Interannual variability of plant phenology in tussock tundra: modelling interactions of plant productivity, plant phenology, snowmelt and soil thaw

We present a linked model of plant productivity, plant phenology, snowmelt and soil thaw in order to estimate interannual variability of arctic plant phenology and its effects on plant productivity. The model is tested using 8 years of soil temperature data, and three years of bud break data of Betula nana. Because the factors that trigger the end of the growing season of arctic vegetation are less well known than those of the start of the growing season, three hypotheses were formulated and tested for their effects on productivity and its sensitivity to climate change; the hypothesised factors determining the end of the growing season were frost, photoperiod and periodic constraints. The performance of the soil thermal model was good; both the onset of soil thaw in spring and the initiation of freezing in autumn were predicted correctly in most cases. The phenology model predicted the bud break date of Betula nana closely for the three different years. The soil thaw model predicted similar growing season start dates under current climate as the models based on sum of temperatures, but it made significantly different predictions under climate change scenarios, probably because of the non‐linear interactions between snowmelt and soil thaw. The uncertainty about the driving factors for the end of the growing season, in turn, resulted in uncertainty in the interannual variability of the simulated annual gross primary productivity (GPP). The interannual variability ranged from ? 25 to + 26% of the mean annual GPP for the frost hypothesis, from ? 20 to + 20% for the photoperiod hypothesis and only from ? 7 to + 7% for the periodic hypothesis. The different hypotheses also resulted in different sensitivity to climate change, with the frost hypothesis resulting in 30% higher annual GPP values than the periodic hypothesis when air temperatures were increased by 3 °C.     

The effects of water table manipulation and elevated temperature on the net CO2 flux of wet sedge tundra ecosystems

In situ manipulations were conducted in a naturally drained lake on the arctic coastal plain near Prudhoe Bay, Alaska (70 °21.98′ N, 148 °33.72′ W) to assess the potential short-term effects of decreased water table and elevated temperature on net ecosystem CO₂ flux. The experiments were conducted over a 2-year period, and during that time, water table depth of drained plots was maintained on average 7 cm lower than the ambient water table, and surface temperatures of plots exposed to elevated temperature were increased on average 0.5 °C. Water table drainage, and to a lesser extent elevated temperature, resulted in significant increases in ecosystem respiration (ER) rates, and only small and variable changes in gross ecosystem productivity (GEP). As a result, drained plots were net sources of ≈ 40 gC m^–2 season^–1 over both years of manipulation, while control plots were net sinks of atmospheric CO₂ of about 10 gC m^–2 season^–1 (growing season length was an estimated 125 days). Control plots exposed to elevated temperatures accumulated slightly more carbon than control plots exposed to ambient temperatures. The direct effects of elevated temperature on net CO₂ flux, ER, and GEP were small, however, elevated temperature appeared to interact with drainage to exacerbate the amount of net carbon loss. These data suggest that many currently saturated or nearly saturated wet sedge ecosystems of the north slope of Alaska may become significant sources of CO₂ to the atmosphere if climate change predictions of increased evapotranspiration and reduced soil water status are realized. There is ample evidence that this may be already occurring in arctic Alaska, as a change in net carbon balance has been observed for both tussock and wet-sedge tundra ecosystems over the last 2–3 decades, which coincides with a recent increase in surface temperature and an associated decrease in soil water content. In contrast, if precipitation increases relatively more than evapotranspiration, then increases in soil moisture content will likely result in greater carbon accumulation.