Hare and vole browsing preferences during winter

The aim with this study was to, under controlled conditions, determine the food preference of mountain haresLepus timidus Linnaeus, 1758 and bank volesClethrionomys glareolus (Schreber, 1780) for a substantial part of the woody plants potentially available for these herbivores during winter. In addition, we compared hare and vole preference patterns. Thirteen woody plant species were simultaneously presented to 9 captive voles and 9 captive hares in preference tests during winter. Consumption by hares from 50 g bundles (one per species) was measured after 3 h, whereas shoot consumption by voles was measured after 12 h. Both hares and voles preferred deciduous species to conifers,Populus tremula, andVaccinium myrtillus being the most preferred species. However, there was considerable variation in palatability among deciduous plant species and only a marginally significant correlation was found between hare and vole preference. One striking differences between hares and voles was that Sorbusaucuparia was the most utilised by voles but the least preferred by hares. In conclusion, deciduous plant species were generally considerably more palatable to hares and voles than conifers, which is consistent with current theories. Nevertheless, the high variation in palatability among deciduous trees and the difference in preference between hares and voles indicate more complex and species-specific patterns with regard to plant— animal interactions. The latter also suggests that hares and voles differ in their dietary adaptations and have different dietary constrains.     

Spatial responses of two herbivore groups to a geometrid larva on mountain birch

Direct or plant-mediated interactions between herbivores may modify their spatial distribution among and within plants. In this study, we examined the effect of a leaf-chewing geometrid, the autumnal moth (Epirrita autumnata), on two different herbivore groups, leaf rolling Deporaus betulae weevils and Eriocrania spp. leafminers, both feeding on mountain birch (Betula pubescens ssp. czerepanovii). The exact locations of herbivores within tree canopies were mapped during three successive summers. In the first 2 years, some trees were artificially colonized by eggs of the autumnal moth to induce both rapid and delayed resistance in the foliage. The natural infection levels of the pathogenic rust fungus (Melampsoridium betulinum), potentially involved in species interactions, were also recorded. At the level of the whole tree, the density of D. betulae leaf rolls was lower in trees infested by the autumnal moth in the same year. However, the feeding locations within trees were partly segregated: D. betulae favoured shadier branches, while E. autumnata preferred the sunny parts of the canopy. The autumnal moth did not affect current- or following-year density of leafminers at the tree or branch level. Trees infected by rust had fewer leafminers in the same summer than noninfected trees. There were no interaction effects between defoliation by the autumnal moth and rust infection, and no delayed effects on the abundance of other herbivores the following year. Taken together, these findings suggest that the autumnal moth has a negative, partially plant-mediated impact on D. betulae, and can reduce the extent of current-year defoliation caused by D. betulae. This may be beneficial for the mountain birch, since the greater part of D. betulae damage occurs around or after the end of the larval period of the autumnal moth, which may be a critical time for tree recovery after moth outbreaks.     

Folivorous larvae on flowers: do autumnal moths benefit from catkins of the mountain birch?

Immature leaves of birches and other deciduous trees and shrubs are the principal food resource for the larvae of the autumnal moth, Epirrita autumnata (Borkhausen) (Lepidoptera: Geometridae). However, the larvae of this geometrid, which displays pronounced population cycles and causes massive forest defoliations in northernmost Europe, are also frequently found on the catkins of mountain birch, Betula pubescens ssp. czerepanovii (Orlova) Hämet-Ahti (Betulaceae). To examine whether autumnal moth larvae visit birch catkins for their high nutritional value, we conducted two growth experiments in which the larval diet was supplemented with male catkins, and a single test where female catkins were used. We found that the presence of male catkins in the larval diet had a positive effect on the larvae, in relation to their pupal mass (reflecting life span fecundity), survival probability, as well as the duration of the larval period. The presence of female catkins in the diet did not affect larval performance. Our findings suggest that young autumnal moth larvae actively move to male catkins, which provide a nutritional benefit and thus lead to a better performance compared with those feeding elsewhere. Another possible benefit for young larvae choosing the catkins instead of leaves may involve predator or parasitoid avoidance.     

