Effects of soil water decline on diurnal and seasonal variations in sap flux density for differently aged Japanese cypress (Chamaecyparis obtusa) trees

摘要

The effects of soil drought on transpiration are often neglected?when predicting transpiration for forests in humid regions under the influence?of the Asian monsoon. These effects have indeed been neglected?for Japanese cypress, Chamaecyparis obtusa, a major plantation species in?Japan and the surrounding area, probably because previous studies have?reported no clear effects of soil drought on transpiration for Japanese cypress?forests. However, a few studies have reported an apparent reduction?in transpiration with soil drought for young Japanese cypress forests. It remains?unclear whether such a reduction in transpiration is limited to young?Japanese cypress forests or if it is not uncommon for mature Japanese cypress?forests, which occupy a large area in Japan. To clarify this point,?we conducted sap flux measurements in a year with soil drought on three?differently aged Japanese cypress stands including mature (43 years old)?and relatively young (23 and 26 years old) trees. In a diurnal time scale, a?cross correlation analysis of sap flux density (Fd) and vapor pressure deficit?(VPD) showed that the time lags between Fd and VPD were 1-3 h in dry?soil conditions. These were larger than those of wet soil conditions (<1 h)?for all sample trees. Fd at a given VPD in dry soil conditions was smaller?than that in wet soil conditions for all sample trees; a 28%–63% reduction?in the rate of change in Fd was observed under dry soil conditions. Because?our results were obtained when the non-exceedance probability of recorded?monthly precipitation was 9%–18%, the results suggest the need to consider?the effects of soil drought more extensively. Those effects should be?considered for not only relatively young but also mature Japanese cypress?when predicting diurnal and seasonal patterns of transpiration in years with?soil drought, and when predicting inter-annual patterns of transpiration for?Japanese cypress despite humid temperate climate.