Leaf production of an overstorey rainforest tree and its effects on the temporal distribution of associated insect herbivores

Summary The host phenology and temporal distribution of insect herbivores associated with a rainforest canopy tree, Argyrodendron actinophyllum (Sterculiaceae), were monitored for 3 and 2 years respectively in an Australian subtropical rain forest near Brisbane. Leaf production of this evergreen species was synchronous among individuals, occurring from October to February, but differed markedly between years for some fast-growing trees. Most herbivore species fed selectively upon young leaves and, consequently, occurred only during windows of young foliage availability, with the exception of some mesophyll-feeders that preferred mature foliage. Fast-growing trees sustained high herbivore activity during periods of intense leaf production but some slow-growing trees, whose availability of young foliage appeared to be more constant and predictable, also supported high herbivore activity in the long term. Host suitability of particular trees during study years was dependent upon the intensity of leaf production and other unidentified factors, and was different for generalist and specialist herbiovores. Certain specialist species, such as psyllids, were able to overcome the changing availability of young foliage between years on vigorous A. actinophyllum trees by feeding upon both vegetative and reproductive meristems. Most of the data suggest that availability of young foliage is an important factor in this insect-plant system.     

Influence of leaf traits on the spatial distribution of arboreal arthropods within an overstorey rainforest tree

Abstract.

• 1 Several attributes of foliage were measured from the Australian rainforest tree Argyrodendron actinophyllum Edlin (Sterculiaceae). These were related to estimates of abundance per leaf area of the most common arthropod guilds and families sampled with restricted canopy fogging.
• 2 When all these arthropod groups were considered, much of the overall variance in arthropod spatial distribution could be attributed to leaf age characteristics, arthropod aggregation patterns, arthropod activity and distance to tree trunk.
• 3 The fraction of variance which could be specifically explained by foliage attributes such as nitrogen-, water- and fibre-content, specific leaf weight, and epiphyll load was small for most arthropod groups (usually <30%). However, an index of food quality explained a higher proportion of variance (50%) in the abundance of phloem-feeders. Leaf size and foliage compactness did not influence significantly the abundance of any arthropod group.
• 4 Most herbivores were more abundant on young foliage than on mature leaves. With the exception of Corylophidae and Chrysomelidae, which were more abundant in the lower and upper canopy respectively, arthropod stratification was not conspicuous within the inner core of tree crowns.
• 5 The results firstly emphasize the distribution of young foliage as a key factor affecting the abundance of many herbivores and, secondly, the importance of the local illumination regime for host leaf production and its indirect effects on the spatial distribution of arboreal arthropods.

     

Comparison of Herbivores and Herbivory in the Canopy and Understory for Two Tropical Tree Species1

The Janzen–Connell model of tropical forest tree diversity predicts that seedlings and young trees growing close to conspecific adults should experience higher levels of damage and mortality from herbivorous insects, with the adult trees acting as either an attractant or source of the herbivores. Previous research in a seasonal forest showed that this pattern of distance‐dependent herbivory occurred in the early wet season during the peak of new leaf production. I hypothesized that distance‐dependent herbivory may occur at this time because the new foliage in the canopy attracts high numbers of herbivores that are limited to feeding on young leaves. As a consequence, seedlings and saplings growing close to these adults are more likely to be discovered and damaged by these herbivores. In the late wet season, when there is little leaf production in the canopy, leaf damage is spread more evenly throughout the forest and distance dependence disappears. I tested three predictions based on this hypothesis: (1) the same species of insect herbivores attack young and adult trees of a given plant species; (2) herbivore densities increase on adult trees during leaf production; and (3) herbivore densities in the understory rise during the course of the wet season. Censuses were conducted on adults and saplings of two tree species, raribea asterolepis and Alseis blackiana. Adults and saplings of both species had largely the same suite of chewing herbivore species. On adults of Q. asterolepis, the density of chewing herbivores increased 6–10 times during leaf production, but there was no increase in herbivore density on adults of A. blackiana. Herbivore densities increased 4.5 times on A. blackiana saplings and 8.9 times on Q. asterolepis saplings during the wet season, but there were no clear trends on the adults of either species. These results suggest that the potential of adult trees as a source of herbivores on saplings depends on the value of new leaves to a tree species' herbivores, which may differ across tree species.     

