Seasonal partitioning of evaporation between trees and understorey in a widely spaced Pinus radiata stand.
Tree transpiration, Et, and understorey evaporation, Eu, were measured hourly on 2-3 fine days during 12 periods throughout a year in a Pinus radiata plantation in Kaingaroa Forest, New Zealand. There were 450 stems/ha and 60% of herbaceous understorey vegetation and forest floor was covered by dead stems, branches and foliage from earlier thinning and pruning operations. Annual rainfall was 1154 mm and trees and understorey vegetation were supplied adequately with water throughout the year. Data are presented and discussed on relations between stomatal conductance, air saturation deficit above the forest canopy, incident quantum flux density, foliage development, night frost, tree canopy leaf area index, canopy conductance, and average daily transmittance of shortwave radiation through the tree canopy. The combination of weather variables, changes in stomatal conductance, increasing leaf area index and the relation between Eu and available energy led to the fraction Et/(Et + Eu) remaining roughly constant throughout the year with an average of 0.52.