The recruitment niche of mountain birch (Betula pubescens ssp. tortuosa) and implications for woodland restoration.

The global climate and biodiversity crisis has led to extensive restoration initiatives, calling for cost‐effective strategies harnessing the potential of natural processes. Natural colonization of target species is a key process in the scaling up of woodland restoration, and effective planning and implementation of restoration strategies requires a mechanistic understanding of colonization processes. In this study, we investigated patterns and processes of natural colonization of mountain birch (Betula pubescens ssp. tortuosa), the only native woodland‐forming tree species in Iceland, into adjacent treeless lands under diverse conditions, and its implications for low‐intensity restoration strategies. Accordingly, we assessed spatial patterns of seedling densities on transects extending from 10 birch woodlands in Iceland and analyzed the impact of local environments on colonization dynamics. Seedling densities generally decreased rapidly with distance from the seed source to around 40 m. In many study areas, however, birch seedlings were registered at high enough densities to potentially form woodlands, up to 140 m from a standing seed source. We identified a range of biotic and abiotic determinants of colonization, with the largest number affecting the early‐establishment phase of birch colonization, including safe site availability, wind speed and dominant direction, and grazing. Our results demonstrate a good potential for natural colonization as a restoration strategy for birch woodlands in subarctic environments, and they can furthermore be used to identify areas where birch colonization is likely to occur naturally and determine appropriate interventions that can facilitate birch colonization in areas where it is less likely.

Typically, European birches are a characteristic product of primary or human-induced secondary succession (Atkinson 1992). The mountain birch (Betula pubescens ssp. czerepanovii) is, however, an exception to this and forms a stable climax forest in north-western Europe. Mountain birch forests are found in parts of the world with relatively low human populations and long distances to major pollution sources (parts of the Kola peninsula are, however, an exception, Kozlov and Barcan 2000). These characteristics, in combination with the low productivity of this birch forest type (Chaps. 4, 5), may give the impression of a very stable environment where dramatic changes or events are rare. Several chapters in this (and the following) section(s) show that this is not the case. This system is highly dynamic at several different scales and aspects. First, the mountain birch seems to be in a phase where its genome is changing relatively fast. It has repeatedly been suggested that in Iceland and continental Europe that the mountain birch formed through introgression between downy and dwarf birch (B. pubescens and B. nana; see e.g. Vare 2001), but the introgression hypothesis has only recently been confirmed by molecular techniques (Thorsson et al. 2001). The Greenland birch seems to be the result of similar introgression between B. pubescens and B. glandulosa (Sulkinoja 1990). The introgression probably contributes to large variability among individual trees in many characteristics and it seems plausible that introgression is an important factor that improves the fitness of the mountain birch in this particular environment. The dwarf birch successfully grows at considerably higher altitudes and latitudes than the downy birch (for distribution data on these species, see e.g. Chap. 1; Lid 1974; Hulten and Fries 1986 ). In fact, when comparing seedling growth performance under the climatic conditions prevailing in the mountain birch forest zone, the mountain birch outperformed downy and dwarf birch in several measures of growth (Karlsson et al. 2000). Similarly, near the tree line, mountain genotypes grow faster than lowland

Mammalian herbivores commonly alter the concentrations of secondary compounds in plants and, by this mechanism, have indirect effects on litter decomposition and soil carbon and nutrient cycling. In northernmost Fennoscandia, the subarctic mountain birch (Betula pubescens ssp. czerepanovii) forests are important pasture for the semidomestic reindeer (Rangifer tarandus). In the summer ranges, mountain birches are intensively browsed, whereas in the winter ranges, reindeer feed on ground lichens, and the mountain birches remain intact. We analyzed the effect of summer browsing on the concentrations of secondary substances, litter decomposition, and soil nutrient pools in areas that had been separated as summer or winter ranges for at least 20 years, and we predicted that summer browsing may reduce levels of secondary compounds in the mountain birch and, by this mechanism, have an indirect effect on the decomposition of mountain birch leaf litter and soil nutrient cycling. The effect of browsing on the concentration of secondary substances in the mountain birch leaves varied between different years and management districts, but in some cases, the concentration of condensed tannins was lower in the summer than in the winter ranges. In a reciprocal litter decomposition trial, both litter origin and emplacement significantly affected the litter decomposition rate. Decomposition rates were faster for the litter originating from and placed into the summer range. Soil inorganic nitrogen (N) concentrations were higher in the summer than in the winter ranges, which indicates that reindeer summer browsing may enhance the soil nutrient cycling. There was a tight inverse relationship between soil N and foliar tannin concentrations in the winter range but not in the summer range. This suggests that in these strongly nutrient-limited ecosystems, soil N availability regulates the patterns of resource allocation to condensed tannins in the absence but not in the presence of browsing.

