The fundamental niche of many species is shifting with climate change, especially in sub-arctic ecosystems with pronounced recent warming. Ongoing warming in sub-arctic regions should lessen environmental constraints on tree growth and reproduction, leading to increased success of trees colonising tundra. Nevertheless, variable responses of treeline ecotones have been documented in association with warming temperatures. One explanation for time lags between increasingly favourable environmental conditions and treeline ecotone movement is reproductive limitations caused by low seed availability. Our objective was to assess the reproductive constraints of the dominant tree species at the treeline ecotone in the circumpolar north. We sampled reproductive structures of trees (cones and catkins) and stand attributes across circumarctic treeline ecotones. We used generalized linear mixed models to estimate the sensitivity of seed production and the availability of viable seed to regional climate, stand structure, and species-specific characteristics. Both seed production and viability of available seed were strongly driven by specific, sequential seasonal climatic conditions, but in different ways. Seed production was greatest when growing seasons with more growing degree days coincided with years with high precipitation. Two consecutive years with more growing degree days and low precipitation resulted in low seed production. Seasonal climate effects on the viability of available seed depended on the physical characteristics of the reproductive structures. Large-coned and -seeded species take more time to develop mature embryos and were therefore more sensitive to increases in growing degree days in the year of flowering and embryo development. Our findings suggest that both moisture stress and abbreviated growing seasons can have a notable negative influence on the production and viability of available seed at treeline. Our synthesis revealed that constraints on predispersal reproduction within the treeline ecotone might create a considerable time lag for range expansion of tree populations into tundra ecosystems.

We applied the leaf-height-seed (LHS) ecology strategy scheme (a combination of three ecologically important traits: specific leaf area (SLA), seed mass and plant height) intraspecifically to two widespread European forest herbs along a latitudinal gradient. The aims of this study were to quantify LHS trait variation, disentangle the environmental factors affecting these traits and compare the within-species LHS trait relationships with latitude to previously established cross-species comparisons. We measured LHS traits in 41 Anemone nemorosa and 44 Milium effusum populations along a 1900-€“2300 €…km latitudinal gradient from N France to N Sweden. We then applied multilevel models to identify the effects of regional (temperature, latitude) and local (soil fertility and acidity, overstorey canopy cover) environmental factors on LHS traits. Both species displayed a significant 4% increase in plant height with every degree northward shift (almost a two-fold plant height difference between the southernmost and northernmost populations). Neither seed mass nor SLA showed a significant latitudinal cline. Temperature had a large effect on the three LHS traits of Anemone. Latitude, canopy cover and soil nutrients were related to the SLA and plant height of Milium. None of the investigated variables appeared to be related to the seed mass of Milium. The variation in LHS traits indicates that the ecological strategy determined by the position of each population in this three-factor triangle is not constant along the latitudinal gradient. The significant increase in plant height suggests greater competitive abilities for both species in the northernmost populations. We also found that the studied environmental factors affected the LHS traits of the two species on various scales: spring-flowering Anemone was affected more by temperature, whereas early-summer flowering Milium was affected more by local and other latitude-related factors. Finally, previously reported cross-species correlations between LHS traits and latitude were generally unsupported by our within-species approach.

Recently, several multiple plot similarity indices have been presented that cure some of the problems associated with the approaches for the calculation of compositional similarity for groups of plots by averaging pairwise similarities. These new indices calculate the similarity between more than two plots whilst considering the species composition on all compared plots. The resulting similarity value is true for the whole group of plots considered (called neighborhood in the following). Here, we review the possibilities for multiple plot similarity calculation and additionally explore coefficients that examine multiple plot similarity between a reference plot (named focal plot in the following) and any number of surrounding plots. The latter represent measures of singularity. Further, we establish a framework for applying these two kinds of multiple plot measures to gridded data including an algorithm for testing the significance of calculated values against random expectations. The capability of multiple plot measures for detecting species compositional gradients and local/regional hotspots within this framework is tested. For this purpose, several artificial data sets with known gradients in species composition (random, gradient, central hotspot, hotspot bottom right) are constructed on the basis of a real data set from a Tundra ecosystem in northern Sweden (Abisko). The coefficients that best reflect the positions of the plots on the realized gradients in species composition are considered as performing best with regard to pattern detection. The tested measures of multiple plot similarity and singularity produced considerably different results when applied to one real and 4 artificial data sets. The newly proposed symmetric singularity coefficient has the best overall performance which makes it suitable for local/regional hotspot detection and for incorporating local to regional similarity analyses in reserve selection procedures.

