岩崎 哲也

本研究では,樹木による防火効果に関連する2つの事項について実験による検証を行った。1点目として,樹葉の温度は上昇する過程において100℃付近で一時的に横ばいの傾向を示すことが明らかになり,100℃を超えるまでの時間は落葉広葉樹が比較的短く,常緑広葉樹は長く,タケ・ササ類は落葉広葉樹より長い傾向があることを確認した。また2点目として,受熱時の葉の形状変化は,樹種の違いによって著しく異なり,広葉樹では常緑樹と落葉樹とに差は見られないこと,針葉樹の形状変化は小さくタケ・ササ類は大きいこと,含水率と正の相関があり特に落葉広葉樹は含水率との正の相関が高いことが明らかとなった。

There were some differences in the results of existing measurements on water content ratio of leaves. Therefore, the ratios of water content in 55 species of leaves were measured under the same conditions when leaf samples were taken. The result of measurement showed that there are differences in the water content between species and leaves. Next, seasonal changes in water content of leaves from 29 evergreen species were compared during summer and winter. As the result this comparison, it was found that the water content of leaves in Tokyo area is highest in summer. In addition to this, results of measurements assessing annual changes in water content of leaves from 6 broadleaf tree species indicated that the water content ratio in evergreen broad-leaved trees was at a minimum before coming into leaf during the winter and spring seasons. After showing the highest ratio of the season just after leaf flush, the ratio began gradually decreasing until the winter season. For deciduous broad-leaved trees, their ratio of water content has a tendency to be highest in April and drops temporarily in midsummer. Moreover, the results in this research and the results of existing measurements were integrated, the water content ratios of 178 species of trees were arranged and the tendency was grasped.

It was confirmed by the existing studies that, when it was exposed by only radiant heat, leaves of many species of trees ignited without flame but Juniperus chinensis cv. Kaizuka flamed lower than the tolerance limit of radiant heat. (It is said the lowest heat flux level of glowing as to coniferous trees was 13.95kW/m^2.) The research on such danger is not progressing but which should be verified, for utilizing the fire prevention function of trees and for planning landscape in narrow residence, such as city. So, in this experiments, the concentration and transition of unburned combustible gas and the temperature under the leaves were measured about 23 species of trees, in this time the hydrocarbon was mainly measured as the unburned gas. The following results were obtained: (1) The states of hydrocarbon generating and temperature transition were obviously different depending on the species. (2) Three species of the family Cupressaceae discharged much hydrocarbon from the first stage of heating as well as J. chinensis cv. Kaizuka. (3) Aucuba japonica, Prunus mume, Phyllostachys bambusoides, and Pleioblastus chino generated hydrocarbon few from beginning to end. (4) Viburnum awabuki, Ginkgo biloba, and Hydrangea macrophylla generated few in first stage compared with the total quantity.

Generally trees have the fire prevention effect. It will ignite, if radiant heat exceeds about 13.95 kw/m^2, it has more fire prevention effect by sprinkling water to the tree crown. It is thought that such an effect is especially effective in the city area where the house crowds. And the experiment for proving this effect was conducted, for three days from August 29, 2001. On the experiment, a tree which sprinkled water, and a tree which did not sprinkle water were heated simultaneously for 30 minutes. And (1) Change of appearance , (2) Temperature, (3) Radiant heat, (4) Crown void ratio , etc. were measured. The strength of radiation was set as 11.63 kw/m^2. As a result, the following effect by sprinkling water was proved. (1) Not only temperature but radiation was decreased by sprinkling water. (2) By watering, temperature was able to be lowered quickly. (3) It was expected that the continuous watering is better than intermittently. (4) When exposed to radiation of 11.63 kw/m^2, the leaf of Osmanthus fragrans was able to avoid damage by a little of watering to the crown. (5) When exposed to the heat of 11.63 kw/m^2 for 30 minutes, there was no big change in the void ratio of the crown. Recently, the danger that the big fire will occur in the city area is very high when the occurrence of the big earthquake is forecasted. When the big fire by the big earthquake will break out in the future, it is thought that it is very effective to strengthen such fire prevention power of trees in order to prevent the early spread of a fire. And it is required to development of the watering equipment not only as the drip irrigation often used now but as the tree crown style. Such equipment is considered to be utilized as a tool for watering at the usual times, and to be utilized as a means for fire prevention in case of a fire. It became clear in this experiment that watering to the tree crown was more effective in order to exert the fire resistance of trees. It will be required, to be proving the effect of watering about the case where higher radiant heat is exposed, in future.