The influence of Salix leaf abscission on leaf-miner survival and life history

Abstract. 1. Early abscission of mined leaves was an important mortality factor of a Phyllonorycter species (Lepidoptera: Gracillariidae) on Salix lasiolepis Benth. (Salicaceae). A larger percentage of mined leaves abscised early (34.4% in 1990; 24.5% in 1991), and Phyllonorycter survival was greatly reduced in these abscised leaves.
2. Leaf-mining by Phyllonorycter was associated with increased early leaf abscission. An egg removal experiment demonstrated that leaf mining induced this increase in leaf abscission.
3. The induction of early leaf abscission was dependent upon the timing of herbivory and simulated herbivory (mechanical damage). Early mechanical damage induced leaf abscission, late mechanical damage did not. Mines which expanded early were more likely to induce leaf abscission than mines which expanded more slowly.     

Leaf abscission phenology of a scrub oak: consequences for growth and survivorship of a leaf mining beetle

Brachys tessellatus is a leaf-mining beetle that attacks Quercus laevis (turkey oak), a deciduous scrub oak in the fall line Sandhills of the southeastern United States. This oak species varies substantially in leaf abscission phenology. In the fall of 1994 we examined leaf abscission patterns at three sites in central South Carolina and found that leaves containing active miners abscised earlier than leaves without active miners. To quantify the effect of leaf abscission phenology on the survivorship and body size of leaf miners within the abscised leaves, we marked naturally and manually abscised leaves, left leaves on the ground to over-winter, and then collected leaves before beetles emerged the following spring. We found that beetles inside leaves that were abscised early in the season were more likely to be killed by predators, and that they produced smaller pupae than beetles within leaves abscised later in the season. We also evaluated the relationship between adult beetle body size and fecundity; body size of B. tessellatus is positively correlated with lifetime fecundity and daily oviposition rate indicating that effects of leaf-abscission phenology on beetle body size likely translate into effects on beetle fitness. We discuss how B. tessellatus has adapted to variation of leaf abscission phenology of oaks by evolving developmental plasticity that permits successful completion of larval development over a range of season lengths.     

High abscission rates of damaged expanding leaves: field evidence from seedlings of a Bornean rain forest tree

Herbivore damage is known to cause the premature loss of mature leaves. However, the effects of herbivory on abscission during the early stages of leaf development remain unexplored, even though herbivores frequently prefer unlignified, immature leaves. In a field experiment, we removed 50% of the tissue from leaves at various stages of development on seedlings of Shorea hopeifolia (Dipterocarpaceae), a dominant rain forest tree in Indonesian Borneo. Four weeks following simulated herbivory, >88% of unlignified expanding leaves had been abscised, compared to only 20% of fully expanded, unlignified leaves and 0% of recently lignified, mature leaves. In a separate experiment over 9 wk, simulated herbivory did not increase abscission rates of mature leaves, even when 75% of leaf tissue was removed. Because most (58%) of S. hopeifolia seedlings under natural conditions had lost 1% or less of the tissue from their mature leaves, herbivore damage probably has little effect on the abscission of mature leaves. In contrast, the tendency for damaged expanding leaves to abscise may explain why 49% of S. hopeifolia seedlings had already lost their youngest leaf. If similar patterns occur in other species, herbivore attacks on developing leaves may contribute substantially to both leaf loss and the cumulative impact of herbivory on the growth and survival of whole plants.     

Leaf dynamics and insect herbivory in a Eucalyptus camaldulensis forest under moisture stress

