颱風擾動時落葉對降低樹木死亡率的效應

Typhoons play a key role in characterizing structure and function of ecosystems in Taiwan. Forests in Taiwan have low tree mortality even when a forest is impacted by multiple strong typhoons. This is very different from many tropical and temperate forests in which tree mortality often exceeds 20% when hit by a strong tropical cyclone. Although typhoon-induced tree mortality is low, canopy defoliation is severe in many low elevation forests of Taiwan. We hypothesize that defoliation minimizes tree mortality because it reduces the effects of wind on trees. However, for defoliation to minimize tree mortality, it must occur before wind velocity exceeds a critical threshold leading to bole failure during a typhoon event. However, conventional litterfall collecting at weekly or longer intervals cannot capture the temporal pattern of defoliation during a typhoon event, which typically lasts for only one to two days. We use sequential litterfall collectors, each equipped with loadcells and a time-lapse auto-camera that records mass of litterfall at 5-minute intervals and captures images of litterfall traps on an hourly basis, to monitor temporal patterns of litterfall at the Fushan Experimental Forest that experiences very frequency typhoon disturbance. Surprisingly, there was no typhoons affected the Fushan Experimental Forest (defined as typhoons with minimum distance of < 100 km) over the past three years, the first time over the past 60 years. The weighing system (made of load cells) was more sensitive to weight changes due do variation in water content on litterfall and litterfall traps than to changes in litterfall quantity so that could not effectively reflect temporal patterns of litterfall production. The cameras, however, are functioning. During Typhoon Infa (July 2021), we capture a series of litterfall images from six litterfall traps (although it does not fit the definition of typhoons that affect Fushan). Based on visual inspection of the number of pieces of leaf litter and branch litter, we found that more than 60% of the litterfall was produced prior to the occurrence of maximum wind speed and rainfall intensity. The early production of litterfall certainly help to reduce the wind load and therefore minimize the risk of tree failure. The result supports our hypothesis that early defoliation contributes to the very low tree mortality at the Fushan Experimental Forest even in seasons with multiple strong typhoons. However, the result should be interpreted with caution because not all typhoons are the same and that Typhoon Infa was far from Fushan Experimental Forest. We need more typhoon data to more rigorously test the hypothesis.