| dc.description.abstracteng | General abstract
Global climate change caused by increased carbon dioxide (CO2) and other greenhouse gases has become one of the most urgent worldwide environmental topics of the 21st century and alleviating climate change has become a core research interest. Forests cover 30% of total global surface and their role in alleviating global climate change has been highlighted because forests act as a continuous carbon sink of 1.9-2.6 Pg C (1 Pg = 1015 g) per year.
China is one of the biggest countries worldwide with large area of forests, and China’s forests act as an important role in alleviating climate change. Plantation forests, as one important forest type in China, have reached an area of 61.7 million ha until 2008 due to massive plantation establishment activities in the past several decades, which amount to 29% of the global plantation area. From 1950 to 2011, plantations in China sequestered 1.686 Pg C by net uptake into biomass and soil organic carbon. Therefore, China’s plantations have made a considerable contribution to reduce atmospheric carbon. However, the stand quality of plantations is relatively low due to the short rotations and a lack of appropriate management. Mean standing volume in plantations is 49 m3 ha-1, which is much lower than the world average of 130.7 m3 ha-1 according to 7th national forest inventory in China. This suggests a huge potential to increase carbon sequestration in China’s plantations through forest management and thereby alleviate future climate change.
To improve the management of China’s plantations, the objectives of this thesis, as a case study, were to provide a methodological basis for stand management of Chinese fir plantations and to develop a method to efficiently estimate regional biomass in Shitai County, China in context of carbon forestry. This thesis contains five main chapters that are prepared as manuscripts. Specifically, firstly, the aboveground biomass (AGB, including stem, branch and leaf biomass) and its dynamics were estimated in a Chinese fir plantation (Cunninghamia lanceolata) and a Castanoposis sclerophylla forest using the generalized algebraic difference approach (GADA) and biomass allometric models. GADA models were developed for both Chinese fir and Castanoposis sclerophylla to predict the diameter at any age of each individual tree. Secondly, a compatible taper function based on Fang et al. (2000) and a stand-level merchantable volume model were developed for Chinese fir plantations to estimate merchantable and total volume at the tree- and stand-levels. Thirdly, stand density management diagrams (SDMDs) were proposed to optimize stand management of Chinese fir plantations for different management purposes. Fourthly, soil organic carbon was analysed to examine the effects of land use change from secondary forest to Chinese fir and Moso bamboo plantations on soil organic carbon (SOC) and nitrogen (N) stocks. Finally, regional biomass was estimated based on inventory data using five different methods, which is expected to contribute to optimize the selection of methods for efficient regional biomass estimation.
This thesis was a part of the Lin4carbon project, which aimed to develop and optimize integrated methodological approaches for the estimation of biomass and carbon stock and improve forest management in China. Forest inventories were conducted to deliver actual estimates of volume, biomass and carbon stock and provide information about the feasibility and costs of sampling and plot designs. To achieve these objectives, three different inventory scales were established: (1) a land use and forest inventory (LUI/FI) that produced information over the whole extent of the sampling frame based on a 3×3 km systematic grid; (2) a forest management inventory (FMI) for those stands with forest management information with a 500×500 m systematic grid, and (3) a stand inventory of selected stands (SI) with a 100×100 m systematic grid. A nested plot design with an inner radius of 6 m and outer radius of 10 m was used. A total of 258 plots were established, 74 of which were dominated by Chinese fir plantations.
In the SI plots, a total of 103 trees of different dominance classes of the five main species were felled for stem analysis and biomass allometric model construction. This included 46 Chinese fir trees. The stems were cut into sections at heights of 0.3 m, 1.3 m, 3 m and 2 m intervals thereafter up to the treetop. Cross-sectional stem discs (about 5 cm thick) were collected at each of these heights, as well as one disc at ground level.
