tended to be more severe for seedlings than for mature trees, although the differences were only significant for deciduous species. This constitutes a confirmation of the strong effects of physical traits on herbivore preferences. The greater resource limitations for defensive mechanisms in seedlings with respect to mature trees would explain that at intraspecific level we do observe a compromise between chemical and physical defences. As a result, seedlings rely on chemical rather than on physical defences.     

Plasticity in the expression of direct and indirect defence traits of young plants of <Emphasis Type="Italic">Mallotus japonicus</Emphasis> in relation to soil nutritional conditions

Although soil nutrients can influence the defence strategy of plants that have multiple defence traits, to date, there have been few studies to examine this. To evaluate the effect of soil nutrients on multiple plant defences, we cultivated Mallotus japonicus under three soil nutritional conditions in the field, and experimentally examined the expression of a physical defence trait (trichomes), chemical traits (pellucid dots), and biotic traits (extrafloral nectaries (EFNs) and pearl bodies) of the plants, and the number of ants visiting them. Under the low soil nutritional condition, plants strongly expressed the physical defence by trichomes and chemical defence by pellucid dots, meaning that the young plants adopted direct defences under the poor soil nutritional condition. Under the high soil nutritional condition, in contrast, the plants strongly expressed the indirect defence traits. They produced abundant EFNs and pearl bodies, and attracted many ants. These results suggest that young plants of M. japonicus use different defence modes in response to different soil nutritional conditions.     

Alpine climate alters the relationships between leaf and root morphological traits but not chemical traits

Leaves and fine roots are among the most important and dynamic components of terrestrial ecosystems. To what extent plants synchronize their resource capture strategies above- and belowground remains uncertain. Existing results of trait relationships between leaf and root showed great inconsistency, which may be partly due to the differences in abiotic environmental conditions such as climate and soil. Moreover, there is currently little evidence on whether and how the stringent environments of high-altitude alpine ecosystems alter the coordination between above- and belowground. Here we measured six sets of analogous traits for both leaves and fine roots of 139 species collected from Tibetan alpine grassland and Mongolian temperate grassland. N, P and N:P ratio of leaves and fine roots were positively correlated, independent of biogeographic regions, phylogenetic affiliation or climate. In contrast, leaves and fine roots seem to regulate morphological traits more independently. The specific leaf area (SLA)–specific root length (SRL) correlation shifted from negative at sites under low temperature to positive at warmer sites. The cold climate of alpine regions may impose different constraints on shoots and roots, selecting simultaneously for high SLA leaves for rapid C assimilation during the short growing season, but low SRL roots with high physical robustness to withstand soil freezing. In addition, there might be more community heterogeneity in cold soils, resulting in multidirectional strategies of root in resource acquisition. Thus our results demonstrated that alpine climate alters the relationships between leaf and root morphological but not chemical traits.     

Correlation between the leaf turnover rate and anti-herbivore defence strategy (balance between ant and non-ant defences) amongst ten species of <Emphasis Type="Italic">Macaranga</Emphasis> (Euphorbiaceae)

We measured variation in the intensities of ant and non-ant anti-herbivore defences amongst ten Macaranga species in Sarawak, Malaysia. Intensities of non-ant defences were estimated by measuring effects of fresh leaves (provided as food) of these Macaranga species on survival of common cutworm larvae [Spodoptera litura (Fabricius), Lepidoptera: Noctuidae]. Intensities of ant defences were estimated by measuring ant aggressiveness in the presence of artificial damage inflicted on plants. As part of our examination of non-ant defences, we measured leaf toughness (punch strength, by penetrometry), and the contents of total phenols and condensed tannin. We demonstrated interspecific variation in intensities of both ant and non-ant defences amongst ten Macaranga species and showed that the rank order of ant defence intensity was negatively correlated with the intensity of non-ant defence. We also found that the balance between ant and non-ant defence intensity was correlated with the rates of leaf turnover and shoot growth. Species investing more in ant defence tended to have higher leaf turnover rates. Macaranga species that occur preferentially in shadier microhabitats had lower leaf turnover rates, suggesting that non-ant defences are more cost-effective in more shade-tolerant species. Our results also suggest that the total intensity of non-ant defences is positively correlated with both leaf toughness and total phenol content.     

Age-dependent shift in response to food element composition in Collembola: contrasting effects of dietary nitrogen

Developing leaves that are soft, with high concentrations of resources, can be particularly vulnerable to herbivore damage. Since a developing leaf cannot be very tough, given the constraints of cell expansion, the major form of protection is likely to be chemical defence. We investigated changes in concentration of herbivore resources (protein, carbohydrates and water) and putative defences (total phenolics, tannin activity, cyanogenic glycosides, alkaloids, cell wall, and leaf mechanics) across five leaf development stages of the soft-leaved Toona ciliata M. Roem. and the tough-leaved Nothofagus moorei (F. Muell.) Krasser. Chemical defences were predicted to be more highly developed in young than expanded leaves of both species, and to decline more in expanded leaves of N. moorei, which become tough and strong at maturity, than in the softer expanded leaves of T. ciliata. Resources and defences were dynamic within the developing leaves. Highest concentrations of protein were recorded in young leaves in both species, and highest levels of non-structural carbohydrate were recorded in young leaves of T. ciliata. Allocation to defence varied in both amount and type across leaf stages. In T. ciliata, there was an increase in chemical defence in expanded leaves (tannin activity, alkaloids). However, in N. moorei, increasing strength and toughness of developing leaves coincided with decreasing chemical defence, consistent with our hypothesis. For phenolics, this decrease was partly due to dilution by cell wall, but cyanogenic glycosides were present in young leaves and absent in fully mature leaves. These results are consistent with leaf toughness acting as an effective anti-herbivore defence, thereby reducing the need for investment in chemical defence.