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.     

水分条件对白桦天然林生长影响的研究

针对全球变暖导致的降水格局变化,选择生长于不同水分条件（湿生立地和中生立地）的中龄天然白桦林为研究对象,研究其生长状况并分析其生长过程。结果表明,水分条件对白桦天然林的密度和平均胸径具有显著影响（p=0.043 3和0.007 9）,中生立地条件下白桦天然林的密度低于湿生立地（降低46.8%）,而平均胸径则前者高于后者（增加52.4%）。中生立地白桦天然林的胸径分布为正态分布（p=0.823 9）,而湿生立地白桦胸径分布呈明显的左偏（p=0.000 8）。水分条件能影响实生天然白桦胸径、树高生长过程,而对材积生长过程影响不大。湿生立地白桦的胸径速生期持续时间长于中生立地,前者速生期在6~15 a,后者在9~12 a,中生立地白桦胸径的数量成熟龄晚于湿生立地1~2 a;湿生和中生立地白桦高生长的速生期均在6 a,中生立地白桦树高的数量成熟龄晚于湿生立地3~6 a;湿生和中生立地白桦材积的速生期均在24 a以后,湿生和中生立地白桦材积的数量成熟龄均为24 a以后。     

Top‐down effects on population dynamics of Eriocrania miners (Lepidoptera) under pollution impact: does an enemy‐free space exist?

In areas disturbed by pollution, populations of herbivorous insects may reach high densities. This study was conducted to test one of the hypotheses attempting to explain this phenomenon – that pollution creates an enemy-free space for herbivores. We monitored the population densities of Eriocrania leaf-mining moths on mountain birch, Betula pubescens subsp. czerepanovii , in the vicinity of the nickel–copper smelter in Monchegorsk (Kola Peninsula, NW Russia) over twelve years (1994–2005) and assessed larval mortality from parasitoids, ants and birds. The mean density (mines/birch leaf area) of Eriocrania populations in severely disturbed habitats (industrial barrens) was about 2.7 times higher, and peak densities 2–4 times higher, than in pristine forests. Temporal population variability (measured as the coefficient of variation of log-transformed densities) increased with an increase in pollution load. The proportion of infested trees was not affected by pollution, but mine distribution among trees was more clumped in the polluted sites. Eriocrania populations in disturbed sites fluctuated independently of each other, whereas populations in forest sites fluctuated in synchrony. Larval mortality caused by parasitoids was lower in disturbed sites only during those years when populations of Eriocrania reached high densities; mortality from ants and birds did not differ between disturbed and undisturbed habitats in either high or low density years. In undisturbed forests the rate of population change correlated negatively with previous-year parasitism, suggesting that parasitoids are the key demographic factor in Eriocrania population dynamics. In the habitats heavily disturbed by pollution no such correlation was found, which means that negative feedback with parasitoids is disrupted: parasitoids fail to follow host population growth, thus creating an enemy-free space for Eriocrania leafminers.     

Stature of sub-arctic birch in relation to growth rate, lifespan and tree form

BACKGROUND AND AIMS: Sub-arctic mountain birch Betula pubescens var. pumila communities in the North Atlantic region are of variable stature, ranging from prostrate scrubs to forests with trees up to 12 m high. Four hypotheses were tested, relating growth and population characteristics of sub-arctic birch woodland and scrub to tree stature; i.e. the variable stature of birch woods is due to differences in (1) the mean growth rate; (2) the age-related patterns of growth rate; (3) the life expectancy of stems; or (4) the tree form. Methods: A stratified random sample of 300 birch trees was drawn from the total population of indigenous birch woodlands and scrub in Iceland, yielding 286 valid sample genets. The population was divided into three sub-populations with dominant trees 0-2, 2-4 and 4-12 m tall, referred to as birch scrub, birch scrub-woodland and birch forest, respectively. KEY RESULTS: Trees in the scrub population were of more contorted growth form than birch in the scrub-woodland and forest populations. Mean growth rates, mean age and median life expectancies increased significantly with sub-population of greater tree stature. At the population level, annual increment and longevity of birch stems was apparently interrelated as the stems in vigorously growing birch sub-populations had a longer life expectancy than those of slower growth. However, no difference was observed between sub-populations in age-related patterns of extension growth rate. CONCLUSIONS: The results were consistent with hypotheses (1), (3) and (4), but hypothesis (2) was rejected. Hence, mountain birch of more vigorous growth attains a greater stature than birch of lesser increment due to faster extension growth rate and a longer lifespan. In addition, the more contorted stem form of scrub populations contributes to their low stature.     