The mountain birch (Betula pubescens ssp. czerepanovii) is considered to have originated through introgressive hybridization between B. pubescens and B. nana. It is intermediate between the putative parent species in terms of growth form and distribution. Consequently, we hypothesized that the mountain birch should have growth characteristics intermediate between the other two birch forms. This hypothesis was tested in an experiment using first-year seedlings. Only in three out of 15 characteristics studied were mountain birch characteristics clearly intermediate between B. pubescens and B. nana. In some cases the mountain birch was most similar to B. pubescens, while in others it resembled B. nana most closely. In certain other respects, B. pubescens and B. nana were more similar to each other than to mountain birch. In three measures of plant productivity, i.e., relative growth rate, leaf area productivity, and plant nitrogen productivity, mountain birch showed the highest values. Cluster analyses of thirteen growth-related characteristics indicate that at a low fertilizer supply, B. pubescens and B. nana are more similar to each other than to the mountain birch. At a high fertilizer supply, mountain birch was more similar to B. pubescens. The results indicate that the growth characteristics of mountain birch seedlings are not inherited from its two ”parent” species in any simple way.

ABSTRACTIn the subarctic environment, shortage of nitrogen is common and may have immediate effects on tree survival via lowered photosynthetic capacity in cold periods. Yet, despite the critical role of nitrogen, the subarctic tree mountain birch [Betula pubescens ssp. czerepanovii (Orlova) Hämet-Ahti] shows remarkable variability in tree architecture and leaf traits, which affect its nitrogen use and its ability to capture light. It is possible that intraspecific variation in these traits exhibits alternative strategies for maintaining efficient nitrogen use, provided it results in equal efficiency despite variation in the underlying traits, but true differences between tree individuals may also exist.Computer simulations were used to investigate how daily photosynthetic nitrogen use efficiency (DPNUE) and the potential rate of photosynthesis (Pmax) of whole birch trees depend on tree architecture, the area or number of leaves per shoot, or nitrogen invested per leaf. The simulations showed that mountain birch has the potential to achieve an equal Pmax or DPNUE by adjusting variation in many traits, but the potential was not realized as a large amount of variation remained when leaf traits measured in the field were used to estimate DPNUE and Pmax . Trade-offs between Pmax , DPNUE, and other tree functions such as resistance to herbivores, growth, maintenance, or reproduction are likely causes for variation.

Subarctic forest‐tundra ecotones dominated by mountain birch (Betula pubescens ssp. czerepanovii) are an important habitat for semi‐domestic reindeer Rangifer tarandus. The seasonal timing of reindeer grazing may direct vegetation trajectories in these systems, because in the summer ranges, mountain birches are subjected to browsing, while in the winter ranges, reindeer feed on understorey vegetation and arboreal lichens but leave the mountain birches intact. Based on earlier research, we predicted that (a) summer browsing dampens ongoing vegetation ‘shrubification’ in semi‐dry and dry mountain birch forests and (b) ‘shrubification’ is accompanied by a decline in lichens. We tested these predictions through re‐analysing forest structure and understorey vegetation after 12 years in areas where winter and summer ranges had been separated since the 1980s. We also tested how changes in lichen abundances align with changes in shrub abundances through correlation analyses. The number of tall mountain birch seedlings had increased twice as fast in winter than summer ranges, while big mountain birches had increased in summer ranges. The dominant evergreen dwarf shrub mountain crowberry (Empetrum nigrum ssp. hermaphroditum) had increased to a greater extent in winter ranges in a semidry habitat, and to a greater extent in summer ranges in a dry habitat. Deciduous dwarf shrub and graminoid biomass had increased similarly in summer and winter ranges. We found no evidence to support that increasing shrub abundances had contributed to a decline in lichens; instead, the lichen cover increased with increasing number of mountain birch seedlings. Synthesis and application. The vegetation trajectories of dry and semi‐dry subarctic mountain birch forests depend greatly on whether the area is used as a winter or a summer range for the reindeer. The recent changes in vegetation are likely to lead to improved summer forage availability for the reindeer, while the opposite may be true for the winter forage availability.

