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     

Phenolic and phenolic-related factors as determinants of suitability of mountain birch leaves to an herbivorous insect

We investigated the role of phenolic and phenolic-related traits of the leaves of mountain birch (Betula pubescens ssp. czerepanovii) as determinants of their suitability for the growth of larvae of the geometrid Epirrita autumnata. As parameters of leaf suitability, we determined the contents of total phenolics, gallotannins, soluble and cell-wall-bound proanthocyanidins (PAS and PAB, respectively), lignin, protein precipitation capacity of tannins (PPC), and leaf toughness. In addition, we examined concentrations of soluble carbohydrates and protein-bound amino acids as background variables describing the nutritive value of leaves. The correlation of the leaf traits of our 40 study trees with the tree-specific relative growth rate (RGR) of E. autumnata showed that the only significant correlation with RGR was that of PAS - the largest fraction of total phenolics - and even that explained only 15% of the variation in E. autumnata growth. The nonlinear estimation of the relationship between RGR and PAS by piecewise linear regression divided the 40 study trees into two groups: (i) 19 trees with good leaves for E. autumnata (RGR ranging from 0.301 to 0.390), and (ii) 21 trees with poor leaves (RGR ranging from 0.196 to 0.296). The suitability of leaves within these two groups of trees was determined by different phenolic traits. Within the good group, the suitability of leaves for larvae was determined by the PPC of extracts, which strongly correlated with gallotannins, and by the total content of gallotannins. In contrast, the leaves of poor trees had significantly higher contents of both PAS and PAB, but leaf toughness correlated only negatively with the RGR of E. autumnata larvae. We also discuss the causes of variation in the phenolic and phenolic-related factors that determine the suitability of leaves for E. autumnata larvae in different groups of trees.     

Covariation of fluctuating asymmetry, herbivory and chemistry during birch leaf expansion

Fluctuating asymmetry (FA) is used to describe developmental instability in bilateral structures. In trees, high FA of leaves has been assumed to indicate the level of environmental or genetic stress, and for herbivores leaves from such trees have been shown to be in some cases (though not invariably) of higher quality compared to trees with symmetrical leaves. We demonstrated that FA of birch leaves correlated positively with growth rate of leaves, and with the amount of leaf biomass consumed by larvae of the geometrid Epirrita autumnata. Since asymmetry per se cannot define leaf quality for a herbivore, we determined the biochemical compounds which covary with the degree of foliage FA, in order to elucidate relationships between leaf FA, chemistry and herbivory. High foliar FA was characteristic of birches with high initial concentrations, and rapid seasonal decline in the concentrations of gallic acid and hydrolysable tannins, and with rapid seasonal changes in the concentrations of flavonoid-glycosides and sugars. In contrast, leaf FA was not related to concentrations of proanthocyanidins, protein-bound amino acids or soluble phenylalanine, the precursor of proanthocyanidins and proteins with aromatic amino acids. The positive correlation between leaf FA and consumption by E. autumnata was presumably related to the previously demonstrated compensatory consumption of E. autumnata to high concentrations of foliar gallotannins. Furthermore, sugars are well-known feeding stimulants. We propose that the variable results in studies correlating leaf FA and herbivory may stem from variable chemical associations of FA in different plants and of species-specific effects of compounds on insects. Received: 15 July 1999 / Accepted: 24 September 1999     

Low nutritive quality as defence against herbivores: induced responses in birch

Summary The effects of artificial defoliation of birch trees in the previous year on the consumption and utilization of food by a geometrid larva, Epirrita autumnata, were studied in laboratory. The leaves were collected from two sites on a slope of a fell. Defoliation had a significant retarding effect on approximate digestibility, efficiency of conversion of ingested food, relative consumption rate and relative growth rate but not on efficiency of conversion of digested food. The effects were to the same direction with leaves from the two sites, but the response was stronger with leaves from the zone defoliated by Epirrita during mid-sixties. The defensive nature of the response(s) of birch to defoliation is discussed in the light of these results. It is concluded that contrary to the hypothesis of Moran and Hamilton (1980) no increase in the consumption incurred by individual trees could be shown in this system.     

