Carbon Balance In Natural Secondary Pinus Tabulaeformis Forest At Huoditang Forest Zone In The Qinling Mountains

Huoditang forest zone is on the south slope, located in middle region of the Qinling Mountains and north margin of subtropical zone. It is a vital region to study on structure and functions of forest ecosystem in ecotone for its special geographical position, unusual conditions, diverse vegetation types and abundant biodiversity. The natural secondary Pinus tabulaeformis forest is one of the typical forest types in middle region of the Qinling Mountains which plays important roles in water conservation, carbon dioxide sequestration and biodiversity maintenance.To estimate carbon budget of P. tabulaeformis community accurately, the instrument of TOC/TON analyzer (TOC-VTH-2000A, Shimadzu Corporation, Japan) was used to measure carbon content ratio (CCR) of arbor, shrub and herbage organs, respectively. The species included P. tabulaeformis, Pinus armandi, Quercus aliena var. acuteserrata, Larix principis, Euonymus phellomas, Lonicera hispida pall, Cerasus stipulacea, Symplocos paniculata, Elaeagnus lanceolata, Carex leucochlora, Deyeuxia sylvatica, Lysimachia christinae, Thalictrum minus, Anemone vitifolia, Rubus parvifolius, Saussurea mutabilis, Anaphalis aureopunctata, Dioscorea nipponica, Rubia cordifolia, Dryopteris laeta and Sinacalia tangutica. Organic carbon content of soil was also analyzed. Carbon density and CO2 sequestration of arbor, shrubs and herbs was estimated by occurred biomass functions, newly established equations and stratified sampling technique, respectively. Carbon dioxide efflux from forest land was continuously and stably measured by means of open-path dynamic chamber technique. The findings are following.(1) CCR among variant plant species and which in the same plant species between their organs was diverse and differed from variant instruments.The significant differences of carbon storage (CS) computing from stems, bark, branches and leaves of P. tabulaeformis, branches of Q. aliena var acuteserrata, bark, branches and leaves of L. principis would be occurred when measuring values and 0.45 were taken as mean CCR of plants under the same biomass. The similar result would also appear in roots of P. tabulaeformis, branches and roots of L. principis as estimating CS with measuring values and 0.50 being mean CCR of plants. The significant differences of CCR still existed among different shrub and herb species and their organs. The most difference of mean CCR of the same organ among different shrub species and variant organs in the same shrub species was 10.58% and 6.47%, respectively. The similar phenomena occurred in herb species. The gap of CCR was 14.46 % (stem to stem) and 26.06% (root to root) in the same organ of diverse shrub species, respectively. For the same shrub species, the disparity of CCR was 11.31% (leaf to stem), 16.83% (stem to root) and 19.86% (root to leaf), respectively.CCR values from reported result by instrument of Elementar Vario EL (Germany) was 4.06%, 6.55%, 9.39%, 5.78% and 4.85% more than ours in root and bark of P. tabulaeformis and L. principis, leaf of Q.aliena var acuteserrata, respectively. But which in root of Q.aliena var acuteserrata was less 3.65% than ours. To compare data from different areas easily in study forest ecosystem carbon balance, the emergent issue is to unify instrument and measuring method.(2) We suggested carbon density (CD) of arbors with DBH <5cm was 12.68% (in 2006) and 16.10% (in 2007) of the total CD in arbor layer, respectively. The carbon dioxide sequestration (CDS) in shrub and herbage layer was 22.49% and13.48% of which in the whole vegetation between 2006 and 2007, respectively. Provided that CDS of understory forest vegetation and CD of arbors with DBH <5cm were neglected, the natural secondary P .tabulaeformis community in experimental area would change into"carbon source"and its carbon sink strength would be -20.947 Mghm-2y-1 to -20.730 Mghm-2y-1.The report of equation regressed among biomass and indices (DBH, height and crown diameter) of shrub species was scarce. Basis on data in field, we established models of organ biomass with DBH, height and crown diameter of mian shrub species in P. tabulaeformis community. The results of statistical test and error analysis demonstrated that the models had higher accuracy and better applicability. But no model could be suitable to describe relationship among organ biomass and their growing indices of fasciculate shrub species (such as Rosa swginzowii, etc.).