The relationship between the climate and budburst of the mountain birch was evaluated for two areas in subarctic (ca 69degreesN) Fermoscandia; at Abisko, Swedish Lapland, and at Kevo, Finnish Lapland. Thermal time (TT, degree-day sums) to budburst was calculated for experimental conditions in the laboratory and for in situ observations of budburst. Two types of models predicting leaf emergence in situ were used: 1) TT to budburst for different threshold temperatures based on daily mean (TTMean) or daily maximum (TTMax) temperatures and 2) ecophysiological budburst models. The obtained models were used to estimate effects of a changed climate. Laboratory experiments of TT to budburst indicated no differences in the thermal requirements at the two areas. Temperature requirements of budburst declined successively during the progression of spring, from ca 250 degree-days ( > + 2degreesC) in January to ca 100 in May. No significant trend in the date of budburst was found over the last 70 (Abisko) or 20 (Kevo) yr. There were some differences in the type of model that best explained the date of budburst in situ at the two areas. For Kevo the best prediction (minimum root mean square error, RMSE) of budburst was obtained by a simple thermal time model (TTMean > 5.5degreesC) from 1 January (RMSE = 2. 1). For budburst at Abisko, models based on daily maximum temperature fitted better than those based on daily means. For Abisko, models based on thermal time accumulation only showed systematic errors in the predicted budburst that were correlated with budburst previous year (BBPY). Including this apparent memory effect in the model decreased the error by 2.4 d. The best prediction for Abisko was thus obtained using T-MAX > 6.5 (RM SE = 3.1) from 1 January. Using these models to predict the effect of a changed air temperature climate indicate 3-8 d earlier budburst for a one-degree increase in temperature, the effect being smaller for Kevo than for Abisko. For both areas a change in May temperature has a larger effect on the date of budburst than changes in any other month.

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 Tornetrask 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 (less than or equal to12%). 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.

Alpine environments are currently relatively free from non-native plant species, although their presence and abundance have recently been on the rise. It is however still unclear whether the observed low invasion levels in these areas are due to an inherent resistance of the alpine zone to invasions or whether an exponential increase in invasion is just a matter of time. Using a seed-addition experiment on north- and south-facing slopes (cf. microclimatic gradient) on two mountains in subarctic Sweden, we tested the establishment of six non-native species at an elevation above their current distribution limits and under experimentally enhanced anthropogenic pressures (disturbance, added nutrients and increased propagule pressure). We found a large microclimatic variability in cumulative growing degree days (GDD) (range = 500.77°C, SD = 120.70°C) due to both physiographic (e.g. aspect) and biophysical (e.g. vegetation cover) features, the latter being altered by the experimental disturbance. Non-native species establishment and biomass production were positively correlated with GDD along the studied microclimatic gradient. However, even though establishment on the north-facing slopes caught up with that on the south-facing slopes throughout the growing season, biomass production was limited on the north-facing slopes due to a shorter growing season. On top of this microclimatic effect, all experimentally imposed anthropogenic factors enhanced non-native species success. The observed microclimatic effect indicates a potential for non-native species to use warm microsites as stepping stones for their establishment towards the cold end of the gradient. Combined with anthropogenic pressures this result suggests an increasing risk for plant invasion in cold ecosystems, as such stepping stones in alpine ecosystems are likely to be more common in a future that will combine a warming climate with persistent anthropogenic pressures.

Biotic interactions are known to affect the composition of species assemblages via several mechanisms, such as competition and facilitation. However, most spatial models of species richness do not explicitly consider inter-specific interactions. Here, we test whether incorporating biotic interactions into high-resolution models alters predictions of species richness as hypothesised. We included key biotic variables (cover of three dominant arctic-alpine plant species) into two methodologically divergent species richness modelling frameworks – stacked species distribution models (SSDM) and macroecological models (MEM) – for three ecologically and evolutionary distinct taxonomic groups (vascular plants, bryophytes and lichens). Predictions from models including biotic interactions were compared to the predictions of models based on climatic and abiotic data only. Including plant–plant interactions consistently and significantly lowered bias in species richness predictions and increased predictive power for independent evaluation data when compared to the conventional climatic and abiotic data based models. Improvements in predictions were constant irrespective of the modelling framework or taxonomic group used. The global biodiversity crisis necessitates accurate predictions of how changes in biotic and abiotic conditions will potentially affect species richness patterns. Here, we demonstrate that models of the spatial distribution of species richness can be improved by incorporating biotic interactions, and thus that these key predictor factors must be accounted for in biodiversity forecasts.

The decline of one farmland bird, the migratory European starling, has been attributed to both agricultural intensification and farmland abandonment and to factors operating both during the winter and during the breeding season. We analysed population data from thirty-three Swedish nestbox colonies over more than two decades to determine if the national decline was caused by a common factor affecting all colonies or by local changes in the breeding grounds affecting starling colonies. We found that numbers of breeding starling had declined significantly, but at different rates in different colonies. The local population sizes were affected by previous years? productivity at both national and local scales, suggesting that changes in habitat quality at both scales could affect local population trends. There were no long-term trends in reproductive output, but fledgling production was lowest at intermediate years. The local population changes were positively related to local changes in reproductive output, but only when including complete nest-failures. A relationship between population declines and low mean local productivity was the result of the association between population sizes and reproductive success over time, since decline rates of starlings were not related to the average success during the first part of the study, but to the average success during the later part of the study. The relationship between population change and changes in reproductive output was evident, but fledgling production showed negative density-dependence. In conclusion this study suggests that the decline of the starling population in Sweden has been affected by processes at small spatial scales during the breeding season affecting reproductive success, but does not exclude an additional role for processes at large spatial scales or outside the breeding season.