Water availability as driver of birch mortality in Hustai National Park,Mongolia

Trees in the forest-steppe ecotones face stress due to reduced water availability as a consequence of more extreme seasonal fluctuations in precipitation and temperature. Together with browsing pressure this can hinder tree growth, tree regeneration and competition between trees and grasses. We studied the impact of both stress factors on the mortality of birch trees in two forest sites at Hustai National Park, Mongolia, by applying tree-ring research to determine growth-limiting factors and assessing browsing pressure on young and adult birch. We expected warm and dry summer conditions as main growth limiting factor. Moreover, we expected a positive relation between deer density and tree mortality with browsing mainly affecting smaller trees with a low diameter at breast height (DBH). We found that the growth in both birch populations is mainly driven by winter precipitation and – to a lesser extent – negatively affected by high summer temperature. This suggests that water availability as defined by soil moisture, especially at the beginning of the growing season is crucial for birch growth in our study area. For mortality we found significant differences between both populations, but no significant relationship with deer density. In plots with high mortality rates mean tree height of the remaining living trees decreased. These results suggest that under expected climate change with declining annual precipitation rates, the birch forest of Hustai National Park is converting into a steppe ecosystem, like described for other forest ecosystems in this ecotone.     

Spontaneous Establishment of Woody Plants in Central European Derelict Sites and their Potential for Reclamation

The performance of woody plants was analyzed in 15 successional seres starting at bare ground in central European manmade habitats. The total cover of woody species after 10 years of succession was significantly related neither to initial soil moisture nor to nitrogen (expressed using Ellenberg indicator values). But the comparison of seres indicates that establishment of woody plants was easier under moderate environmental conditions and retarded in extreme habitats (dry, nutrient-poor, or acid). The arrival of the first woody plants was delayed in dry sites. No significant differences were found between primary and secondary seres, either with respect to the total cover of woody plants reached after 10 years of succession or considering the time of their arrival. In total, 24 woody species (10 shrubs and 14 trees) appeared in the series investigated. Their successional performance (in terms of the number of seres in which the species occurred and maximum cover reached in any sere) was not related to species traits (life strategy, type of mycorrhizae, mode of dispersal, diaspore weight), except for the regeneration strategy, species with seasonal regeneration by seeds were capable of creating higher cover. Betula pendula (European birch) was the most successful species in spontaneous succession, especially on moist sites. Practical suggestions for the management of particular habitats (sites disturbed by mining, sites reclaimed after acid rain deforestation, urban sites, abandoned fields) are provided regarding the establishment of woody plants.     

Growth models of silver birch (Betula pendula Roth.) on two volcanic mountains in the French Massif Central

Silver birch woodlands of two volcanic mountains (altitude 850 m and 1450 m) were studied in order to establish a growth model of birch. Height, radial increment and crown width were measured on both sites taking into account two situations: isolated birch or birch within a stand. For the latter case two categories were sampled considering the competition status of the tree: birch suffering the most severe competition (suppressed tree) were distinguished from trees facing the lowest competition (dominant tree). Measures of competition were also made using plots where each tree was located by its coordinates and its height, trunk circumference and crown width recorded. Examination of radial growth curves showed similar patterns for the two sites for the same category of tree. Radial growth was always inferior for the site located at the highest altitude but tree lifespan was about two-fold longer. Radial increment data were used to calculate circumference as function of tree age. Models predicting circumference with time were then established for each category of tree (dominant, suppressed or isolated) using Chapman–Richards'model. Height and crown width models were produced using circumference as the predictor. Competition indices based on vertical or horizontal angles weighted by the distance were calculated for birches in stands. Indices using vertical angles appeared to be more resistant and robust to characterize local competition. Competition index was then associated with the parameters of Chapman-Richards' growth circumference model for the two sites and models predicting the circumference from the age of the tree and its competition index are proposed.     

