| Chinese fir (Cunninghamia lanceolata (lamb.) Hook) is a native fast-growing timber species in China with the characteristics of easy to breeding, fast-growth, excellent timber quality and high productivity, and has been planted and cultivated for more than 1000 years. Chinese fir plantation is one of the most managed forest types in Southern China and is cultivated almost subtropical regions in the country. The planted area of this species accounts for about one fourth of the total plantation areas in the nation. Because of over successive rotations of this species with multiple generations on the same sites in the past years, many studies have reported apparent yield decline and soil fertility degradation in Chinese fir plantations. Thus, it is necessary and important to study the changes of biomass production and net primary productivity (NPP) of Chinese fir plantations over successive rotations. The results from such projects will provide valuable information and references for sustainable management of the Chinese fir plantations in order to maintain long-term soil fertility and stand productivity for the forest ecosystems.In the present study, the biomass and NPP of two successive rotation generations of Chinese fir plantations were examined at Huitong Ecosystem Research Station of the Central South University of Forestry and Technology in Huitong County, Hunan Province, one of the National Field Stations for Scientific Observation and Experiment in China, using a chronosequence approach on the same forested small watersheds. Additionally, biomass partitioning in Chinese fir tree, understory vegetation ground dead vegetation, and little-fall layers was investigated and compared at fast-growing stage (7-11 year-old), timbering stage (14-18 year-old) and mature stage (20 year-old) in the two rotation stands. The major results showed:1. The mean individual tree biomass, stand biomass and NPP of a 7-year-old stand in second rotation of Chinese fir plantations were 13.17 kg (ranging 11.75-14.69 kg),31.10 t·hm-2 (ranging 27.03-38.48 t·hm-2), and 4.44 t·hm-2·a-1 (ranging 3.86-5.50 t·hm-2·a-1), respectively. The mean individual tree biomass, stand biomass and NPP was lower 2.46 kg (reduced by 15.74%),14.18 t·hm-2 (31.47%), and 2.04 t·hm-2·a-1 (31.47%) in the second rotation when compared to first rotation. In addition, the wood economic coefficient (wood biomass/total tree biomass) was reduced by 40% in the second rotation compared to the first rotation.2. For a 11-year-old stand, the mean individual tree biomass was declined by 7.49% in second rotation (34.73 kg) than first rotation (37.54 kg). The stand biomass and NPP were reduced by 16.5 and 16.6% in the second rotation (71.45 t·hm-2 and 6.49 t·hm-2·a-1) than the first rotation (85.60 t·hm-2 and 7.78 t·hm-2·a-1). The stem had the highest proportion of the total stand biomass, and it accounted for 61.1 and 58.2% of the total stand biomass in the two rotations, respectively. The biomass of other organs was in an order:root> leaf> branch.3. The mean individual tree biomass, stand biomass and NPP of a 14-year-old stand of the first and second successive rotations were 45.22 kg, 104.45 t·hm-2 and 7.47 t·hm-2·a-1, and 42.07 kg,97.18 t·hm-2 and 6.95 t·hm-2·a1, respectively. It meant the mean individual tree biomass, stand biomass and NPP decreased 3.15 kg,7.27 t·hm-2 and 0.52 t·hm-2·a-1 in second rotation compared to first rotation, respectively.4. The mean individual tree biomass of a 18-year-old stand was reduced by 1.2% in second rotation (62.75 kg) compared to first rotation (63.51 kg). The stand biomass and NPP were reduced 1.19%(144.96 and 146.70 t·hm-2) and 20.1%(9.49 and 11.88 t·hm-2·a-1), respectively, in second rotation when compared to first rotations.5. The mean individual tree biomass, stand biomass and NPP of a 20-year-old stand were declined by 4.55,4.56 and 14.56% in second rotation (86.56 kg,199.94 t·hm-2,9.21 t·hm-2·a-1) compared to first rotation (90.69 kg,209.50 t·hm-2 and 10.78 t·hm-2·a-1). The wood economic coefficient was lower by 17.12% in the second rotation stands than the first rotation stands. 6. The generation effects of biomass production between the two successive rotations showed that mean individual tree biomass and stand biomass of a 7-year-old stand were declined by 16.44%(13.06 and 15.63kg) and 16.48%(37.90 and 45.38t·hm-2) in second rotation compared to the first rotation; by 16.93 and 16.94%(31.11 and 37.45 kg, and 90.31 and 108.73 t·hm-2) of a 11-year-old stand; by 7.0 and 6.96%(42.07 and 45.22kg, and 97.18 and 104.45t·hm-2) of a 14-year-old stand; by 1.20 and 1.19%(62.75 and 63.51kg, and 144.96 and 146.70t·hm-2) of a 18-year-old stand; and by 4.55% and 4.56%(86.56 and 90.69kg, and 199.94 and 209.50t·hm-2) of a 20-year-old stand.The generation effects of NPP between the two successive rotations showed that NPP was 1.15 times higher in first rotation than in second rotation for 7-11 year-old stands, and 1.25 times and 1.17 times higher for 14-18 and 18-20 year-old stands, respectively.Generally speaking, the proportion of branch, leaf and bark components in the total stand biomass was slightly higher in second rotation than first rotation stands. Particularly, the percentage of root biomass in the total stand biomass was 2-4 times higher in second rotation than first rotation stands. But the ratio of wood biomass in the total stand biomass in second rotation was 20% less than that in first rotation stands, meaning the wood economic coefficient was lower in second rotation than first rotation of Chinese fir plantations.7. Different dynamic patterns of understory vegetation biomass were found in the fast-growing stage (7-11 years) and timbering stage (14-18 years) for the two successive rotations. Understory vegetation biomass increased with stand aged in first rotation, but decreased in send rotation during the fast-growing stage. In contrast, understory vegetation biomass decreased with increasing stand ages in first rotation, but increased in send rotation during the timbering stage.The dead vegetation biomass on the forest floor increased with increasing stand ages in the two successive rotations of Chinese fir plantations. The ground dead vegetation biomass of a 7-year-old stand was lower in second rotation than that in first rotation. However, the ground dead vegetation biomass in 11-,14-, and 18-year-old stands was higher in second rotation than that in first rotation stands.The average amount of little-fall was 4479.31 kg·hm-2 in first rotation, which was about 4 times as that in second rotation (1109.86 kg·hm-2). The difference was mainly derived from the dissimilar growth rate between the two rotation stands. In addition, the difference of stand density between the two rotation stands also made contribution to the difference of little-fall amount between the two rotation stands. The stand density was lower in second rotation stands was lower than that in first rotation. As a consequence, the competition of resources (such as light, water and nutrient elements) among the individual tree was weaker in second rotation compared to first rotation, and the natural pruning phenomenon was less in second rotation than first rotation stands.8. The biomass of tree stratum had the highest proportion of the total biomass in the two successive rotations of Chinese fir plantation ecosystem. The Chinese fir tree stratum accounted for 97.2-97.9% of the total biomass in first rotation, and 91.8-96.0% in second rotation. Understory vegetation stratum accounted for 0.28-0.55% and 1.21-2.88% in first and second rotations. The ground dead vegetation stratum accounted for 1.55-2.27% and 0.18-5.92% in first and second rotations, respectively. The results indicated that the Chinese fir as the planted tree species occupied almost spatial area in the forests.
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