Long-term carbon storage in a semi-natural British woodland

摘要

Atmospheric levels of CO2 are currently 395 ppm (dry air mole fraction measured at Mauna Loa, Hawaii), their highest concentration in 420,000 years. Forests play a major role in the global carbon (C) cycle by taking up inorganic C as CO2 through photosynthesis, converting it to organic compounds (biomass), and either storing it in living and dead organic matter (above and below ground: including trees, dead wood, litter, and soil) or returning it to the atmosphere by respiration, decay or fire. Globally, forests cover around 4.1 billion ha of the Earth’s surface and are estimated to contain up to 80% of all aboveground C and around 40% of all belowground (soils, litter, roots) terrestrial C. Forest C stocks have been reported to be increasing over the past 50 years in Europe and over the past 17 years in the United States. However, national forest inventories used to provide these data are often biased towards managed plantations, thereby leaving a knowledge gap regarding the dynamics of unmanaged, semi-natural forests. There are significant uncertainties about changes in C flux through time and the relative contributions of drivers such as land use, climate and atmospheric CO2. Decomposition of tree root C represents a potentially large C flux and contribution to the soil C sink when the input of dead and decaying root tissue, and root exudates, are greater than the output from respiration of roots, their symbionts, and the soil decomposer organisms. Therefore, quantifying decomposition rates and identifying primary controls of root decomposition are important for evaluating ecosystem function and possible responses to environmental change. This thesis explores long-term C dynamics in Lady Park Wood (LPW), an ancient semi-natural woodland situated in the counties of Monmouthshire and Gloucestershire, UK. We calculated changing tree biomass C stocks in LPW from 1945 to 2010. Separate estimates of tree biomass C, soil C and dead wood C were obtained to verify how C is apportioned among these types of forests. We used the dynamic vegetation model LPJ-GUESS to explore the likely contributions of temperature, CO2 and management to forest C stocks in this region during the last 65 years. A 30 month field experiment was conducted in LPW using oak roots of different diameter classes (