Abstract The influence of soil moisture content on leaf dynamics and insect herbivory was examined between September 1991 and March 1992 in a river red gum (Eucalyptus camaldulensis) forest in southern central New South Wales. Long-term observations of leaves were made in trees standing either within intermittently flooded waterways or at an average of 37. 5m from the edge of the waterways. The mean soil moisture content was significantly (P≤0.05) greater in the waterways than in the non-flooded areas. Trees in the higher soil moisture regime produced significantly larger basal area increments and increased canopy leaf area. This increase in canopy leaf area was achieved, in part, through a significant increase in leaf longevity and mean leaf size. Although a greater number of leaves was initiated and abscissed per shoot from the non-flooded trees, more leaves were collected from litter traps beneath the denser canopies of the flooded trees. Consumption of foliage by insects on the trees subjected to flooding compared to the non-flooded trees was not significantly different. However, the relative impact of insect herbivory was significantly greater on the non-flooded trees. Leaf chewing was the most common form of damage by insects, particularly Chryso-melidae and Curculionidae. No species was present in outbreak during this study. Leaf survival decreased as the per cent area eaten per leaf increased. In addition, irrespective of the level of herbivory, leaf abscission tended to be higher in E. camaldulensis under moisture deficit. The influence of soil moisture content on the balance between river red gum growth and insect herbivory is discussed.     

Acclimation of crown structure to drought in Quercus robur L.—intra- and inter-annual variation of abscission and traits of shed twigs

In pedunculate oak (Quercus robur L.) the architecture of the crown is strongly influenced by age and vigour of the tree. In old oak trees cladoptosis is a major mechanism on the transformation of crown architecture. Although it can be seen quite regularly, the causes and timing of shedding of twigs as well as the quantity of affected branches remain unclear. Because abscission is often used as an indicator of reduced vigour or stress in the assessment of stand and ornamental tree health, it deserves detailed investigation, especially in the context of oak decline. We studied the inter- and intra-annual variation of abscission in six stands across the eastern part of Germany in order to identify possible triggering events and controlling factors of abscission.On average, the number of twigs abscised per year increased from 1999 to 2001. While in 1999 approximately 100 abscised twigs per m² per year were shed, this number increased to 250 per m² in 2001. The majority of twigs was actively shed, a significant proportion of the remaining twigs was partly abscised. Only a small part of the abscised twigs had leaves attached to them. From June to September 20 per cent of the twigs had leaves, in the remaining months of the year less than 10 per cent.The analysis of almost 30,000 twig fragments over the course of 3 years demonstrated at least partial control by the tree of the process of abscission. The loss of the terminal bud and the formation of a male flower are traits correlated with abscission.Our data do not support the widely held belief that cladoptosis is mainly an immediate reaction to drought stress that reduces transpiring leaf area. The proportion of leafy twigs was quite low and the main peak of abscission occurred in late autumn, thus having little immediate effect on transpiration. In addition, we observed a time lag of 3 weeks between the onset of drought stress and increased levels of abscission.     

Phenological responses in maple to experimental atmospheric warming and CO2 enrichment

Evidence that global warming has altered the phenology of the biosphere, possibly contributing to increased plant production in the northern hemisphere, has come from a diversity of observations at scales ranging from the view of the back yard to satellite images of the earth. These observations, coupled with an understanding of the effects of temperature on plant phenology, suggest that future changes in the atmosphere and climate could alter plant phenology with unknown or unpredictable consequences. We assessed the effects of simulated climatic warming and atmospheric CO₂ enrichment on the spring and autumn phenology of maple trees (Acer rubrum and A. saccharum) growing for four years in open‐top field chambers. CO₂ enrichment (+300 ppm) had no consistent effects on the timing of budbreak and leaf unfolding in the spring or leaf abscission in the autumn. Warming (+4°C) usually had predictable effects: in two of the three years of assessment, budbreak occurred earlier in warm chambers than in ambient temperature chambers, and leaf abscission always occurred later. The lengthening of the growing season could contribute to increased productivity, although effects of temperature on other physiological processes can concurrently have negative effects on productivity. In 1995, budbreak was unexpectedly delayed in the warmer chambers, apparently the result of advanced budbreak leading to injury from a late‐spring frost. Likewise, there was increased risk associated with longer leaf retention in the autumn: in 1994, leaves in the warm chambers were killed by freezing temperatures before they had senesced. These observations support the premise that global warming could increase the length of the growing season. Phenological responses should, therefore, be part of any assessment of the possible consequences of global change, but our results also suggest that those responses may not always have positive effects on production.     

