Carbon pools of semi-arid Picea crassifolia forests in the Qilian Mountains (north-eastern Tibetan Plateau)

Abstract

The vast alpine grasslands of the Tibetan Plateau are lined by a belt of oroboreal coniferous forests in the transition zone to the Central Asian semi-deserts and deserts which may play important roles in the hydrology and biogeochemistry of the region. Many of these forests exist under semi-arid climates, are exposed to rapid climate warming and suffer from intensive human impact. We investigated the carbon stocks in biomass and soil in the Picea crassifolia (Qinghai spruce) forests of the Qilian Mountains, north-eastern Tibetan Plateau. In 18 forest plots at 2600–2800m elevation, we found ecosystem carbon stocks of 348MgCha−1 with carbon densities of 43MgCha−1 in the live and dead aboveground biomass, 12MgCha−1 in roots, 3MgCha−1 in litter and 305MgCha−1 in the soil (SOC; 0–100cm). The belowground carbon pools exceed averages reported for northern boreal forests and even more clearly for zonal forest vegetation in the temperate zone. The high SOC density in the P. crassifolia forests are probably the result of slow decomposition rates due to low soil temperatures in combination with low soil moisture. The widespread degradation of mountain spruce forests to shrubland reduces the ecosystem carbon stock by >85MgCha−1, and is partly caused by a 15%-reduction in SOC. We conclude that the remaining mountain forests of spruce and other conifers at the northern fringe of the Tibetan Plateau play an important role in the regional carbon budget and need urgent conservation.