Variability of litter carbon stocks in Croatia

Litter stores around 5% of total carbon (C) stocks in the World's forests (Pan et al. 2011) and is one of five forest ecosystem C pools in national greenhouse gas (GHG) inventory reports, for which reporting is mandatory. Litter is known for its high spatial heterogeneity at different scales. Litter mass, and therefore its C stock, varies with respect to climate region, forest type, and various site and stand characteristics. Litter in the context of GHG reporting in Croatia corresponds to the forest floor (undecomposed leaf organic layer – OL, and fragmented and humified organic layer – OFH), while emissions and removals from the Forest land category, which includes the Litter pool, are stratified into Broadleaves and Conifers. Although a relatively small country in Europe, the biogeographical diversity of Croatia is high, which leads to the question if the existing stratification for the litter should be refined. We tested the hypothesis that litter C stocks within specific tree species groups (Broadleaves and Conifers) differ between biogeographical regions (BGR). From available national data sources, we compiled a database on litter, soil and forest stand variables at 276 plots distributed across three BGRs in Croatia: Alpine, Continental and Mediterranean. Litter data includes height, dry mass, C stock and C/N. Soil data includes soil organic C (top 30 cm), soil texture and bulk density. Stand variables include main tree species, stand basal area and tree density. Additionally, the database includes information on mean annual temperature (MAT), mean annual precipitation (MAP) and elevation at the plot level. Data were analysed at different scales regarding three BGR and two tree species groups (Broadleaves and Conifers). Litter C stocks showed high variability (CV>30%) at the regional scale, with the Mediterranean BGR having the highest variability (CV of 43%). When looking at the specific tree species group, coniferous forests in Mediterranean BGR have the highest averaged litter C stocks (8.13 tC ha-1), while the broadleaf forests in Continental BGR have the lowest averaged litter C stocks (4.37 tC ha-1). Litter C stocks significantly differ between Alpine and Mediterranean BGR in coniferous forests, while in broadleaf forests significant difference in litter C stocks was observed between Alpine and Continental BGR. Our results indicate that the stratification of the Litter with respect to BGRs may improve the accuracy of the national carbon inventory.