We made a comparative study of the carbon content, carbon storage, and carbon accumulation of a 13-year-old stand of Betula alnoides and a 12-year-old stand of mixed Betula alnoides × Castanopsis hystrix located at the Fubo experimental farm of the Experimental Centre of Tropical Forestry in Pingxiang, Guangxi, China. The purpose was to evaluate the potential carbon storage and content of large-sized trees in plantations, and to provide more perspective about commercial forest plantations that are being adaptively managed for timber production objectives in conjunction with carbon storage and biodiversity conservation objectives.
The variation in mean carbon content of all components ranged from 481.11 to 600.79 g/kg for Betula alnoides, and from 451.24 to 543.42 g/kg for Castanopsis hystrix. These results are similar to the carbon content found in other tree species in China's southern subtropical area. The carbon content of the understorey vegetation was ranked as trees > shrubs. The average carbon content of the shrub layer was 437.15 g/kg in the Betula alnoides stand and 436.98 g/kg in the mixed Betula alnoides × Castanopsis hystrix stand. The average carbon content of the herbaceous layer in the mixed Betula alnoides × Castanopsis hystrix stand was higher than that in Betula alnoides stand; the difference was significant (P<0.05). The carbon content of the soil decreased as the depth of soil increased. The average carbon content of the soil in the Betula alnoides stand was more than that in the mixed Betula alnoides × Castanopsis hystrix stand; the difference was not significant (P > 0.05).
The carbon storage of the tree layer in the mixed Betula alnoides × Castanopsis hystrix stand (29.144 t/hm²) was a little higher than that in the Betula alnoides stand (28.541 t/hm²). The total carbon storage in the mixed Betula alnoides × Castanopsis hystrix stand (276.486 t/hm²) was less than in the Betula alnoides stand (305.514 t/hm²). The carbon storage in the vegetation of the Betula alnoides stand was 29.444 t/hm², and in the mixed Betula alnoides × Castanopsis hystrix stand it was 29.265 t/hm², comprising 9.64% and 10.58% of total carbon storage respectively. The litterfall plays a very important role in the carbon cycle, although in this study litterfall stored relatively little carbon, accounting for only 0.19% in the single-species stand and 0.56% in the mixed stand.
In the case of the soil, the carbon was stored mainly in the 0 to 20 cm layer, which accounted for 38.05 to 45.76% of the carbon stored in all the soil. Carbon storage declined as the depth of soil increased. The carbon storage in the 0 to 60 cm soil layer of the Betula alnoides stand (275.488 t/hm²) was greater than that in the mixed Betula alnoides × Castanopsis hystrix stand (245.688 t/hm²), accounting for 90.17 and 88.86% of the carbon stored in the respective stands.
The annual carbon accumulation rates in the tree layer of the Betula alnoides stand and the mixed Betula alnoides × Castanopsis hystrix stand were 2.196 t·hm²·a^-1 and 2.428 t·hm²·a^-1 respectively, which indicates that the rate of carbon accumulation was greater in the mixed stand than in single-species stand.