Abstract:

Aims Water consumption of single trees can be estimated by measuring the sap flow rate in trunk sapwood. Previous studies of sap flow had problems researching temporal correlations between environmental factors and sap flow because there is a lag time between environment and sap flow due to stomatal regulation in the leaf and water capacitance in the inner tissue of the trunk. Although this method has been used extensively for forest trees, it has not been reported for Quercus acutissima.

Methods We used a micro-meteorological station and thermal diffusion probes to measure daily meteorological factors such as total solar radiation (R_s), net solar radiation (TBB), air humidity (RH_a), air temperature (TP_a), wind speed (W_s), soil temperature (TPs), soil relative humidity (RHs) and the diurnal course of sap flow at lower (1.3 m), mid (4.5 m) and upper (8.0 m) heights of 40 a Quercus acutissima trunk in May 2005. The research site was on a south-facing hillside of Tai Mountain at the Forestry Centre, Forestry Science Academy of Taishan. Weather factors were sampled at 30 s intervals and recorded as 10 min averages. Sap flow velocity (SFV) was recorded by a Delta-T data logger at 10 min intervals. The temporal response of SFV to climate forcing factors was investigated using cross-correlation analysis over a range of time lags from -100 min to +180 min.

Important findings Patterns of daily and diurnal SFV fluctuation were different at the three trunk heights. SFV in upper trunk sapwood changed quickly and peaked >0.002 cm·s ^-1. SFV in the lower trunk changed slowly and was no more than 0.001 cm·s^-1. SFV in the mid-trunk was intermediate. The main environmental factors correlated with SFV were TBB, TP_a, RH_a, although their effects were not similar to each other (TPs and RHs were not significantly correlated to SFV). TBB showed the strongest (positive) correlation with SFV. TP_a and RH_a had weaker correlations: positive for TP_a and negative for RH_a. Correlations ranged from 0.265 to 0.944 for TBB versus SFV, from 0.409 to 0.869 for TP_a versus SFV and from -0.406 to -0.159 for RH_a versus SFV. The correlation of sap flow and environmental factors indicated that there were lags between SFV and TBB, TP_a and RH_a. Upper, mid and lower trunk lag times were about 80, 20 and 30 min, respectively, for SFV versus TBB, 60, 130 and 110 min for SFV versus TP_a and 170, 160 and 90 min for SFV versus RH_a.

Key words: Quercus acutissima, sap flow, lag effect, environmental factor