Phenolic compounds of willow bark as deterrents against feeding by mountain hare

Summary Overwintering mountain hares (Lepus timidus) fed selectively on the shoots of a number of northern willow (Salix spp.) species. The hares preferred certain species over others and mature shoots over juvenile ones. There was a negative correlation between the phenolic glycoside concentration in the twigs and the hares' feeding. This correlation was substantiated by feeding experiments in which oat grains treated with purified phenoglycoside and catechin fractions of willow bark were offered along with untreated control oat grains to free-ranging mountain hares. Both fractions in concentrations normally found in willow twigs inhibited hare feeding. The results suggest that these phenolic compounds play a crucial role in the building up of resistance patterns among the willows. The decline in resistance in mature shoots of tall willows indicates that the juvenile resistance can be, perhaps secondarily, an adaptation against mammals browsing from ground level. Accordingly, low willow species retain a high level of resistance also in maturity.     

Do Insectivorous Birds use Volatile Organic Compounds from Plants as Olfactory Foraging Cues? Three Experimental Tests

Some insectivorous birds orient towards insect‐defoliated trees even when they do not see the foliar damage or the herbivores. There are, however, only a few studies that have examined the mechanisms behind this foraging behaviour. Previous studies suggest that birds can use olfactory foraging cues (e.g. volatile organic compounds (VOCs) emitted by defoliated plants), indirect visual cues or a combination of the two sensory cues. VOCs from insect‐defoliated plants are known to attract natural enemies of herbivores, and researchers have hypothesized that VOCs could also act as olfactory foraging cues for birds. We conducted three experiments across a range of spatial scales to test this hypothesis. In each experiment, birds were presented with olfactory cues and their behavioural responses or foraging outcomes were observed. In the first experiment, two different VOC blends, designed to simulate the volatile emissions of mountain birch (Betula pubescens ssp. czerepanovii) after defoliation by autumnal moth (Epirrita autumnata) larvae, were used in behavioural experiments in aviaries with pied flycatchers (Ficedula hypoleuca). The second experiment was a field‐based trial of bird foraging efficiency; the same VOC blends were applied to mountain birches, silver birches (B. pendula) and European white birches (B. pubescens) with plasticine larvae attached to the trees to serve as artificial prey for birds and provide a means to monitor predation rate. In the third experiment, the attractiveness of silver birch saplings defoliated by autumnal moth larvae versus intact controls was tested with great tits (Parus major) and blue tits (Cyanistes caeruleus) in an aviary. Birds did not orient towards either artificial or real trees with VOC supplements or towards herbivore‐damaged saplings when these saplings and undamaged alternatives were hidden from view. These findings do not support the hypothesis that olfactory foraging cues are necessary in the attraction of birds to herbivore‐damaged trees.     

Micro‐fungi and invertebrate herbivores on birch trees: fungal mediated plant–herbivore interactions or responses to host quality?

We studied interactions between microfungi and herbivores sharing a host tree. In a series of experiments and field observations over a 3‐year period, we compared phenotypic and genetic correlations of fungal frequencies and performance of invertebrate herbivores growing on mature half‐sib progenies of mountain birches (Betula pubescens ssp. czerepanovii) in two environments, a forested river valley and an adjacent higher‐elevation mountain birch woodland. We found little support for direct relation between fungal frequencies and performance of herbivore species. Instead, genetic correlations, particularly between autumnal moth (Epirrita autumnata) and rust fungus (Melampsoridium betulinum), suggest that herbivore performance may be caused by (1) genetic differences in plant quality for fungi and herbivores, or (2) genetic differences in responses to environmental conditions.     

