Models to predict changes in annual runoff with thinning and clearcutting of Japanese cedar and cypress plantations in Japan

Forest management (thinning and clearcutting) can reduce evapotranspiration and increase catchment runoff. By summarizing data on the increase in annual runoff (ΔQ) due to forest management at various sites and analysing data using linear regression, traditional studies have reported large unexplained variability among data for different sites. To improve the predictability of ΔQ, it might be useful to model ΔQ for specific species and regions while considering underlying processes. This study performed such modelling for Japanese cedar and cypress plantations in Japan. Model 1 predicts ΔQ assuming that ΔQ equals the decrease in canopy interception loss (ΔEi), which was further modelled by stem density using 46 data for interception loss. Model 2 predicts the potential maximum of ΔQ (ΔQmax) assuming that ΔQmax equals the sum of ΔEi and the decrease in canopy transpiration (ΔEt). Here, ΔEt was calculated using a model developed in our previous study. ΔQ predicted using Model 1 approximated ΔQ observed for seven catchments, and the errors in prediction were less than those derived from traditional linear-regression analysis. ΔQmax predicted using Model 2 was greater than the observed ΔQ for all catchments. Thus, Models 1 and 2 would be respectively useful in assessing the effectiveness and limitations of managing Japanese cedar and cypress plantations to secure water resources, which have been controversial in Japan. Furthermore, the concept of the models gives implications for studies on other species and regions, because the models have demonstrated how to improve predictability of ΔQ considering underlying processes with the input of commonly available data

A model relating transpiration for Japanese cedar and cypress plantations with stand structure

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 (G[c]) as a product of reference G[c] (G[cref]) when vapor pressure deficit is 1.0 kPa and functions expressing the responses of G[c] to meteorological factors. We determined G[cref] 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 G[cref] and the parameters among the sites. However, E observed for the sites was reproduced well when using G[cref] determined for each site and mean values of the parameters of the functions among the sites, similar to the case when using G[cref] and the parameters of the functions determined for each site. These results suggest that considering variations in G[cref] among the sites was important to reproduce variations in E, but considering variations in the parameters of the functions was not. Our analysis revealed that G[cref] linearly related with the sapwood area on a stand scale (A) and that Alinearly related with stem density (N) and powers of the mean stem diameter (d[m]). Thus, we proposed a model relating E with A (or N and d[m]), where G[cref] was calculated from A (or N and d[m]) and the parameters of the functions were assumed to be the mean values among the sites. This model estimates changes in Ewith changing structure from commonly available data (N and d[m]), and therefore helps improve our understanding of the underlying processes of the changes in the water balance for Japanese cedar and cypress plantations

Effects of soil water decline on diurnal and seasonal variations in sap flux density for differently aged Japanese cypress (Chamaecyparis obtusa) trees

The effects of soil drought on transpiration are often neglected when predicting transpiration for forests in humid regions under the influence of the Asian monsoon. These effects have indeed been neglected for Japanese cypress, Chamaecyparis obtusa, a major plantation species in Japan and the surrounding area, probably because previous studies have reported no clear effects of soil drought on transpiration for Japanese cypress forests. However, a few studies have reported an apparent reduction in transpiration with soil drought for young Japanese cypress forests. It remains unclear whether such a reduction in transpiration is limited to young Japanese cypress forests or if it is not uncommon for mature Japanese cypress forests, which occupy a large area in Japan. To clarify this point, we conducted sap flux measurements in a year with soil drought on three differently aged Japanese cypress stands including mature (43 years old) and relatively young (23 and 26 years old) trees. In a diurnal time scale, a cross correlation analysis of sap flux density (Fd) and vapor pressure deficit (VPD) showed that the time lags between Fd and VPD were 1-3 h in dry soil conditions. These were larger than those of wet soil conditions (&lt;1 h) for all sample trees. Fd at a given VPD in dry soil conditions was smaller than that in wet soil conditions for all sample trees; a 28%–63% reduction in the rate of change in Fd was observed under dry soil conditions. Because our results were obtained when the non-exceedance probability of recorded monthly precipitation was 9%–18%, the results suggest the need to consider the effects of soil drought more extensively. Those effects should be considered for not only relatively young but also mature Japanese cypress when predicting diurnal and seasonal patterns of transpiration in years with soil drought, and when predicting inter-annual patterns of transpiration for Japanese cypress despite humid temperate climate. </p

