Productivity overshadows temperature in determining soil and ecosystem respiration across European forests

This paper presents CO2 flux data from 18 forest ecosystems, studied in the European Union funded EUROFLUX project. Overall, mean annual gross primary productivity (GPP, the total amount of carbon (C) fixed during photosynthe sis) of these forests was 1380 +/- 330gC m(-2) y(-1) (mean +/- SD). On aver age, 80% of GPP was respired by autotrophs and heterotrophs and released ba ck into the atmosphere (total ecosystem respiration, TER=1100 +/- 260 gC m( -2) y(-1)). Mean annual soil respiration (SR) was 760 +/- 340 gC m(-2) y(-1 ) (55% of GPP and 69% of TER). Among the investigated forests, large differences were observed in annual S R and TER that were not correlated with mean annual temperature. However, a significant correlation was observed between annual SR and TER and GPP amo ng the relatively undisturbed forests. On the assumption that (i) root resp iration is constrained by the allocation of photosynthates to the roots, wh ich is coupled to productivity, and that (ii) the largest fraction of heter otrophic soil respiration originates from decomposition of young organic ma tter (leaves, fine roots), whose availability also depends on primary produ ctivity, it is hypothesized that differences in SR among forests are likely to depend more on productivity than on temperature. At sites where soil disturbance has occurred (e.g. ploughing, drainage), so il espiration was a larger component of the ecosystem C budget and deviated from the relationship between annual SR land TER) and GPP observed among t he less-disturbed forests. At one particular forest, carbon losses from the soil were so large, that in some years the site became a net source of car bon to the atmosphere. Excluding the disturbed sites from the present analy sis reduced mean SR to 660 +/- 290 g C m(-2) y(-1), representing 49% of GPP and 63% of TER in the relatively undisturbed forest ecosystems.