| The main focus of this study was to estimate carbon sequestration in two 30a rubber plantations in Hainan (Clone PR107 and clone RRIM600). Biomass method was introduced to study carbon sequestration in tree layer, understory vegetation and litterfall in this ecosystem. The major results and conclusions are summarized below.The rate of carbon content in various organs of rubber tree fluctuated within the range of 45.16%-53.73%, with the order as followed:leaf>trunk> branches>bark>roots. Whereas, the order of carbon stock in various organs were as followed,PR107:trunk(55%)>branch (18%)>roots (17%)>bark (8%)>leaf (2%);RRIM600:trunk (43%)>branch (28%)>roots (16%)>bark (11%)>leaves (2%). SAS9.0 was used to establish two types of regression models for rubber tree, namely, the first model between carbon storage in various organs and girth (G), the second model between carbon stock and tree height plus diameter at breast height (D2H), all the first regression model with girth as a variable was used to estimate carbon stock in the rubber tree in each level, so that carbon stock in every girth intervals would be obtained. The average rubber tree density in Hainan is 450/hm2, which lead to PR107:153.51t·hm-2, RRIM600:131.13t·hm-2 carbon sequestration in rubber tree during their 30 years life cycle.Annual located monitoring suggested that carbon stock in litterfall was PR107:110.81 t·hm-2; RRIM600:120.26 t·hm-2, throughout the life cycle 30 years, and carbon stock in understory vegetation was PR107:12.93t·hm-2; RRIM600:11.17t·hm-2. Carbon content in litterfall and understory vegetation ranges from 44.83%-49.17%, with the sequence fruit>leaves>flowers>understory vegetation>deadwood. Each year, PR107&RRIM600 carbon stock in litterfall under 8a-22a rubber tree varies from 2801.59 kg·hm-2-5288.01kg·hm-2,2897.26 kg·hm-2-5770.14 kg·hm-2, with the average value of 4124.70 kg·hm-2,4381.07kg·hm-2. The order for carbon stock in litterfall appeared to be:dead leaves>deadwood>understory vegetation>fruit>flower. In other words, along with increasing in tree age, carbon stock in litter and understory vegetation of rubber plantation is gradually increasing. For that, the total value of carbon sequestration during 30year's life cycle for rubber plantation ecosystem are:PR107:302.4t·hm-2; RRIM600:287.35t·hm-2, which means the average annual fixed amount are 10.07t·hm-2 and 9.58t·hm-2. According to these, the total value of C sequestration and O2 release for plant subsystem in rubber plantation (PR107&RRIM600) ecosystem could be calculated. The results show that: PR107:45,306 U.S. dollars for C sequestration per hectare; 39,934.19 U.S. dollars for O2 release per hectare, that is to say,85,240.19 U.S. dollars for the total value of C sequestration and O2 release per hectare.It is therefore concluded that rubber plantation forest can sequester large amount of green house gas CO2 and store in its biomass during its economic life span and product's life span while producing latex and timber. It is on this context that rubber plantation forest is considered a large canban sink that helps the slow down of global climate change. Rubber plantation forest development can offer direct and indirect eocological services. The present result is of certain significance so as to proper valuation of rubber plantation forest in regional and global climate change studies.
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