For about a century, Abisko Scientific Research Station in northern Swedish Lapland has served as a logistic base for high-quality geoecological research in subalpine/subarctic environments. In recent years, and driven by the prospect of alleged man-made global warming, much of the scientific focus has been on dynamics of the treeline ecotone. In this context, field observations, analyses and interpretations emanating from research carried out in the Abisko region are discussed in perspective of recent observations and analyses. Local mountain birch (Betula pubescens ssp. czerepanovii) treeline rise by maximum 230 m during the past 100 years conforms quantitatively to data obtained further south in the Scandes. This broad-scale inter-regional coincidence indicates that a common operative agent has been responsible. The most likely candidate is recorded secular climate warming by 2.5 °C. This contention is further supported by age structure analysis in the birch treeline advance zone, indicating that the vegetative initiation of new trees peaked during the warm 1930s, when reindeer number were high and reached a nadir during the relatively cold 1960s and 1970s, coincident with smaller reindeer herds. These data suggest, contrary to previous hypotheses, stating that, relative to climate change, intensity of reindeer browsing has been of minor importance for birch treeline dynamics. The upper limit of closed stands of mountain birch and pine has shifted relatively insignificantly in elevational position during the predominantly warm past 100 years. Over the same period or longer, common aspen (Populus tremula) has frequently occurred as low-growing krummholz (stunted growth forms) over the entire mountain birch region. During the warm 1930s, and just like birch, rapid height increment was initiated and has continued up to the present day. Thereby, many individuals have attained tree-sized in recent decades. Accordingly, aspen (Populus tremula) has, presumably in response to climate warming, become a more conspicuous element in the mountain birch forest. The current analyses refute prior claims that aspen regeneration is accomplished by seed regeneration rather than phenotypic adjustment of old-growth creeping individuals. Picea abies and Larix sp. are recorded as new species in the Abisko area. In accordance with prior analyses in other parts of the Scandes, megafossil data show that the treelines of Scots pine (Pinus sylvestris), mountain birch (Betula pubescens ssp. czerepanowii) and grey alder (Alnus incana) peaked in the early Holocene. Based on the elevational difference between early Holocene and present treeline positions (adjusted for 100 m land uplift) it may be inferred that the summer temperatures exceeded those of the last few decades by about 3.0 °C.

