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Vegetation changes following agricultural land abandonment at high elevation --- which is frequent in Europe --- could have a major impact on carbon (C) sequestration. However, most information on the effects of vegetation changes on ecosystem C stocks originates from low-elevation studies on reforestation or early successional forests, and little is known about how these stocks change during long-term secondary forest succession and at high elevation. We assessed aboveground, belowground, and ecosystem organic matter and C stocks in high-elevation ecosystems that represent the long-term development (centuries) following land abandonment: short- and tall-grass pastures, Swiss mountain pine (Pinus mugo Turra), mixed-conifer, and Swiss stone pine (Pinus cembra L.) -- European larch (Larix decidua P. Mill.) forests. Aboveground C stocks were lowest in the short-grass pastures (0.1 Mg C·ha-1) and reached a maximum in the mixed-conifer and stone pine - larch forests (166 Mg C·ha-1). Belowground C stocks did not differ among the ecosystems studied. We only detected ecosystem C sequestration during reforestation; whereas no significant differences in ecosystem C stocks were found during long-term secondary forest development. Our calculations showed that only an additional 1733-3032 Mg C·year-1 would be sequestered owing to natural reforestation in high-elevation Switzerland, which likely can be considered negligible compared with total annual C sequestration calculated for Swiss forests in other studies.
