Inter-annual variation in the response of leaf-out onset to soil moisture increase in a teak plantation in northern Thailand

To understand the impact of inter-annual climate change on vegetation-atmosphere mass and energy exchanges, it has become necessary to explore changes in leaf-out onset in response to climatic fluctuations. We examined the response of leaf-out and transpiration onset dates to soil moisture in a teak plantation in northern Thailand based on a 12-year leaf area index and sap flow measurements. The date of leaf-out and transpiration onset varied between years by up to 40 days, and depended on the initial date when the relative extractable water in a soil layer of 0-0.6 m (Theta) was greater than 0.2 being consistent with our previous results. Our new finding is that the delay in leaf-out and transpiration onset relative to the initial date when Theta>0.2 increases linearly as the initial date on which Theta>0.2 becomes earlier. The delay spans about 20 days in years when Theta>0.2 occurs in March (the late dry season)-much earlier than usual because of heavy pre-monsoon rainfalls-while there is little delay in years when Theta>0.2 occurs in May. This delay indicates the influence of additional factors on leaf-out onset, which controls the delay in the response of leaf-out to soil moisture increase. The results increased our knowledge about the pattern and extent of the changes in leaf phenology that occur in response to the inter-annual climate variation in tropical regions, where, in particular, such research is needed.