Foliage phenols and nitrogen in relation to growth,insect damage,and ability to recover after defoliation,in the mountain birch Betula pubescens ssp tortuosa

We studied growth of the mountain birch, and the role of foliage phenols, nitrogen, and variance in the timing of bud burst, as potential defensive characters, in Finnish Lapland in 1975–1979. Annual and local variation both in phenol and nitrogen concentration of foliage were significant. Individual trees retained their position in the foliage and nitrogen distribution of the population in successive years, as well as in the order of leaf flush in spring. Growth of twigs, mature leaf size, and ability of trees to recover in the year following artificial defoliation correlated positively with the sum of degree days in the previous growing season. Foliage nitrogen correlated negatively with foliage phenols in within-site comparisons. Twig growth correlated negatively with foliage phenols, particularly in growing seasons following cool summers, but did not correlate with foliage nitrogen. Birches flushing early did not grow more than birches flushing late. Between-site differences in foliage phenol content were mainly determined by abiotic conditions, like temperature and nutrient availability. In a between-site comparison insect chewing marks in leaves correlated positively with foliage phenols as well as with nitrogen; intensity of invertebrate predation presumably explained variable herbivory between the sites. In a within-site comparison trees with the highest foliage phenol content had few herbivores only at the site with the highest average phenol level.     

Communities of insect herbivores foraging on saplings versus mature trees of Pourouma bicolor (Cecropiaceae) in Panama

The arthropod fauna of 25 saplings and of three conspecific mature trees of Pourouma bicolor (Cecropiaceae) was surveyed for 12 months in a tropical wet forest in Panama, with particular reference to insect herbivores. A construction crane erected at the study site provided access to tree foliage in the upper canopy. A similar area of foliage (ca. 370 m²) was surveyed from both saplings and trees, but samples obtained from the latter included 3 times as much young foliage as from the former. Arthropods, including herbivores and leaf-chewing insects with a proven ability to feed on the foliage of P. bicolor were 1.6, 2.5 and 2.9 times as abundant on the foliage of trees as on that of saplings. The species richness of herbivores and proven chewers were 1.5 (n=145 species) and 3.5 (n=21) times higher on trees than on saplings, respectively. Many herbivore species preferred or were restricted to one or other of the host stages. Host stage and young foliage area in the samples explained 52% of the explained variance in the spatial distribution of herbivore species. Pseudo-replication in the two sampling universes, the saplings and trees studied, most likely decreased the magnitude of differences apparent between host stages in this forest. The higher availability of food resources, such as young foliage, in the canopy than in the understorey, perhaps combined with other factors such as resource quality and enemy-free space, may generate complex gradients of abundance and species richness of insect herbivores in wet closed tropical forests.     

Facilitative interactions between two lepidopteran herbivores of Asimina

Summary Insect herbivores that require young foliage for successful larval development are often restricted to a single generation during a year by the scarcity of suitable food over most of the growing season. The major specialist herbivore attacking shrubs in the genus Asimina in Florida, Eurytides marcellus, requires young foliage for successful larval development. Field manipulations were used to investigate the role of the young foliage produced by Asimina in response to defoliation by the late-season feeder Omphalocera munroei, a second specialist herbivore of Asimina in Florida, in maintaining Eurytides populations during the summer months when young foliage is otherwise scarce. Defoliation by Omphalocera proved to be the major inducer of young growth during the summer because Omphalocera defoliated Asimina shrubs so frequently and severely. When compared to young leaves produced in the absence of damage, the teaves produced by Asimina in response to defoliation were equally as suitable as food for Eurytides larvae and as acceptable as oviposition sites by Eurytides females. The availability of young foliage in an Asimina population was correlated with the size of the associated Eurytides population. The combination of regular, severe defoliation by Omphalocera and lack of a defensive response to damage by Asimina lead to a positive affect of Omphalocera on Eurytides population size, and may be central to other facilitative interactions between herbivores as well.     

