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 (Θ) 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 Θ > 0.2 increases linearly as the initial date on which Θ > 0.2 becomes earlier. The delay spans about 20 days in years when Θ > 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 Θ > 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.
Relationship of the medians of the dates of transpiration onset determined by all heat pulse sensors with the initial date of the longest period in which the LAI was greater than 0.15, 0.2, 0.25, 0.3, 0.4, and 0.5. Broken lines indicate 1:1 relationship (JPEG 55 kb)
Relationship between the dates of leaf-out (solid triangles) and the median of the dates of transpiration onset determined by all heat pulse sensors (open circles) with the initial dates when relative extractable water in the 0-0.6-m soil layer.(Θ) became greater than 0.1, 0.15, 0.2, 0.25, 0.3, and 0.4. (JPEG 427 kb)