(3)Basis on stratified sampling method, soil organic carbon density (SOCD) in depth of 40cm in experimental area was estimated as 60.492±6.73 Mghm-2 under the accuracy of 95% with relative error value±11.13%. Due to uncertain mean value of SOCD in national evergreen coniferous forest from literature in China, SOCD in experimental area was 33.64% and 90.67% of the national average value, respectively. SCOD was 64.92% of which in the whole forest ecosystem in 2007.(4) To some extent, soil mechanical and physical properties related to soil respiration in 0-20cm depth in our findings. Moreover, soil respiration rate positively and negatively related to content of gravel and cosmid in 0-10cm depth.During measuring period, significant exponential relation occurred between soil respiration rate and soil temperature in some scale. Relationship between soil respiration rate and soil volumetric moisture was pretty complex. An exponential relation between soil respiration rate and soil volumetric moisture with small determination coefficient (R20-5=0.440, R25-10 =0.521) in growing period. During dormant period, no function could be fit to show relationship between soil respiration rate and soil volumetric moisture.Basis on exponential models regressed by soil respiration rate and soil temperature in depth of 0-5 cm and 5-10 cm, adding to field measurement beyond proper temperature for models, the soil CO2 efflux estimating value was 33.307±0.166 Mghm-2y-1 to 33.326±0.400 Mghm-2y-(1α=0.05)in growing period and 9.774±1.772 Mghm-2y-1 to 9.972±1.057 Mghm-2y-1(α=0.05)during dormant period, respectively. Soil CO2 efflux during dormant period was 22.69% to 23.03% of total soil CO2 efflux in the whole year.In different position on the slope in experimental area, soil respiration rate revealed more special variance and its C.V (coefficient of variance) was more than 42%. The general trend that soil respiration rate at night and during growing period was generally more than which during daytime and dormant period.Hindered by low temperature in winter and complex environmental conditions in forest, soil respiration data in winter could hardly be obtained, but much effort should be paid on improving method and instrument to observe soil respiration during winter in the near future. Values of Q10 varied in periods and soil depth. Seeking for inherent factors and mechanism of Q10 value varying greatly will help to reveal sensitivity of components of ecosystem to global warming.(5) Basis on CD of vegetation and soil CO2 efflux, the natural secondary P. tabulaeformis forest in experimental area showed as a carbon sink and its carbon sink strength was 1.783-2.000 Mghm-2y-1 between 2006 and 2007. Comparing similar findings to ours, CO2 sequestration in the natural secondary P. tabulaeformis forest in experimental area almost euquals to Pinus sylvestnis var. mongolica forest, but our findings is less than L. principis forest, spruce-fir forest, broad-leaved Korean pine forest,temperate coniferous forest, coniferous forest, coniferous and broad-leaved forest, deciduous broad-leaved forest, sclerophyllous evergreen broad-leaved forest, evergreen broad-leaved forest ,tropical forest and even less than their weighted average value. P. tabulaeformis forest in experimental area demonstred weak carbon sinks.(6)We found great differences of CCR occurred among plant species and their organs. Plant species with high CCR should be chosen in the process of foresting and reforesting here. Variant management measures according to CCR differences of plant organs must be adapted to cultivated forest for high CO2 storage. Due to high CO2 storage in understory forest vegetation, proper management measures should be taken to arbors basis on biological traits of shrub and herb species so that appropriate niche could be provided for shrub and herb species. Moreover, measures as burning and clear-cut must be abandoned. We broached models of soil respiration rate and soil temperature in some temperature scale. To get high benefit from forest CDM (clean development mechanism) projects, soil CO2 efflux should be taken into account besides factors, such as, fire , thunder , drought , debris flow , individual growth and artificial interference when baseline is enacted and future forest CO2 storage is predicted.