Publications
>> internal Site (SSL)
|
| , : Evaporation, Xylem Sap Flow, and Tree Transpiration in a New Zealand Broad-Leaved Forest, Agricultural and Forest Meteorology, 62, 53-73 (1992) | |
Abstract:
Total evaporation (E), forest floor evaporation (Ef), tree xylem sap flow (F), and environmental parameters were measured on 6 consecutive late-summer days under different weather conditions in a well-watered, temperate broad-leaf forest. Two tree species, Nothofagus fusca (Hook. f.) Oerst. (red beech) and N. menziesii (Hook. f.) Oerst. (silver beech), formed a vertically structured, complex canopy with a one-sided leaf area index of 7. The forest comprises 30-40 trees ha-1 of emergent red beech up to 36m tall and 1.7m diameter, above a mixed species canopy of 200 trees ha-1 about 20 - 30m tall and 0.4m average diameter, and approximately 900 trees ha-1 of sub-canopy, mostly silver beech < 20m tall and 0.1m average diameter.Agreement of E (determined by eddy covariance) and the difference between available energy and sensible heat flux densities was generally within 10% on half-hourly and daily bases. On clear days, the Bowen ratio obtained a broad plateau of about 1-2 for most of the time with much lower and even negative values around sunrise and sunset. Variable cloudiness caused substantial variation in available energy and the Bowen ratio. After rain, daytime Bowen ratios were somewhat lower and relatively constant at about 0.8 when the tree canopy was partially wet.Lysimeter measurements indicated Ef was a significant evaporation component and accounted for 10-20% of E, with rates up to 0.5mm day-1. Agreement between F (measured by a xylem sap flow method in a representative 337m2 plot of 14 trees) and tree canopy transpiration (E - Ef) was reasonable, with an average disparity of order 10-20% or 0.3 ± 0.1 mm day-1 (standard deviation) when the tree canopy was dry.Within the plot, F typically varied by more than an order of magnitude. Tree social position, assessed by emergence of crown from the general canopy level, strongly affected an individual"s contribution to plot sap flux density. About 50 % of daily plot F emanated from only three emergent trees.Diurnal variation in coupling of the tree canopy to its aerial environment reflected changes in humidity and wind speed corresponding with changes in stomatal and aerodynamic conductances. Consequently, varying proportions of radiative and advective energy were involved in determining tree transpiration rate. Wet canopy evaporation rate was also examined because it significantly influences when transpiration takes place as rain falls on about 200 days of the year in the forest studied. Effects of leaf size and plant nutrition on tree transpiration and the partitioning of available energy are also discussed. |
|
