The impact of shrub browsing by mountain hare and reindeer in subarctic Sweden

Background: Climate warming has been causing an increase in tall shrub cover around the Arctic, however, mammalian herbivory has been shown to inhibit shrub expansion. Though the effect of reindeer (Rangifer tarandus) and many other mammals has been widely studied in this context, the role of the mountain hare (Lepus timidus) in subarctic Scandinavia remains unknown.

Aims: To quantify browsing from mountain hare and reindeer on tall shrubs in different vegetation types and to investigate differences in shrub preference between the two.

Methods: In the summers of 2013 and 2014, we counted signs of browsing by hare and reindeer on tall shrub species in 31 study plots at three alpine locations in the Scandes range, Sweden.

Results: Hare browsing was significantly more frequent than that by reindeer in two (dry-mesic heath and dry meadow) out of seven vegetation types studied. Reindeer browsing was significantly higher in the low herb meadow and Långfjället shrub heath. Two shrub species, Betula nana and Salix hastata, were significantly more browsed by hare, while reindeer browsing was significantly higher on S. phylicifolia and S. lapponum.

Conclusions: Our results show that mountain hares can cause extensive damage to tall shrubs in the subarctic and may have a stronger impact on shrub communities than previously recognised.     

Rapid evolution towards heavy metal resistance by mountain birch around two subarctic copper-nickel smelters

Adaptations to pollution among long‐lived trees have rarely been documented, possibly because of their long reproductive cycles and the evolutionarily short timescales of anthropogenic pollution. Here, I present the results of a greenhouse experiment that suggest rapid evolutionary adaptation of mountain birch [Betula pubescens subsp. czerepanovii (Orlova) Hämet‐Ahti] to heavy metal (HM) stress around two copper–nickel smelters in NW Russia. The adaptation incurs a cost with reduced performance of adapted seedlings in pristine conditions. The industrial barrens around the studied smelters are extremely high‐stress sites with low seed germination and survival. It is likely that strong natural selection has eliminated all sensitive genotypes within one or two generations, with only the most tolerant individuals persisting and producing adapted seeds in the individual barrens. The results were similar from around both smelters, suggesting parallel evolution towards HM resistance.     

Individual and combined effects of disturbance and N addition on understorey vegetation in a subarctic mountain birch forest

Questions: What are the effects of repeated disturbance and N‐fertilization on plant community structure in a mountain birch forest? What is the role of enhanced nutrient availability in recovery of understorey vegetation after repeated disturbance? How are responses of soil micro‐organisms to disturbance and N‐fertilization reflected in nutrient allocation patterns and recovery of understorey vegetation after disturbance? Location: Subarctic mountain birch forest, Finland. Methods: We conducted a fully factorial experiment with annual treatments of disturbance (two levels) and N‐fertilization (four levels) during 1998–2002. We monitored treatment effects on above‐ground plant biomass, plant community structure and plant and soil nutrient concentrations. Results: Both disturbance and N‐fertilization increased the relative biomass of graminoids. The increase of relative biomass of graminoids in the disturbance treatment was over twice that of the highest N‐fertilization level, and N‐fertilization further increased their relative biomass after disturbance. As repeated disturbance broke the dominance of evergreen dwarf shrubs, it resulted in a situation where deciduous species, graminoids and herbs dominated the plant community. Although relative biomass of deciduous dwarf shrubs declined with N‐fertilization, it did not cause a shift in plant community structure, as evergreen dwarf shrubs remained dominant. Both disturbance and N‐fertilization increased the N concentration in vascular plants, whereas microbial biomass N and C were not affected by the treatments. Concentrations of NH₄⁺, dissolved organic N (DON) and dissolved organic C (DOC) increased in the soil after N‐fertilization, whereas concentrations of NH₄⁺ and DON decreased after disturbance. Conclusions: Disturbances caused by e.g. humans or herbivores contribute more to changes in the understorey vegetation structure than increased levels of N in subarctic vegetation. Fertilization accelerated the recovery potential after repeated disturbance in graminoids. Microbial activities did not limit plant growth.     

