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Abstract: Spatial variability of throughfall (TF) isotopic composition, used as tracer input, influences isotope hydrological applications in forested watersheds. Notwithstanding, identification of the dominant canopy factors and processes that affect the patterns of TF isotopic variability remains ambiguous. Here, we examined the spatio‐temporal variability of TF isotopic composition in a Japanese cypress plantation, in which intensive strip thinning was performed and investigated whether canopy structure at a fine resolution of canopy effect analysis is related to TF isotopic composition and how this is affected by meteorological factors. Canopy openness, as an index of canopy structure, was calculated from hemispherical photographs at different zenith angles. TF samples were collected in a 10 × 10 m experimental plot in both pre‐thinning (from July to November 2010) and post‐thinning (from May 2012 to March 2013) periods. Our results show that thinning resulted in a smaller alteration of input δ18O of gross precipitation, whereas the changes in deuterium excess varied in both directions. Despite the temporal stability of spatial patterns in TF amount, the spatial variability of TF isotopic composition was not temporally stable in both pre‐ and post‐thinning periods. Additionally, after thinning, the isotopic composition of TF was best related to canopy openness calculated at the zenith angle of 7°, exhibiting three different relationships, that is, significantly negative, significantly positive, and nonsignificant. Changes in meteorological factors (wind speed, rainfall intensity, and temperature) were found to affect the relationships between TF δ18O and canopy openness. The observed shifts in the relationships reveal different dominant factors (partial evaporation and the selection), and canopy water flowpaths control such differences. This study provides useful insights into the spatial variability of TF isotopic composition and improves our understanding of the physical processes of interception through canopy passage. [ABSTRACT FROM AUTHOR]

Throughfall ( TF ) is a critical component of the hydrological and biogeochemical cycles and is greatly influenced by stand structures in forested watersheds. Previous studies have examined the relationships between TF and stand structures for different species and regions. However, there remains acknowledged difficulty in estimation of TF with changing stand structures due to forest management (e.g., thinning) for specific species and regions. This study conducted intensive thinning with 43–50% stem removal at seven experimental plots with various structures (e.g., stand density, canopy cover and basal area) across Japan that were covered by Japanese cypress ( Chamaecyparis obtusa Endl.) and cedar ( Cryptomeria japonica D. Don) plantations. TF was measured by a set of 20 tipping-bucket rain gauges placed in a lattice-like pattern within the plots before and after thinning. Results showed that during the study periods, thinning caused an increase in the mean TF rate from 63.2 ± 7.0% to 75.4 ± 6.0%. The TF rate was significantly negatively correlated with stand density, canopy cover, and basal area on a stand scale. Additionally, the slope ( a TF ) of event-based linear TF - P g (gross rainfall) equation was significantly higher in post-thinning (0.782 ± 0.077) than that in pre-thinning (0.663 ± 0.078), whereas it’s intercept ( b TF ) showed no significant difference (−0.707 ± 0.810 mm in pre-thinning and −0.441 ± 0.607 mm in post-thinning). The a TF significantly decreased with increasing stand density and canopy cover, whereas the b TF was not significantly correlated with any forest-structure variables. Further, the TF rate and a TF were estimated, respectively, by their related stand-structural variables in multiple regression models with high determination coefficients and moderate relative errors. The b TF was assumed to be a constant and its mean value obtained from all the experimental plots in each (pre- and post-thinning) period. Thus, the models of stand-scale TF rate and event-based TF amount were developed with input of commonly forest inventories. These models were validated by using the dataset of this study and earlier publications for Japanese cedar and cypress plantations, and showed good fit between the estimated and measured values. These models are practical tools that can be readily used for assessing the changes in TF with changing stand structures at both stand and event-based scales, and have also potential implications in evaluating the spatial TF patterns at catchment scale and exploiting similar models in other species and regions. [ABSTRACT FROM AUTHOR]