Recycling of nitrogen in herbivore feces: plant recovery,herbivore assimilation,soil retention,and leaching losses

Herbivores directly and indirectly affect ecosystem functioning in forests. Feces deposition is a direct effect that supplies ephemeral N pulses to soils. Herbivore-mediated changes in plant N allocation and uptake are indirect effects that can also influence soil N availability. These effects may interact if defoliation influences the ability of plants to recover fecal N, and this may affect subsequent generations of herbivores. We added ¹⁵N-enriched insect feces (frass) to a series of replicated red oak, Quercus rubra, mesocosms that had been damaged experimentally and then followed the frass N over the course of 2 years. In the first season, some frass N was mineralized in the soil and leached in organic form from the mesocosms within 1 week of deposition. Within 1 month, frass N had been acquired by the oaks and enriched the foliage; late-season herbivores assimilated the frass N within the same growing season. In the second season, herbivore damage from the previous year lowered total leaf N contents and ¹⁵N recovered in the foliage. A subsequent cohort of early-season herbivores fed on this foliage consequently derived less of their N from the previous year’s frass, and feral leaf rollers colonized fewer of these saplings. The 0- to 5-cm soil fraction was the largest N sink measured, and 42% of the frass N was recovered in the soil. The results demonstrate that: (1) some frass N can be recycled rapidly into foliage and assimilated by successive cohorts of herbivore within the same season; (2) damage can affect N allocation in the following year’s foliage, influencing N availability to and host selection by herbivores; and (3) leaching losses occur soon after deposition but are buffered by soil pools, which are the largest sinks for frass N.     

Herbivore attack in Casearia nitida influenced by plant ontogenetic variation in foliage quality and plant architecture

Traits influencing plant quality as food and/or shelter for herbivores may change during plant ontogeny, and as a consequence, influence the amount of herbivory that plants receive as they develop. In this study, differences in herbivore density and herbivory were evaluated for two ontogenetic stages of the tropical tree Casearia nitida. To assess plant ontogenetic differences in foliage quality as food for herbivores, nutritional and defensive traits were evaluated in saplings and reproductive trees. Predatory arthropods were quantified and the foraging preferences of a parasitoid wasp of the genus Zacremnops were assessed. In addition, survival rates of lepidopteran herbivores (Geometridae) were evaluated experimentally. Herbivore density was three times higher and herbivory was 66% greater in saplings than in reproductive trees. Accordingly, concentrations of total foliar phenolics were higher in reproductive trees than in saplings, whereas leaf toughness, water and nitrogen concentration did not vary between ontogenetic stages. Survival rates of lepidopteran larvae exposed to natural enemies were equivalent in reproductive trees and saplings. Given the greater herbivore density on saplings, equal survival rates implied a greater foraging effort of predators on reproductive trees. Furthermore, observed foraging of parasitoid wasps was restricted to reproductive trees. I propose that herbivore density, and as a consequence, leaf damage were lower in reproductive trees than in saplings due to both traits influencing food quality, and architectural or unmeasured indirect defensive traits influencing foraging preference of natural enemies of herbivores.     

Insect herbivores and their frass affect Quercus rubra leaf quality and initial stages of subsequent litter decomposition

Defoliation‐induced changes in plant foliage are ubiquitous, though factors mediating induction and the extent of their influence on ecosystem processes such as leaf litter decomposition are poorly understood. Soil nitrogen (N) availability, which can be affected by insect herbivore frass (feces), influences phytochemical induction. We conducted experiments to test the hypotheses that insect frass deposition would (1) reduce phytochemical induction following herbivory and (2) increase the decomposition and nutrient release of the subsequent leaf litter. During the 2002 growing season, 80 Quercus rubra saplings were subjected to a factorial experiment with herbivore and frass manipulations. Leaf samples were collected throughout the growing season to measure the effects of frass deposition on phytochemical induction. In live foliage, herbivore damage increased tannin concentrations early, reduced foliar N concentrations throughout the growing season, and lowered lignin concentrations in the late season. Frass deposition apparently reduced leaf lignin concentrations, but otherwise did not influence leaf chemistry. Following natural senescence, litter samples from the treatment groups were decomposed in replicated litterbags for 18 months at the Coweeta Hydrologic Laboratory, NC. In the dead litter samples, initial tannin concentrations were lower in the herbivore damage group and higher in the frass addition group relative to their respective controls. Tannin and N release rates in the first nine months of decomposition were also affected by both damage and frass. However, decomposition rates did not differ among treatment groups. Thus, nutrient dynamics important for some ecosystem processes may be independent from the physical loss of litter mass. Overall, while lingering effects of damage and even frass deposition can therefore carry over and affect ecosystem processes during decomposition, their effects appear short lived relative to abiotic forces that tend to homogenize the decomposition process.     

