Interannual changes in PAR and soil moisture during warm season may be more important than temperature fluctuations in directing annual carbon balance in Tundra

Abstract

The results of 16-year instrumental monitoring (1996–2011) of the state of permafrost, soils, and vegetation influenced by environmental factors in the shrub tundra subzone of northeastern European Russia (Bol’shezemel’skaya Tundra) are described. The field measurement data for CO2 fluxes in 1996 and 1998–1999 have been used to construct regression equations and to simulate intraanual carbon fluxes with a step of 3 h. Over the observation period, the average annual and summer air temperatures have been considerably elevated relative to the local climate norms as of a climatically stable period of 1980–1996. According to our instrumental observations, the average winter and annual temperatures, total number of positive degree days, the input of photosynthetically active radiation (PAR), and average seasonal soil moisture increased during the next period (1996–2011). Correspondingly, the average annual depth of seasonal soil thawing, and seasonal maximum of vascular leaf phytomass stock also increased considerably. According to the model, the net ecosystem exchange of carbon (NEE) increased from an equilibrium state in 1996–2002 to a considerable elevation in the carbon stock in 1998–2011. Thus, the total stock over the 16-year period amounted to 109 g C m−2. An independent confirmation of the effect assumed by the model is an increase in the aboveground stocks of the live and dead organic matter in ecosystem. A statistical analysis of the model has demonstrated that, in addition to an increase in PAR and temperature, acting over the long-term period in the observed range in a mode of mutual compensation relative to NEE, the main reason underlying the observed increase in long-term carbon stock is the amount of vascular plant leaf stock, which has emerged as a major predictor for NEE and, correspondingly, acts as an independent parameter of the carbon balance. This increase is most likely determined by an increase in soil moisture.