Forests in the middle and high latitudes of the Northern Hemisphere function as a significant sink for atmospheric carbon dioxide (CO2). This carbon (C) sink results from two major processes: age-related growth (regrowth) after land use change and growth enhancement due to environmental changes (simply called as `growth enhancement` thereafter), such as elevated CO2, nitrogen (N) deposition, and climate change. These two processes have contrasting implications for predicting the magnitude of future C sequestration in forest ecosystems. Therefore, understanding the relative contribution of tree regrowth and growth enhancement to C accumulation in forest ecosystems across broad geographical scale is critical for predicting future C sequestration in land ecosystems and planning strategies to mitigate the increase of atmospheric CO2 concentration. During the past decade, the relative contribution of these two mechanisms to C accumulation in forest ecosystems has received considerable attention, but attribution between these two is largely controversial, because of a lack of available data and appropriate methodology.
Recently, Dr. Jingyun Fang＊s group first presented a conceptual model and developed a new approach to successfully distinguish the relative contribution of tree regrowth and growth enhancement to biomass C accumulation for major planted forest types in Japan from 1980 to 2005. The core is that forest types with the same age class and similar management regimes across different inventory periods should have a similar biomass density (biomass per area) under equilibrium environmental conditions (i.e. atmospheric CO2 concentration, N deposition, and climate). Thus, if the biomass density of forests with the same age class varies through time (or across different inventory periods), then the variation, either increasing or decreasing biomass density, should be induced by environmental changes. Thus, we can assess the impact of environmental changes on forest growth by examining differences in forest biomass density for the same age class but over different inventory periods.
In this study, using a unique time series of age-class dataset from six national forest inventories in Japan and a new approach developed in this study (i.e., examining changes in biomass density at each age class over the inventory periods), Fang et al quantify the growth enhancement due to environmental changes and its contribution to biomass C sink in Japan＊s forests. Results show that the growth enhancement for four major plantations was 4.0~7.7 Mg C ha-1 from 1980 to 2005, being 8.4-21.6% of biomass C sequestration per hectare and 4.1-35.5% of the country`s total net biomass increase of each forest type. Among four environmental variables (CO2, N deposition, mean annual temperature, and annual precipitation), elevated CO2 explains the largest proportion of the variations in the enhanced growth for all four forest types. In addition, the growth enhancement differs among forest types, age classes, and regions.
The research work was entitled with ※Evidence for environmentally enhanced forest growth§ and recently published online in PNAS (www.pnas.org/cgi/doi/10.1073/pnas.1402333111). The study was funded by National Natural Science Foundation of China, National Basic Research Program of China on Global Change, and Strategic Priority Research Program of the Chinese Academy of Sciences.
In addition, a couple months ago, Fang＊s group also developed a new methodology to quantitively explore the relative contribution of changes in forest area and biomass C density (biomass C stock per area) to Biomass C sink and assessed the dynamics of forest biomass C stocks and the characteristics of C sinks or sources for the entire East Asia in the past three decades (1970s~2000s). (see details in Fang et al. 2014, Global Change Biology, 20: 2019每2030)
Fig. Environmentally induced annual changes in biomass density (biomass per area) with various age classes for four major plantations in Japan during the period 1980-2005.