Delayed induced responses of birch glandular trichomes and leaf surface lipophilic compounds to mechanical defoliation and simulated winter browsing

Changes in morphology and chemistry of leaf surface in response to herbivore damage may increase plant resistance to subsequent herbivore attack; however, there is lack of studies on induced responses of glandular trichomes and their exudates in woody plants and on effects of these changes on herbivores. We studied delayed induced responses in leaf surface traits of five clones of silver birch (Betula pendula Roth) subjected to various types of mechanical defoliation and simulated winter browsing. Glandular trichome density and concentrations of the majority of surface lipophilic compounds increased in trees defoliated during the previous summer. This induced response was systemic, since control branches in branch defoliated trees responded to the treatments similarly to defoliated branches, but differently from control trees. In contrast to defoliation treatments, simulated winter browsing reduced glandular trichome density on the following summer and had fewer effects on individual surface lipophilic compounds. Moreover, constitutive density of glandular trichomes was negatively correlated with induced total amount of lipophilic compounds per trichome, indicating a trade-off between constitutive and induced resistance in silver birch. Induced changes in leaf surface traits had no significant effect on leaf damage by chewers, miners and gall mites, but increased susceptibility of birch trees to aphids. However, leaf damage by chewers, miners and gall mites in defoliated (but not in control) trees was correlated with concentrations of some fatty acids and triterpenoids, although the direction of relationships varied among herbivore species. This indicates that induction of surface lipophilic compounds may influence birch resistance to herbivores. Our study thus demonstrated both specificity of elicitation of induced responses of birch leaf surface traits by different types of damage and specificity of the effects of these responses on different types of herbivores.Electronic Supplementary Material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Anti-herbivore strategies in mountain birch at the tree-line

Birch forest on the slope of the Jesnalvaara fell forms a gradient ranging from 7–8 m high trees (at 75 m a.s.l.) to birch shrubs less than 1 m high at the top of the fell (330 m a.s.l.). Upper parts of the forest (above 220 m) were defoliated by Oporinia autumnata (Lep., Geometridae) about ten years ago and, with the exception of the top, only a fraction of birches have recovered. Climatic conditions explain the limitation of the damaged area – Oporinia eggs were killed during cold winters. Predators may also limit the damage. Defensive mechanisms in the birch leaves – especially a low nitrogen content and high concentrations of phenols inhibiting trypsin – can slow down an increase in herbivore density by reducing their reproductive capacity. Birch leaves from the lower limit of the damaged area are least suitable for the growth of herbivores. They have the highest concentration of phenols, too. Birches at the foot of Jesnalvaara are able to increase leaf phenols after mechanical damage of nearby leaves. This is enough to retard the growth of several herbivore species. Ability to recover after defoliation is an important part of the anti-herbivore strategy of the birch. This ability is higher in shrub birches than in tall trees and better after a warm than a cold summer. Birch leaves in the year following defoliation are less usable for herbivores and cause increased mortality and lowered reproduction. These properties are the weakest in birches at the foot of Jesnalvaara where the probability of being defoliated is also lowest.     

Rapid induced resistance of silver birch affects both innate immunity and performance of gypsy moths: the role of plant chemical defenses

In this study we tested the effects of rapid induced resistance of the silver birch, Betula pendula, on the performance and immune defense of the gypsy moth, Lymantria dispar. We also measured the effects of defoliation on the concentrations of plant secondary metabolites, particularly on phenolics and terpenoids. It was found that severe natural defoliation (by moth larvae) of silver birch led to an increase in lipophilic flavonoids on the leaf surface. The concentration of some simple phenolics and monoterpenes (linalool and geraniol) also increased, while that of several glycosides of quercetin decreased. The female pupal weights and survival rates of moths decreased, and larval development time increased, when the insects fed on defoliated trees. However, the feeding of caterpillars with the leaves of defoliated trees led to an increase in lysozyme-like activity in their hemolymph, with an increase in their ability to encapsulate potential parasites. Our data show that the silver birch deploys a rapid chemical defense against gypsy moth larvae. We suggest that lipophilic flavonoids are important compounds in the direct silver birch defense against L. dispar caterpillars. The increased strength of immune defense of insects exposed to trees that had deployed a rapid induced resistance may be an adaptation of the herbivores to resist the rising density of parasites when host population density is high.     

