Isolation and characterization of microsatellite markers in the tetraploid birch,Betula pubescens ssp. tortuosa

The mountain birch, Betula pubescens ssp. tortuosa is a tetraploid tree species which forms the tree limit in northern Fennoscandia. We identified nine polymorphic microsatellite loci in order to characterize the genetic structure of populations at different elevations close to the treeline. The microsatellite loci were highly polymorphic, with 14–42 alleles per locus and an average expected heterozygosity of 0.73 ± 0.25 under random chromosome segregation and 0.68 ± 0.23 under random chromatid segregation.     

Demography of Betula pubescens ssp. tortuosa sown in contrasting habitats close to the birch tree-limit in Central Sweden

Seeds of mountain birch (Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman) were sown on permanent plots in three contrasting habitats close to the birch tree-limit in Central Sweden. The habitats differed mainly in regard to the length of the growth period, as determined by the pattern of local snow-accumulation. Germination success, seedling demography (1981–1983) and related aspects of the early stages were studied. Germination was found to start shortly after snowmelt in the year after ripening, in all three habitats. The conclusion is that germination is not the crucial phase of the life-cycle. In the habitat with the longest growth period, and where spontaneous birch stands are present, mortality mainly occurred in summer, evidently caused by soil drought. With shortening of the growth period, however, mortality during the winter became increasingly important. Survival was clearly correlated with the length of the growth period and the autumn height of seedlings. Factors which affect ecesis determine the altitude of the tree-limit and the long-term stability of the mountain birch forest ecosystem.     

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.     

The influence of ants on the survival of mountain birches during an Oporinia autumnata (Lep., Geometridae) outbreak

Summary The present study reports the influence of ants on the survival of mountain birch (Betula pubescens ssp. tortuosa) during an Oporinia autumnata (Lep., Geometridae) outbreak. Undamaged green islands with a radius of 15–20 m were observed around Formica aquilonia mounds in a defoliated area in Finnish Lapland. The herbivore densities and grazing pressure were shown to be low within this radius. The foraging strategy of ants and their influence on birch growth are discussed.     

The influence of hybridization on epidermal properties of birch species and the consequences for palaeoclimatic interpretations

The Fennoscandian birch population primarily consists of Betula nana, B. pendula and B. pubescens ssp. czerepanovii, the Mountain birch. Frequent hybridization between the Mountain birch and B. nana generates a wide range of genotypic and phenotypic plasticity in the subarctic birch zone of Fennoscandia. Phases of subarctic conditions prevailed during the Late Glacial in large parts of NW Europe, and palynological as well as macrofossil analysis provide some evidence for the occurrence of birch hybrids during these intervals. Leaves from genetically controlled specimens of Betula pendula, B. pubescens ssp. czerepanovii, B. nana and the hybrids B. pubescens ssp. czerepanovii × nana and B. nana × pubescens ssp. czerepanovii are investigated for their specific characteristics of the epidermis morphology. Frequency and size of epidermal cells and stomata reveal a close affinity of both hybrids to B. nana and allow a differentiation of the intermediate forms between B. nana and the Mountain birch. With respect to palaeoatmospheric CO₂ reconstructions based on stomatal index, epidermal analysis shows that a possible occurrence of hybrids in fossil leaf assemblages has no profound consequences for combined species records. However, the significant differences observed in B. nana demand the separation of this species. A comparison of the cuticle properties of B. pendula and B. pubescens from Finnish Lapland and leaf material from The Netherlands reveals a divergence of the stomatal index that may be due to differences in day light length.     

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 start of the birch pollen season in Finnish Lapland: separating non‐local from local birch pollen and the implication for allergy sufferers

Determining the start of the birch pollen season requires the reliable separation of non‐local from locally produced birch pollen. The research was undertaken close to the latitudinal birch tree line at the Kevo Subarctic Research Institute (69°45′N 27°01′E) in northern Finland. By comparing phenological and aerobiological observations, the proportion of birch pollen present in the air before local anthesis commences can be delimited. We coupled this with data of pollen deposition monitored by means of a modified Tauber trap. The dominant birch species at Kevo is the mountain birch Betula pubescens ssp. czerepanovii, whereas B. pubescens ssp. pubescens is very rare, hence we consider the proportion of the southerly B. pubescens‐type pollen deposited in the pollen trap to be non‐local in origin.

We did not observe any trend towards an earlier start of the phenologically observed mountain birch anthesis at Kevo as predicted from work elsewhere. Moreover, the fixed 2.5% threshold method for determining the birch pollen season proved not to be applicable since in many years this threshold was reached before the end of continuous snow cover. The results indicate that in some years non‐local birch pollen contributes considerably to the allergen load in Lapland with up to 57% of the total birch pollen sum being recorded before the day on which local anthesis commenced, and up to 70% of the annual birch pollen deposited being of the southerly birch type.     

