西南高山地区净生态系统生产力时空动态

西南高山地区生态系统类型丰富、地形复杂,是响应全球气候变化的重点区域,对全球气候变化具有重要的指示作用.应用生态系统模型(Carbon Exchange between Vegetation,Soil,and the Atmosphere,CEVSA)模型估算了1954-2010年西南高山地区净生态系统生产力(NEP)的时空变化,分析了其对气候变化的响应.结果表明:(1)1954-2010年西南高山地区NEP平均为29.7gC·m-2·a-1,其中低海拔地区常绿针叶林和常绿阔叶林NEP较高,而高海拔地区的草地覆盖类型NEP较低.(2)西南高山地区NEP总量的变动范围为-8.36-29.4Tg C/a,平均每年吸收碳15.4Tg C;NEP年际下降趋势显著(P＜0.05),平均每年减少0.187Tg C,下降显著的区域占研究地区总面积的35.2％ (P＜0.05),其中草地(-0.526 g C·m-2·a-2,P＜0.01)和常绿针叶林(-0.691g C·m-2·a-2,P＜0.01)下降趋势极为显著.(3)年NEP总量的年际变化与年平均温度呈负相关(r=-0.454,P＜0.01),与年降水量呈正相关(r=0.708,P＜0.01),与温度显著负相关的区域占60.3％ (P＜0.05),与降水显著正相关的区域占52.1％(P＜0.05),其中草地和常绿针叶林均与温度极显著负相关(r=-0.603,P＜0.01；r=-0.485,P＜0.01),而与降水量极显著正相关(r=0.554,P＜0.01; r=0.749,P＜0.01).(4)西南高山地区是明显的碳汇区,但是由于土壤异养呼吸(HR,heterotrophic respiration)的增长速度大于净初级生产力(NPP,net primary production)的增长速度,最近20a有部分地区开始由碳汇转为碳源.

Temporal-spatial variations of net ecosystem productivity in alpine area of southwestern China

There are various types of ecosystems and complex landform in alpine area of southwestern China, which make it very sensitive to global climate change. Quantifying the spatial and temporal variation of terrestrial ecosystem carbon sink in this place requires a detailed understanding of carbon exchange between vegetation, soil, and the atmosphere. A process-based biogeochemical model CEVSA(Carbon Exchange between Vegetation, Soil, and the Atmosphere) was used to estimate temporal and spatial variations of net ecosystem production (NEP) in alpine area of southwestern China from 1954 to 2010. In this study, the set of independent environmental variables to run the model consisted of climate (ten days mean value of temperature, precipitation, relative humidity, cloudiness), atmospheric CO₂ concentration, soil and vegetation types at a spatial resolution of 0.1°. We first ran CEVSA model with average climate data during the period 1954-2010 until an ecological equilibrium was reached, then made dynamic simulations from 1954 to 2010 with transient changes in climate at a time-step of ten days.To achieve the results, various kinds of computer software were applied, such as ANUSPLIN4.1, Arcgis9.3, SPSS18.0, Fortran 90. The results showed: (1)The mean value of NEP was 29.7 g C·m^-2·a^-1 during the period 1954-2010 in alpine area of southwestern China, with higher NEP in the evergreen needle-leaved and evergreen broadleaved tree cover at low altitude and lower NEP in herbaceous cover at high altitude of study area. (2) Annual total NEP ranged from -8.36 to 29.4 Tg C/a with a mean value of 15.4 Tg C/a. NEP showed a statistically significant decreasing trend with a reduction rate of 0.187 Tg C/a (P<0.05) during the period 1954-2010 and the significant decreasing area of NEP accounted for 35.2% (P<0.05). The decreasing trend was highly significant in herbaceous cover(-0.526 g C·m^-2·a^-2, P<0.01) and evergreen needle-leaved tree cover (-0.691 g C·m^-2·a^-2, P<0.01). (3)Annual total NEP was significantly negatively correlated with annual mean temperature (AMT) (r=-0.454, P<0.01) and positively correlated with annual total precipitation (ATP)(r=0.708, P<0.01). The negative correlation (P<0.05) of annual NEP and AMT existed in 60.3% of the study region while the positive correlation (P<0.05) of annual NEP with ATP existed in 52.1% of the study region, respectively. Herbaceous cover and evergreen needle-leaved tree cover as the two main vegetation types in study area were both correlated negatively with AMT(r=-0.603, P<0.01; r=-0.485, P<0.01) and positively with ATP(r=0.554, P<0.01; r=0.749, P<0.01), reaching an extremely remarkable level. (4)Alpine area of southwestern China acted as a carbon sink, however, a small fraction of terrestrial ecosystems here shifted from carbon sinks into carbon sources in recent 20 years, for that the NPP (net primary production) growth rate was less than that of the soil HR (heterotrophic respiration).These estimates indicate that the carbon sequestration capacity in alpine area of southwestern China was weakened by the ongoing climate change.