Determining Ideal Timing of Row Thinning for a Cryptomeria japonica Plantation Using Event History Analysis

Effective time of thinning is essential for determining a silvicultural operation schedule. One of the most commonly used methods is the percentage of radial increase to assess the effect of thinning. However, it is difficult to determine the ideal time point due to variation in tree growth rates. Event history analysis was used to quantify the optimal timings for different row thinning types for a 45-year-old Cryptomeria japonica plantation in the mountainous region of Taiwan. The increase in tree-ring size was measured and converted to the basal area increment (BAI) to estimate annual tree growth; derived time-series data were entered into event history analysis to calculate the time to 50% probability of survival. Additionally, an accelerated failure time regression was applied to test the effects of thinning and its timing; model validation was carried out to examine the influence of thinning time variation on plant growth through time. Results showed that thinning modified the temporal dynamics of the BAI, and, in general, a positive trend was observed between strip-width and time of thinning. Simulated tree growth in the model validation corroborated that accurate timing may optimize thinning effects. Combining tree-ring measurement and event history analysis may facilitate determining the timing of row thinning, which can improve carbon sequestration of forest stands.