Within-tree variability of mountain birch leaves causes variation in performance for Epirrita autumnata larvae

Within-tree variability in leaf characteristics of the mountain birch (Betula pubescens ssp. tortuosa) was bioassayed for the autumnal moth (Epirrita autumnata) by rearing larvae on birch leaves in a laboratory and measuring their growth, consumption rate, approximate digestibility, efficiency of conversion of ingested food and efficiency of conversion of digested food. Only short shoot leaves, i.e. leaves of the same age, were used. The highest hierarchical level, which included trees and ramets within trees, accounted for most of the total variance in almost all the measured traits. Short shoots (within branches) accounted for more of the variance than branches (within trees/ramets) in most of the traits. The results suggest that differences in leaf quality were reflected in larval growth mainly by differences in food utilization efficiencies (postingestive effects) and less by differences in consumption rate (preingestive effects). The observed within-tree variation is probably a consequence of the modular structure, sectoriality and partial functional independence of tree parts.     

Recovery of a Betula pubescens forest in northern Sweden after severe defoliation by Epirrita autumnata

Abstract. Mountain birch (Betula pubescens ssp. czerepanovii) forest in the Abisko valley of northern Sweden was completely defoliated by Epirrita autumnata caterpillars during an outbreak in 1954–1955. The defoliation resulted in an 80–90% mortality of the leaf‐carrying shoots of birches in 1956 and triggered a rejuvenation of stands. The subsequent regrowth of foliage was studied in two damaged birch stands and in one unattacked stand. The number of leaves approximately doubled in the damaged stands between 1961 and 1987, while the number on the reference plot fluctuated without significant increase. Regrowth started with increased production of long shoots from surviving shoots and basal sprouts. Basal sprouts were a substantial source of new shoots in the recovery of the foliage, especially on the most damaged plot. Trees of seed origin constituted a minor fraction of the regrowth. Initial rapid growth of foliage reduced gradually and the annual leaf production in 1986/1987 was 75% of that of the reference plot. Comparison between the recovery curve and data from the reference plot indicates that the shoot population of the damaged forest will, after more than 30 years, need many more years to reach the assumed size of a mature forest. The degree of rejuvenation varied between stands, with different consequences for future dynamics of E. autumnata populations.     

Effects of host shading on consumption and growth of the geometrid Epirrita autumnata: interactive roles of water, primary and secondary compounds

Shading is assumed to reduce allocation to plant phenolics and to defense in general. We here report the results of experimental shading of individual branches or whole canopies in mountain birch on foliar chemistry and on the growth and consumption of a geometrid, Epirritaautumnata. Branch‐wide shading tended to have at least as strong effects on both leaf chemistry and herbivore performance as canopy‐wide shading, indicating local responses of the host to shading. Responses to shading varied among the key leaf traits. Leaf water content was higher and toughness lower in shaded than in non‐shaded leaves. Leaf sugars were lower and protein‐bound and free amino acids higher in shaded than in control leaves. Sucrose and galactose were at high levels in unshaded branches adjacent to shaded ones, suggesting that partial shading enhanced translocation of sugars within canopies. Total phenolics and soluble proanthocyanidins were low in both shading treatments. Of the other phenolic groups, concentrations of gallotannins and cell‐wall‐bound proanthocyanidins did not differ between shaded and non‐shaded leaves. Epirrita larvae grew better in both types of shading treatments compared to either unshaded control trees or to unshaded branches in the branch‐shading trees. By far the most important correlate of larval growth was the amount of water consumed with leaf mass (r=0.94). When variance in water intake was standardized (also largely eliminating parallel variation in proteins), fructose and glucose still had significant positive correlations and proanthocyanidins negative with larval growth on control but not on shade leaves. Concentrations of several phenolic compounds correlated negatively with intake of dry matter and especially water, and different phenolics were important in shaded (gallotannins) and in control (flavonoids) leaves. Our findings strongly suggest that the effects of putatively defensive leaf traits on insect consumption and growth interact with nutritive leaf traits, particularly with water.     

Delayed inducible resistance against a leaf-chewing insect in four deciduous tree species

Summary Both mechanical damage to mountain birch foliage and rearing of moth larvae on the trees reduced the growth of Epirrita autumnata larvae reared on these trees in the following year. The effects of physical damage and some other cues from insects were additive. On bird cherry the performance of Epirrita larvae was equal on untreated trees and on trees artificially defoliated in the previous year, but larval growth was reduced on previously insect-damaged branches. With mountain ash just physical damage per se reduced the performance of Epirrita larvae. On Salix phylicifolia there were no significant differences in the growth or survival of Epirrita on untreated control bushes and on bushes with partial larval damage during the previous year. Among untreated control trees the growth and survivorship of Epirrita were higher on fast-growing willow and bird cherry than on the slow-growing mountain birch. Mountain birch and mountain ash, the two deciduous tree species adapted to nutrient-poor soils, showed delayed inducible resistance triggered by defoliation (artificial or insect-made). This supports the hypothesis that delayed inducible resistance may be a passive response due to nutrient-stress caused by defoliation. On the other hand, the additional increase in the resistance of mountain birch triggered by specific cues from insects suggests that this response may be an evolved defense against leaf-eating insects.     