Precipitation on abscised leaves modulates leaf‐mining beetle (Trachys yanoi) survivorship and population dynamics

1. We clarified the effects of early leaf abscission on the survivorship of the leaf‐mining beetle Trachys yanoi Y. Kurosawa (Coleoptera: Buprestidae) and the underlying mechanism in relation to weather conditions in Japan. Trachys yanoi is an insect pest of zelkova trees [Zelkova serrata (Thunb.) Makino (Rosales: Ulmaceae)]. Larvae burrow into zelkova leaves and feed on leaf tissue, causing early leaf abscission. 2. This study investigated the relationship between the beetle population and weather conditions over 10 years in a zelkova forest. The effects of moisture and temperature on adult emergence from early abscised leaves were examined in the laboratory and in the field. 3. The beetle population in the studied forest was negatively affected by high precipitation levels when the beetles still inhabited early abscised leaves. Fewer adults emerged from early abscised leaves under wet conditions than under dry conditions, in both laboratory and field tests. 4. These results demonstrate that early leaf abscission plays an important role in leaf‐mining beetle survivorship and population dynamics, and that the amount of precipitation when leaf‐mining beetles still inhabit early abscised leaves modulates this effect. 5. Because precipitation when leaf‐mining beetles still inhabit early abscised leaves was mainly driven by an East Asian rainy season front, the beetle population dynamics were affected by the activity of the front.     

Reddish spring colouring of deciduous leaves: a sign of ecotype?

Several deciduous woody plant species produce anthocyanins during leaf development in spring and again during leaf senescence in autumn. The leaves of Betula pendula Roth (silver birch) commonly exhibit transient reddening in juvenile leaves under northern growing conditions, with the intensity of the red colour varying among individual trees. The objective of our study was to test the hypothesis that the accumulation of foliar anthocyanins during spring in leaves of B. pendula is an ecotypic response. Chlorophyll fluorescence ratio (Fv/Fm), leaf reflectance and anthocyanin concentrations were measured, in relation to phenology in spring, summer and autumn from birches used for landscaping with either red or green-emergent leaves. The results suggest that (1) the trees with green- or red-emergent juvenile leaves represent different populations, and (2) that the red-emergent leaves senesced earlier, indicating that (3) trees with red-emerging leaves belong to a more northern ecotype than the trees with green-emerging leaves. The role of anthocyanin synthesis in a northern radiation environment is discussed.     

Delayed greening,leaf expansion,and damage to sympatric<Emphasis Type="Italic"> Shorea</Emphasis> species in a lowland rain forest

The function of delayed greening in the seedlings of canopy tree species in a lowland tropical rain forest was examined in terms of its potential defensive value against herbivory. To explore the ecological and evolutionary backgrounds for delayed greening, we chose eight sympatric congeneric (Shorea) dipterocarp species that were either normal-greening or delayed-greening species. Expansion and toughening of leaves took approximately 30 days for all species, and did not differ between the normal- and delayed-greening species. The main factors that affected leaf damage during expansion were insect herbivory and fungal infection. Levels of leaf damage were significantly lower for delayed-greening species than for normal-greening species, but proportions of heavily damaged leaves and leaf abscission during expansion did not differ. In addition, no significant difference was found in damage levels on leaves (aged 1–2 months) of naturally occurring seedlings between normal- and delayed-greening species. Therefore, delayed greening may effectively reduce the level of leaf damage in young expanding leaves, but may not necessarily reduce leaf abscission and damage to mature leaves. The existence of delayed greening could not be simply explained by the phylogenetic and ecological backgrounds of the trees. Consequently, delayed greening may have a function in reducing damage during expansion, but more information (such as knowledge of the secondary metabolites involved in this phenomenon) is needed to explain fully why these species exhibit delayed greening.     

