Climate warming is expected to shift bioclimatic zones and plant species distribution. Yet, few studies have explored whether seedling establishment is a possible bottleneck for future migration and population resilience. We test how warming affects the early stages of seedling establishment in 10 plant species in subarctic tundra. To zoom into the life phases where the effects of warming actually take place, we used a novel approach of breaking down the whole-season warming effect into full factorial combination of early-, mid-, and late-season warming periods. Seeds were sown in containers placed under field conditions in subarctic heath and were exposed to 3 1C elevation of surface temperature and 30% addition of summer precipitation relative to ambient. Heating was achieved with Free Air Temperature Increase systems. Whole-season heating reduced germination and establishment, significantly in four out of 10 species. The whole-season warming effect originated from additive effects of individual periods, although some of the periods had disproportionally stronger influence. Early-germinating species were susceptible to warming; the critical phases were early summer for germination and mid summer for seedling survival. Graminoids, which emerged later, were less susceptible although some negative effects during late summer were observed. Some species with intermediate germination time were affected by all periods of warming. Addition of water generally could not mitigate the negative effects of wholeseason heating, but at individual species level both strengthening and amelioration of these negative effects were observed. We conclude that summer warming is likely to constrain seedling recruitment in open micro sites, which is a common seed regeneration niche in tundra ecosystem. Importantly, we described both significant temporal and species-specific variation in the sensitivity of seedling establishment to warming which needs to be taken into consideration when modelling population dynamics and vegetation transitions in a warmer climate.

Arctic ecosystems are known to be extremely vulnerable to climate change. As the Intergovernmental Panel on Climate Change scenarios project extreme climate events to increase in frequency and severity, we exposed High Arctic tundra plots during 8 days in summer to a temperature rise of approximately 9 1C, induced by infrared irradiation, followed by a recovery period. Increased plant growth rates during the heat wave, increased green cover at the end of the heat wave and higher chlorophyll concentrations of all four predominating species (Salix arctica Pall., Arctagrostis latifolia Griseb., Carex bigelowii Torr. ex Schwein and Polygonum viviparum L.) after the recovery period, indicated stimulation of vegetative growth. Improved plant performance during the heat wave was confirmed at plant level by higher leaf photochemical efficiency (F v /F m ) and at ecosystem level by increased gross canopy photosynthesis. However, in the aftermath of the temperature extreme, the heated plants were more stressed than the unheated plants, probably because they acclimated to warmer conditions and experienced the return to (low) ambient as stressful. We also calculated the impact of the heat wave on the carbon balance of this tundra ecosystem. Below-and aboveground respiration were stimulated by the instantaneous warmer soil and canopy, respectively, outweighing the increased gross photosynthesis. As a result, during the heat wave, the heated plots were a smaller sink compared with their unheated counterparts, whereas afterwards the balance was not affected. If other High Arctic tundra ecosystems react similarly, more frequent extreme temperature events in a future climate may shift this biome towards a source. It is uncertain, however, whether these short-term effects will hold when C exchange rates acclimate to higher average temperatures.

A new technique, called Free Air Temperature Increase (FATI), was developed to artificially induce increased canopy temperature in field conditions without the use of enclosures. This acronym was chosen in analogy with FACE (Free Air CO2 Enrichment), a technique which produces elevated CO2 concentrations [CO2] in open field conditions. The FATI system simulates global warming in small ecosystems of limited height, using infrared heaters from which all radiation below 800 nm is removed by selective cut‐off filters to avoid undesirable photomorpho‐genetic effects. An electronic control circuit tracks the ambient canopy temperature in an unheated reference plot with thermocouples, and modulates the radiant energy from the lamps to produce a 2.5°C increment in the canopy temperature of an associated heated plot (continuously day and night). This pre‐set target differential is relatively‐constant over time due to the fast response of the lamps and the use of a proportional action controller (the standard deviation of this increment was <1°C in a 3 week field study with 1007 measurements). Furthermore, the increase in leaf temperature does not depend on the vertical position within the canopy or on the height of the stand. Possible applications and alternative designs are discussed.

BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.

Summary• Extreme temperature events are projected to increase in frequency in a future climate. As successive extremes could occur more frequently, patches of vulnerable tundra vegetation were exposed to two consecutive heat waves (HWs) of 10 d each, with a 5-d recovery period in between.• Surface temperatures during the HWs were increased approximately 6 ° C using infrared irradiation sources.• In three of the four target species ( Pyrola grandiflora , Polygonum viviparum and Carex bigelowii ), plant conditions improved upon the first exposure. Depending on species, leaf relative growth, leaf chlorophyll content or maximal photochemical efficiency was increased. In P. grandiflora the positive effects of the heat on the photosynthetic apparatus led to augmented net photosynthesis. By contrast, Salix arctica responded mainly negatively, indicating species-specific responses.• During the second HW, leaf mortality suddenly increased, indicating that the heat stress induced by the extreme events lasted too long and negatively influenced the species resistance to high temperature. After the HWs, when plants were exposed to (low) ambient temperatures again, plant performance deteriorated further, indicating possible loss of cold resistance.

BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.

Summary1. Climate projections suggest that extreme events will increase in frequency during this century. As tundra is recognized to be among the most vulnerable biomes, we exposed patches of arctic tundra vegetation to an experimental heatwave (by infrared irradiation), followed by a recovery period. The heating increased the surface temperature with an average of 7·6 ° C during 13 days, which slightly exceeded the longest climatic episode with such a temperature deviation since 1961. 2. The heatwave decreased stomatal conductance ( g s ) and PSII maximum efficiency ( F v /F m ), although there were differences in response among the four target species. Salix arctica Pall. (shrub) was affected during the heatwave and could not recover. In Carex bigelowii Tor. ex Schwein (sedge) and Pyrola grandiflora Radius (forb), on the other hand, the effects on g s and F v /F m became clear, particularly in the aftermath of the heatwave, whereas Polygonum viviparum L. (forb) was never stressed. 3. Effects of the heat on g s were mainly indirect, through increased desiccation, whereas effects on F v /F m were more related to leaf temperature (although not in all species). The observed changes can therefore probably be ascribed to a combination of heat and drought causing dysfunctions that ultimately led to senescence. 4. Two conclusions of this study, species-specific responses and increased leaf mortality, indicate that more frequent extreme temperature events accompanied by desiccation might alter/endanger tundra communities in a future climate. Predictions of global change effects on arctic ecosystems should therefore take into account the impact of extremes.

No abstract