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Clarifying the influence of disturbances by biotic and abiotic agents on long‐term changes in forest structure and demographic rates is an important task of forest ecology. Here, we investigated changes in forest structure and demographic rates and the effects of biotic and abiotic factors (sika deer, ambrosia beetle, neighborhood trees, and indices of strong wind and wetness) in a 4‐ha long‐term ecological study plot in a lowland warm temperate rain forest on Yakushima Island, Japan, from 1996 to 2019 for three regeneration groups related to gap dynamics (gap demander: GD, light shade tolerant: LS, and strong shade tolerant: SS). GD had relatively lower recruitment and survival than LS and/or SS. Changes in demographic rates were not significantly correlated with those in the index of strong wind in any regeneration group. The effects of sika deer on demographic rates did not differ among groups. Stems of Fagaceae species belonging to LS attacked by ambrosia beetles recorded in 2013 did not significantly differ in RGR and survival rates. Our results indicate that despite the relatively long monitoring period of 23 years, we did not detect significant effects of strong winds on demographic rates among regeneration groups, and shade‐tolerant species (LS and SS) showed better demographic performance than light‐demanding species (GD) and an increase in total basal area over time. We need longer monitoring of forest dynamics to understand forest responses to disturbances considering several factors, including the time lag of demographic responses to disturbances and the long‐term effects of disturbances. [ABSTRACT FROM AUTHOR]

In recent years, a decrease in dissolved oxygen in the lake bottom has been observed in the northern part of Lake Biwa, the largest freshwater lake in Japan. The main physical controlling factor is considered to be changes in stratification, which are influenced by weather and climatic conditions. As atmospheric temperature increases, lake surface temperature rises, and stratification is formed. The strength of stratification has a significant impact on the transport of various material cycles, such as dissolved oxygen and nutrients, from the lake surface to the bottom layer. Therefore, in this study, the analysis of the effect of strong winds during the stratification collapse period (autumn) on the stratification structure of Lake Biwa was performed. Baseline simulations using actual meteorological data and experimental simulations using meteorological data with modified wind speed and direction were conducted. The numerical experiments showed that collapse of stratification was more likely to occur in autumn than in summer, and vertical mixing was enhanced. When strong winds blew more in one direction, vertical mixing was further enhanced when the duration of strong winds was longer during periods of weak stratification, as well as the results during the stratification period. When winds of the same magnitude as during the stratification period blew during periods of decreasing water temperature, a decrease in bottom water temperature was observed prior to the full-layer circulation in the following year. This suggested that strong winds during the decreasing temperature period had a strong influence on the water temperature environment next year. [ABSTRACT FROM AUTHOR]

Water temperature near the surface of a lake increases with increasing air temperature, which results in stratification. The strength of stratification substantially influences the transport of dissolved oxygen from the surface to the bottom water of a lake. In recent years, the decrease in dissolved oxygen at the bottom of the northern part of the Lake Biwa, the largest freshwater lake in Japan, has been observed. The main cause of this is considered to be the change in stratification, which depends on weather and climate conditions. In the present study, numerical simulations were carried out to investigate the effect of strong wind on the structure of stratification in Lake Biwa. The baseline simulation was conducted using actual meteorological data, and experimental simulations were conducted using meteorological data with modified wind speed and direction. The numerical experiments showed that if the magnitude of the wind is strong enough, the stratification collapses and the wind can enhance the vertical mixing in the bottom layer even in the summer season with strong thermal stratification. In a stratified season, when the strong wind blows predominantly in one direction, the rate of the vertical mixing changes by the wind direction. Moreover, as the duration of the strong wind extends, vertical mixing easily occurs. [ABSTRACT FROM AUTHOR]