The effect of establishing native woodland on habitat selection and ranging of moorland mountain hares (Lepus timidus), a flexible forager

Mountain hares Lepus timidus L. typify species that occupy a broad geographic range and have flexible foraging and nutritional strategies. Such species may show a range of responses to habitat modification. This study aimed to provide a basis for prediction of the impact of mountain hares on woodland establishment, and of woodland establishment on mountain hare distribution. The selection of and the extent of incorporation of new woodland into the home range of mountain hares was investigated in an area where Scots pine Pinus sylvestris L. woodland was establishing within their usual habitat in Britain, upland heather moorland. Seasonal home, day and night-range sizes of radio-tracked mountain hares were determined using the multinuclear probability polygon technique and analysed using residual maximum likelihood (REML). Habitat selection was analysed using compositional analysis. Three main habitat types were available to hares: heather moorland with trees, heather moorland and grassland-mire. Mean home-range size of mountain hares in summer was 10.3 ha and in winter 9.6 ha. There were no significant seasonal or sex differences in home-range size. Females selected grassland-mire habitat in summer and showed no strong selection for any habitat in winter. Males selected heather moorland in both summer and winter. Heather moorland with trees was not selected preferentially by mountain hares of either sex in summer or winter. The absence of selection for areas of newly establishing-Scots pine woodland suggests that any browsing damage to trees by hares is most likely to be a function of the local abundance of mountain hares, rather than a result of active preference of hares for the modified habitat.     

Does application of methyl jasmonate to birch mimic herbivory and attract insectivorous birds in nature?

Earlier studies have suggested that insectivorous birds, similar to invertebrate predators and parasitoids, may be guided by herbivore-induced plant volatiles (HIPVs) to damaged, herbivore-rich trees. Recent studies have also shown that birds use olfaction more than previously thought, underlying the potential for HIPVs to be sensed by insectivorous birds and utilised during foraging for prey. The HIPV production in plants is mediated, at least partly, by the jasmonic acid signalling pathway, and similar HIPVs to those induced by herbivores can often be induced by exposing plants to methyl jasmonate (MeJa). We studied the effects of MeJa on volatile emission and bird attraction using mature mountain birches (Betula pubescens ssp. czerepanovii) under natural conditions in northern Finland. Experimental trees were assigned to four treatment groups: herbivore-damaged [autumnal moth (Epirrita autumnata)], higher dose of MeJa (30 mM), lower dose of MeJa (15 mM) and control. All trees had three branches covered with mesh bags, but there were larvae inside the bags only of the herbivore-damage treatment. Bird predation rate was monitored with artificial plasticine larvae which were checked daily for peck marks. Birds most often pecked the larvae in the herbivore-damaged trees, but the attractiveness of MeJa-treated trees did not differ from the control. High within-treatment variation in systemic HIPV emissions probably masked MeJa treatment effects. The bird predation rate was high in birches that emitted large amounts of α-pinene. Thus, α-pinene may be one cue used by birds to find herbivore-rich birches.     

Expansion of the winter moth outbreak range: no restrictive effects of competition with the resident autumnal moth

1. Both direct and indirect competition can have profound effects on species abundance and expansion rates, especially for a species trying to strengthen a foothold in new areas, such as the winter moth (Operophtera brumata) currently in northernmost Finland. There, winter moths have overlapping outbreak ranges with autumnal moths (Epirrita autumnata), who also share the same host, the mountain birch (Betula pubescens ssp. czerepanovii). Competitive interactions are also possible, but so far unstudied, are explanations for the observed 1–3 years phase lag between the population cycles of the two moth species. 2. In two field experiments, we studied host plant‐mediated indirect inter‐specific competition and direct interference/exploitation competition between autumnal and winter moths. The experimental larvae were grown either with the competing species or with the same number of conspecifics until pupation. Inter‐specific competition was judged from differences in pupal mass (reflecting lifespan fecundity), larval development time and larval survival. 3. Larval performance measurements suggested that neither direct nor indirect inter‐specific competition with the autumnal moth reduce the growth rate of winter moth populations. Winter moths even had a higher probability of survival when reared together with autumnal moths. 4. Thus, we conclude that neither direct nor indirect inter‐specific competition is capable of suppressing the spread of the winter moth outbreak range and that both are also an unlikely cause for the phase lag between the phase‐locked population cycles of the two moth species.     