Herbivory-induced overcompensation and resource-dependent production of extrafloral nectaries in Luffa cylindrica (Cucurbitaceae)

Extrafloral nectaries (EFNs) are nectar secretory structures involved in the indirect defense of plants. In the sponge gourd (Luffa cylindrica), EFNs commonly occur on the lower surface of leaf blades and stipules and remain functional until leaf senescence. To test the hypothesis that the development of EFNs is influenced by herbivore damage and resource availability, we grew Luffa cylindrica under different concentrations of Hoagland's nutrient solution (nutrient-poor conditions: 10%, 50%; and control condition: 100%) and two herbivory treatments (damaged and undamaged leaves). We collected ten leaves from treated plants to quantify leaf area and EFN density. Overall, leaf area increased and EFN decreased in damaged plants, but this significantly depended on nutritional status. In undamaged plants, EFN density tended to remain constant, whereas foliar area increased with nutrient input. Under herbivory, foliar area increased at 10% but decreased at 50 and 100% of nutrients in relation to undamaged plants, whereas EFN density tended to increase with nutrient availability to exceed undamaged plants under control concentrations. Plants under nutrient-poor conditions subjected to herbivory exhibited an increased foliar area, characterizing a compensatory mechanism. Our results suggest that herbivore-induced indirect defense is a damage- and resource-dependent response in Luffa cylindrica. These findings contribute to understanding the factors that modulate indirect defenses and plant-herbivore-environment interactions.     

Herbivory pattern and induced responses to simulated herbivory inQuercus mongolica var.grosseserrata

Seasonal changes in leaf traits and the herbivory pattern ofQuercus mongolica var.grosseserrata were studied, and simulated herbivory experiments were carried out in order to evaluate leaf trait responses. Leaves ofQ. mongolica emerged simultaneously in spring and most were retained until autumn. Nitrogen concentration was highest when leaves first emerged and decreased rapidly with leaf age. Leaf mass per area (LMA) increased with leaf age. Herbivore attack was concentrated in the first 20 days after bud-break, which corresponded to the high nutritional value of the leaves for herbivores at this time. Simulated herbivory experiments indicated that LMA increased with artificial leaf damage, suggesting an increase in leaf toughness, and that nitrogen concentration decreased later in the season in comparison with intact leaves. As a result, herbivore attack following artificial leaf damage decreased with increasing initial leaf damage. However, leaf longevity was not affected by initial leaf damage. These responses were considered to be a strategy to disperse herbivory damage among leaves.     

Association of red coloration with senescence of sugar maple leaves in autumn

We evaluated the association of red coloration with senescence in sugar maple (Acer saccharum Marsh.) leaves by assessing differences in leaf retention strength and the progression of the abscission layer through the vascular bundle of green, yellow, and red leaves of 14 mature open-grown trees in October 2002. Computer image analysis confirmed visual categorization of leaves as predominantly green, yellow or red, and chemical quantification of leaf pigment concentrations verified that leaf color reflected underlying differences in leaf biochemistry. Significantly lower chlorophyll concentrations within red and yellow leaves indicated that senescence was more advanced in leaves from these color categories relative to green leaves. Among leaf types, only red leaves contained high concentrations of anthocyanins. There were significant differences in leaf retention capacity among color categories, with the petioles of green leaves being the most firmly attached to twigs, followed by red and then yellow leaves. Microscopic analysis indicated that yellow leaves had the most advanced extension of the abscission layer through the vasculature, with green and red leaves having significantly less abscission layer progression than yellow. A more limited progression of the abscission layer through vascular bundles may be evidence of delayed leaf senescence that could extend resorption of mobile leaf constituents. Together, results from this study suggest an association between leaf anthocyanin content and functional delays in senescence.     

Modification of Vegetative Phenology in a Tropical Semi‐deciduous Forest by Abnormal Drought and Rain1

The control of vegetative phenology in tropical trees is not well understood. In dry forest trees, leaf abscission may be enhanced by advanced leaf age, increasing water stress, or declining photoperiod. Normally, it is impossible to dissect the effects of each of these variables because most leaves are shed during the early dry season when day length is near its minimum and leaves are relatively old. The 1997 El‐Niño Southern Oscillation caused a ten‐week long, severe abnormal drought from June to August in the semi‐deciduous forests of Guanacaste, Costa Rica. We monitored the effect of this drought on phenology and water status of trees with young leaves and compared modifications of phenology in trees of different functional types with the pattern observed during the regular dry season. Although deciduous trees at dry sites were severely water stressed (Ψstem < ‐7MPa) and their mesic leaves remained wilted for more than two months, these and all other trees retained all leaves during the abnormal drought. Many trees exchanged leaves three to four months earlier than normal during the wet period after the abnormal drought and shed leaves again during the regular dry season. Irrigation and an exceptional 70 mm rainfall during the mid‐dry season 1998/1999 caused bud break and flushing in all leafless trees except dormant stem succulents. The complex interactions between leaf age and water stress, the principal determinants of leaf abscission, were found to vary widely among trees of different functional types.     

