Martin Hallinger

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra

Establishing a missing link: warm summers and winter snow cover promote shrub expansion into alpine tundra in Scandinavia

*Shrub expansion in alpine and arctic areas is a process with possibly profound implications for ecosystem functioning. The recent shrub expansion has been mainly documented by remote sensing techniques, but the drivers for this process largely remain hypotheses. *Here, we outline a dendrochronological method, adapted to shrubs, to address these hypotheses and then present a mechanism for the current shrub expansion by linking recent climate change to shrub growth performance in northern Sweden. *A pronounced increase in radial and vertical growth during recent decades along an elevational gradient from treeline to shrubline indicates an ongoing shrub expansion. Age distribution of the shrub population indicates the new colonization of shrubs at high elevations. *Shrub growth is correlated with warm summers and winter snow cover and suggests the potential for large-scale ecosystem changes if climate change continues as projected.

Landscape Heterogeneity of Shrub Expansion in Arctic Alaska

The expansion of shrubs into tundra areas is a key terrestrial change underway in the Arctic in response to elevated temperatures during the twentieth century. Repeat photography permits a glimpse into greening satellite pixels, and it shows that, since 1950, some shrub patches have increased rapidly (hereafter expanding), while others have increased little or not at all (hereafter stable). We characterized and compared adjacent expanding and stable shrub patches across Arctic Alaska by sampling a wide range of physical and chemical soil and vegetation properties, including shrub growth rings. Expanding patches of Alnus viridis ssp. fruticosa (Siberian alder) contained shrub stems with thicker growth rings than in stable patches. Alder growth in expanding patches also showed strong correlation with spring and summer warming, whereas alder growth in stable patches showed little correlation with temperature. Expanding patches had different vegetation composition, deeper thaw depth, higher mean annual ground temperature, higher mean growing season temperature, lower soil moisture, less carbon in mineral soil, and lower C:N values in soils and shrub leaves. Expanding patches—higher resource environments—were associated with floodplains, stream corridors, and outcrops. Stable patches—lower resource environments—were associated with poorly drained tussock tundra. Collectively, we interpret these differences as implying that preexisting soil conditions predispose parts of the landscape to a rapid response to climate change, and we therefore expect shrub expansion to continue penetrating the landscape via dendritic floodplains, streams, and scattered rock outcrops.

Local adaptations to frost in marginal and central populations of the dominant forest tree Fagus sylvatica L. as affected by temperature and extreme drought in common garden experiments.

Local adaptations to environmental conditions are of high ecological importance as they determine distribution ranges and likely affect species responses to climate change. Increased environmental stress (warming, extreme drought) due to climate change in combination with decreased genetic mixing due to isolation may lead to stronger local adaptations of geographically marginal than central populations. We experimentally observed local adaptations of three marginal and four central populations of Fagus sylvaticaL., the dominant native forest tree, to frost over winter and in spring (late frost). We determined frost hardiness of buds and roots by the relative electrolyte leakage in two common garden experiments. The experiment at the cold site included a continuous warming treatment; the experiment at the warm site included a preceding summer drought manipulation. In both experiments, we found evidence for local adaptation to frost, with stronger signs of local adaptation in marginal populations. Winter frost killed many of the potted individuals at the cold site, with higher survival in the warming treatment and in those populations originating from colder environments. However, we found no difference in winter frost tolerance of buds among populations, implying that bud survival was not the main cue for mortality. Bud late frost tolerance in April differed between populations at the warm site, mainly because of phenological differences in bud break. Increased spring frost tolerance of plants which had experienced drought stress in the preceding summer could also be explained by shifts in phenology. Stronger local adaptations to climate in geographically marginal than central populations imply the potential for adaptation to climate at range edges. In times of climate change, however, it needs to be tested whether locally adapted populations at range margins can successfully adapt further to changing conditions.

Can shrubs help to reconstruct historical glacier retreats

In the 21st century, most of the world’s glaciers are expected to retreat due to further global warming. The range of this predicted retreat varies widely as a result of uncertainties in climate and glacier models. To calibrate and validate glacier models, past records of glacier mass balance are necessary, which often only span several decades. Long-term reconstructions of glacier mass balance could increase the precision of glacier models by providing the required calibration data. Here we show the possibility of applying shrub growth increments as an on-site proxy for glacier summer mass balance, exemplified by Salix shrubs in Finse, Norway. We further discuss the challenges which this method needs to meet and address the high potential of shrub growth increments for reconstructing glacier summer mass balance in remote areas.

Shrubs tracing sea surface temperature—Calluna vulgaris on the Faroe Islands

The climate of Central and Northern Europe is highly influenced by the North Atlantic Ocean due to heat transfer from lower latitudes. Detailed knowledge about spatio-temporal variability of sea surface temperature (SST) in that region is thus of high interest for climate and environmental research. Because of the close relations between ocean and coastal climate and the climate sensitivity of plant growth, annual rings of woody plants in coastal regions might be used as a proxy for SST. We show here for the first time the proxy potential of the common and widespread evergreen dwarf shrub Calluna vulgaris (heather), using the Faroe Islands as our case study. Despite its small and irregular ring structure, the species seems suitable for dendroecological investigations. Ring width showed high and significant correlations with summer and winter air temperatures and SST. The C. vulgaris chronology from the Faroe Islands, placed directly within the North Atlantic Current, clearly reflects variations in summer SSTs over an area between Iceland and Scotland. Utilising shrubs like C. vulgaris as easy accessible and annually resolved proxies offers an interesting possibility for reconstruction of the coupled climate-ocean system at high latitudes.

Publisher Correction to: Background invertebrate herbivory on dwarf birch (Betula glandulosa-nana complex) increases with temperature and precipitation across the tundra biome

The above mentioned article was originally scheduled for publication in the special issue on Ecology of Tundra Arthropods with guest editors Toke T. Høye . Lauren E. Culler. Erroneously, the article was published in Polar Biology, Volume 40, Issue 11, November, 2017. The publisher sincerely apologizes to the guest editors and the authors for the inconvenience caused.

Does it pay to concentrate conservation efforts for dead-wood dependent insects close to existing reserves: a test on conservation planning in Sweden

Forestry has significantly changed many boreal forest landscapes with negative effects on biodiversity. Main reasons are habitat loss and deterioration, due to a simplified forest composition, including a reduction in old trees and dead wood. In Sweden, measures to counter the loss of biodiversity are taken at forest harvesting, and include forest harvest retention. Areas of high conservation values (AHCV) have been identified in Sweden to prioritise conservation efforts, but so far these areas have not been used when making decisions about forest harvest retention. In this study, we tested if dead wood from forest harvest retention is used by more dead wood‐dependent insect species (24 beetles and four other insects, redlisted or indicating high biodiversity) on clearcuts within than outside of AHCVs and whether the composition of the surrounding landscape influences species occurrence. There were not more dead wood‐dependent insect species per clearcut within AHCVs than outside. The surrounding landscape had an influence on the species richness on clearcuts, but in several cases the pattern was the opposite of our hypotheses. Increased standing volume of relevant tree species, however, tended to increase the occurrence or number of associated species. The designation of ACHVs in Sweden is not related with occurrence of dead wood or dead wood‐dependent organisms. We also found that landscape context can have effects on species richness. The result urges for better planning so that ACHV areas are designated in landscapes where their potential for conservation of dead wood‐dependent organisms is highest.