Summer grazing by voles and lemmings upon subarctic snow-bed and tall herb meadow vegetation – an enclosure experiment

Ther hypothesis of the exploitation of ecosystems predicts that plant communities in productive habitats should experience a low grazing pressure, while communities in less productive habitats should be structured by intense grazing. Thus, if a high population density of rodents was to be established in these communities and they could graze without predation, the plant community in the productive habitat should be more affected than the community in the less productive habitat. This was tested by building enclosures for grey-sided voles Clethrionomys rufocanus and lemmings Lemmus lemmus in a productive tall herb meadow and a less productive snow-bed on Finnmarksvidda, northern Norway. The simulated rodent population densities varied from 150 to 750 ind. ha^-1, and the rodents were allowed to graze for 18 to 55 d. The shoot mortality of relatively abundant plant species were monitored together with plant community structure, and the above-ground biomass were harvested at the end of the experiment. The shoot mortality were generally low, the significantly grazed species were either known preferred species or species with a grazing-sensitive morphology. The community structure did not show any major changes. The aboveground biomass of woody plants was significantly lower in the enclosures on the snow-bed, but not in the tall herb meadow. The total biomass did not differ significantly. These results are somewhat ambiguous with reference to the predictions from the food chain hypothesis, and the conclusion were drawn that intense grazing by voles and lemmings during 18 to 55 d during the growing season does not seem to be sufficient to greatly influence shoot mortality and structure of the studied plant communities.     

马拉维Nyika高原三种溪流鱼类的生境类型

本研究调查了生活在非洲马拉维北部Nyika高原三种溪流鱼类的栖息地.分3个时期选择3条河流的24个地点进行取样,每个样点长100 m左右;分急流和回水湾两种生境类型作为独立的栖息地单元取样,据此观察研究对象和物理环境间的关系;用喜好指数和逻辑回归分析每种鱼重要栖息地是否稳定.瞻星平鳍鲇(Amphilius uranoscopus)通常喜欢生活在水深小于50 cm、流速0.4-1 m/s的缓流河段,这里水色较深,河底多卵石.黄鱼(Labeobarbus johnstonii)常被发现在水深大于50 cm的河段,河水流速中等或较急.引入种虹鳟(Oncorhynchus mykiss)的活动范围常局限在水温低于20℃的河段,这些河段水深中等,流速较快,沿河多植物.虹鳟栖息地与土著种相似,可能与同域分布的土著种,特别是与广泛分布的瞻星平鳍鲇存在生存竞争.     

Winter diet, habitat selection and fluctuation of a mountain hare Lepus timidus population in Finnish Forest Lapland

Composition of the winter diet, habitat selection and population fluctuations in the mountain hare Lepus timidus were studied in the Värriötunturi fell area, Eastern Finnish Forest Lapland, during the winters 1968/69–1984/85. The three population lows recorded during this 17-year period followed each other at intervals of 4 and 8 years. During the lows the hares occurred only in the most favoured (forest-covered) habitats and in two of them they behaved according to the concept of the refuge theory. The mountain birch Betula pubescens ssp. tortuosa appeared to be the most important, but not the most favoured winter food item. When the population crashed, the proportion of birch in the diet decreased, and was replaced especially by juniper which is one of the secondary food items (and is for this reason often discarded although cut). It is suggested that the quality and/or quantity of the winter food (i.e. winter pastures) are one of the driving forces in the population fluctuation of the mountain hare in this area.     

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.     

The demography of a clonal herbaceous perennial plant, the longleaved sundew Drosera intermedia, in different heathland habitats

Eight populations of longleaved sundew Drosera intermedia Hayne ramets were studied in three hydrologically and phytosociologically differing heathland habitats over a 4 yr period a pool edge, an old path through the wet heath, a seepage area At the extreme sites, with high levels of water table fluctuations (pool edge and dry path), both survival and recruitment were low, resulting in small populations The pool edge populations went extinct At the less extreme sites (wet paths and seepage area), populations were stable, but ramet and genet turnover was higher at the path sites than in the seepage area Demographic variation between populations was explained by differences in adult survival and sexual recruitment (asexual recruitment did not differ clearly), which, in turn, were related to specific biotic and abiotic factors Likely adult mortality causes were summer drought (all habitats), winter frost (path habitat), cover with a crust of iron bacteria after heavy summer rainfall (seepage area) and shading by Sphagnum and algae (pool edge) Low seedhng recruitment was caused by a low germination succes (pool, seepage area) and seedling survival (seepage area, dry path)     

Changing demography and dispersal behaviour: ecological adaptations in an alpine butterfly

High mountain ecosystems are extreme habitats for all organisms and therefore demand specific adaptations. In this context, we studied the ecology of the butterfly Euphydryas aurinia debilis in the High Tauern (Austria) and compared the obtained data against the ecology of the species in lower elevation habitats. We performed mark-release-recapture studies over the entire flight periods (end of June to end of July) in 2007 and 2008 to analyse the fundamental ecological parameters of a population. The demography of males and females was similar in both years, and no indication of typical protandry was detected. We observed a generally low dispersal of the individuals in both years, but males dispersed significantly more than females in 2008; this finding of low vagility was supported by allozyme analyses. Furthermore, butterflies survived periods of several days of continuously closed snow cover without any indication of increased mortality rates. In these three traits, this alpine population of E. aurinia apparently has ecological and physiological adaptations to the extreme requirements of high-altitude habitats and strongly deviates from the lower elevation populations.     