Scaling-up from tree to stand transpiration for a warm-temperate multi-specific broadleaved forest with a wide variation in stem diameter

<p>Previous studies have demonstrated a clear relationship between diameter at breast height (DBH) and tree transpiration (<i>Q</i>_T) in multi-specific broadleaved forests. However, these studies were conducted with a limited range of tree sizes and species, and thus many multi-specific broadleaved forests fall outside these conditions. Therefore, this study examined the relationship between DBH and <i>Q</i>_T in a warm-temperate multi-specific broadleaved forest (<i>n</i> = 12 species) with a wide range of tree sizes (5.0–70.0 cm DBH) using the Granier-type heat dissipation method. The results showed that, although sap flow density varied between individual trees and species, there was a significant relationship between log <i>Q</i>_T and log DBH (<i>r</i>² = 0.66, <i>P</i> < 0.001) because of the strong dependence of sapwood area on DBH. This study confirmed the applicability of the relationship for the stand transpiration (<i>E</i>_C) estimates even in a multi-specific broadleaved forest with a wide variation in DBH. Our results also revealed that selecting the sample trees in descending order of DBH effectively reduced potential errors in <i>E</i>_C estimates for a specific sample size, as larger trees contribute more to <i>E</i>_C. This information should be useful for future studies investigating the transpiration of multi-specific broadleaved forests, reducing errors during the scaling-up procedure.</p

sap flux and census data

This file includes 30-min and daily data of sap flux for 11 lianas and 10 trees, as well as meteorological variables (precipitation, temperature, vapour pressure deficit and solar radiation) over 1 year period. This also contains census data for lianas and trees rooted in our study plot

首都圏上流域のダム貯水池が渇水時流量に与える影響

Flow duration curves (FDCs) using inflow (Qin) and outflow (Qout) data does not accurately determine the flow contribution of reservoir dams. Analysis based on FDCs could lead to an incorrect conclusion that a reduction in flow occurred due to misinterpretation of zero Qout data. Zero Qout was often recorded when high precipitation occurred because the dam gate was operating to prevent flood flow. To avoid this problem (i.e., misinterpretation of zero Qout data), we proposed an alternate method that evaluates the increase in water flow induced by reservoir dams (Qout - Qin) in relation to Qin which indicates natural water flow without modification by reservoir dams. Based on this method, we evaluated 7 reservoir dams that were located upstream of the Tokyo metropolitan area. For 6 of the 7 reservoir dams, Qout ? Qin was > 0 when Qin was at its lowest. These results showed clear contribution of the 6 reservoir dams to increasing flow which contrasts to the results when FDC analysis is applied. As Qin and Qout data for the main reservoir dams in Japan are readily available, our method can be easily applied to evaluate the contribution of reservoir dams to increasing low flow.ダム貯水池への流入量（Qin）と流出量（Qout）をもとにした流況曲線による解析は, ダム貯水池の渇水時流量の調節への貢献を正確に表現しないことがある. 流況曲線を用いた解析において, Qout = 0のデータはダム貯水池による渇水時流量の低下と解釈されるが, 現実にはQout = 0のデータはしばしば降水量の大きいときに記録されており, このことはQout = 0のデータが洪水緩和のためのダム制御によるものであることを意味している. このように, 流況曲線による解析はQout = 0のデータを誤って解釈するという問題があるため, 本論は, この問題を避けるための代替的な方法を考案した. この方法は, ダム貯水池による流量増加（Qout - Qin）を, ダム貯水池がなかった場合の流量に相当するQinとの関係から評価するものである. この方法を, 首都圏上流域にある7つのダム貯水池に適用したところ, 6つのダムにおいて, Qinの値が解析期間中でもっとも小さいときに, Qout ? Qinがゼロより大きかった. この結果は, この6つのダムが渇水時流量の増加に貢献していたことを意味している. この結果は, 流況曲線による解析結果とは異なるものであった. 日本の主要なダム貯水池におけるQinとQoutのデータは公表されているため, 本論で提案した方法は, 日本のダム貯水池が渇水時流量の増加に貢献しているか否かを評価するのに役立つものである