Environmental manipulation studies are integral to determining biological consequences of climate warming. Open Top Chambers (OTCs) have been widely used to assess summer warming effects on terrestrial biota, with their effects during other seasons normally being given less attention even though chambers are often deployed year-round. In addition, their effects on temperature extremes and freeze-thaw events are poorly documented. To provide robust documentation of the microclimatic influences of OTCs throughout the year, we analysed temperature data from 20 studies distributed across polar and alpine regions. The effects of OTCs on mean temperature showed a large range (-0.9 to 2.1 °C) throughout the year, but did not differ significantly between studies. Increases in mean monthly and diurnal temperature were strongly related (R(2)  = 0.70) with irradiance, indicating that PAR can be used to predict the mean warming effect of OTCs. Deeper snow trapped in OTCs also induced higher temperatures at soil/vegetation level. OTC-induced changes in the frequency of freeze-thaw events included an increase in autumn and decreases in spring and summer. Frequency of high-temperature events in OTCs increased in spring, summer and autumn compared with non-manipulated control plots. Frequency of low-temperature events was reduced by deeper snow accumulation and higher mean temperatures. The strong interactions identified between aspects of ambient environmental conditions and effects of OTCs suggest that a detailed knowledge of snow depth, temperature and irradiance levels enables us to predict how OTCs will modify the microclimate at a particular site and season. Such predictive power allows a better mechanistic understanding of observed biotic response to experimental warming studies and for more informed design of future experiments. However, a need remains to quantify OTC effects on water availability and wind speed (affecting, for example, drying rates and water stress) in combination with microclimate measurements at organism level.

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Questions: How do population structure and recruitment characteristics of Betula saplings beyond the treeline vary among climatic regions, and what is the potential for development into tree-sized individuals with interacting grazing pressure?Location: Scandes Mountains.Methods: Sapling characteristics of Betula pubescens subsp. tortuosa, their topographic position above the treeline, growth habitat and evidence of recent grazing was investigated in three areas with a long continuous grazing history, along a latitudinal gradient (62-691N).Results: Saplings were common up to 100 m above the treeline in all areas. The northern areas were characterised by small (o30 cm) and young (mean 14 years old) saplings in exposed micro-topographic locations unfavourable to long-term survival. In the southern area, broad height (2-183 cm) and age (4-95 years; mean 32 years) distributions were found in sheltered locations. Age declined with altitude in all areas. Sapling growth rate varied within and between areas, and the ageÂheight interaction was significant only in the southern area. Growth rates decreased from south to north and indicated a considerable time required to reach tree size under prevailing conditions. Conclusions: Regional differences can be attributed to climatic differences, however, interacting biotic and abiotic factors such as micro-topography, climate and herbivory, mutually affect the characteristics of birch saplings. In view of the long time needed to reach tree size, the generally expected evident and fast treeline advance in response to climate warming may not be a likely shortterm scenario. The sapling pool in the southern region possesses strongest potential for treeline advance.

Background: Tree encroachment of arctic tundra and alpine vegetation is a generally predicted response to climate warming. However, herbivory plays an important role in structuring these ecosystems and their responses to warming. Aims: To experimentally test how grazing and increased growing season temperature influence growth, physiognomy and stature of birch in the alpine zone. Methods: Trait responses of naturally regenerated birch saplings to warming (open-top chambers), and changed grazing regime (exclosures) were compared with those growing in unmanipulated conditions over a 10-year period (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008). The effect of treatment over time and differences between treatments were analysed with repeated measures GLM (Generalised Linear Model) and simple contrasts in GLM. Results: Warming alone had no major effect on trait responses, however, significantly smaller leaves and an increased number of short shoots indicated warming-related growth constraints. Grazing showed a strong controlling effect on most traits, conserving low stature sapling stage characterised by fewer shoots and larger leaves, compared with non-grazed treatments. Conclusions: Although derived from one experimental site, the results point to a grazing-controlled response to environmental change, with climate (warming) as a secondary driver. This herbivore-driven masking of expected climate-driven tree expansion emphasises the necessity to consider changes in grazing regimes along with climate change, in order to avoid misleading interpretations regarding climate-driven tundra encroachment.

Questions: Vegetation in the forest-tundra ecotone faces changes in both climate and land-use. While climate warming is an important driver of vegetation growth and composition, herbivory may have opposing effects. In the present study, we experimentally test how removal of sheep herbivory affects the vegetation in an alpine forest-tundra ecotone, and how responses are manifested at higher temperatures. Location: Dovre Mountains, Central Norway. Methods: Shrub growth (height and cover) and ground layer composition were analysed each third year over an 18-year period in a nested, three-factorial experiment (ambient temperature and herbivory; ambient temperature and no herbivory; increased temperature and no herbivory). Fencing and open-top-chambers were used as expedients. Treatment effects and interactions over time were analysed using linear mixed effects models and ordination.Results: Shrub height and cover increased over time due to reduced herbivory, but without additional warming effect. Lichen cover declined in all treatments over time, but more rapidly and earlier under warming treatment (significant after three years).Contrary to expectations, there was no statistically significant increase in woody species due to warming, although evergreen woody species displayed a trend shift after six years, comprising a sharp decline towards year twelve. Litter accumulated in all treatments, but at higher rates under warming (significant after nine years). Conclusions:Our results disclose removal of sheep herbivory as a prominent driver of shrub growth, with warming as a subordinate driver in the studied alpine vegetation. The warming-driven increased litter abundance may, however, be caused by the decrease of wind inside chambers and the subsequent absence of wind-driven removal of litter. This chamber effect and the displayed timing differences in vegetation responses call for the critical use of short-term experimental data in predictions of long-term consequences of environmental change.