Forest Carbon Storage Dynamics At The Upper Stream Of Minjing River

As one of the most important terrestrial ecosystems, forest ecosystem plays an irreplaceable role in balancing the global carbon budget and mitigating negative impact of global climate change. Among different approaches of measuring forest carbon cycling and carbon storage, the volume-derived method using forest resources inventory data (FID) and the relationship between dominant tree species (or species association) and its biomass have been widely applied with high accuracy. Therefore this method was recommended by the Intergovernmental Panel on Climate Change (IPCC) to calculate carbon storage at the national and regional scales. With the typical forest type, forest composition, dynamics and carbon storage size in the Valley of the Upper Minjiang River (UMR), the local at the sub-alpine area, is similar to those in the middle-high latitude of the Northern Hemisphere and sub-alpine mountainous area. By studying the dynamics and composition of carbon storage in tree layer, biomass density change of dominant tree species and biomass dynamics in old-growth forests, the factors influencing carbon dynamics in the UMR were determined and the carbon allocation features were addressed. The results provide useful information for regional carbon estimating. The major results are summarized as follows:The total forest carbon storage has increased by 0.32×10⁶ Mg·C in the UMR from 1992 to 2006 [check whether this is your original meaning] with the average annual forest growth rate of 1.52 %, which converted to the annual increment of 0.1².0×10⁶ Mg·C·a^-1, forest area increasing of 38.82 % while biomass carbon density(BCD) reducing from 80.06 Mg·C·hm^-2 to 69.97 Mg·C·hm^-2 resulting from an average annual reduction rate of -0.9 %. The proportions of area and carbon storage of fir (Abies spp.) forest were declined,and the proportions for Picea spp. forest were increased. Timber forest area and its carbon storage were reduced, and the protected forest area and its carbon were increased. In the special purposed forest area, the carbon storage and BCD have observed an increase. BCD in the other forest types showed a reduction under the natural forest protection and forest classification management. Along with age, the proportion of carbon storage in the mature and over mature forest was declining, while it was increasing in young, middle-age and pre-mature forests. Currently, the mature and over-mature forest carbon storage still dominant in the UMR and the forest coverage rate has increased by 9.66 %. BCD of the middle-age and pre-mature forests showed an upward trend, while the young, mature and over-mature forests showed a downward trend.The higher BCD was found in the mature and over-mature dark coniferous forests (DCF) and the lower BCD in the young and middle aged DCF; For the same age class, the BCD values of the mature and over-mature DCF were higher than that of the national average level, while the BCD values of the middle age and pre-mature DCF were lower than that of the national average level. The BCD value of the young forests was similar to the national average. The annual increment rates of BCD in the middle-age, young and over-mature DCF were 1.3%, 0.8% and 0.7%, respectively. The higher BCD values occurred at the sites where the elevations range from 3600 to 3800 m. The annual increment rate of BCD was found to be the highest (1.03%) at the site with the elevation between 3 200 and 3 600 m. The higher BCD and the annual increment rate of BCD occurred on both north facing aspects and south-west and south-east aspects, while the lower values occurred on south facing aspects; The BCDs of DCF in the UMR have been increasing over the past 20 years, and the annual increment rate of BCD was 1.15% from 1997 to 2002, which was the highest among the five inventory periods.The net increase in the above-ground biomass density (AGBD) was 27.311±15.580 Mg·hm^-2 while the mean annual growth rate and the mean annual mortality rate were 1.930±1.091 Mg·hm^-2·a^-1 and 2.271±1.424 Mg·hm^-2·a^-1 during 1988-2002, respectively. The above-ground biomass (AGB) depended largely on the growth and mortality rate of the remaining trees of different diameter at breast height (DBH) classes and recruitment rate from one DBH class to another as well. The largest increment component of AGB came from DBH class at 20 to 40 cm, whereas the minimum increment component of AGB was above 80 cm. The net negative increment of AGB occurred at DBH ranging from 40 60 to 60～80 cm. There were temporal and spatial variations of AGB in the alpine old-growth forests as AGB changed over time for the same sampling plot and AGB varied with location or sites for the same period. The variations not only reflected in numerical value but also in positive or negative biomass increment.Based on factor analysis and other analysis methods, we analyzed relative contribution of the biotic and abiotic factors which include climate (annual precipitation, mean annual temperature), elevation, population density, mean forest age and BCD to forest carbon storage in the UMR. Our results indicated that forest carbon storage was sensitive to changes in annual precipitation and mean annual temperature, with annual precipitation more sensitive than mean annual temperature. There was significant logarithmic relationship between population density and BCD, and BCD decreased along with the increase of population density. Forest age was the internal factors to forest carbon storages, and the total forest carbon storage of all age groups were increased; however, BCD in the young, mature and over-matures forest were decreased in the whole region. The forest carbon storage and BCD in every age group were increased in the permanent sampling plots. Forest area and BCD were major factors influencing forest carbon storage, and the increased forest carbon storage was resulted mainly from the contribution of increased forest area, and the BCD showed an increasing trend in the later period of this study.