| The stems not only are the largest storages of the biomass in forest ecosystems,but also represent a huge and long-term carbon sink. Trees metabolic breath can release the CO2 and consume more than 50% of their fixed. The research on stem respiration mainly in temperate forests and tropical forests, It is rare for subtropical degraded forest reports. The soil respiration chamber method was used to measure in situ stem respiration rates. The experiment was conducted with an automatic portable photosynthesis system(LC PRO-SD, ADC)connecting with a PVC respiration collar in Qianyan zhou, Jiangxi Province. The measured 29 year old Pinus elliottii was divided into three DBH(17.9~22.9cm, 23.6~26.8cm, 27.5~32.5cm), and 29 year old Pinus massoniana was divided into three DBH(15.9~19.5cm、20.5~21.9cm、23.7~28.3cm). We measured stem respiration rates of situ Pinus elliottii and Pinus massoniana for a year. To investigate the individual difference and the seasonal variation of the stem respiration of the two tree species, and to investigate the response of the stem respiration to the stem temperature. Revealing patterns of tree respiration variation and its impact factors could help us predict the caron sink function of the global climate change of 2 dominant tree species(Pinus elliottii and Pinus massoniana) in southern subtropical degraded red soil region and the future role of the global warming, and could also provide basic data and scientific basis for the establishment of the forest carbon cycle model. The results as followed:(1) Daily variations of stem respiration rates of The Pinus elliottii showed single-peak patterns, which were consistent with the stem temperature. The response of stem respiration had 1-2 hours’ delay. The peak value of the stem respiration appeared between 16:00 and 17:00, and the average value appeared during11:00-14:00 and 22:00-2:00. The stem temperature could explain the day and night of the stem respiration of the 16.6%~43.1%, and this change was more sensitive to the stem temperature in the non-growing season.(2) In the upward phase of the sap flow rate, there was no significant correlation between the sap flow rate and stem respiration in growing season and non-growing season. In the downward phase of the sap flow rate, there was significant positive correlation between the sap flow rate and stem respiration in non-growing season. In the stable phase of the sap flow rate, regression analysis showed that the regression model of the growth of different growth stages and the stem temperature at different growth stages was better than that in the growing season.(3) Stem respiration rates of Pinus elliottii and Pinus massoniana showed obvious seasonal variation, the highest rate occurred in July, the minimum rate occurred in February(Pinus elliottii), January(Pinus massoniana). The percentage of stem maintenance respiration of Pinus elliottii is 41.4%~53.8%, and for Pinus massoniana,it is 57.2%~67.8%. The percentage of the large DBH was significantly higher than that of the small and medium DBH trees.(4) The sensitivity of Pinus elliottii of the medium DBH(23.6~26.8cm)trees to the stem temperature was significantly better than that of the large DBH(27.5~32.5cm)trees(p<0.05), large DBH(27.5~32.5cm)trees was significantly better than that of the small DBH(17.9~22.9cm)(p<0.05).While the sensitivity of Pinus massoniana of the large(27.5~32.5cm) and medium DBH(20.5~21.9)trees to the stem temperature was significantly better than that of the small DBH(15.9 ~19.5cm)trees(p<0.05). However, no significant difference was found between the large DBH(23.7~28.3cm)trees and the medium DBH(20.5~21.9cm)trees(p>0.05).(5) The stem respiration rate of Pinus elliottii and Pinus massoniana increased with the increasing DBH, which showed a very significant linear correlation with the increasing DBH(P<0.01) and suggested that the DBH could be a simple and practical proxy for predicting and extrapolating stem respiration of Pinus elliottii and Pinus massoniana. |
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