Do warmer growing seasons ameliorate the recovery of mountain birches after winter moth outbreak?

Subarctic mountain birch (Betula pubescens ssp. czerepanovii) forests in northern Fennoscandia have shown a slight recovery from recent severe defoliation by the winter moth (Operophtera brumata). This development in trees is hypothesized to be a result of ameliorated growing conditions through increased summer temperatures. We examined if accumulated thermal sum affects the ability of mountain birches to tolerate foliage losses. We quantified the number of leaf-bearing short shoots, the emergence of inflorescences and the seasonal height growth of long shoots in both intact and defoliated trees. We also determined the concentrations of carbon and nitrogen in leaves and carbohydrates in roots. Our results show that defoliation constrained the growth of long shoots, as well as the emergence of inflorescences regardless of thermal sum accumulation. However, the number of leaf-bearing short shoots did not differ between intact and defoliated trees. In the both tree groups, the amounts of emerging leaves increased as a response to thermal sum accumulation. Also the leaf carbon concentration increased in defoliated trees at higher thermal sums, whereas it decreased in intact controls. Generally, the mean carbohydrate concentrations were greater in roots of defoliated than intact trees. However, with increased thermal sums, root carbohydrates increased in intact trees but remained the same in defoliated trees. We conclude that thermal sum accumulation does not greatly promote the recovery of mountain birches. Although the damaged trees produced more leaves at warmer growing sites, this did not increase their height growth or carbohydrate gain in roots.     

Rapid wound-induced resistance in white birch (Betula pubescens) foliage to the geometrid Epirrita autumnata: a comparison of trees and moths within and outside the outbreak range of the moth

Two strains of a geometrid defoliator, Epirrita autumnata, were used in bioassays to test existence and relative efficacy of rapid, wound-induced foliage resistance in two provenances of the white birch. One birch and one moth strain originated in the outbreak range of the moth and another outside it. Both birch provenances responded to manual leaf damage by changes in foliage quality which significantly retarded growth of the insects, reducing their pupal weights and protracting larval periods. Leaves which were previously damaged were lower quality as Epirrita food than adjacent intact leaves. Both of them were lower quality than intact leaves without damaged leaves nearby. Because of variance between years in the efficacy of the response, and because of different transfer distances of the provenances to the common garden where the experiments were performed, we could not ascertain whether there is any overall difference in the efficacy of rapid inducible responses between the provenances. Both moth strains were affected by wound-induced deterioration in foliage quality. There were no differences in how the moth strains experienced inducible resistance in the two birch provenances. Moths achieved relatively higher pupal weights on the birch provenance matching their origin. Moths from the outbreak range completed their larval period in a shorter time and pupated in a smaller size and, due to dependence of fecundity on size, had a lower potential rate of increase than insects outside the outbreak range.     

Effects of host-plant shift on immune and other key life-history traits of an eruptive Geometrid, Epirrita autumnata (Borkhausen)

Abstract.  1. Population density of Epirrita autumnata (Lepidoptera: Geometridae) reaches outbreak densities regularly in northernmost Scandinavia. During these outbreak years, the most abundant host species, the mountain birch ( Betula pubescens ssp. czerepanovii ), is regularly exhausted, although larvae may rescue themselves from starvation by using alternative host species.
2. In this paper, the effects of the shift of host species on the immune defence and other life-history traits of E. autumnata were investigated, and possible consequences for population dynamics were briefly discussed. Moth larvae were reared on the leaves of the main host, mountain birch, until larvae reached their third instar. After this, larvae were allocated randomly to five treatments: larvae were either allowed to finish larval stage on the mountain birch or were shifted onto four alternative host species that are typical species for the area.
3. As expected, the host species had a major effect on fitness traits: body weight, development, and survival rate of the moths. The pupal weight was lower and development rates slower on the three alternative host species, Salix myrsinifolia Salisb., Vaccinium uliginosum L., and Betula nana L., than on the main host, mountain birch.
4. The immunity was, however, the same or better on the alternative hosts than on the main host. The immunity and pupal weights were negatively related, suggesting a trade-off between body size and immunocompetence.
5. The decreased body size and fecundity of E. autumnata during outbreak years may be partly due to the shift to alternative host species whereas the host-plant species probably does not affect markedly the rate of parasitism.     