Survival types of high mountain plants under extreme temperatures

Extreme temperatures are a main factor limiting plant growth in high mountain habitats. During winter, the risk of frost damage is highest at windblown and often snow-free sites. During summer, actively growing plants are particularly endangered by episodic cold spells, but also by short-term overheating. The current review gives an overview of extreme temperatures in the European Alps and observations of temperature damage on plants in their natural habitats. Furthermore, seasonal time courses of frost and heat resistance derived from laboratory tests on different plant growth forms are presented. Study species were the cushion plants Silene acaulis, Minuartia sedoides, Saxifraga oppositifolia and Carex firma collected on wind-exposed ridges; the rosette plant Soldanella alpina collected on snow-protected sites, and three Sempervivum species collected in xerothermic habitats.     

Environmental correlates of multiple-stem formation in Betula pubescens ssp. tortuosa

Mountain birch (Betula pubescens ssp. tortuosa (Ledeb.) Nyman) shows extensive variation in the number of trunks per genet. Previous investigations have suggested that polycormism (multiple-stem formation) is associated with soils that have a low nutrient content and also with sites that have relatively shallow snow cover. These studies however lack quantitative measurements on the number of stems per genet. Canonical correspondence analysis, regression analysis and correlation analysis were used to investigate the environmental correlates of polycormism in mountain birch in Swedish Lapland. The best predictor for polycormism is the pH of the B and C soil horizons: pure monocormous stands are expected in habitats with pH 5.7 or higher, and polycormism increases with decreasing soil pH. Multiple-stem formation at high pH values is associated with waterlogging of the soil and the composition of the vegetation of the field layer is correlated with polycormism along a pH gradient. Betula pubescens ssp. tortuosa regenerates mainly via sprouts from the stem base: root sprouts and seedlings are uncommon. Decreased vigour of the tallest stems in polycormous units indicated that selfthinning in the classical sense (i.e. mortality of small stems due to suppression by larger ones) does not play a role in the dynamics of polycormous stands.     

Determinants of mountain birch growth in situ: effects of temperature and herbivory

Variation in in situ growth performance of the mountain birch as indicated by the widths of annual rings was analysed and related mainly to temperature and herbivory using ring width series from five heath forest sites in the Lake Torneträsk area, northern Sweden. Climate explained 48–64% of the variation in age-corrected mean ring width series. In general, the effect of current year July followed by June temperature was most important at all sites. A warm May resulted in wider rings due to an earlier budburst. Short-term (inter-annual) responses to increased temperature were in most cases not reflected into long-term responses (decades). A large proportion of the variation in stem mean ring width was due to variation among stems within trees (81%) in these polycormic trees, while variation among sites was marginal (0.4%). Within trees, main stems grew faster and were more responsive to climate variation than subordinate stems. No effect of insect herbivory on ring width was found at low defoliation levels (≤12%). At a defoliation level of ca 84% a one-year reduction in stem growth was observed while the growth reduction (ca 50% reduction in ring width) lasted for 4 yr after ca 93% defoliation. After outbreaks resulting in complete defoliation and some stem mortality, ring widths of surviving stems mainly responded with increased growth. Basal sprouts, emerging just after a severe insect outbreak with a high mortality of old stems, grew faster than sprouts occurring during other periods. It is concluded that the mountain birch is well adapted to recover from Epirrita outbreaks; the ability to produce basal sprouts, that can benefit from an existing root system for fast initial growth, is one important mechanism for this.     