Food competition between a large ruminant and a small hindgut fermentor: the case of the roe deer and mountain hare

In this study, we demonstrate that the mountain hare and roe deer compete with each other. This was determined using "natural experiments" of populations found in sympatry and allopatry on the islands along the west coast of Norway. We demonstrate that both species occupy the same habitats, share the same food resources and that food availability is limited. Two browsing species as different in size as roe deer and mountain hare might be expected to partition the available vegetation (e.g. woody scrub) in terms of height above ground level. However, from the evidence collected, the feeding-height-separation hypothesis must be rejected as an explanation for ecological separation between roe deer and mountain hares because there was extensive height overlap in resource utilisation by both species and neither species changed its feeding height in response to the presence of the other. Total browse utilisation did not increase when both species were together; rather, species-specific browse utilisation declined when the other species was present. However, the foraging behaviour of each herbivore varied significantly between the allopatric and sympatric sites. When both herbivores were present, the clip diameter of shoots browsed by mountain hares declined to match those selected by roe deer, while roe deer switched from a browse-dominated diet to a diet dominated by winter-green gramineae. The change to smaller-diameter shoots likely resulted in the hare increasing its intake of digestion-inhibiting or toxic secondary metabolites, while the alternative choice of digging through the snow like roe deer to reach the winter-green gramineae is a practice considered energetically too costly for hares. On this basis, we conclude that the enforced switch to a nutritionally inferior diet by mountain hares at the sympatric sites may result in changes to growth rate and body size which consequently impact on mortality and may explain the competitive superiority of the roe deer.     

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.     

The diet of the mountain hare (Lepus timidus hibernicus) on coastal grassland

Across most of their range in Europe, mountain hares are usually restricted to upland areas with poor food quality. In these areas they generally feed on browse species such as heather or twigs and barks of trees. On lowland areas in Europe, with better food quality, the mountain hare is replaced by the brown hare ( Lepus europaeus ) which feeds predominantly on greasses. This khas led some authors to conclude that mountain hares are primarily adapted for browsing. In the absence of brown hares in Ireland, mountain hares are found on a wide variety of habitats including grassland. On grassland, their diet consists almost exclusively of grasses, up to 94% of their annual diet, which is more than has been reported for brown hares on similar habitat. Based on this evidence, and other work, it is proposed that the mountain hare in primarily a grazing animal and competitive exclusion by brown hares may underlie much of their present distribution in Europe.     

Testing the enemies hypothesis in forest stands: the important role of tree species composition

Numerous studies conducted in agro-ecosystems support the enemies hypothesis, which states that predators and parasites are more efficient in controlling pest densities in polycultures than in monocultures. Few similar studies, however, have been conducted in forest ecosystems, and we do not yet have evidence as to whether the enemies hypothesis holds true in forests. In a 2-year study, we investigated whether the survival of autumnal moth (Epirrita autumnata) larvae and pupae differs between silver birch monocultures and two-species mixtures of birch with black alder, Norway spruce and Scots pine. We placed young larvae on birch saplings and monitored their survival until the end of the larval period, when we checked whether they had been parasitized. After the larvae had pupated, pupal survival was tested in a field trial. In 2002, the larvae disappeared earlier and their overall survival was lower in birch–pine mixtures than in other stand types. In 2003, survival probability was lowest in birch–pine stands only during the first week and there were no differences between stands in overall survival. Larval parasitism was not affected by tree species composition. Pupal weight and pupal survival were likewise not affected by stand type. Among the predators, wood ants were more abundant on birches growing in birch–pine mixtures than in other stand types probably because colonies of myrmecophilic aphids were common on pines. In contrast, spider numbers did not differ between stand types. Ant exclusion by means of a glue ring around the birch trunk increased larval survival, indicating that ants are important predators of the autumnal moth larvae; differences in larval survival between stands are probably due to differential ant predation. Our results provide only partial support for the enemies hypothesis, and suggest that it is both tree species composition and species diversity which affect herbivore survival and predation.     

