Relationship between the budburst timing of Castanopsis sieboldii and thermal factors.

Abstract

Differences of budburst timing of Castanopsis sieboldii (Makino) Hatusima ex Yamazaki et Mashiba (incl. ssp. lutchuensis (Koids.) H. Ohba) reported in Japan, were discussed from the point of view of air temperatures around the budburst period, frost period and the optimum for maximizing annual photosynthetic gain. Firstly, daily mean, maximum, and minimum temperatures during and immediately before the budburst were compared. It revealed that air temperatures were generally different between mainland Japan and the Ryukyu Islands, and within the Ryukyu Islands themselves. This suggests that later budburst in the north cannot be fully ascribed to the delayed arrival of the same warm temperature conditions. Secondly, actual budburst timing of Castanopsis sieboldii and the normal frost period of the areas were compared. Budburst timing generally corresponded to the end of frost period in mainland Japan. Thirdly, a model for estimating the annual photosynthetic gain was proposed using the maturation and aging process of Castanopsis sieboldii leaves, and the relationship between photosynthetic rate and temperature. By simulating this model using the air temperature data at six different locations in Japan, optimal timing of budburst for maximizing the annual photosynthetic gain was obtained. The resultant optimal timings of budburst coincided with the actual timing in Ryukyu Islands where no frost occurs. In mainland Japan, the optimal timings fell within the frost period and did not coincide with the actual budburst timing. However, actual budburst timings in mainland Japan led to the largest annual photosysnthetic gain in the simulation outside the frost period. Therefore, budburst timing of Castanopsis sieboldii may be scheduled to maximize the annual photosynthetic gain, while avoiding frost damage.