Effects of warming and increasing litterfall
The experimental warming increased isoprene emissions from the Subarctic heath, as hypothesized. A close to significant tendency was detected in 2006, and significantly higher emissions were observed under warming in 2007, after eight growing seasons under the exposure. The effect of warming was not long term or indirect (i.e. a result of changed vegetation cover) but rather a direct response to increased temperature, which indicates a rapid physiological effect of current temperature on isoprene production (Lerdau & Gray, 2003). This was demonstrated by the similar isoprene emissions across the treatments in a measurement campaign carried out on plots directly after suspension of the long-term warming.
In contrast to our hypothesis, isoprene emission was not increased by litter addition, although it brought extra nutrients, especially phosphorus, to the soil and increased the greenness of the vegetation (indicated by the normalized differential vegetation index, NDVI) (Rinnan et al., 2007, 2008). This is in contrast with earlier studies on isoprene emissions under nutrient fertilization treatments in the laboratory. Increased isoprene emissions have been detected under enhanced nutrient availability in velvet bean (Mucuna sp.) (Harley et al., 1994), aspen (Populus sp.), white oak (Quercus alba) (Litvak et al., 1996) and English oak (Quercus robur) (Possell et al., 2004). However, the present study was carried out at the plant community level, which limits comparison with earlier studies. Laboratory incubation of the added litter, both dry and moist, showed that it does not release isoprene.
The heath ecosystem was a sink of carbon during the growing seasons 2006 and 2007. However, the net ecosystem CO2 assimilation was significantly reduced by warming and litter addition in 2007. The result is in accordance with earlier studies in the same region as our experiment (Christensen et al., 1997) and at the same experimental site in which ecosystem respiration was significantly increased under warming (A. Michelsen et al., unpublished). Furthermore, the net CO2 assimilation became negative under the combined treatment in the late season of 2007 as ecosystem CO2 fluxes were controlled more by ecosystem respiration than by photosynthesis. Occasions of negative NEE are not unusual in Subarctic areas with large soil carbon stocks which have accumulated during previous colder periods. For instance, at a similar nearby heath ecosystem, NEE was negative on four out of eight occasions during the growing season (Illeris et al., 2004).
The heath as an isoprene source
We showed here that the Subarctic heath is a significant source of isoprene. The average emissions were similar, 36 and 58 µg m−2 h−1, in the two growing seasons. The largest emissions were detected in both years in August, although periods of warm weather were experienced in June and July. This suggests that fully developed plants have the largest capacity for isoprene emission in the heath. A similar seasonal trend was previously observed in Eriophorum russeolum-dominated Subarctic peatland (Tiiva et al., 2007b) as well as in, for eample, Salix phylicifolia (Hakola et al., 1998), Populus tremuloides (Fuentes et al., 1999) and in several other broadleaved species (Lerdau & Gray, 2003).
The mean isoprene emissions from the Subarctic heath were comparable to emissions from a Subarctic peatland (Tiiva et al., 2007b). However, the emissions from the peatland increased 10-fold during an exceptionally warm period (Tiiva et al., 2007b) while less radical fluctuations were observed for the heath in the present study.
Our results show that Subarctic heaths are more important sources of isoprene than Subarctic forests. A low isoprene flux, 14 µg m−2 h−1, was observed by Rinne et al. (2000) from a Betula pubescens ssp. czerepanovii- and Picea abies ssp. obovata-dominated Subarctic forest with the gradient technique in mid-July 1996. Tarvainen et al. (2007) have estimated isoprene emission from northern boreal forests at 45 kg km−2 per growing season. According to our results, the total growing season isoprene emission from Subarctic heaths is substantially larger, 96 kg km−2 (with a growing season of 111 d and 2041 h of daylight; Pearsall & Newbould, 1957). Emissions from the total heath area in the Lake Torneträsk catchment area in Northern Sweden (1858 km2; 47% of the catchment area; Christensen et al., 2007) are 178 tonnes of isoprene during a growing season.
The mean flux of CO2 into the heath ecosystem was 234 and 220 mg m−2 h−1 in 2006 and 2007, respectively. Therefore, the loss of carbon as isoprene to the atmosphere was in general < 0.1% of the net ecosystem carbon assimilation in the heath. During periods of the highest isoprene emissions, however, the carbon loss reached 1% of the mean net ecosystem carbon assimilation rate. This amount of carbon is at the same level as previously detected from single species under nonstressed conditions (e.g. Monson & Fall, 1989; Sharkey et al., 1991; Pegoraro et al., 2004). However, direct comparison with single-species studies is impossible as carbon exchange in the heath also includes soil respiration, and not all plant species in the heath are isoprene emitters.
Isoprene emissions from the heath were associated with the abundance of C. vaginata and T. pusilla. Significant emissions of isoprene have previously been observed from peatlands with abundant Carex sp. (Janson & De Serves, 1998; Haapanala et al., 2006; Hellén et al., 2006) whereas there are no earlier observations of isoprene emissions from ecosystems with Tofieldia sp. or other species in the family Liliaceae.
Several moss species have been reported to emit isoprene in laboratory trials (Hanson et al., 1999; Tiiva et al., 2007b). In the present study the abundance of mosses was not distinctly associated with isoprene emission, although the emission in general correlated more with mosses than with lichens and other species in drier microsites. Some of the moss species present in the plots are strong isoprene emitters, such as Sphagnum sp. (Hanson et al., 1999), which would explain the positive correlation with the emission. However, the positive correlation between the emission and the vascular species C. vaginata and T. pusilla in both years indicated that these species contribute more than mosses to isoprene emission in the heath.
To conclude, we have shown that heaths are significant sources of isoprene in the Subarctic. The emissions are on the scale of those from Subarctic peatlands but smaller than those from forests in the same region. Under long-term field manipulation, the emissions were substantially increased by warming. Thus, the predicted climatic warming in the Arctic will increase isoprene emissions from the heath ecosystem. As Arctic ecosystems form a substantial store of carbon (Post et al., 1982), the increasing carbon loss as isoprene may have implications for terrestrial ecosystem carbon accumulation if periods of high emissions become more frequent as a result of the warming climate.