Flowers for bees,autumn colours for whom? An experimental test with autumn colouration in mountain birches (Betula pubescens ssp. czerepanovii) as a signal against the moth Epirrita autumnata

In 2001, Hamilton and Brown proposed a controversial hypothesis of handicap signalling to potential insect parasites as an adaptive explanation for autumn leaf colouration. In subsequent studies there has been little attention to the costs and benefits of early autumnal colour change. Yet, in an observational study by Hagen et al. (2003) it was demonstrated that birch trees [Betula pubescens ssp. czerepanovii Ehrhart (Betulaceae)] turning yellow early in autumn had less damage from insects chewing on leaves the subsequent summer. Here, two experiments are presented which test the mechanisms in this model. The first addresses the proposed defence of leaves of B. pubescens ssp. czerepanovii by letting caterpillars of Epirrita autumnata Borkhausen (Lepidoptera: Geometridae), the birches’ most common insect parasites, choose between leaves from trees that either turned yellow late or early the foregoing autumn. The second experiment addresses whether adult female E. autumnata choose between early or late senescent (i.e., yellow or green) ‘twigs’ when ovipositing in autumn. We could not find evidence of preferences in either larvae or females, suggesting that timing of colour change in B. pubescens ssp. czerepanovii is not a warning signal that elicits a response in E. autumnata.     

Variability in the efficacy of delayed inducible resistance in mountain birch

This paper summarizes the results from experiments during 12 years on effects of delayed inducible resistance of mountain birches (Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman) on performance of a geometrid, Epirrita autumnata (Bkh.). Manual defoliation usually reduced significantly pupal mass of E. autumnata the next summer. The years interacted significantly with the defoliation treatment indicating variation among years in the responses of trees of larvae, or that the methodological differences between different experiments may have affected the results. Potential methodological sources for the annual variability were discussed and found unlikely. In addition, the efficacy of defoliation treatment in reducing the pupal mass was greatest when foliage quality in control trees was best, suggesting a contribution of biological causes to the annual variability.     

Developmental plasticity in birch leaves: defoliation causes a shift from glandular to nonglandular trichomes

The structures on leaf surfaces, e.g. trichomes, can act as effective antiherbivory mechanisms as chemical repellents. Structural defences usually represent constitutive resistance, but there are also a few cases of inducible morphological defences. We tested whether defoliation may induce changes in trichome production in white birch (Betula pubescens). The studied birches were either 0, 50 or 100% defoliated during the previous or current summer, and we measured the alterations in the production of glandular vs. nonglandular leaf trichomes, developmental instability (fluctuating asymmetry, FA) and leaf and shoot growth. We detected a clear shift from glandular to nonglandular leaf trichomes following previous‐year defoliation but not after current‐year defoliation. Furthermore, the density of nonglandular trichomes around the mid‐vein of leaves increased following previous‐year defoliation but decreased after current‐year defoliation. While leaf and shoot growth showed a distinct decrease in response to defoliation, FA turned out to be less sensitive. Consequently, previous‐year defoliation can induce the production of nonglandular trichomes in birch leaves. Because this response was accompanied by a reduction in glandular trichomes, the present results may suggest a trade‐off between the different trichome types of birch leaves.     

Interpopulation differences in pupal size and fecundity are not associated with occurrence of outbreaks in Epirrita autumnata (Lepidoptera, Geometridae)

Abstract.

