Interannual variation in transpiration onset and its predictive indicator for a tropical deciduous forest in northern Thailand based on 8-year sap-flow records

In deciduous forests, leaf-out and leaf-fall determine the durations of canopy transpiration and photosynthesis and affect interannual variations in energy, water, and carbon exchanges. Despite numerous studies on the interannual variation in vegetation phenology and its modelling for boreal and temperate deciduous forests, few studies have examined these topics for tropical deciduous forests. We quantified the extent of interannual variation in the transpiration onset dates in a teak plantation in northern Thailand, one of the major deciduous forests in Thailand, on the basis of 8 years of sap-flow measurements and developed an empirical model to predict their interannual variation through correlation analysis with environmental variables. Interannual variation in the transpiration onset date at this site spanned 41 days over the 8-year observation period. The transpiration onset date corresponded to the date when the leaf area index reached 0.2-0.4. Interannual variation in the transpiration onset date was reproduced by the date when relative extractable water in the soil layer of 0-0.6 m depth became greater than 0.2 with a root mean square error of 10 days. The interannual variation in the transpiration onset dates for the studied tropical deciduous forest site was greater than that of deciduous forests in temperate and boreal regions, implying the considerable potential effect on annual energy, water, and carbon exchanges between vegetation and the atmosphere. Our results also suggest that the use of soil moisture would allow predictions of interannual variations in the transpiration onset and the leaf-out for deciduous forests in tropical regions. Copyright (C) 2011 John Wiley & Sons, Ltd.