Induced resistance in mountain birch: defence against leaf-chewing insect guild and herbivore competition

Summary The effect of leaf damage simulating the feeding of early season insect herbivore species, e.g. Epirrita autumnata, to mountain birch, Betula pubescens ssp. tortuosa, on the performance of insect larvae was studied with eleven leaf-chewing sawfly species. I found variation in the results that was due to short- and long-term inducible responses and to the phenology of herbivore species. In general, early and mid-season species were more strongly affected by induced reactions than late-season species. This finding is in accordance with earlier results but I could show that the persistance of induced reactions rather than the influence of timing of damage is responsible for the result. The growth of the larvae of mid-season sawfly species was affected by both short- and long-term induced reactions. This result shows that early season species may escape short-term induced reactions of mountain birch in current year but may not avoid long-term effects. It is supposed that seasonal deterioration of leaf quality either masks the effects of induced defences or late-season species are better adapted to low-quality leaves. Some species show variation in their response to induced defence in different years. This may be due to yearly differences in induced reactions as well as to species-specific responses. Induced defence reactions may play a role in competitive interactions between herbivore species in leaf-chewing guild of mountain birch.     

Estimation of canopy density and irradiance in 20- to 40-year-old birch stands (Betula pubescens Ehrh. andBetula pendula Roth)

Three different experiments in birch (Betula pubescens Ehrh. andBetula pendula Roth) stands were conducted during summer 1991: an evaluation of the accuracy of the vertical tube method measuring crown free projection (CFP) in four birch stands; a study of relative irradiance (RI; photosynthetically active radiation, PAR), in birch stands differing in stem density and basal area; and an estimation of the CFP in different types of birch stands and the correlation between RI and CFP. The results indicate that the number of observation points well distributed over an area of 0.1 ha can vary between 50 and 200 to achieve sufficient accuracy. There were no significant differences in the CFP estimates made on the same plot by different observers including an untrained observer. CFP was related to basal area, stem number and diameter sum in a non-linear regression, CFP = A^*e^-BX. The correlations between RI and basal area, density and diameter sum, respectively, were 0.54, 0.57 and 0.67 for PAR. The coefficients of determination for RI (PAR) and CFP were high (0.86).     

A review of the sub-species carpatica and tortuosa within the species betula pubescens Ehrh.

Summary

The classifications employed during the last decade for defining sub-specific variation within the species B. pubescens Ehrh. have produced a situation in which the terms carpatica and tortuosa, originally and usually applied to separate taxa, could be regarded as synonymous. This has been caused by the association of both terms, at different times, with B. odorata Bechst. Recent research at Merlewood supports the use of the term ssp. tortuosa and its distinction from ssp. carpatica whose presence in the British flora requires further verification.     

Sources of variation in rapidly inducible responses to leaf damage in the mountain birch-insect herbivore system

Summary Studies on rapidly inducible resistance in trees against insect herbivores show substantial variation in the strength of responses. Here we report the results of a study which examined causes of this variation. We bioassayed the quality of leaves of two developmental phases (young vs. mature) of the mountain birch Betula pubescens ssp. tortuosa by measuring the growth of two instars of Epirrita autumnata larvae. We used only short shoot leaves from trees of a natural stand, uniform in size and age. Damage was caused by larvae and artificial tearing of leaf lamina, varying the scale and time. We separated seasonal changes in plants from instar-dependent effects of the animals by testing experimental larvae in two subsequent growth trials. We found that only larval-made damage induced responses in leaves that made the leaves significantly poorer quality for the test larvae. Artificial damage induced only weak responses, and artificial canopy-wide damage even caused slight improvement of leaf quality. Cumulative leaf damage did not strengthen birch responses. Leaves that were in the expansion phase responded to damage while fully-expanded, mature leaves showed no response. The pattern of responses indicated that there might be physiological constraints: small-scale damage induced resistance against the larvae but largescale damage did not. Prevalent weather conditions might have modified these responses. Larvae of two instars and sexes, of low- and high-density populations responded to leaf damage similarly. However, prior experience of larvae with the host plant may have affected subsequent larval performance. Variation in rapidly inducible responses in birches was caused by plant characters rather than by test animals.     