In chapter III, GADA models based on stem analysis of 18 C. lanceolate and 15 C. sclerophylla trees were developed. The application of the GADA model yielded a very good performance in terms of diameter prediction for each individual tree. Combining the allometric models, estimated AGB increased with stand age, increasing from 69.4 ± 7.7 (mean ± standard error) in 2010 to 102.5 ± 11.4 Mg ha-1 in 2013 for Chinese fir plantations, compared to 136.9 ± 7.0 in 2010 to 154.8 ± 8.0 Mg ha-1 in 2013 for C. sclerophylla forests. AGB increment was 10.6 ± 1.2 for 2010-2011, 11.0 ± 1.2 for 2011-2012 and 11.5 ± 1.3 Mg ha-1 a-1 for 2012-2013 for C. lanceolate, respectively. They were significantly higher than those of C. sclerophylla forests with 5.8 ± 0.3, 5.9 ± 0.3 and 6.3 ± 0.4 Mg ha-1 a-1. Stems contributed 76% to total AGB and AGB increment, highlighting the importance of stems in ecosystem biomass stocks. By combining biomass allometric models, stand-level AGB dynamics were successfully estimated, which can contribute to our understanding of net primary production and carbon sequestration dynamics in these forest ecosystems.
Based on a statistical sample of n = 46 non-forked Chinese fir trees, a compatible taper function proposed by Fang et al. (2000) and a stand-level merchantable volume model were developed for Chinese fir plantations in chapter IV. A second-order continuous autoregressive error structure corrected the inherent serial autocorrelation of different observations in a given tree. The taper function and volume equations were fitted simultaneously after autocorrelation correction. The model developed by Fang et al. (2000) fitted the data well and had a very good performance in terms of diameter and individual tree volume prediction. The stand-level merchantable volume equation based on the ratio approach was developed using basal area, dominant height, quadratic mean diameter and top diameter (ranging from 0 to 30 cm) as independent variables. The high correlation between measured and predicted total and merchantable stand volume from this model highlighted the efficiency of stand-level merchantable model in stand-level merchantable volume and total stand volume estimation. Lastly, a total stand-level volume table using stand basal area and dominant height was proposed for local forest managers to simplify the stand volume estimation.
To provide a simple tool that guide the stand management for different management purposes, SDMDs for stand volume, stem biomass and total AGB were developed for Chinese fir plantations in chapter V. The number of the trees per ha was set on the y-axis and dominant height was set on the x-axis, while stand volume, stem biomass or AGB was presented on the isolines. To develop the SDMD, a system of four equations was fitted simultaneously using the data collected from 74 inventory plots. The relative spacing index was used to characterize the growing stock. Two practical examples with the same management target were also proposed to explain the use of SDMD for stand management optimization. The proposed SDMDs can be an easy tool for local forest managers to estimate the stand volume, stem biomass and AGB, and could be a reference to determine optimum thinning schedules. It is an effective planning tool based on relatively low input with reliable data.
Conversion of secondary forests to Chinese fir and Moso bamboo plantations represents an important land use change in subtropical China. The effects of this conversion on soil organic carbon (SOC) and nitrogen (N) stocks have been examined in chapter VI. Soil samples were collected from six plots in the Castanopsis sclerophylla forest, five plots in the Chinese fir and three plots in the Moso bamboo plantations. Three profiles starting from the north direction, lying at 0°, 120° and 240°, and 3 m from the plot center, were sampled in each plot. Soil samples were collected down to 50 cm in four layers: 0-10 cm, 10-20 cm, 20-30 cm and 30-50 cm. Mineral soil was air-dried at room temperature and sieved through 2-mm and 0.15-mm sieves for total SOC and N concentration analysis. Over 0-50 cm, SOC and N stocks in the secondary forests were significantly higher than those of Chinese fir and Moso bamboo plantations with values of 203.68, 127.34 and 118.25 Mg ha-1 for SOC stock and 9.24, 5.10 and 6.35 Mg ha-1 for N stock. The results indicated that converting the secondary forests to Chinese fir and Moso bamboo plantations significantly decreased the SOC and N stocks at a depth of 0-50 cm. These reductions were associated with prescribed burning, site preparation, belowground metabolism and stand age. Therefore, it is expected that increasing the rotation age (and improving establishment techniques) could not only increase the AGB accumulation, but also increase SOC stock in the Chinese fir plantations. | de |