Tetsuya Iwasaki

Controlling the effect of rising temperature in the water contained in leaves was performed by continuously measuring leaf temperature on the reverse side of the leaf at intervals of two seconds during the heating experiments. Time period to maintain nearly 100 degrees C or less at the reverse side of leaves for 17 species of trees was also shown as the water retention time. It was observed that trees such as Phyllostachys bambusoides, Viburnum awabuki, Hydrangea macrophylla, Camellia japonica spp., etc. had long water retention time, and Acer palmatum var. matumurae, Zelkova serrata, Mellotus japonicus, Prunus jamasakura, etc. had short water retention time. Moreover, the form shrinkage of leaves by heat was measured in relation to the heat interception effect of a leaf. As a result of the heating experiments for 55 species of trees, it was found that the rate of shrinkage of leaves varies from one species of trees to another. It was also found that there is a high positive correlation between the ratio of water content and rate of shrinkage. As a result, it was established that, within the limits of this experiments, evergreen needle-leaf trees demonstrated the smallest shrinkage rate. Evergreen broad-leaved trees such as Photinia glabra, Osmanthus fragrans var. aurantiacus, Pasania edulis, Hedera helix and Euonymus japonicus and deciduous broad-leaved trees such as Ulmus parvifolia, Acer buergerianum, Aphananthe aspera and Cercis chinensis also show smaller shrinkage rate. It is clear that the higher the ratio of form shrinkage, the higher the ratio of water content in the broadleaf trees, especially in deciduous broad-leaved trees. In addition, an opposite relationship can be seen between high water content ratio and the high durability of heat interception in a leaf.

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.