Influence of planting density and thinning on timber productivity and resistance to wind damage in Japanese larch (Larix kaempferi) forests

Abstract

In recent years, forest damage caused by typhoons has occurred frequently in Hokkaido, northern Japan. According to predictive reports, a typhoon's intensity increases and it then maintains its intensity as it moves north. The relationship between this prediction and forest damage is not clear, but the importance of dealing with forest damage is increasing. Therefore, to consider the countermeasures in forest management, we evaluated the influence of planting density and thinning on timber yield and resistance to wind damage in Japanese larch (Larix kaempferi), which is one of the main tree species used for afforestation in northern Japan. In this study, the following three management types were investigated: sparse (ST: relative yield index < 0.8), middle (MT: relative yield index < 0.9), and dense (DT: unthinned). To assess resistance to wind damage, the critical wind speed required to overturn and break the trunks of trees was calculated using a mechanistic model. Furthermore, timber volumes were estimated from a stand age of 10-50 years using the Yield Prediction System. The management type and planting density (1500-2500 trees ha^-1) affected resistance to wind damage. Scenarios with ST and low planting density (1500 trees ha^-1) showed a high resistance. In contrast, total timber volumes for scenarios varied from approximately 440 to 630 m³ depending on the site index (SI = 22, 25, and 28) at a stand age of 50 years. The merchantable log volume of the final cutting varied from approximately 210 to 470 m³ depending on the management type, planting density, and SI. There was a negative linear correlation between the log volume and resistance to wind damage. Therefore, it is important to balance both the decreased wind damage risk and higher timber yield or to prioritize them.

Keywords: Forest; Log volume; Planting density; Thinning; Wind damage.