Spatio-temporal variation in Lepidopteran larval assemblages associated with oak, Quercus crispula: the importance of leaf quality

Abstract.  1. Lepidoptera larval abundance and diversity in the canopies of oak ( Quercus crispula ) trees and saplings were surveyed in a cool-temperate, deciduous broadleaf forest in northern Japan.
2. In general, newly developed leaves were soft, rich in water and nitrogen, and low in tannin, whereas they became tough, poor in water and nitrogen, and high in tannin as the season proceeded. Leaf quality also varied among forest strata, such variations resulting in seasonal and among-strata differences in the structure of the Lepidoptera larval assemblage.
3. The greater Lepidoptera larval abundance and species richness may related to the higher leaf quality on spring foliage compared with summer foliage. On the other hand, diversity (Shannon's H' ) and evenness (Pielou's J' ) were greater on summer foliage than on spring foliage. Strengthened defences of the host plants against herbivory may cause these differences by filtering the larvae of Lepidoptera species and by constraining the super-dominance of a few species on summer foliage.
4. Canonical Correspondence Analysis (CCA) ordination also revealed a stratified structure of the Lepidoptera larval assemblage in the forest. In both spring and summer, the assemblage composition was more similar between sunlit and shaded canopies than between canopies and saplings. Such assemblage stratification was highly correlated with toughness and tannin content (in spring and summer) or water content (in summer).
5. This study emphasised the importance of spatio-temporal variations in leaf quality, even within the same host plant species, for promoting herbivore diversity in forests.     

Multiplicity of biochemical factors determining quality of growing birch leaves

Due to rapidly changing physical and biochemical characteristics of growing leaves, correlations between traits of foliage biochemistry and the performance indices of flush feeding herbivores may vary considerably following relatively minor changes in experimental conditions. We examined the effects of the seasonal and inter-tree variation of a comprehensive array of biochemical compounds on the success of an early season geometrid, Epirrita autumnata, feeding on maturing foliage of mountain birch, Betula pubescens ssp. czerepanovii. We monitored the concentrations of individual phenolics, sugars, total nitrogen, nitrogen of proteins, and nitrogen of soluble compounds, water and acetone-insoluble residue. Simultaneously we recorded larval consumption, physiological performance, growth, and pupal mass of E. autumnata. We found significant phenological changes in almost all leaf traits measured. In bioassays with half-grown leaves, leaf gallotannin concentrations showed a nonlinear effect: in trees with high foliar gallotannin concentrations (over 10 mg g⁻¹), physiological performance was strongly reduced by high gallotannin concentrations. In trees with lower gallotannin concentrations, on the other hand, larval growth was reduced by soluble proanthocyanidins, not gallotannins. Differences between high and low gallotannin trees largely depended on phenology, i.e., on the age of leaves. However, not all the differences in leaf traits between late (with high gallotannin concentrations at the time of the bioassay) and early flushing trees disappeared with leaf maturation, indicating that there is also phenology-independent variance in the tree population. In the full-grown leaves of all the study trees, low concentrations of water and of nitrogen of proteins (but not nitrogen of soluble compounds) were the main factors reducing pupal masses of E. autumnata, while neither gallotannin nor proanthocyanidins now played a significant role. The observed change in the factors underlying leaf quality (from gallotannins and proanthocyanidins to nitrogen and water) relate to the activity of the shikimate pathway and the formation of cell walls: gallotannins and proanthocyanidins are both produced in the pathway, and these tannins are assumed to contribute – via binding into cell walls – to tough and durable cell walls. Interestingly, low quality of leaves did not automatically translate into low foliar consumption (i.e., benefits to the tree). On the trees with young, high gallotannin leaves, larvae actually increased consumption on low quality foliage. In the group of trees with slightly more developed, low gallotannin leaves, the quality of leaves did not clearly modify amounts consumed. In full-grown leaves, low leaf quality strongly reduced leaf consumption. These results emphasize the strong influence of tree phenology on the relationships between biochemical compounds and the herbivore. Received: 30 November 1998 / Accepted: 1 March 1999     