Thinning results in more open-stand canopies and then immediately modifies the environmental factors that influence forest floor evaporation ( E f ). Thus, the changes in E f induced by thinning would play an important role in the forest water cycle, whereas few studies have reported this topic. This study analyzes the effect of strip thinning on E f , its spatial variation and identifies the factors influencing it in a Japanese cypress ( Chamaecyparis obtusa Endl.) plantation in Tochigi Prefecture, Japan. Strip thinning, which removed 50% of the stems, was conducted in a headwater basin during the period of October 11–November 5, 2011. The E f was monitored by weighing lysimeters before and after thinning. The daily E f was strongly correlated with the daily solar radiation ( R 2 = 0.62) followed by vapor pressure deficit ( R 2 = 0.41) below the canopy, whereas the soil water content had a poor effect on it in post-thinning. After thinning, the daily E f among the measuring points had no significant difference, indicating that the daily E f had a small spatial variation. This responded to the small spatial variability in the daily solar radiation under the canopy. Additionally, on an annual scale, thinning resulted in the daily mean E f increasing from 0.34 ± 0.23 to 0.68 ± 0.47 mm d −1 . The total E f increased by 97.6% from 124.0 to 245.0 mm. These findings indicate that E f composes a significant part of the forest water budget after thinning and emphasize the importance of E f measurements for management practices. This study also provides useful information for modeling the changes in hydrological processes at the forest floor and for evaluating the E f response to different management practices. [ABSTRACT FROM AUTHOR]

Summary We examined the effect of strip thinning on rainfall interception ( E i ) in a 32-year-old Japanese cypress plantation in central Japan. Strip thinning was conducted in the catchment in October 2011; that removed 50% of the stems. The gross precipitation ( P g ), throughfall ( TF ) and stemflow ( SF ) were monitored in a 12-m × 13-m plot before and after thinning. E i was calculated as the difference between P g and the sum of TF and SF . The interception processes were illustrated using the revised Gash model with quantifying interception parameters. The results showed that the Gash model successfully predicted E i on a rainy-season basis in both pre- and post-thinning periods. Thinning altered the interception components whereas the largest part during and after rainfall accounted for similar proportion in both periods. Additionally, after thinning, the annual TF rate was increased from 61.4% to 73.0%, whereas the annual SF rate was decreased from 9.8% to 6.1%, and the annual E i rate was decreased from 28.7% to 20.8%. The summarized findings of previous studies indicate that the degree of decline in the E i caused by thinning is related to P g and the thinning ratio. These results provide useful information for understanding the changes in interception processes induced by thinning, and for acquiring a more accurate forecast of the effects of forest management practices on water resources in the watershed. The response in rainfall partitioning to strip thinning can also help us to acquire an integrated understanding of the changes in canopy water balance under different forest practices. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigated the contribution of three evapotranspiration (ET) components including canopy interception ( Ei), tree transpiration ( Et) and forest floor evaporation ( Ef) and identified the sources depth of evaporated water in a Japanese cypress stand. Monitoring primarily focused on the growing season of July-October 2011. In a 12 × 13 m plot with 28 trees with a DBH of 19.1 ± 3.9 cm, Ei was calculated as the difference between precipitation on an open field and precipitation under a canopy as the sum of throughfall and stemflow, and Et was measured using the Granier method. Ef was measured by weighing lysimeters. Total ET during the monitoring period was 446.3 mm, accounting for 47.5% of the total precipitation of 938.8 mm. Ei was the dominant evaporation flux and accounted for 53.6% of ET or 25.5% of rainfall, followed by Et with 33.7% of ET or 16.0% of rainfall. The average Et was 1.5 ± 0.6 mm d−1. It was well correlated with soil moisture at a depth of 5-15 cm, which reflected the forest properties, i.e. tree roots were exposed on the forest floor. Ef accounted for 12.7% of ET or 6.0% of rainfall with a daily mean of 0.55 ± 0.26 mm d−1, and was best correlated with soil moisture in the upper 5 cm of soil. These results can improve the understanding of water budget in forested watershed and also be used to build and validate hydrologic models for water resource managements. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]