Limited decadal growth of mountain birch saplings has minor impact on surrounding tundra vegetation

Temperatures over the Arctic region are increasing at three times the rate of the global average. Consequently, Arctic vegetation is changing and trees are encroaching into the tundra. In this study, we examine the establishment and growth of mountain birch (Betula pubescens ssp. tortuosa), which forms the treeline in subarctic Europe, and its impact on community composition across the treeline ecotone nearby Abisko, Sweden. Birch advancement along elevational gradients was studied by comparing data collected in 2016 with data collected 10 and 15 years previously. Species identity, cover, and phylogenetic relatedness were used to assess the impact of birch encroachment on community composition. Our results show that birch occurrence above the treeline did not affect plant community composition, probably owing to the observed lack of significant growth due to herbivore browsing, nitrogen limitation, or a reduction in snow cover. Independent of birch performance, the tundra community structure shifted toward a novel community dissimilar from the forest plant community found below the treeline. Taken together, our findings are explained by species‐specific responses to climate change, rather than by a linear forest advance. Future treeline advancements are likely more restricted than previously expected.     

Tree limit dynamics of Betula pubescens ssp. tortuosa in relation to climate variability: evidence from central Sweden

Abstract. The tree limit of Betula pubescens ssp. tortuosa (mountain birch) in the southern Swedish Scandes was monitored during 1972–1992. The study included various aspects of growth, vigour and reproduction of Betula. The main focus was on the character and mechanics of the tree limit/climate equilibrium system. The tree limit, which changed in response to a temperature rise early this century, remained constant in position and tree physiognomy did not change, although the past 50 yr or so have been colder. Indeed, growth and reproductive effort and capacity decreased in this period. Obviously, most resources in Betula were used to resist stress in the mature phase, resulting in delayed recession of trees. Unless drastic warming occurs, stem dieback is predicted for the near future. Even a minorclimatic disturbance would have an effect, because resources are gradually being depleted. Radial growth correlated most closely with the mean temperature in July. Particularly at the present-day tree limit, the variance in annual growth could be largely explained by climatic factors. The hypothesis is that long-term tree limit dynamics during the late Holocene is merely a matter of fluctuations in vegetative vigour and stature of old individuals. At the population level, response to climatic variability appears to be greatly delayed.     

Simulated acid rain affects birch leaf endophyte populations

Endophytes were frequently isolated from mountain birch (Betula pubescens var. tortuosa (Ledeb.) Nyman) leaves at a subarctic site where natural air pollution is low. We tested whether simulated acid rain had any influence on the occurrence of endophytes. Dry controls with only ambient rain and irrigated controls treated with spring water of pH 6 were compared with acid treatments at pH 3 and pH 4, prepared by adding both sulphuric and nitric acids. Treatments began in 1985 and leaf samples were taken twice during the summer of 1992. Leaves were surface sterilized, five leaf disks from each leaf placed on malt extract agar, and growing colonies were counted and identified. The most frequently isolated endophyte from birch leaves was a Fusicladium anamorph of Venturia sp. (88% of all the isolates in July and 75% of all the isolates in August), followed by a sterile mycelium and Melanconium sp. The number of endophytes isolated and the species number increased from July to August. Endophytes were most frequently isolated from the basal part of the midrib. The percentage of colonization by endophytes was similar in short and long shoots. More endophytes were isolated from leaves of branches taken at 1 m height than at 2 m height. The stronger acid rain treatment (pH 3) reduced by approximately 25% the number of isolated endophytes in August. Treatments did not have any effect on species composition of endophyte assemblages in birch leaves. Offprint requests to: Marjo L. Helander.     

Leaf surface traits: overlooked determinants of birch resistance to herbivores and foliar micro-fungi?

Previous studies on the mechanisms of birch resistance to herbivores and foliar micro-fungi (both pathogenic and endophytic) have focused mainly on the role of internal leaf chemistry. In the present study, we examined genetic correlations between leaf surface traits (glandular trichome density and total concentrations of surface flavonoid aglycones) and occurrence of three species of foliar micro-fungi, one pathogenic rust (Melampsoridium betulinum) and two endophytic fungi (Fusicladium sp. and Melanconium sp.), and performance of autumnal moth larvae (Epirrita autumnata) in two birch species, Betula pubescens ssp. czerepanovii and B. pendula. The performance of autumnal moth larvae on B. pubescens ssp. czerepanovii was negatively correlated with density of glandular trichomes (RGR: r=–0.855; pupal mass: r=–0.709). In addition, rust infection was negatively correlated with trichome density in B. pendula (r=–0.675) and with epicuticular flavonoid aglycones in B. pubescens ssp. czerepanovii (r=–0.855). The frequency of the endophytic fungus Fusicladium sp., was related to epicuticular flavonoid aglycones (r=–0.782), while another endophytic fungus, Melanconium sp., showed no associations with any of the studied variables in B. pubescens ssp. czerepanovii. Our results indicate that leaf surface traits may be at least as important determinants of herbivore performance and micro-fungi abundance in birch as leaf internal chemistry.     