Effects of different leaf traits on growth rates of insect herbivores on willows

We examined relative effects of traits of leaf quality of ten willow species (Salix: Salicaceae) on growth rates of five species of insect herbivores found in interior Alaska (a willow sawfly, Nematus calais; the tiger swallowtail butterfly, Papilio canadensis; and three species of chrysomelid beetles, Gonioctena occidentalis, Calligrapha verrucosa, and Chrysomela falsa). Leaf traits examined were water content, toughness, total nitrogen contnet, pubescence, and presence or absence of phenolic glycosides. Of ten Salix species, four species contain phenolic glycosides in their leaves. We examined relative effects of water content, toughness, and nitrogen content of the Salix leaves on larval growth rates at three different levels, i.e., on a single host species, between different host species, and between herbivore species. The within-host analyses showed that effects of water content, toughness and/or nitrogen content on herbivore growth rates were generally significant in early-season herbivores but not in late-season herbivores. For each herbivore species, differences in growth rates between hosts were not explained by differences in water content, toughness, or nitrogen content. The between-herbivore analysis showed that the interspecific difference in larval growth rates were related to difference in water and nitrogen content of the hosts. Pubescence of Salix leaves had little effects on herbivore growth rates. Presence of phenolic glycosides had a positive effects on growth rates of a specialist, N. calais, but no effect on the other specialist, Ch. falsa. Presence of phenolic glycosides had, in general, negative effects on growth rates of nonspecialists, G. occidentalis, C. verrucosa, and P. canadensis.     

Stress magnitude matters: different intensities of pulsed water stress produce non‐monotonic resistance responses of host plants to insect herbivores

Abstract 1. Water stress may increase or reduce the suitability of plants for herbivores. The recently proposed ‘pulsed stress hypothesis’ suggests consideration of stress phenology (pulsed vs. continuous stress) to explain these conflicting effects of plant water stress on herbivore performance. 2. This hypothesis was tested for the effect of differing stress intensity on performance and preference of insect herbivores belonging to different feeding guilds, namely leaf‐chewing insects (Spodoptera littoralis caterpillars) and phloem‐feeding insects (Aphis pomi aphids), on apple plants (Malus domestica). The plants were non‐stressed or exposed to a low or high intensity of pulsed water stress. 3. Plant responses to the different stress levels were generally monotonic. Growth, stomatal conductance (g_s), leaf water, and old‐leaf nitrogen concentration decreased, whereas young‐leaf nitrogen concentration and leaf mass per area (LMA) increased with increasing stress intensity. The stable isotope composition of foliar carbon (δ¹³C) responded non‐monotonically to the drought treatments. The δ¹³C values were highest in low‐stress plants, intermediate in high‐stress plants, and lowest in non‐stressed plants. 4. The preference and performance responses of the caterpillars were also non‐monotonic. Non‐stressed plants were intermediately, low‐stress plants least, and high‐stress plants most attractive or suitable. Aphid population growth was highest on non‐stressed plants and lowest on low‐stress plants. 5. The results highlight the importance of water stress intensity for the outcome of interactions between herbivores and drought‐affected plants. They show that pulsed water stress may enhance or reduce insect herbivore performance and plant resistance, depending on stress intensity.     

Seasonal variation in the nutritional value of woody plants along a natural gradient in Eastern Africa

Several hypotheses relate a negative relationship between foliar concentration of phenolic compounds and nitrogen to physiological processes such as leaf development, seasonal variation in allocation priorities, nutrient, light and water related growth limitation, as well as herbivore attack. We sampled four common deciduous woody species of central Tanzania monthly during the growing season to assess changes in this relation and their nutritional value to ruminants. We found a negative relationship between leaf N and phenolic compounds within and among species and sites that weakens during the course of the growing season that was consistent for total phenolics, but not for condensed tannins. Leaf N concentration decreased throughout the season, its withdrawal being positively related with leaf N at first sampling date. Secondary compounds concentration showed no consistent seasonal trend. Concentrations of leaf N and phenolics were correlated with ¹³C discrimination in the two shrub species and with soil P in the two tree species. Digestibility was positively correlated with foliar N and negatively correlated with secondary compounds. We conclude that phenolic compounds may serve as reliable clues for selecting foliage rich in N at site and species level only during the first months of the growing season.     