Effects of reindeer grazing on understorey vegetation in dry Pinus sylvestris forests

Abstract. Data on floristic composition and environmental variables were collected in floristically homogeneous oligotrophic pine (Pinus sylvestris) forests with heath-like under- storey vegetation in eastern Fennoscandia, and ordinated by non-linear multidimensional scaling (NMDS) in order to study the effect of lichen grazing by reindeer on the understorey vegetation. The study sites included areas with varying grazing pressure, as well as 50-yr old grazing exclosures. Sites rich in respectively bryophytes and lichens were placed at opposite ends of the ordination axes, and heavily grazed sites were placed in between them. Reindeer grazing increased the abundance of bryophytes, especially Dicranum spp. and Pleurozium schreberi. Grazing changed the vegetation to the extent that it resembled more mesotrophic sites, but this did not show any relationship with tree volume or other site productivity indicators. This was observed both in the ordination and, in a more compelling way, when exclosures with adjacent grazed areas were compared. No such signs were evident at ungrazed sites, where especially Cladina spp. spatially replace Cladonia spp. and tiny bryophytes like Barbilophozia spp., Polytrichum spp. and Pohlia nutans during succession. Cladina stellaris had almost disappeared from the most intensively grazed sites. The soil at ungrazed sites was characterized by high Al and Fe concentrations and bryophyte-rich sites by high Mn concentrations. Shannon's diversity index, depth of humus layer and proportion of bare ground also increased in sites getting richer in bryophytes.     

Growth of reindeer lichens and effects of reindeer grazing on ground cover vegetation in a Scots pine forest and a subarctic heathland in Finnish Lapland

Reindeer lichens are an important component of northern ecosystems. The aim of this study was to measure the growth rate of terricolous lichens as it is a key parameter involved in productivity of these ecosystems and an important part of lichen tolerance to reindeer grazing. Furthermore, the natural succession and the long-term effects of reindeer grazing on lichen community characteristics in two contrasting habitats were investigated as well as the interactions between lichen cover and mosses and vascular plants. Biomass and coverage measurements were conducted in a lichen woodland and in a subarctic heath with grazed and ungrazed areas in northern Finland. Measurements spanning over 13 yr of undisturbed development show that the growth rate of Cladina stellaris can be as high as >0.17 g g⁻¹ produced annually, although in average growth rates were much lower. During the succession of ground vegetation, C. stellaris , C. rangiferina , C. mitis and Cetraria nivalis increased in biomass in fenced areas and were reduced most in biomass by reindeer in unprotected areas. Reindeer grazing and trampling seem to change the vegetation towards a type that is dominated by small dwarf shrubs, bare soil and minute-cup lichens ( Cladonia spp.). Removing the lichen layer by reindeer may reduce natural regeneration of pine trees as implied by increasing numbers of pine seedlings with increasing lichen cover.     

Seasonal vegetation changes in mountain pastures due to simulated effects of cattle grazing

Cattle influence grassland dynamics in three ways: herbage removal, dung deposition and trampling. The objective of this study was to assess the effects of these factors, separately or in combination, and to compare them with cattle grazing over a one year period in a field experiment conducted in the Jura Mountains of northwestern Switzerland. A set of controlled treatments simulating the three factors was applied in a fenced area: (1) repeated mowing — three levels; (2) intensive trampling — two levels; (3) manuring with a liquid mixture of dung and urine — three levels. All treatments were applied homogeneously to the entire surface of each of the 40 plots inside the exclosure. Additionally, ten plots outside the fenced area represented reference plots with regular cattle pasturing. The multivariate response of species composition was assessed three times with the point‐intercept method: in spring before the treatments, in autumn after one season of treatments and at the beginning of the following year after winter rest. Multivariate analyses of vegetation data in the first year showed an overwhelming seasonal shift and significant differences induced by treatments. Abandoned and manured plots showed the largest deviation from the cattle grazed reference. Herbage removal, simulated by repeated mowing, appeared to be the most important factor for maintaining vegetation texture. Seasonal treatment effects were only partially carried over to the next spring, showing an unexpected resilience of the plant community, probably due to life‐history traits and competition release following climatic disturbance in winter.     

