Hirokazu Kon

Masting behavior of Fagus crenata in northern Japan : spatial synchrony and pre-dispersal seed predation

An 11-year study of total seed and seed production in five natural Japanese beech (Fagus crenata) forests in Hokkaido, northern Japan, estimated the spatial synchrony of masting and examined whether variation in annual total seed production resulted in fluctuations in pre-dispersal seed-predator populations. We found less synchrony in total seed production than in sound seed production among sites. The coefficients of variation of sound seeds (1.3–2.1) were greater than those of total seeds (0.8–1.3) in each site. Pre-dispersal seed predation by moth larvae has a major influence on sound seed production in F. crenata. To test whether variation in annual total seed production resulted in fluctuations in predator populations, we examined the relationship between the ratio of current year total seed production to previous year total seed production, and the percentage of pre-dispersal seed predation in the current year. We found a negative correlation between insect predation and the ratio of current to previous year total seed production, suggesting that poor total seed years resulted in a decrease in predator populations. The pre-dispersal seed predators of F. crenata appeared to cope well with up to 10-fold variation in total seed production between years, but when the total seed production exceeded 20 times that of the prior year, the predators were satiated.

Experimental investigation on weather cues for mast seeding of Fagus crenata

Although a recent study has suggested that the minimum temperature from late April to mid-May in the year preceding flowering causes mast seeding in Fagus crenata, no direct evidence is available to support this finding. The aim of the present investigation was, therefore, to test – in a field experiment – whether the minimum temperature determines mast seeding in F. crenata. We examined the effect of nighttime temperatures on flower-bud initiation in F. crenata by enclosing fruit-bearing branches in heated bags at night, thereby maintaining average nighttime temperatures of approximately 2°C above the ambient temperatures. Heating was applied at night from 21 April to 20 May, 21 May to 19 June, and 21 April to 19 June in 2001. Female inflorescence initiation was inhibited by the nocturnal heating in the period 21 April to 20 May and 21 May to 19 June. However, nocturnal heating from 21 April to 20 May was the more important based on the odds ratio of the former being much lower than that of the latter in a logistic regression model. Male inflorescence initiation was also inhibited by nocturnal heating from 21 April to 20 May. We therefore conclude that flower-bud initiation in F. crenata was controlled by nighttime temperatures between 21 April and 20 May.

Mortality of planted Pinus thunbergii Parlat. saplings subject to coldness during winter and soil types in region of seasonal soil frost

To assess and improve methods for the development of coastal forests in Hokkaido, northern Japan, this study examined the factors affecting mortality of planted Pinus thunbergii Parlat. saplings in a region with seasonal soil frost. The needles of pine saplings at the study site turn red in late spring, suggesting xylem embolism, which can lead to death. Sapling mortality was strongly correlated with the degree of cold just after planting and with the occurrence of repeated severe winters. Volcanic ash soil had been supplied to improve the soil nutrients, and the volcanic ash soil always thawed later than the original coastal sand. Saplings in volcanic ash were significantly more damaged than those in original coastal sand. This suggests that hydraulic stress caused by cuticular transpiration under frozen soil conditions during spring may also accelerate damage due to embolisms. Hence, volcanic ash soil should not be applied to soils in regions with seasonal soil frost. A covering of woodchips can help prevent frost from reaching greater soil depths, and, indeed, fewer saplings at the study site died in areas with such coverage. Planting location can also affect pine sapling mortality. For example, shading caused by wood fencing may affect the soil freezing profile during winter; therefore, planting close to a fence should also be avoided in regions with seasonal soil frost. These findings should be used to improve the development of forests in regions with seasonal soil frost.

Seasonality of sprouting in the exotic tree Robinia pseudoacacia L. in Hokkaido, northern Japan

Robinia pseudoacacia L., one of the major invasive tree species in Japan, has been used for coppice wood production worldwide. However, we have little knowledge of the species’ sprouting characteristics. Here, we tested seasonality in R. pseudoacacia sprout development. We removed aboveground parts of R. pseudoacacia at different times throughout the year (spring, summer, autumn, and winter) and censused sprout demographics for three consecutive growing seasons thereafter. The most vigorous sprouting was observed among trees cut in winter, while sprouting was lowest among those cut in summer. Sprout abundance in the spring- and autumn-cut treatments was intermediate between that of winter- and summer-cut treatments. Both trunk diameter and season affected stump sprout biomass after three growing seasons, following the same seasonal pattern as above. We found a negative relationship between the herbaceous vegetation cover height and the biomass of root suckers. Both trunk removal in summer and consecutive sprout-clipping treatments contributed to the decline of living stumps. Our results show that R. pseudoacacia trees should be cut during the dormant season to facilitate coppicing and in the summer to facilitate control or removal. Conservation of herbaceous vegetation cover, together with consecutive sprout clipping, will aid the complete removal of this species.