Ontogenetic and temporal variations in herbivory and defense of Handroanthus spongiosus (Bignoniaceae) in a Brazilian tropical dry forest

We compared the richness and abundance of free-feeding herbivore insects (sap-sucking and leaf-chewing), leaf herbivory damage, leaf toughness and total phenolic content between two ontogenetic stages (juvenile and reproductive) of Handroanthus spongiosus (Rizzini) S. O. Grose (Bignoniaceae) throughout the rainy season in a Brazilian seasonally dry tropical forest. Twenty marked individuals of H. spongiosus were sampled per ontogenetic stage in each period of the rainy season (beginning, middle, and end). Herbivore richness and abundance did not differ between ontogenetic stages, but higher percentage of leaf damage, higher concentration of phenolic compounds, and lower leaf toughness were observed for juvenile individuals. The greatest morphospecies abundance was found at the beginning of the rainy season, but folivory increment was higher at the end, despite the fact that leaf toughness and total phenolic content increased in the same period. No significant relationships between leaf damage and both total phenolic content and leaf toughness were observed. These results suggest that insect richness and abundance do not track changes in foliage quality throughout plant ontogeny, but their decrease along rainy season confirms what was predicted for tropical dry forests. The general trends described in the current study corroborate those described in the literature about herbivores and plant ontogeny. However, the lack of relationship between herbivore damage and the two plant attributes considered here indicates that the analyses of multiple defensive traits (the defense syndrome) must be more enlightening to determine the mechanisms driving temporal and spatial patterns of herbivore attack.     

Clumped distribution of oak leaf miners between and within plants

Leaf miners typically show non-random distributions both between and within plants. We tested the hypothesis that leaf miners on two oak species were clumped on individual host trees and individual branches and addressed whether clumping was influenced by aspects of plant quality and how clumping and/or interactions with other oak herbivores affected leaf-miner survivorship. Null models were used to test whether oak herbivores and different herbivore guilds co-occur at the plant scale. Twenty individual Quercus geminata plants and 20 Quercus laevis plants were followed over the season for the appearance of leaf miners and other herbivores, and foliar nitrogen, tannin concentration, leaf toughness and leaf water content were evaluated monthly for each individual tree. The survivorship of the most common leaf miners was evaluated by following the fate of marked mines in several combinations that involved intra- and inter-specific associations. We observed that all leaf miners studied were clumped at the plant and branch scale, and the abundance of most leaf-miner species was influenced by plant quality traits. Mines that occurred singly on leaves exhibited significantly higher survivorship than double and triple mines and leaves that contained a mine or a leaf gall and a mine and damage by chewers exhibited lowest survivorship. Although leaf miners were clumped at individual host trees, null model analyses indicated that oak herbivores do not co-occur significantly less than expected by chance and there was no evidence for biological mechanisms such as inter-specific competition determining community structure at the plant scale. Thus, despite co-occurrence resulting in reduced survivorship at the leaf scale, such competition was not strong enough to structure separation of these oak herbivore communities.     

Herbaceous production in South India—limiting factors and implications for large herbivores

This study’s goal was to better understand the growth pattern and limitations of the herbaceous production that supports South India’s rich large herbivore grazer assemblage. We conducted a fully factorial nitrogen and water (three levels each) treatment field experiment in the herbivore rich South Indian Western Ghats region to determine the seasonal pattern and the extent to which nitrogen and water availability limit herbaceous production. Graminoid production was found to be nitrogen limited. Despite low rainfall, additional water did not significantly increase overall biomass production nor extend growth in the dry season. Accumulated standing biomass was highest in the late wet season (November) and lowest in the dry season (May). Leaf nitrogen was highest in the early wet season (June) and lowest in the late dry season (March). Grazing had a positive effect on grass production by extending the growing season. Biomass production and graminoid leaf nitrogen concentration levels in the study area were similar to other tropical areas in the world. Also similar to other tropical large herbivore areas, the dry season poses an annual challenge for large herbivores in the study area —particularly the smaller bodied species—to satisfy their nutrient requirements.     

The effects of light acclimation during and after foliage expansion on photosynthesis ofAbies amabilis foliage within the canopy