The forgotten season: the impact of autumn phenology on a specialist insect herbivore community on oak

1. Variation in spring phenology – like tree budburst – affects the structure of insect communities, but impacts of autumn phenology have been neglected. Many plant species have recently delayed their autumn phenology, and the timing of leaf senescence may be important for herbivorous insects. 2. This study explored how an insect herbivore community associated with Quercus robur is influenced by variation in autumn phenology. For this, schools were asked to record, across the range of oak in Sweden, the autumn phenology of oaks and to conduct a survey of the insect community. 3. To tease apart the relative impacts of climate from that of tree phenology, regional tree phenology was first modelled as a function of regional climate, and the tree‐specific deviation from this relationship was then used as the metric of relative tree‐specific phenology. 4. At the regional scale, a warmer climate postponed oak leaf senescence. This was also reflected in the insect herbivore community: six out of 15 taxa occurred at a higher incidence and five out of 18 taxa were more abundant, in locations with a warmer climate. Similarly, taxonomic richness and herbivory were higher in warmer locations. 5. Trees with a relatively late autumn phenology had higher abundances of leaf miners (Phyllonorycter spp.). This caused lower community diversity and evenness on trees with later autumn phenology. 6. The findings of the present study illustrate that both regional climate‐driven patterns and local variation in oak autumn phenology contribute to shaping the insect herbivore community. Community patterns may thus shift with a changing climate.     

Leaf damage and functional traits along a successional gradient in Brazilian tropical dry forests

Herbivory has significant impacts on individual plants and plant communities, both at ecological and evolutionary time scales. In this context, this study aims to evaluate herbivore damage and its relationship with leaf chemical and structural traits, nutritional status, and forest structural complexity along a successional gradient. We predicted that trees in early successional stages support conservative traits related to drought tolerance (high specific leaf mass and phenolics), whereas trees in light-limited, late successional stages tend to enhance light acquisition strategies (high nitrogen content). We sampled 261 trees from 26 species in 15 plots (50 × 20 m; five per successional stage). From each tree, twenty leaves were collected for leaf trait measures. Phenolic content increased whereas specific leaf mass and nitrogen content decreased from early to late stages. However, leaf damage did not differ among successional stages. Our results partially corroborate the hypothesis that early successional plants in tropical dry forests exhibit leaf traits involved in the conservative use of water. The unexpected decrease in nitrogen content along the chronosequence is likely related to the fact that thinner leaves with low specific leaf mass could have less nitrogen-containing mesophyll per unit area. Mechanisms affecting herbivory intensity varied across scales: at the species level, leaf damage was negatively correlated with tannin concentration and specific leaf mass; at the plot level, leaf damage was positively affected by forest structural complexity. Herbivory patterns in tropical forests are difficult to detect because abiotic factors and multiple top-down and bottom-up forces directly and indirectly affect herbivores.     

The alkaloidal responses of wild tobacco to real and simulated herbivory

Summary I compared the induced alkaloidal response in undamaged leaves of plants subjected to herbivory by the larvae of Manduca sexta and to different simulations of this herbivory; all herbivory treatments removed similar amounts of leaf mass. Although larval feeding induced a significant increase (2.2x) in alkaloid concentrations compared to undamaged plants, the alkaloid responses to larval feeding were significantly lower than the responses to an herbivory simulation (4x controls) which involved removing the same amount of leaf area from the same positions on the leaf, over a similar time period. Moreover, another herbivory simulation, identical in amount of leaf mass removed and duration of damage to the larval feeding, but without regard to spatial array of leaf damage, resulted in an alkaloidal response (5.5x controls) higher still than the previous herbivory simulation. In a second experiment the importance of leaf vein damage on the induced alkaloidal response was examined. Here, leaf removal that involved cutting leaf tissues from between secondary veins before removing the midrib, resulted in alkaloidal responses that were significantly lower (1.7x controls) than responses from leaf removal that involved cutting both veins and midribs along with the intervein tissues (2.6x controls). Vein damage alone did not produce a significant response. These results indicate that herbivory is difficult to simulate: that how a leaf is damaged can be as important as the magnitude of leaf damage in determining a plant's response to damage.     