Ecological condition of coral reef assemblages in the Cuban Archipelago

The condition of coral reefs in the Cuban Archipelago is poorly known. We aimed to analyse coral assemblages across 199 reef sites belonging to 12 localities. Crest and fore reefs were assessed using six metrics: species richness, density, coral cover, mortality, coral size and reef complexity. The condition of reefs varied across the archipelago from healthy to depleted reefs. The localities with best scores were Cienfuegos, Bahía de Cochinos and Cazones. These reefs have values of living coral cover (>20%) and complexity (>50?cm) similar to the best preserved Caribbean reefs. However, the majority of crest biotopes suffered important deterioration with old mortality of Acropora palmata populations and moderate coral cover (15%); although crest reefs still maintained their structural complexity. Despite moderate levels of coral cover in fore reefs (18%), their condition was alarming because 25% of the sites had cover below the recovery threshold of 10%, accumulated mortality and structural flattening. Compared with the 1980s, the species richness was roughly the same (42) for crest and fore reefs, although dominance has changed to widespread tolerant species. Coral reef assemblages varied at local and regional scales in similar magnitude, suggesting the combined effects of natural and anthropogenic drivers.     

Tree pollen viability in areas subject to high pollutant deposition

Summary The wet deposition process, including fog, is a major pathway of acidic pollutants from the atmosphere to the biosphere. Pollen grains are known to be sensitive biological indicators of pollution at this interface. In southern Québec, pollen viability of three hardwood species (sugar maple, white and yellow birch), was significantly reduced above the mean cloud base height (600 m) for two successive years (1987–1988) at two mountain sites (Roundtop and Tremblant). Ozone concentrations, fog water and precipitation pH values over the sampling periods are discussed.     

Silver birch shows nonlinear responses to moisture availability and temperature in the eastern Baltic Sea region

Silver birch (Betula pendula Roth.) is a widespread species with a high potential for aiding sustainability and multifunctionality of European forests, as evidenced in Finland and the Baltics. However, under increasing relevance of climate change for tree growth, the meteorological sensitivity of the species is largely unknown, presuming it to be weather tolerant (low sensitivity). Considering local adaptations of populations of widespread species, climatic changes are subjecting trees to extreme conditions, thus testing their adaptability. Accordingly, information on the plasticity (variability) of responses across a gradient of meteorological conditions is crucial for reliable predictions of tree growth. Tree-ring width network was established to assess the plasticity of growth responses of silver birch to meteorological conditions across the eastern Baltic climatic gradient. Time series analysis in combination with generalized additive modelling were applied to assess responses of birch from 21 naturally regenerated conventionally managed stands scattered from southern Finland to northern Germany. Despite the presumed tolerance, explicit meteorological sensitivity of silver birch was estimated. A gradient of local linear weather-growth relationships was estimated, as growth limitation shifted from temperature during the dormancy to water availability during vegetation period in southern Finland and northern Germany, respectively. However, these relationships were nonstationary, as the effect of summer water shortage was intensifying and sensitivity to it has likely been subjected to local adaptation. The regional generalization revealed presence of stationary, yet nonlinear and plastic growth responses, implying disproportional effects of climatic changes. Such responses also explained the nonstationarities, as the local climates shifted along the regional gradient. At the regional scale, summer water shortage was the main driver of increment, while winter conditions had a secondary role; temperature of the preceding vegetation season also had an effect on increment. Accordingly, increased variability of increment of silver birch is expected under changing climate; still, sensitivity and plasticity of increment can be considered as an adaptation to shifting environments.     

Interpreting Space Use and Behavior of Blue Tang, <Emphasis Type="Italic">Acanthurus coeruleus</Emphasis>, in the Context of Habitat,Density, and Intra-specific Interactions

Synopsis We hypothesized that blue tang, Acanthurus coeruleus, territories on sites with low biogenic structure would be larger than territories on sites with relatively high biogenic structure due to differences in the amount and distribution of resources. We tested this hypothesis by tracking blue tang over uncolonized pavement and reef crest, two habitat types at opposite ends of the habitat structure spectrum. We recorded density, feeding rates and aggression events in order to evaluate our findings in the context of a territory model and the ideal free distribution model. Territories of A. coeruleus averaged nearly four times larger on pavement sites than on reef crest sites. Conversely, densities of A. coeruleus were significantly lower on pavement sites. While there was no significant difference in the average rates of movement between habitats, average turning angles were significantly higher on reef crest. There were no significant differences in feeding rates between habitats, suggesting that higher territory sizes and lower densities may allow fish on uncolonized pavement to match resource acquisition of fish on reef crest. The insignificant difference of aggression encounters between habitats suggests that movement and density differences among habitats are not solely legacies of differential settlement.