Interactions between mountain birch seedlings from differentiated populations in contrasting environments of subarctic Russia

So far very few experiments have accounted for the combined effect of two phenomena co-occurring in stress gradients: local adaptation to stress and the increase in facilitation with increasing stress (predicted by the stress-gradient hypothesis, SGH). Mountain birch (Betula pubescens subsp. czerepanovii) facilitates conspecific seedlings in subarctic high stress sites and is capable of rapid evolutionary adaptation, being therefore a good model species for a study combining local ecotypes and SGH. A within-species experiment was conducted to test SGH in three stress gradients, detect potential local adaptations between low and high stress populations, and assess their effects on seedling-seedling interactions. Although no evidence for local adaptation was detected, high and low stress populations showed some differentiation, possibly explained by decreasing phenotypic plasticity in high stress conditions and/or neutral evolutionary mechanisms. Weak support for SGH was detected. While facilitation was unaffected by seedling origin, low stress populations showed better competitive ability.     

Long- and short-term effects of reindeer grazing on tundra wetland vegetation

We studied long-term (50 years) and short-term (4 years) effects of summer grazing of reindeer on subarctic tundra wetland vegetation. The long-term effects of summer grazing were studied by comparing vegetation on Finnish and Norwegian sides of the fence line separating reindeer grazing regimes. The Finnish side was intensively grazed and trampled throughout the year, whereas the Norwegian side was grazed in winter. Experimental fences were erected to examine short-term effects of grazing exclusion. Both in the long- and short-term, summer grazing decreased the height of Salix lapponum whereas the short-term effects on willow cover were less clear than the long-term effects. In contrast, Carex spp. benefited from grazing. Long-term grazing had little effect on total bryophyte cover. Grazing had negligible effects on the nutrient content of leaves of S. lapponum and Eriophorum angustifolium. We conclude that tundra wetlands can withstand moderately high grazing pressure sustained over several decades.     

Spatial and temporal patterns of artificial nest predation in mountain birch forests fragmented by spruce plantations

Studies of ground-nesting birds stress the importance of high nest losses as a factor influencing population dynamics. In particular, nest predation has been found to be accentuated in human-modified forest landscapes. In boreal ecosystems, the assemblage of nest predators is likely to be temporally variable. Thus, multi-year predation studies are required in order to highlight the temporal aspects of habitat and edge-specific ground-nest predation. On this basis, we conducted a 3-year predation study in Northern Norwegian mountain birch forests which had been fragmented by spruce plantations. Track boards were used to identify predators in different habitat and edge types. We used logistic regression to assess the importance and consistency of spatial and temporal predictors for the predation rate of six predator species. Total predation rates were high and were higher in the second and third year (range 89.9–96.7%) compared to the first year of the study (range 57.1–75.3%). Mammalian predation decreased while avian predation increased over the 3 years. Red fox (Vulpes vulpes) and hooded crow (Corvus cornix) were the dominant predators, followed by raven (Corvus corax) and magpie (Pica pica). Pine marten (Martes martes) and stoat (Mustela erminea) predation was low and almost absent in two of the years. Within the study years, predator species exhibited different temporal trends, probably owing to species-specific functional responses. While some edge types were preferred consistently over time by the dominant predators, the spatial pattern of predation was mainly due to unexplained large-scale differences among landscape blocks. This large-scale pattern was constant over the three study years despite the strong temporal fluctuation in predation rates within and between years.     

Increasing intraspecific facilitation in exposed environments: consistent results from mountain birch populations in two subarctic stress gradients

The stress‐gradient hypothesis predicts that the importance of facilitation relative to competition should increase with increasing stress. The hypothesis has received support from several environments, but multi‐gradient studies on the generality of the hypothesis are exceptionally rare. A within‐species experiment with mountain birch Betula pubescens subsp. czerepanovii was conducted to test the hypothesis in the extreme ends of two subarctic stress gradients (elevation and seashore) in the Kola Peninsula, northwestern Russia. The high stress sites were characterized by strong winds, temperature extremes and potentially drought. The negative effects of abiotic stress on the study seedlings were verified from performance characteristics. Effects of adult hosts as well as seedling–seedlings interactions were studied. Positive host–seedling interactions dominated in each study site, and three out of four performance variables indicated stronger positive net effects in the high stress sites. In the seashore gradient also seedling survival gave similar interpretations. Also a temporal shift towards host–seedling competition was detected in a low stress site after two study years. In seedling–seedling interactions competition dominated, but the effects were weak, likely due to the ‘noise’ caused by genetic and environmental factors. Our results support the stress‐gradient hypothesis and its generality in subarctic environments, as the interpretations were similar for both stress gradients and several fitness‐related variables. The temporal variation in host–seedling interactions and the difference between host–seedling and seedling–seedling effects hint on size‐dependency of plant interactions: facilitation might dominate when the benefactor is substantially larger than the beneficiary, while competition may be stronger when the plants are of similar size and developmental status.     

