西藏原始林芝云杉林雨季林冠降水分配特征

Canopy interception and throughfall in a primary Picea likiangensis var. linzhiensis forest of Tibet

利用2006-2007年对西藏米林县南伊沟原始林芝云杉(Picea likiangensis var.linzhiensis)林林外降水、穿透水和树干茎流定位观测数据,对林芝云杉林的林冠降水再分配特征进行研究。结果表明:西藏南伊沟的年降水量为716.4mm,主要集中在4-9月份,占全年降雨量的86.95%。在林芝云杉的生长季节(4-10月份),林冠截留量为338.6mm,占同期林外降水量的51.60%;林内穿透水量为316.3mm,占同期林外降水量的48.21%;树干茎流量仅为1mm,仅占0.19%。林内穿透水(Tp)、树干茎流(Sf)、林冠截留量(Ip)及林冠截留率(PIp)与林外降水量(p)之间的关系分别为:Tp=0.8622p-3.5229,r=0.9964;Sf=0.0004p1.4586,r=0.9458;Ip=1.2222p0.6341,r=0.874;PIp=253.6p-0.7008,r=0.9732。林芝云杉林雨季林冠降水的分配规律与该森林结构复杂、林分年龄高、胸高断面积大密切相关,说明该森林在涵养水源和保持水土等方面发挥着重要的作用。

Rainfall partitioning in forest canopy plays an important role in hydrological cycles. Because the process that rainfall is partitioned by canopy into interception,throughfall and stemflow affects the volume and routine of rainfall that reaches the ground surface,the surface runoff causing soil erosion,and soil water supply for plant growth are altered. Although extensive researches have been conducted to examine the canopy interception,throughfall and stemflow in forest ecosystems across China,lacking of the information of rainfall partitioning in the forests in Tibet impedes our understanding their hydrologic ecosystem service in this region. We measured the precipitation,canopy interception,throughfall and stemflow in a primary Picea likiangensis var. linzhiensis forest in Nanyigou of Tibet during a period from October 2006 to September 2007. Total 105 time rain events were observed within the study period. The annual precipitation was 716.4 mm,of which 86.95% occurred during the period from April to September. During the growing season throughfall amounted about 316.3mm that accounted for 48.21% of the precipitation occurring in the rainy season. No throughfall was found when rainfall was lower than 3.5 mm,but beyond this range of the rainfall intensity throughfall increased linearly with rainfall amount. Throughfall percentage was lower compared to the data observed in both China forests and temperate and boreal forest in world. Canopy interception was estimated to be 338.6 mm that was 51.60% of the annual precipitation. Canopy interception showed a curvilinear increase tendency with increase in rainfall when rainfall was within 10 mm per time,and then tended to be asymptotic relationship with rainfall. Canopy interception percentage was higher than the data reported in China. The low throughfall percentage and high canopy interception percentage in the primary P. likiangensis var. linzhiensis forest were closely related to the complexity of forest structure,older stand age （210 years） and high stem basal area （126.7m^2/hm^2）,as well as the characteristics of lower rainfall intensity in the research region. The amount of stemflow was estimated to be about 1mm. Stemflow exponentially increased with rainfall. The low amount of stemflow could be attributed to the high stem roughness and bark thickness,in addition to canopy structure. The relationship between throughfall （Tp）,stemflow （Sf）,canopy interception （Ip）,canopy interception percentage （PIp） and precipitation （p） could be expressed as following formula,respectively：Tp=0.8622 p^-3.5229,r=0.9964; Sf=0.0004 p1.4586 ,r =0.9458; Ip=1.2222 p^0.6341,r=0.874; PIp=253.6 p^-0.7008,r=0.9732. The improve Horton model was also used to simulate the canopy interception. The two parameters in Horton model were calculated to be Icm^＊=4.800 and α= 0.091,r=0.9400. The high I＊cm value indicated that the forest canopy in this study had a high capacity of rainfall interception. The Horton model showed a good fitness for predicting canopy interception and could provide an insight into the canopy interception mechanism. Our results suggest that primary P. likiangensis var. linzhiensis forests have a great potential in the functions of water conservation and soil erosion control in this region.