Sapwood cross-sectional area (As, cm2) was estimated from sapwood crosssectional area and diameter at breast height (DBH) data collected in the Beijing Teaching Botanical Garden and Jiu Feng mountain in suburban Beijing using the relationship:
As = 0.3786 x (DBH)2’1419 (n = 22;R2 = 0.9772;p < 0.0001). (4.1)
Whole-tree leaf area index (LAI) was measured using a plant canopy analyzer (LAI2000; USA), and it was measured every 2 or 3 days during leaf expansion and defoliation and once a week during other periods under diffuse light conditions on cloudy days or at dusk.
Three Chinese pine trees of uniform size that were 45 years old were selected for sap flow measurements. The characteristics of the sampled trees are summarized in Table 4.1. Thermal dissipation probes (Dynamax, USA) were horizontally inserted in the sapwood of the trunk at breast height on both the north and south side of every sampled tree. Based on an empirical relationship (Granier 1987), sap flux density was derived from the temperature difference between the upper, constant-heated probe and the lower, unheated probe, which acted as a reference. Measurements of sap flow were taken every 10 s, and 10-min averages were stored in a datalogger (CR1000; Campbell Scientific, UK). Sap flux density measurements made in stems were scaled to each individual tree by its As (Granier et al. 1992):
Et = 52 (((Js-north + Js-south) x As x 3600)/(2 x 1000)). (4.2)