Study of the rate of stem sap flow in Pinus tabulaeformis and Quercus variabilis by using the TDP method
油松、栓皮栎树干液流速率比较

This study assessed the sap flow velocity of pine (Pinus tabulaeformis) and oak (Quercus variabilis) using Thermal Dissipation Probe (TDP), one of methods (others incliding the injection of dyes, radioactive tracers, heat pulse, water balance and nuclear magnetic resonance imaging) to estimate sap flow velocity. Then, the trans-evaporation of the stand and related factors were estimated based on the sap flow velocity. The study site was in the Experimental Station of Beijing Forestry University (about 800 ha), which locates on the north end of Taihang mountain, and closes to the east end of Yanshan mountain in Beijing. Longitude is 116(28'E and latitude is 39(54'N. The original vegetation was temperate mixed conifer-deciduous broad leaf forest with the pine and oak forest as the main forest type. The sampled plot was a 31 year old uniform mixed conifer-deciduous broad leaf forest. The crown density was 0.8 while the tree density was 1433 trees/hm~2 with a stand composition of 70% of pine and 30% of oak. In average, the diameters at breast height (DBH) were 11.4 cm and 20.6 cm, the heights (H) were 8.9 m and 12.8 m, and crown widths were 2 to 3 m and 3 to 4 m with the density of 1051 trees/ha and 382 trees/ha for the pine trees and oak trees respectively. The edge of the plot (30 m2) was trenched to the hard rock with asphalt felt buried to prevent soil water horizontal exchange from nearby. In the plot, one mean pine tree (H=9 m, DBH=11.3 cm) and one mean oak tree (H=13 m, DBH=21.2 cm) were selected for the study. A pair of TDP-50 probes connected with a data logger was inserted into the stem of each tree at a height of 1.5 m to monitor the sap flow velocity using the Stem Flow System (ICT Corporation, Australia) for about one year.The monthly average of sap flow velocity in oak stems was five to ten times higher than that in pine stems. In the dry season there was still sap flow in oak stems during daytime, but no in pine stems. However, remarkable sap flow did happen in pine stems during nighttime. The sap flow velocity in pine and oak stems were about 0.0001 cm/s and 0.0006 cm/s respectively. In the wet season, the patterns of sap flow velocity in stems of both pine and oak followed the patterns of the total solar radiation, but mono-peak found in pine while multi-peak in oak. Low total solar radiation decreased the sap flow velocity in pine stems, but did not affect it in oak stems. The sap velocities in the stems of both pine and oak were about 0.0015 cm/s, which were about 10 and 2.5 times higher than the sap flow velocities in stems of pine and oak in dry season respectively.