The homologies of the Fennoscandian mountain and coastal birch forests in Eurasia and North America

Summary The Fennoscandian mountain birchwoods and the ecologically and physiognomically closely related oceanic coastal birchwoods are found in all the boreal zones (northern boreal, middle boreal, southern boreal and hemiboreal) in Europe. In general, they are characteristic and commonly dominant (not necessarily in pure stands) in the oceanic to suboceanic, cool and windy western sections of the boreal zones.The same pattern is clearly repeated on the east coast of Eurasia, despite of its less pronounced oceanity. There the most oceanic boreal sections are dominated by forests of Betula ermanii s. lat. (ranging from the northern boreal to the hemiboreal zone) and Alnus maximowiczii (essentially middle and northern boreal).The western North American Alnus sinuata woods may be interpreted as homologous to the Fennoscandian birchwoods. They are found mainly in the middle and northern (upper) boreal zones in southern Alaska (also in the mountains of British Columbia and SW Yukon). In southern Alaska there are also some fragments of oceanic birchwoods.In eastern North America such homologous deciduous forests are poorly developed.Among the true inland mountain birchwoods only those of the northern Ural Mts., some Transbaikalian mountains, the Himalaya and (with reservation) the Caucasus are referred to the distinct homologies of the Fennoscandian birchwoods.The widespread timber-line alder scrubs consisting of Alnus crispa and related taxa in North America and Eurasia are not included in such homologies.The occurrence of treeless boreal maritime heaths and grasslands is closely related to the distribution of birch and alder forests. The lowlands of Iceland, the Aleutian Islands and the Kuril Islands, for instance, are referred to the boreal heath sections rather than to the arctic.

---

Zusammenfassung Die fennoskandischen Gebirgsbirkenwälder und die ökologisch und physiognomisch nahe verwandten ozeanischen Küstenbirken-wälder kommen in allen borealen Zonen vor. Sie sind typisch und allgemein dominant (nicht immer in reinen Beständen) in den ozeanisch-subozeanischen, kühlen und windigen Sektionen der borealen Zonen.Dasselbe Phänomen wiederholt sich deutlich auch an der Ostküste Eurasiens, ungeachtet ihrer geringeren Ozeanität. In den meist ozeanischen borealen Sektionen herrschen die Wälder von Betula ermanii s. lat. (von der nordborealen zur hemiborealen Zone) und von Alnus maximowiczii (eigentlich mittel- und nordboreal).Im westlichen Nordamerikakönnen die Alnus sinuata-Wälder als homolog mit den fennoskandischen Birkenwäldern erklärt werden. Sie kommen hauptsächlich in den mittel- und nordborealen Zonen in Südalaska vor (auch in den Gebirgen von Britisch-Kolumbien und im südwestlichen Yukon). Auch in Südalaska gibt es einige Fragmente von ozeanischen Birkenwäldern.Die homologen Laubwälder im östlichen Nordamerika sind schwach entwickelt.Nur die Binnengebirgsbirkenwälder, die in Nordural, in Trans-baikalien, im Himalaya und (unter Vorbehalt) in Kaukasien vorkommen, werden für homolog mit den fennoskandischen Birkenwäldern angesehen.Die an der Waldgrenze weitverbreiteten Erlengebüsche (Alnus crispa und nahe verwandt) in Nordamerika und Eurasien werden nicht in solche Homologien aufgenommen.Das Vorkommen der baumlosen borealen maritimen Heiden und Wiesen ist sehr nahe verwandt mit den Birken und Erlenwäldern. Zum Beispiel die Tiefländer von Island, Aleuten und Kurilen werden besser zu den borealen Heide-Sektionen als zur arktischen Zone gezählt.

