Carbon budgets in fertile grey alder (Alnus incana (L.) Moench.) stands of different ages

Abstract Estimation of the carbon (C) storages and fluxes in different forest ecosystems is essential for understanding their C sequestration ability. Grey alder ( Alnus incana (L.) Moench) is a fast growing tree species with a great potential for short-rotation forestry in the Nordic and Baltic countries and its stands are considered C accumulating ecosystems. We hypothesized that grey alder stands growing at fertile sites act as C sinks in the young and middle-age stages, while mature stands become C sources as a consequence of declined net primary production (NPP). Net ecosystem production (NEP) was studied in five grey alder stands aged between 9 and 40 years. It was found that the NEP of the studied grey alder stands of different ages varied from −1.98 to +4.14 t C ha −1  yr −1 . The oldest grey alder stand proved to be a weak C source (−0.77 t C ha −1  yr −1 ). However, also young alder stands regenerated in a clear-cut area may emit C in the earlier stage, owing to previous cutting and decomposition of organic residues matter. In this aspect, the land use history is of great significance. Leaf litter accounted for the largest (44–61%) annual organic C input to soil, the fine roots contributed with 12–32% and the herbaceous understorey (aboveground + belowground), with 14–26% to it. Annual cumulative soil heterotrophic respiration (Rh) in the studied stands ranged between 2.6 and 5.8 t C ha −1  yr −1 ; C leaching was negligible, being 10–30 kg C ha −1  yr −1 . The C sequestration to soil depended on previous land use; in the first forest generation, i.e. a stand growing on previous agricultural land, the annual C org input into soil exceeded Rh, which indicated C accumulation in soil. Although grey alder stands are generally C accumulating ecosystems, the annual biomass production of the studied stands was highly variable; thus annual NEP depended on the effect of the year rather than on stand age. The current study demonstrates that use of repeated time-series measurements applied for stands of different development stages can provide more detailed estimation of C accumulation in forest ecosystems.