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Abstract:Evaporation from the forest floor (Ef ) of a secondary broad-leaved forest was monitored for 1 year at 30 min intervals using a closed chamber system. The diurnal and seasonal variation of Ef and forest structure were analysed for days undisturbed by rainfall. The diurnal change in Ef reached a maximum at about 14 : 00 and gradually decreased towards midnight along with the vapour pressure deficit (D). Although Ef comprised about 20% of evapotranspiration from the dry canopy (Et ), it had only a small influence on diurnal evaporation efficiency (ˇ) characteristics above the canopy because its diurnal range was much smaller than that of Et. Although leaf emergence and leaf fall clearly affected available energy (Ae) beneath the canopy, the influence was not clear with Ef. In contrast, seasonal variation in Ef was strongly correlated with D and Ae above the canopy, and the determination coefficient (R 2 ) changed with the Bowen ratio (B). At night, Et was almost equal to Ef when the friction velocity (u Ł ) ranged between 0Ð2 and 0Ð4 m s 1 . The ratio of Ef to Et was exponentially correlated with leaf area index (LAI ) when the soil was not dry. The ratio of Ef to Et was mainly influenced by LAI and soil moisture, but the existence of understory vegetation did not have a strong influence.

Abstract:Evaporation from the forest floor (E FF ) in a deciduous broadleaf forest was measured using microlysimeter and closed-chamber systems. The microlysimeter was used at six points in the experimental basin, and measurements gave different E FF values at different points. This could be attributed to the local photoenvironment of each sampling point, rather than to litter conditions, if the spatial variation in air temperature (Ta) or vapour pressure deficit (VPD) at the forest floor was small within this basin. A detachable microlysimeter measured condensation in the litter layer during the night, indicating that the litter layer, as well as the mulch layer, played a role in preventing evaporation from the soil layer. The closed-chamber system made it possible to continuously measure long-term E FF . E FF was closely related to VPD; even during the night, when solar radiation was zero, E FF amounted to 14Ð0% of the daily E FF . The daily E FF was 0Ð20 š 0Ð13 mm day 1 during the study period, with two seasonal peaks: in late spring (0Ð31 mm day 1 in April) and early fall (0Ð22 mm day 1 in September). The former peak has been reported from two deciduous forests in Japan and is strongly related to the solar radiation reaching the forest floor when the trees are dormant.

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We explored the relationship between directional variation (changes in direction from a reference point) in vegetation and wind profiles, and propose an empirical wind profile model that may reproduce the wind profile within the canopy (such as secondary wind maximum) and reduce calculation loads. Based on the results of our observations in secondary broad-leaved forest, we clarified the variation in secondary wind maximum and aerodynamic parameters in wind direction, and assessed the influence of forest structural heterogeneity on the wind profile. Wind blew from specific directions depending on the time of day and season, and secondary wind maxima were observed in particular wind directions. Outlier estimations of aerodynamic parameters were determined for wind directions of 45°, 120-210°, and 300°, and these did not show a logarithmic distribution. Our proposed model reproduced the wind profile within and above the canopy, closely resembling K-theory and closure models, and reduced the required number of calculations and observations. The influence of forest structural heterogeneity was greatest in the upper part of the canopy (9.14 m), and reproducibility of the wind profile was improved by 0.05 m/s (3.4% on average) by changing the forest structural parameters of the model according to the wind direction. It was clear that forest structural heterogeneity did not cause critical errors in estimation of the wind profile, even at our study site, which had complex forest structure.