六盘山森林植被碳密度空间分布特征及其成因

Spatial distribution of carbon density for forest vegetation and the influencing factors in Liupan Mountains of Ningxia,NW China

深入了解干旱缺水地区森林植被碳密度的空间分布特征是定量评价森林固碳能力、合理协调林水矛盾的重要基础。然而,目前有关干旱缺水地区的植被碳密度的研究仅限于典型样地上的碳储量、碳密度的比较,对区域尺度上森林植被碳密度的空间分布特征了解较少。为此,利用宁夏六盘山自然保护区2005年森林资源一类清查数据,计算了森林植被碳密度,并分析了其与林分结构特征和环境因子的关系。结果表明,六盘山的森林植被碳密度(t/hm2)平均为26.17(0.67—120.63),其中天然次生林为30.2(7.6—120.6),显著高于人工林的15.7(0.67—66.7)。森林植被碳密度随林龄增加而线性增大,天然林和人工林的平均增速分别为1.11和2.48 t hm-2a-1,而且,部分未成熟林的林分植被碳密度已接近甚至超过全国同类森林类型成熟林的植被碳密度平均值。随林分密度增加,森林植被碳密度增大,但在林分密度>1000株/hm2后,森林植被碳密度不再增大,达到其最大值,其中,天然林为75.4 t/hm2,人工林为34.6 t/hm2;林冠郁闭度对森林植被碳密度的影响与林分密度相似,森林植被碳密度增长的郁闭度拐点为0.5。水分条件是影响六盘山森林植被碳密度的重要因素,森林植被碳密度(t/hm2)由700 mm以上地点的32.5(7.6—120.6)下降至年降水量500—600 mm地点的10.9(0.67—42.9),而且随年降水量减少,最大森林植被碳密度所对应的海拔高度呈增加趋势,如在年降水量为>700、600—700和<600 mm的地区,最大碳密度所在海拔高度分别为1900—2100、2100—2300和2300—2500 m。综上所述,研究区森林植被还有较大的固碳潜力,从提高森林固碳功能角度来看,林分郁闭度不宜超过0.5。

In arid regions, the conflict between forest carbon sequestration and its water consuming has severely limited regional sustainable development. Deeply understanding of the spatial distribution of vegetation carbon density for forest vegetation is essential for quantitative evaluation of forest carbon sequestration capacity and the carrying out of water-forest integrated watershed management. However, most former studies mainly focused on the comparison of carbon density among typical sample plots. The spatial distribution of carbon density for forest vegetation and its causes were still unclear. In present study, the vegetation carbon density for forest and its variation with forest structure and site conditions were analyzed based on the data of forest resources inventory in 2005 for Liupan Mountains, northwest China and the data of carbon content and the biomass regression models from published papers. The results showed that the average of carbon density for forests in Liupan Mountains was 26.7 t/hm^2 with a variation from 0.7 to 120.6 t/hm^2, of which 69.4% （8.86%-99.2%） was stored in tree layer, 25.6% （1.6%-88.1%） in shrub layer, and 4.7% （0.12%-32.3%） in herb layer. The ratio of carbon density for tree layer to the total vegetation carbon density rose with the increase of total vegetation carbon density,while the ratio for shrub layer and herb layer sharply declined. The average carbon density was 35.1 t/hm^2 for natural secondary forest. The average carbon density among the key natural secondary forest varied from 43.4 t/hm^2 for Quercus liaotungernsis forest, 35.9 t/hm^2 for Salix cathayana forest, 28.4 t/hm^2 for Betula spp. forest to 22.3 t/hm^2 for Populus davidiana forest. The carbon density of artifical plantation was very low with a mean value of 15.7 t/hm^2 （0.67-66.7 t/hm^2） which was significantly lower than 35.1 t/hm^2 of natural secondary forest. The carbon density of forest stand increased linearly with the increase of stand age by the rate of 1.11 and 2.48 t hm^-2 a^-1 for natural secondary forest and artificial plantation, respectively. There was a tree density threshold of 1000 trees·hm^-2 and a threshold of 0.5 for canopy density of forest vegetation. When either tree density or canopy density increased beneath the values of two thresholds, the carbon density of forest stand would increase sharply. Once either tree density or canopy density of forest stand reached and exceeded these threshold values, the carbon density of forest stand would meet its maximum and increase no more. The maximum carbon density was 75.4 t/hm^2 and 34.6 t/hm^2 for natural secondary forests and plantations, respectively. The carbon density of forest stand increased sharply with the increase of annual precipitation. The maximum of carbon density for forest stand occured on the altitude of 1900 -2100 m, 2100 -2300 m, 2300 -2500 m in the areas of Liupan Mountains with annual precipitation 〉 700 mm, 600-700 mm, 〈 600 mm, respectively. It was concluded that water condition was the determinant for forest vegetation carbon density in Liupan Mountains. The canopy density of forest stands in the areas like Liupan Mountains should be kept about 0.5 for carbon sequestration.