The Importance of Winter for Controlling the Growing Season Net Ecosystem Exchange (NEE) of Boreal Forests

It is becoming increasingly apparent that the winter season of high latitudes can be important for controlling a range of ecological and biogeochemical properties of northern ecosystems. Here we evaluate the importance of winter conditions on the carbon exchange between boreal forest systems and the atmosphere during the following growing season in order to elucidate any influence of inter-seasonal “memory” effects on carbon exchange properties of boreal forest ecosystems. The study is based on 5 years of continuous eddy covariance measurements at two ca 50 year old Norway spruce stands situated in mid- and northern Sweden, respectively (a total of 10 site years).

The growing season net ecosystem exchange (NEE) ranged from -530 to -60 g C m-2 (negative values indicates carbon sinks). Environmental conditions during the growing season (e.g. temperature, radiation, length) only weakly explained the year-to-year variability in NEE. In contrast, up to 75% of the variation could be explained by the severity of the preceding winter (defined as the lowest observed average weekly air temperature) using an exponential response function. After warm winters the carbon sink properties were high as compared to those observed after cold winters. The winter conditions markedly affected the systems potential for carbon uptake in early summer.

This presentation will address the potential mechanisms underpinning the observed correlations linking growing season carbon exchange to the conditions of the preceding winter. The influence of winter on the partitioned carbon fluxes of ecosystem respiration and gross primary productivity, respectively, will also be addressed. The results strongly indicate that controls on boreal forest carbon exchange can transcend across seasons. Understanding these mechanisms are integral for understanding the environmental drivers of atmospheric carbon exchange, allowing for accurate predictions of boreal forest NEE under both present and future climates.