Immediate Effects of Thinning with a Small Patch Clearcut on Understory Light Environments in a Cryptomeria japonica Plantation in Central Taiwan

【Summary】 Thinning is an important forest management practice; however, its effects on the microenvironment have rarely been investigated in tropical and subtropical ecosystems. We examined the effect of 25% (25 of 100 10 x 10 m subplots were clearcut) and 50% (1/2 of the subplots were cut) thinning on understory light environments characterized using hemispherical photography at a Cryptomeria japonica plantation, Taiwan’s most common plantation species. Thinning had a greater effect on the variability than on the availability of understory light, with the former increasing 40 and 120% under 25 and 50% thinning intensities, respectively, and the variance increasing 2.4~9.9-fold. The > 40% increase in understory light availability following 25% thinning was much greater than the 25% increase in light at a forest in northeastern Taiwan following the 1996 category-3 typhoon Herb, which decreased variability of understory light. Typhoons are the most important natural disturbance in Taiwan. Thus, mechanical thinning at intensities > 25% increases light availability and variability to levels that rarely exist following typhoon disturbance. In addition, mechanical thinning of 0.01-ha patches as applied in this study led to an understory light environment that was decoupled from the pre-thinning condition. In contrast, typhoon disturbances are typically more evenly spread, resulting in positively correlated pre- and post-typhoon light environments. Long-term monitoring is required to understand differences in successional trajectories following mechanical thinning and typhoon disturbance.