Pre-study regarding potential SOC stocks in central European old-growth beech forests: a comparative analysis

Abstract

Soils have long been identified as the most important terrestrial carbon sinks – and thus, as one of the preliminary sources of atmospheric CO2. About 80% of the terrestrial carbon actively involved in the global carbon cycle are bound in soils, whereas only 20% are bound in vegetation. According to the Thünen Institute, approximately 2.5 billion tons of carbon are stored in German forests; more than half of it can be found belowground. Here, the accumulation and mineralization of soil organic matter (SOC) form a dynamic equilibrium. Although the understanding of organic carbon turnover is ever increasing, little is known about maximum carbon storage capacities in forest soils, respectively about accumulation rates. This is partly because natural forests in Central Europe are very rare due to historic land use, and at the same time there has been no systematic approach to investigate the forest soils’ potential to store SOC. Additionally, forest soils are spatially very heterogenous, making it exceedingly difficult to systematically investigate SOC stocks. For instance, organic carbon storage and turnover depend on a variety of natural factors, such as climate, parent material and terrain – but, equally important, forest management practices. Studies have shown that there is a negative correlation between the intensity of forest management and SOC stocks. Harvesting trees and biomass can significantly deplete carbon stocks in the long term. However, for a better understanding of the global carbon cycle and more informed land-use management decisions, this pre-study aims to answers following questions: What are the maximum SOC stocks that can be accumulated in Central European natural forests and which time spans are needed to reach these? How do different forest management practices effect the storage of SOC? To help answer these questions, we decided to investigate SOC stocks on seven sites in mesophilic beech forests along a chronological and management gradient. Other influencing parameters, such as climate, potential natural vegetation and soil types were kept as constant as possible. Hence, the only variable impact factors are the age of the forest site and the management type. The gradients range from a regularly managed forest site over an ecologically managed forest site to a protected zone of a biosphere reserve which has seen no forest management for almost 500 years. We hypothesize that the longer forest sites can grow under natural, unmanaged circumstances, the more SOC is accumulated, while eventually approaching a maximum. The soil samples are currently being analyzed in the laboratory. The final results are due by the end of February 2018. The results of this pre-study identify reference values of SOC in very old unmanaged forest sites and support the discussion around the impact of different management practices, which is important for the discussion of natural climate solutions with respect to climate change.