Variation in the photosynthetic function ofAbies amabilis foliage within a canopy was examined and related to three different processes that affect foliage function: foliage aging, sun-shade acclimation that occurred while foliage was expanding, and reacclimation after expansion was complete. Foliage produced in the sun had higher photosynthesis at light saturation (A _max, mol·m^-2·s^-1), dark respiration (mol·m^-2·s^-1), nitrogen content (g·m^-2), chlorophyll content (g·m^-2), and chlorophylla:b ratio, and a lower chlorophyll to nitrogen ratio (chl:N), than foliage produced in the shade. As sun foliage becomes shaded, it becomes physiologically similar to shade foliage, even though it still retains a sun morphology. Shaded sun foliage exhibited lowerA _max, dark respiration, nitrogen content, and chlorophylla:b ratio, and a higher chl:N ratio than sun foliage of the same age remaining in the open. However, shaded sun foliage had a higher chlorophyll content than sun foliage remaining in the open, even though true shade foliage had a lower chlorophyll content than sun foliage. This anomaly arises because as sun foliage becomes shaded, it retains a higher nitrogen content than shade foliage in a similar light environment, but the two forms have similar chl:N ratios. Within the canopy, most physiological indicators were more strongly correlated with the current light environment than with foliage age or leaf thickness, with the exception of chlorophyll content.A _max decreased significantly with both decreasing current light environment of the foliage and increasing foliage age. The same trend with current light and age was found for the chlorophylla:b ratio. Foliage nitrogen content also decreased with a decrease in current light environment, but no distinct pattern was found with foliage age. Leaf thickness was also important for predicting leaf nitrogen content: thicker leaves had more nitrogen than thinner leaves regardless of light environment or age. The chl:N ratio had a strong negative correlation with the current light environment, and, as with nitrogen content, no distinct pattern was found with foliage age. Chlorophyll content of the foliage was not well correlated with any of the three predictor variables: current light environment, foliage age or leaf thickness. On the other hand, chlorophyll content was positively correlated with the amount of nitrogen in a leaf, and once nitrogen was considered, the current light environment was also highly significant in explaining the variation in chlorophyll content. It has been suggested that the redistribution of nitrogen both within and between leaves is a mechanism for photosynthetic acclimation to the current light environment. Within theseA. amabilis canopies, both leaf nitrogen and the chl:N ratio were strongly correlated with the current light environment, but only weakly with leaf age, supporting the idea that changing light is the driving force for the redistribution of nitrogen both within and between leaves. Thus, our results support previous theories on nitrogen distribution and partitioning. However,A _max was significantly affected by both foliage age and the current light environment, indicating that changes in light alone are not enough to explain changes inA _max with time.     

The effects of ants on herbivory and herbivore numbers on foliage of the mallee eucalypt,Eucalyptus incrassata Labill.*

Although there has been much recent interest in ant-plant mutualisms, few data are available on the effects of foraging ants on herbivore numbers and levels of herbivory on plants that do not offer specific inducements to attract ant visitation. In forestry plantations and tropical crops, ants have erratic but sometimes dramatic effects on the numbers of insect herbivores but, in more natural habitats, their effects on levels of herbivory appear to be largely unknown. In Australia, where ants and Eucalyptus woodlands are ubiquitous and abundant and where considerable debate has occurred regarding levels of herbivory in Eucalyptus forests, very little work has been done to examine the effects of ants on densities of insect herbivores on eucalypts. In this study, ants were experimentally excluded from mature and immature foliage of saplings of the mallee Eucalyptus incrassata in South Australia, and herbivore numbers and levels of leaf herbivory were assessed during the next 6 months. No significant differences in herbivory were found between ant-access and ant-exclusion treatments. In spring and early summer, ants were found in associations with aggregations of eurymelid bugs on young foliage, and the effects of ants on bug densities were experimentally investigated. Bug densities decreased rapidly in ant-exclusion treatments compared with ant-access controls. Ants also quickly removed seeds of E. incrassata from experimental caches. The potential of ants to limit the numbers of insect herbivores on eucalypts seems limited given their tendency to form mutualistic associations with sap-feeding Homopterans and because of a lack of other herbivores that are particularly vulnerable to ant predation.     

The seasonal dynamics of leaf resin,nitrogen, and herbivore damage in Eriodictyon californicum and their parallels in Diplacus aurantiacus

Summary The chaparral shrub Eriodictyon californicum produces a flavonoid leaf resin with a chemically similar composition to that previously reported for the sympatric shrub Diplacus aurantiacus. We determined the phenology, resin content, and nitrogen content of Eriodictyon leaves and the leaf area lost to herbivores. Nitrogen content and resin content were both negatively correlated with leaf age at each sampling date, but nitrogen decreased during the growing season while resin increased. The fraction of leaf area lost during the growing season averaged less than 7% and was highest on the oldest leaves. The seasonal pattern of resin production in Eriodictyon corresponds to that in Diplacus, indicating that the similar environments of Eriodyctyon and Diplacus have led to convergent leaf resins. This convergence in these two plants has implications for chemicals of similar function in other chaparral shrubs.