Effects of elevated ultraviolet-B radiation on a plant–herbivore interaction

Enhanced ultraviolet-B (UV-B) radiation may have multiple effects on both plants and animals and affect plant–herbivore interactions directly and indirectly by inducing changes in host plant quality. In this study, we examined combined effects of UV-B and herbivory on the defence of the mountain birch (Betula pubescens ssp. czerepanovii) and also the effects of enhanced UV-B radiation on a geometrid with an outbreak cycle: the autumnal moth (Epirrita autumnata). We established an experiment mimicking ozone depletion of 30% (a relevant level when simulating ozone depletion above Northern Lapland). Both arctic species responded only slightly to the enhanced level of UV-B radiation, which may indicate that these species are already adapted to a broader range of UV-B radiation. UV-B exposure slightly induced the accumulation of myricetin glycosides but had no significant effect on the contents of quercetin or kaempferol derivatives. Mountain birch seedlings responded more efficiently to herbivory wounding than to enhanced UV-B exposure. Herbivory induced the activities of foliar oxidases that had earlier been shown to impair both feeding and growth of moth larvae. In contrast, the contents of foliar phenolics did not show the same response in different clones, except for a decrease in the contents of tannin precursors. The induction of foliar phenoloxidase activities is a specific defence response of mountain birches against insect herbivory. To conclude, our results do not support the hypothesis that the outbreak cycle of the autumnal moth can be explained by the cycles of solar activity and UV-B.     

The occurrence of mountain hare mitochondrial DNA in wild brown hares

If interspecific hybrids are fertile and backcross to either parental species, transmission of mitochondrial DNA over the species barrier can occur. To investigate if such transmission has occurred between the brown hare Lepus europeus Pall and the mountain hare L. timidus L. in Scandinavia, an analysis of genetic variation in mitochondrial DNA from 36 hares, collected from 15 localities, was performed. Sequence divergence of mtDNA between species was estimated at 8 ± 1% (SD). Intraspecific mtDNA sequence divergence varied between 0.09 and 0.38% in brown hares and 0.10 and 1.44% in mountain hares. In six out of 18 brown hares examined, two different haplotypes of mountain hare origin were detected, demonstrating a transmission of mtDNA haplotypes from mountain hares to brown hares. The results indicate that interspecific hybridization between the two species occurs in wild populations.     

Factors related to the occurrence of hybrids between brown hares Lepus europaeus and mountain hares L. timidus in Sweden

Hybridization occurs among many species, and may have implications for conservation as well as for evolution. Interspecific gene flow between brown hares Lepus europaeus and mountain hares L. timidus has been documented in Sweden and in continental Europe, and has probably to some extent occurred throughout history in sympatric areas. What local factors or ecological relationships that correlate with or trigger hybridization between these species has however been unclear. We studied spatial distribution of hybrids between brown hares and mountain hares in Sweden in relation to characteristics of the sampled localities (hunting grounds). In a sample of 70 brown hares collected from 39 populations in south‐central Sweden during 2003–2005, 11 (16%) showed introgressed mtDNA from mountain hares. Among the brown hares from their northern range, i.e. in general the most recent establishments, the corresponding figure was 75% (9/12). The frequency of samples with hybrid ancestry increased significantly with latitude, altitude and hilliness, and were higher (p<0.1) in recently established populations and/or where the proportion of arable land was low. Several site‐specific parameters were correlated, e.g. latitude as expected to hilliness, and no parameter explained the occurrence of hybrids exclusively. Instead, the appearance of mountain hare mtDNA among brown hares was associated with a conglomerate of parameters reflecting landscapes atypical for the brown hare, e.g. forest dominated and steep areas where the species quite recently was established. We suggest that these abiotic factors mirror the main aspect influencing hybridization frequency, namely the density or relative frequency of the two species. In atypical brown hare landscapes with recent establishment, mountain hares are probably relatively more common. When one species dominate in numbers, or when both species display low densities, increased frequency of hybridization is expected due to low availability of conspecific partners, a phenomenon referred to as Hubbs’ principle.     