For about a century, Abisko Scientific Research Station in northern Swedish Lapland has served as a logistic base for high-quality geoecological research in subalpine/subarctic environments. In recent years, and driven by the prospect of alleged man-made global warming, much of the scientific focus has been on dynamics of the treeline ecotone. In this context, field observations, analyses and interpretations emanating from research carried out in the Abisko region are discussed in perspective of recent observations and analyses. Local mountain birch (Betula pubescens ssp. czerepanovii) treeline rise by maximum 230 m during the past 100 years conforms quantitatively to data obtained further south in the Scandes. This broad-scale inter-regional coincidence indicates that a common operative agent has been responsible. The most likely candidate is recorded secular climate warming by 2.5°C. This contention is further supported by age structure analysis in the birch treeline advance zone, indicating that the vegetative initiation of new trees peaked during the warm 1930s, when reindeer number were high and reached a nadir during the relatively cold 1960s and 1970s, coincident with smaller reindeer herds. These data suggest, contrary to previous hypotheses, stating that, relative to climate change, intensity of reindeer browsing has been of minor importance for birch treeline dynamics. The upper limit of closed stands of mountain birch and pine have shifted relatively insignificantly in elevational position during the predominantly warm past 100 years. Over the same period or longer, common aspen (Populus tremula) has frequently occurred as low-growing krummholz (stunted growth forms) over the entire mountain birch region. During the warm 1930s and just like birch, rapid height increment was initiated and has continued up to the present day. Thereby, many individuals have attained tree-sized in recent decades. Accordingly, aspen (Populus tremula) has, presumably in response to climate warming, become a more conspicuous element in the mountain birch forest. The current analyses refute prior claims that aspen regeneration is accomplished by seed regeneration rather than phenotypic adjustment of old-growth creeping individuals. Picea abies and Larix sp. are recorded as new species in the Abisko area. In accordance with prior analyses in other parts of the Scandes, megafossil data show that the treelines of Scots pine (Pinus sylvestris), mountain birch (Betula pubescens ssp. czerepanovii) and grey alder (Alnus incana) peaked in the early Holocene. Based on the elevational difference between early Holocene and present treeline positions (adjusted for 100 m land uplift) it may be inferred that the summer temperatures exceeded those of the last few decades by about 3.0°C. This study challenges recent proposals that aspen is currently spreading upslope and westwards in the birch forest belt by seed establishment of new individuals. In response to recent warming they have attained tree size and have become a more conspicuous element of the landscape.

Cuticle analysis performed on fossil Betula nana (L.) leaves provides a strong proxy to reconstruct past growing season thermal properties expressed as growing degree days (GDD5). This proxy is so far available for the dwarf birch only and, therewith, restricted to regions or past periods of subarctic climatic conditions. In this study, we analysed modern leaf samples of mountain birch ( Betula pubescens spp. czerepanovii (N. I. Orlova) Hämet-Ahti), which has a wider temperature range than the dwarf birch B. nana. The strong latitudinal climate gradient over Fennoscandia provides a unique opportunity to track growing season temperature imprints in the epidermis cell morphology of the modern mountain birch. We quantified the GDD5-dependent epidermal cell expansion, expressed as the undulation index (UI), over a 10° latitudinal transect translating to a range from ~1500°C to ~600°C GDD5 in 2016. Our results indicate that even in mountain birch the UI is positively correlated to GDD5 and, moreover, is largely independent of regional habitat conditions such as daylight length and precipitation. These results imply that in addition to the earlier studied (sub-)arctic dwarf birch, the closely related mountain birch can also be utilized in GDD5 reconstructions. The abundant presence of fossil mountain birch leaves in sediments from warmer than (sub)arctic palaeoclimates enables the reconstruction of growing season climate dynamics over past phases of climate change, overcoming earlier restrictions of the proxy related to spatial and temporal species occurrence as well as local light regimes.

Changing climate in the Arctic is expected to have significant effects on the pattern and distribution of terrestrial vegetation. Species characteristic of specific zones in the mountains of northern Sweden have been shown to migrate up- and down-slope with changes in climate over the Holocene. This study evaluates the potential for Scots pine (Pinus sylvestris L.) to become a treeline dominant at Fennoscandian treelines, replacing mountain birch (Betula pubescens subsp. czerepanovii (Orlova) Hämet-Ahti). Data from paired mountain birch and Scots pine tree-ring chronologies for eight locations in northern Sweden are used to develop climate – tree ring width index (RWI) relationships. Modeled climate–RWI relationships are then used to predict the relative RWI values of the two species under a suite of climate-forcing scenarios using an ensemble of three global climate models. Results indicate that mountain birch and Scots pine RWI are both correlated with summer temperatures, but Scots pine is more likely than mountain birch to be influenced by moisture conditions. Predictions of RWI under future climate conditions indicate that mountain birch is unlikely to be replaced by Scots pine within the next century.

