Estimation of Sensible and Latent Heat Fluxes of an Isolated Tree in Japanese Summer

Abstract

Considering that the characteristics of the heat balance between an isolated tree and the urban atmosphere have not yet been sufficiently clarified, we quantify both the sensible heat flux ($H_{T}$) and latent heat flux ($lE_{T}$) to and from an isolated Z. Serrata in the Japanese summer. To estimate the whole-tree transpiration rate (E_T) and $lE_{T}$ values, we apply a previously developed method using a weighing lysimeter. To estimate the boundary-layer heat conductance ($g_{hT}$) of the total leaf area of the tree crown and the sensible heat flux ($H_{T}$), we apply a scaling-up approach using the heat-balance two-state (HB-TS) method, which previously only targeted single leaves, to the tree crown. Two sample trees with similar crown shape and total leaf area are selected for the HB-TS method but under different irrigation conditions, and $g_{hT}$ is estimated. The E_T values of the tree change three times during the dry-down experiment (during which time irrigation was halted). We also compare H_T and lE_T values between two different days under different irrigation and soil–water conditions. The most important result from this comparison is that the tendencies of H_T and lE_T were reversed on these days, and the Bowen ratio ($\beta = H_{T} /lE_{T}$) dramatically varies between 0.29 and 2.2. These results indicate that the Bowen ratio of isolated trees at urban sites can vary between the previously reported values for forest sites and those for artificial urban sites within a short period, owing to urban-unique conditions (e.g., limited water supply and rooting space, and artificial sealing).

Appendix

As shown in Fig. 7, the soil–water content corresponded well with the transpiration rate, as there was no intake nor drainage of water without transpiration when the irrigation was stopped. Figure 9 shows the measurement results of soil–water content and soil–water content change per hour for tree A′. For the irrigated period until 23 August, the soil–water content change was over 1.5% per hour during the daytime owing to the transpiration. The value decreased after the cease in irrigation on 24 August. On 27 August, the value became one-tenth of that on the irrigated days. To make sure the transpiration was completely (bio-physiologically) limited and negligible for g_hT and H_T estimation, we waited three more days. On 30 August, the value became one-twentieth of that on the irrigated days, as we expected. Therefore, we judged this day to be applicable to the estimation.

Fig. 9

Measurement results of soil water content (W) and soil water content change per hour (∆W) for tree A′. Gray bands show the period of irrigation