ABSTRACT
While NEP may approach zero with suffi cient time in terrestrial ecosystems it is unlikely that it remains zero for an extended period of time. A more useful concept is that it averages zero, but given variation in climate, small scale disturbances, and other factors it is rarely exactly zero as described by classical theory.[13] Some have questioned whether NEP ever approaches zero in forests (Fig. 3A).[10] Given that NEP is not the same as NECB, NEP could remain positive for extended periods and carbon need not accumulate in terrestrial ecosystems; for example if there is a loss that is not associated with respiration such as erosion, signifi cant solution losses, fi re, or harvest (Fig. 3B). Because these processes remove carbon that has been produced, it is no longer available as substrates for respiration and cannot contribute to NEP; hence NPP > Rh. Without these losses an extended period of positive and constant NEP would mean ecosystem carbon stores would increase indefi nitely in a straight line manner (Fig. 3C). This might be associated with forms of carbon that cannot be respired such as forms of soil organic matter.[14] However, classical studies of soil carbon increases over time using chronosequences indicate that while soil carbon accumulation can take long periods of time, the overall accumulation curve is not a straight line, but decreases in slope over time.[15,16] If there are major losses not associated with Rh, then it is possible that carbon accumulation is quite similar to the classical curve (Fig. 3C), albeit for different reasons than originally proposed. Additional research is required to measure long-term trends in NEP and to document the specifi c mechanisms responsible for extensive periods of constant, positive NEP.