Mountain birch forest covers large areas in Eurasia, and their ecological resilience provides important ecosystem services to human societies. This study describes long-term stand dynamics based on permanent plots in the upper mountain birch belt in SE Norway. We also present forest line changes over a period of 70 years. Inventories were conducted in 1931, 1953, and 2007. Overall, there were small changes from 1931 up to 1953 followed by a marked increase in biomass and dominant height of mountain birch throughout the period from 1953 to 2007. In addition, the biomass of spruce (Picea abies) and the number of plots with spruce present doubled. The high mortality rate of larger birch stems and large recruitment by sprouting since the 1960s reveal recurrent rejuvenation events after the earlier outbreak of the autumnal moth (Epirrita autumnata). Our results demonstrate both a high stem turnover in mountain birch and a great ability to recover after disturbances. This trend is interpreted as regrowth after a moth attack, but also long-term and time-lagged responses due to slightly improved growth conditions. An advance of the mountain birch forest line by 0.71 m year-1 from 1937 to 2007 was documented, resulting in a total reduction of the alpine area by 12%. Most of the changes in the forest line seem to have taken place after 1960. Regarding silviculture methods in mountain birch, a dimension cutting of larger birch trees with a cutting interval of c. 60 years seems to be a sustainable alternative for mimicking natural processes.

Fluctuating asymmetry (FA) has been suggested as a useful indicator of elevation stress and, hence, distribution limits in plants. However, no plant studies have been carried out to test (i) whether FA shows a gradual increase towards the alpine distribution limit and (ii) whether FA responds to elevation stress independent of other stressors which is necessary for FA to be a useful indicator in this context. To test these two hypotheses, this 2-year field study investigated the dose–response relationship between elevation stress and FA in mountain birch (Betula pubescens) under contrasting levels of insect attack in northern Norway. The results showed that FA increased linearly from sea level towards the tree line in both years independent of insect attack, which had no observable effect on FA, i.e. insect attack did not appear to disturb the FA-elevation relation. Thus, in mountain birch, FA appeared to be a reliable indicator of elevation stress. Further investigation is now needed in order to develop this hypothesis.

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

Most of the Earth's surface has now been modified by humans. In many countries, natural and semi‐natural ecosystems mostly occur as islands, isolated by land converted for agriculture and a variety of other land‐uses. In this fragmented state, long‐distance dispersal may be the only option for species to adapt their ranges in response to changing climate. The order of arrival of species may leave a lasting imprint on community assembly. Although mostly studied at and above the species level, such priority effects also apply at the intraspecific level. We suggest that this may be particularly important in subarctic and arctic ecosystems. Mountain birch (Betula pubescens ssp. tortuosa) is characterized by great intraspecific variation. We explored spatio‐temporal patterns of the first two mountain birch generations on a homogeneous, early successional glacial outwash plain in SE Iceland that was the recipient of spatially extensive long‐distance dispersal ca. 30 years ago. We evaluated the decadal progress of the young population by remeasuring in 2018, tree density and growth form, plant size, and reproductive effort on 30 transects (150 m2) established in 2008 at four sites on the plain and two adjacent sites ca. 10 km away. All measured variables showed positive increases, but contrary to our predictions of converging dynamics among sites, they had significantly diverged. Thus, two of the sites (only 500 m apart) could not be distinguished in 2008, but by 2018, one of them had much faster growth rates than the other, a higher growth form index reflecting more upright tree stature, greater reproductive effort, and much greater second‐generation seedling recruitment. We discuss two hypotheses that may explain the diverging dynamics, site‐scale environmental heterogeneity, and legacies of intraspecific priority effects.

The North American lodgepole pine (Pinus contorta) has been widely introduced globally and is now considered invasive in several countries. It was first planted in subarctic Iceland in the 1950s. Recently, the forestry sector has strongly promoted it as an attractive means of carbon capture to mitigate global climate change. It is now the most extensively planted tree species in Iceland. We describe the expansion of the lodgepole pine from a mid-20th-century plantation in Steinadalur, southeast Iceland, and decadal changes between 2010 and 2021. The extent of occurrence expanded nearly tenfold, with tree number and population density reflecting exponential growth patterns. The lodgepole pine colonised diverse habitats, including native birch woodlands and heathland, and was associated with significant reductions in vascular plant species richness and diversity. We conclude that lodgepole pine has the characteristics of an invasive species in Steinadalur and that this will also apply to many native ecosystems across most lowland regions of Iceland. Our study highlights the urgent need for management strategies to mitigate the long-term ecological impacts of lodgepole pine invasion in subarctic environments.