Impact of host plant quality on geometrid moth expansion on environmental and local population scales

The warming climate has enabled a rapid expansion of many forest pests. The adaptation potential of the invaders affects largely on how well the invasive species can spread to new areas and in what extent can they have an impact on the invaded ecosystem. To measure the adaptation potential of an invasive (winter moth) and a potentially invasive defoliating moth species (scarce umber moth), we examined life history parameters in two environments on a set of genetically diverse host trees and compared the traits with those of a resident moth species (autumnal moth). In addition, variations in life history parameters due to host genotype were calculated and compared. The bioassay was executed by rearing moth larvae on 11 half‐sib families of the host tree in two large tree line gardens. The use of half‐sib families allowed us to calculate the variances due to tree genotype and also to examine if the new arrivals can affect the selection pressure on the genetic population structure of the host. According to our results, the natural genetic variation in host plant quality and small environmental differences are not sufficiently effective to restrict the spread of the already established invasive species. The invading moth species may even be better at adapting to variation in food quality than the resident moth species. The overall effect of the natural variation in tree quality was similar for all three moth species. Therefore, the newcomers are likely to only inflict a quantitative rather than qualitative change in the selection pressure on the host.     

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.     

Reduction in size and fecundity of the autumnal moth, <Emphasis Type="Italic">Epirrita autumnata</Emphasis>, in the increase phase of a population cycle

Increasing fecundity with increasing density has been observed for many cyclic herbivore populations, including some forest Lepidoptera. We monitored population density, body size and reproductive capacity of the cyclic lepidopteran, the autumnal moth (Epirrita autumnata, Geometridae), from the early increase phase to the devastating outbreak density in northernmost Norway. Larval density of the species increased exponentially from 1998 to 2002 and remained at the outbreak level also in 2003. Within the same period, the body size and fecundity of individuals reduced as analysed from several parallel datasets on larvae, pupae and adults. In another study area in northernmost Finland, the density increase of the autumnal moth was moderate only, and true outbreak density was not attained during the study. Despite that, a reduction was again detected in the size and fecundity of individuals. Possible factors responsible for the reduced size and fecundity of individuals in the Norwegian population were quantitative shortage of foliage, rapid and delayed inducible resistances of the host, mountain birch (Betula pubescens ssp. czerepanovii), as well as crowding-induced responses of larvae. These factors likely acted in concert, although non-delayed responses to the density were emphasized. Our findings did not support the hypotheses of climatic release, inducible susceptibility of the host tree and mast depression (i.e. lowered chemical defence of the host tree after its mast seeding) as promoters of the fecundity-based density increase of the autumnal moth, since the reduced fecundity in relation to increased density was strongly against the predictions of these hypotheses. Therefore, we suggest that the density increase of autumnal moth populations is promoted by high survival rather than exceptionally high fecundity.     

The effect of bark phenols upon mountain hare barking of winter-dormant Scots pine

Due to high numbers of mountain hares in recent winters in northern Finland the barking of winter dormant Scots pines was widespread. The hares fed selectively upon upper crown bark of grafted pines, nitrogen fertilized trees and trees in poor condition. Bark of preferred upper crown branches and physiologically mature scions contained less total phenols than lower parts. However, there were no differences between total phenols concentrations of graft bark preferred by hares and nearby unbarked juvenile phase Scots pine. Neither decrease in resistance after nitrogen fertilization n- or stress was correlated with bark total phenolic concentration. Thus, total phenolic concentration is not a reliable predictor of the susceptibility of Scots pine to winter barking by the mountain hare.