Previous studies have revealed that changes in forest structure due to management (e.g., thinning, aging, and clearcutting) could affect the forest water balance. However, there are unexplained variability in changes in the annual water balance with changing structure among different sites. This is the case even when analyzing data for specific species/regions. For a more advanced and process-based understanding of changes in the water balance with changing forest structure, we examined transpiration (E) observed using the sap-flux method for 14 Japanese cedar and cypress plantations with various structure (e.g., stem density and diameter) in Japan and surrounding areas and developed a model relating E with structural parameters. We expressed E using the simplified Penman-Monteith equation and modeled canopy conductance (Gc) as a product of reference Gc (Gcref) when vapor pressure deficit is 1.0 kPa and functions expressing the responses of Gc to meteorological factors. We determined Gcref and parameters of the functions for the sites separately. E observed for the 14 sites was not reproduced well by the model when using mean values of Gcref and the parameters among the sites. However, E observed for the sites was reproduced well when using Gcref determined for each site and mean values of the parameters of the functions among the sites, similar to the case when using Gcref and the parameters of the functions determined for each site. These results suggest that considering variations in Gcref among the sites was important to reproduce variations in E, but considering variations in the parameters of the functions 3 was not. Our analysis revealed that Gcref linearly related with the sapwood area on a stand scale (A) and that A linearly related with stem density (N) and powers of the mean stem diameter (dm). Thus, we proposed a model relating E with A (or N and dm), where Gcref was calculated from A (or N and dm) and the parameters of the functions were assumed to be the mean values among the sites. This model estimates changes in E with changing structure from commonly available data (N and dm), and therefore helps improve our understanding of the underlying processes of the changes in the water balance for Japanese cedar and cypress plantations.

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Summary Lianas (woody climbers) have a greater amount of leaves relative to basal area or standing biomass than trees, and very wide vessels that permit efficient water transport. These features suggest that lianas possibly consume proportionally more water through transpiration than trees. Despite their potential importance, researchers have made only limited attempts to evaluate effects of lianas on forest water dynamics. We conducted sap flow measurements for 1 year using a thermal dissipation method for four species each of lianas and trees in a liana‐rich, warm‐temperate forest in Japan and estimated the contribution of lianas to stand canopy transpiration. Based on a calibration measuring water uptake rates from cut‐stem ends, the actual sap flux (Fd) in liana stems was several times greater than those estimated from the original calibration provided for the method. In the field, lianas showed an average of 2–4 times greater Fd than trees throughout the year. Except for this difference, diurnal and seasonal patterns of relative changes of Fd were similar in both groups. The whole‐plant transpiration (Qt) of sample plants was exponentially related to basal diameter for both lianas and trees; Qt of lianas increased more steeply with basal diameter than that of trees. By extrapolating the relationships between Qt and basal diameter to the inventory data of the study plot, we estimated that lianas contributed 12·8% to the annual stand canopy transpiration while comprising 2·3% of stand basal area, which probably reflected the top‐heavy architecture of lianas. Our results indicate that the contribution of lianas to forest water dynamics may be several times greater than their contribution to forest basal area. This implies that a slight increase of liana abundance might have greater effects on water dynamics and, through competitions with trees for limited water, the carbon sequestration capacity of forests than expected from the increase in basal area. This study underlines the necessity of evaluating the relative importance of lianas to forest water dynamics in forests world‐wide. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12881/suppinfo is available for this article.

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