Introduction

Abstract

Forest ecosystems cover the largest part of ice-free land surface among all terrestrial ecosystems. Trees, the main component of forest ecosystems, contain the largest stock or absolute quantity of the living forest biomass. The total forest biomass is about 677 petagram (Pg), and trees constitute 80% of the world’s biomass (Kindermann et al. 2008). Forest ecosystems absorb large amounts of CO₂ from the atmosphere via photosynthesis, and return a large part of the fixed carbon (C) back to the atmosphere through auto- and heterotrophic respirations. However, a small fraction of assimilated C is stored in above- and belowground biomass, litter, and soil. About half of the terrestrial C sink is located in forests (Canadell et al. 2007; Fig. 1.2). Based on FAO statistics, about 234 Pg C are stored aboveground in forests, 62 Pg C belowground, 41 Pg C in dead wood, 23 Pg C in litter, and 398 Pg C in forest soils (Kindermann et al. 2008). Forest C data are, however, highly uncertain as, for example, up to 691 Pg C may be stored in forest plant biomass and up to 968 Pg C in forest soils to 1-m depth (Fig. 1.2). Yet, more C is stored in forests than in the atmospheric pool which is estimated to contain about 817 Pg C. In particular, pristine, undisturbed, old-growth forests accumulate large amounts of C and are, therefore, important components of the terrestrial C cycle. Historically, the conversion of forest ecosystems to other land uses (e.g., agricultural and urban) and forest degradation have been major threats to the forest C stock. However, with unprecedented increases in atmospheric CO₂ emissions from burning of fossil fuel and deforestation accompanied by unprecedented global population growth, direct and indirect human-induced pressures on the C stock of forests are dramatically increasing. Specifically, global climate change may weaken the C uptake by forest ecosystems and render forests into a C source which will then have a positive feedback on the global climate. While C sequestration in forest ecosystems cannot stop increases in atmospheric CO₂ originating from fossil fuel combustion, enhancing and strengthening C-fluxes into stable forest C pools can offset anthropogenic CO₂ emissions and minimize risks of abrupt climate change (ACC). Thus, C sequestration, the transfer and secure storage of atmospheric CO₂ into long-lived C pools such as forest ecosystems, buys time for the development and implementation of low-C technologies and de-carbonization of the global economy.