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.     

Physical Sheltering and Liming Improve Survival and Performance of Mountain Birch Seedlings: A 5-Year Study in a Heavily Polluted Industrial Barren

Denuded landscapes adjacent to big polluters represent an extremely harsh environment for plants due to a unique combination of soil toxicity and physical stress. In a 5‐year experiment we tested whether survival and performance of seedlings of Mountain birch (Betula pubescens ssp. czerepanovii) planted in two barren sites around the nickel–copper smelter at Monchegorsk, northwestern Russia, can be enhanced by physical sheltering and how large the supposed benefits to survival and performance are in relation to liming. Seedling performance was assessed by measuring growth parameters and chlorophyll fluorescence. Physical sheltering was found to be beneficial in some conditions: when soil characteristics were not so harsh as to cause 100% mortality, sheltering increased both survival and performance of birch seedlings. Although the benefits of liming on seedling performance and survival were stronger than the benefits of sheltering, sheltering may still have its uses in restoration when large‐scale liming is not applicable, for example, when the ecological side effects of liming are to be avoided.     

A High Concentration of Glucogallin, the Common Precursor of Hydrolyzable Tannins, Does Not Deter Herbivores

The diversity of structures of plant phenolic compounds suggests that their interactions with insect herbivores may be compound specific. In this study, we modified the natural covariances observed in mature leaves of mountain birch, Betula pubescens ssp. czerepanovii (Orlova) Hämet-Ahti, by supplying gallic acid, the common precursor of gallotannins, through the stems of cut branches. Only one gallotannin, glucogallin, was consequently increased, and responses to this change on larvae of Epirrita autumnata Bkh. were evaluated by choice and nonchoice experiments. Glucogallin-increased leaves were consumed equally to control leaves in a nonchoice situation and they were preferred by E. autumnata larvae when they had to choose. No other short-term postingestive effects in E. autumnata larvae were observed and therefore our studies did not suggest a defensive role for glucogallin.     

Potassium nutrition and free polyamines of Betula pendula Roth and Betula pubescens Ehrh.

Potassium and free polyamine concentrations in the leaves of silver birch (Betula pendula Roth) and downy birch (Betula pubescens Ehrh) were followed during three successive growing seasons 1996, 1997 and 1998 in order to define K deficiency levels.The highest foliar K concentrations were found in June. In August, the K concentrations were lower and remained quite stable in Betula pendula but varied in Betula pubescens. In addition to a common diamine, putrescine, and the polyamines, spermidine and spermine, also a less common diamine and polyamine, 1,3-diaminopropane and norspermidine were found in the birch leaves. The accumulation of both diamines, putrescine and 1,3-diaminopropane, was used to define the critical levels of K nutrition in birch leaves. Foliar K concentrations below 7–8 mg g ^–1 DW were found to correlate with the accumulation of diamines at most sampling dates.     

Growing seasons of Nordic mountain birch in northernmost Europe as indicated by long-term field studies and analyses of satellite images

The phenophases first greening (bud burst) and yellowing of Nordic mountain birch (Betula pubescens ssp.tortuosa, also called B. p. ssp. czerepanovii) were observed at three sites on the Kola Peninsula in northernmost Europe during the period 1964–2003, and at two sites in the trans-boundary Pasvik-Enare region during 1994–2003. The field observations were compared with satellite images based on the GIMMS-NDVI dataset covering 1982–2002 at the start and end of the growing season. A trend for a delay of first greening was observed at only one of the sites (Kandalaksha) over the 40 year period. This fits well with the delayed onset of the growing season for that site based on satellite images. No significant changes in time of greening at the other sites were found with either field observations or satellite analyses throughout the study period. These results differ from the earlier spring generally observed in other parts of Europe in recent decades. In the coldest regions of Europe, e.g. in northern high mountains and the northernmost continental areas, increased precipitation associated with the generally positive North Atlantic Oscillation in the last few decades has often fallen as snow. Increased snow may delay the time of onset of the growing season, although increased temperature generally causes earlier spring phenophases. Autumn yellowing of birch leaves tends towards an earlier date at all sites. Due to both later birch greening and earlier yellowing at the Kandalaksha site, the growing season there has also become significantly shorter during the years observed. The sites showing the most advanced yellowing in the field throughout the study period fit well with areas showing an earlier end of the growing season from satellite images covering 1982–2002. The earlier yellowing is highly correlated with a trend at the sites in autumn for earlier decreasing air temperature over the study period, indicating that this environmental factor is important also for autumn phenophases.