     

Non-parallel changes in soil microbial carbon and nitrogen dynamics due to reindeer grazing in northern boreal forests

Reindeer grazing has a considerable influence on mineralization processes in northern Fennoscandian boreal forests, but the mechanisms underlying the observed differences between grazed and ungrazed areas are not well understood. We studied the below-ground impacts of reindeer grazing by comparing the carbon and nitrogen mineralization rates inside and outside long-term fenced reindeer exclosure areas in five oligotrophic, lichen-dominated and five mesotrophic, dwarf-shrub dominated forests. The soil C mineralization rates and microbial metabolic activity (qCO₂) were significantly lower in the grazed than the ungrazed areas in both oligotrophic and mesotrophic forests. The reductions occurred irrespective of the impact on soil moisture. We conclude that reindeer grazing causes a reduction in the supply of labile C substrates to microbes, resulting in reduced organic matter decomposition rates through changes in the activity of the microbial biomass. Simultaneously, grazing had no consistent effect on the microbial N dynamics, but the impact ranged from no change to increased or decreased in N mineralization rates at the different study sites. The impact of grazing on the N mineralization potential thus seems to be site-specific and uncoupled from the impact of grazing on soil C mineralization. Reciprocal transplant incubations showed no interactions between N mineralization rates and the reindeer-mediated impact on the soil microclimate. We suggest that plant root damage due to trampling by reindeer may be an important mechanism for the deceleration of soil C cycling. In some cases, however, the impact of grazing on the soil active N pool may be strong enough to outweigh the reduction in soil organic matter decomposition, and by these means uncouple soil N dynamics from soil C quality.     

Effects on heather of heavy grazing by mountain hares

Continual heavy grazing of small patches of heather by mountain hares produces conspicuous, short, lawn-like bushes. Their probable development is described. The shoots of such heather bushes maintained a juvenile appearance, contained more nitrogen and phosphorus than the surrounding heather and, to judge from their chemical composition, were more digestible. The generalization that tree and shrub twigs of juvenile growth form are more unpalatable to hares than twigs of mature growth form did not apply in this case.     

Altitude effects on spatial components of vascular plant diversity in a subarctic mountain tundra

Environmental gradients are caused by gradual changes in abiotic factors, which affect species abundances and distributions, and are important for the spatial distribution of biodiversity. One prominent environmental gradient is the altitude gradient. Understanding ecological processes associated with altitude gradients may help us to understand the possible effects climate change could have on species communities. We quantified vegetation cover, species richness, species evenness, beta diversity, and spatial patterns of community structure of vascular plants along altitude gradients in a subarctic mountain tundra in northern Sweden. Vascular plant cover and plant species richness showed unimodal relationships with altitude. However, species evenness did not change with altitude, suggesting that no individual species became dominant when species richness declined. Beta diversity also showed a unimodal relationship with altitude, but only for an intermediate spatial scale of 1 km. A lack of relationships with altitude for either patch or landscape scales suggests that any altitude effects on plant spatial heterogeneity occurred on scales larger than individual patches but were not effective across the whole landscape. We observed both nested and modular patterns of community structures, but only the modular patterns corresponded with altitude. Our observations point to biotic regulations of plant communities at high altitudes, but we found both scale dependencies and inconsistent magnitude of the effects of altitude on different diversity components. We urge for further studies evaluating how different factors influence plant communities in high altitude and high latitude environments, as well as studies identifying scale and context dependencies in any such influences.     

长白山北坡阔叶红松林及其次生白桦林高等植物物种多样性比较

应用Ｓｉｍｐｓｏｎ和Ｓｈａｎｎｏｎ多样性指数及其相应的均匀度,对阔叶红松林及其次生白桦林的高等植物物种多样性进行了对比研究．结果表明,对于木本植物,次生白桦林物种多样性高于阔叶红松林;而对于草本植物,情况正好相反．但在次生白桦林中,无论木本植物还是草本植物,占优势的物种都是一些常见种,而许多阔叶红松林中的珍稀或特有物种在次生白桦林中数量很少,有些甚至完全消失．对群落各种群多度分布的检验表明,２种森林类型中,无论木本植物还是草本植物,各种群的多度分布都遵从对数级数分布．     

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.