Leaf Abscission in Varieties of Phaseolus vulgaris (L. ) and Glycine max (L. ) Merrill/emdash a--Correlation with Propensity to Produce Adventitious Roots

In field trials of Phaseolus vulgaris large differences wereobserved between varieties in the rate at which the leaves abscised.Similar differences were found in the rate of decline of thechlorophyll content of excised leaf discs. A grafting experimentshowed that the differences in leaf abscission depended on thegenotype of the scion and on that of the rootstock. Scion andstock effects of each genotype were similar, and additive. Rootstock/scioncombinations which conferred enhanced leaf retention producedgreater yields of seed and of seed nitrogen. When shoots ofdelayed-senescence genotypes of P. vulgaris were held in waterthey produced more adventitious roots than did shoots of rapid-senescencegenotypes. This relationship between senescence pattern andadventitious rooting was also observed among varieties of Glycinemax, and between isogenic lines of G. max differing in the leafabscission alleles Ab/ab. These results are discussed in relationto current theories of leaf senescence, abscission, and theproduction of yield.     

Herbivory of wild Manduca sexta causes fast down-regulation of photosynthetic efficiency in Datura wrightii: an early signaling cascade visualized by chlorophyll fluorescence

Plants experiencing herbivory suffer indirect costs beyond direct loss of leaf area, but differentially so based on the herbivore involved. We used a combination of chlorophyll fluorescence imaging and gas exchange techniques to quantify photosynthetic performance, the efficiency of photochemistry, and heat dissipation to examine immediate and longer-term physiological responses in the desert perennial Datura wrightii to herbivory by tobacco hornworm, Manduca sexta. Herbivory by colony-reared larvae yielded no significant reduction in carbon assimilation, whereas herbivory by wild larvae induced a fast and spreading down-regulation of photosynthetic efficiency, resulting in significant losses in carbon assimilation in eaten and uneaten leaves. We found both an 89?% reduction in net photosynthetic rates in herbivore-damaged leaves and a whole-plant response (79?% decrease in undamaged leaves from adjacent branches). Consequently, herbivory costs are higher than previously estimated in this well-studied plant-insect interaction. We used chlorophyll fluorescence imaging to elucidate the mechanisms of this down-regulation. Quantum yield decreased up to 70?% in a small concentric band surrounding the feeding area within minutes of the onset of herbivory. Non-photochemical energy dissipation by the plant to avoid permanent damage was elevated near the wound, and increased systematically in distant areas of the leaf away from the wound over subsequent hours. Together, the results underscore not only potential differences between colony-reared and wild-caught herbivores in experimental studies of herbivory but also the benefits of quantifying physiological responses of plants in unattacked leaves.     

Leaf flushing phenology and herbivory in a tropical dry deciduous forest,southern India

Patterns of leaf-flushing phenology of trees in relation to insect herbivore damage were studied at two sites in a seasonal tropical dry forest in Mudumalai, southern India, from April 1988 to August 1990. At both sites the trees began to flush leaves during the dry season, reaching a peak leaf-flushing phase before the onset of rains. Herbivorous insects emerged with the rains and attained a peak biomass during the wet months. Trees that flushed leaves later in the season suffered significantly higher damage by insects compared to those that flushed early or in synchrony during the peak flushing phase. Species whose leaves were endowed with physical defenses such as waxes suffered less damage than those not possessing such defenses. There was a positive association between the abundance of a species and leaf damage levels. These observations indicate that herbivory may have played a major role in moulding leaf flushing phenology in trees of the seasonal tropics.