Hiroyuki Torita

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.

Effective tree distribution and stand structures in a forest for tsunami mitigation considering the different tree-breaking patterns of tree species

The effectiveness of coastal forests to mitigate a tsunami has received increased attention. However, many trees are broken, overturned, or washed out in large tsunami events like the 2011 Great East Japan tsunami (GEJT). For quantitatively estimating the advantages and disadvantages of a coastal forest, it is important to reproduce the forest breakage, especially paying attention to the production of driftwood and the trapping ability of standing trees. The objective of this study was to analyze the tree-breaking mode in detail, considering the stand structure of trees, to demonstrate an energy reduction even when trees are broken, and to utilize the information about the breaking pattern to design and manage a coastal forest properly. In this regard, one location, Misawa, the forest of which was affected by the GEJT, was selected for model validation, and coastal forests in two locations, Shiranuka Town and Taiki Town, in Hokkaido, Japan, were selected because a large tsunami is expected to occur there in the future. A numerical simulation of two tsunami magnitudes at the two Hokkaido sites demonstrated that a tree whose crown is far from the ground tends to be broken at the tree trunk. Dahurian larch (Larix gmelinii) and Daimyo oak (Quercus dentata) tend to be overturned and broken at the tree trunk, respectively. However, the tendency changed with the tsunami magnitude. In addition, even when trees with a dense crown were overturned, they contributed to tsunami resistance to some extent. The fluid force was reduced not only with the forest thickness but also with the tree species and the destruction mode. To maintain the fluid-force reduction and to reduce secondary damage by driftwood, mixed planting is recommended in which tree stand structures are different and large diameter trees at the landward side of forest are planted to trap the driftwood produced from the sea side.

Spatial pattern of windbreak effects on maize growth evaluated by an unmanned aerial vehicle in Hokkaido, northern Japan

Effects of tree windbreaks on crop production vary from field to field. However, considerable labor is required to evaluate the effects of windbreaks by conducting yield surveys in multiple fields. The present study aimed to clarify whether the spatial pattern of windbreak effects on maize growth can be evaluated using an unmanned aerial vehicle (UAV) in a field with a windbreak in Hokkaido, northern Japan. We mapped normalized difference vegetation index (NDVI) distribution in the field using a UAV and conducted micrometeorological observation and survey of growth and yield. We found that the windbreak positively affected maize growth by increasing soil temperature at 3–5 H (H = windbreak height) and negatively by shading at approximately 1 H. NDVI estimated using the UAV was high in June and low in September, suggesting that the growth rate of maize at 3–5 H was higher than that at 6–12 H. NDVI was significantly correlated to dry matter content of maize, although the area shaded by the windbreak during observation could not be used for analysis. These results indicate that a UAV was successful to grasp spatial pattern of windbreak effects on maize growth. This study shows the potential of a UAV to be useful for saving time and money when we evaluate the effects of windbreaks on crop growth.