• 1 Among-population differences in pupal mass were studied in a geometrid, Epirrita autumnata. Some Epirrita autumnata populations regularly reach outbreak densities while others are never known to do so. Because adults do not feed, pupal mass of females correlates strongly with fecundity.
• 2 Larvae were collected from twelve field sites. Ten of our sample populations originated within the outbreak range of the species and represented different phases of outbreaks. Two populations originated outside the outbreak range.
• 3 Pupal mass of field-collected E. autumnata varied significantly among populations. The peak phase populations had the smallest pupae and the biggest were found in low density populations outside the outbreak range.
• 4 Offspring of moths from each population were reared under identical conditions in two larval densities. Significant differences were not found in pupal mass among populations. That is, the inherent size, correlated with fecundity of moths, was not different between populations originating within and outside the outbreak range, nor among collections from different densities or phases of the outbreaks.
• 5 Rearing density did not interact in a consistent way with population.
• 6 As far as size and fecundity are concerned, the results do not support Chitty's hypothesis that differences in genetic composition of the population at low and high density phases generate cyclic fluctuations of population density.
• 7 Because no hereditary or maternal differences were found in size and fecundity between E.autumnata originating within and outside the outbreak range, variation in reproductive capacity cannot explain why outbreaks occur only in some populations.

     

Variation in shoot mortality within crowns of severely defoliated <Emphasis Type="Italic">Betula maximowicziana</Emphasis> trees in Hokkaido,northern Japan

To clarify mortality patterns of current-year shoots within the crown of Betula maximowicziana Regel after severe insect herbivory in central Hokkaido, northern Japan, we investigated the degree of defoliation, pattern of shoot development, shoot mortality, and leaf tissue-water relations. One hundred current-year long shoots growing in a B. maximowicziana plantation were observed for defoliation and mortality in June 2002. An outbreak of herbivorous insects (Caligula japonica and Lymantria dispar praeterea) occurred in the stand in mid-to-late June, and the monitored shoots were defoliated to various degrees. Within 1 month of defoliation, some of the severely defoliated shoots had produced new leaves on short shoots that had emerged from axillary buds. Stepwise logistic regression revealed that the probability that current-year long shoots would put out axillary short shoots with leaves is closely related to the degree of defoliation. To evaluate the water relations of the leaves, we determined pressure–volume curves for the leaves that survived the herbivorous insect outbreak and the new leaves that emerged after defoliation. The water potential at turgor loss (Ψ_l,tlp) and the osmotic potential at full turgidity (Ψ_π,sat) were higher for the new leaves than for the surviving leaves, indicating a lower ability to maintain leaf cell turgor against leaf dehydration in the new leaves. Of the 100 shoots, 13 died after the emergence of new leaves. Stepwise logistic regression revealed that the probability that the long shoots would die generally increased with the emergence of new leaves, with increasing shoot height. This result suggests that the combined effect of the vulnerability of newly emerged leaves and low water availability, associated with higher shoot positions within the crown, caused shoot mortality. Based on our results, some possible mechanisms for mortality in severely defoliated B. maximowicziana are discussed.     

Defoliation increases carbon limitation in ectomycorrhizal symbiosis of <Emphasis Type="Italic">Betula pubescens</Emphasis>

Boreal forest trees are highly dependent on root-colonizing mycorrhizal fungi. Since the maintenance of mycorrhizal symbiosis implies a significant carbon cost for the host plant, the loss of photosynthetic leaf area due to herbivory is expected to reduce the host investment in mycorrhizae. We tested this hypothesis in a common garden experiment by exposing ectomycorrhizal white birch (Betula pubescens Ehrh.) seedlings to simulated insect defoliation of 50 or 100% intensity during either the previous or the current summer or repeatedly during both seasons before harvest. The shoot and root growth of the seedlings were distinctly reduced by both 100% defoliation and repeated 50% defoliation, and they were more strongly affected by previous-year than current-year defoliation. The root to shoot ratio significantly decreased after 100% defoliation, indicating reduced proportional allocation to the roots. Ergosterol concentration (i.e. fungal biomass) in the fine roots decreased by 100% defoliation conducted either in the year of harvest or in both years. No such decrease occurred following the 100% defoliation conducted in the previous year, indicating the importance of current photosynthates for fungal symbionts. The trend was similar in the colonization percentage of thick-mantled mycorrhizae in the roots, the most marked decline occurring in the repeatedly defoliated seedlings. The present results thus support the prediction that the plant investment in ectomycorrhizae may decline as a response to foliage loss. Moreover, the colonization percentage of thick-mantled mycorrhizae correlated positively with the ratio of leaf to heterotrophic plant biomass in the defoliated birch seedlings, but not in the control ones. This tends to indicate a stronger carbon limitation of ectomycorrhizal colonization in defoliated seedlings.