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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]

• TF or SF base cations fluxes had no significant differences between UP and LP. • Relative contributions of DD and CE to NTF were different at the two slope plots. • NTF K+ flux was dominated by CE regardless of slope position. • NTF Ca2+ and Mg2+ fluxes were dominated by DD in UP, while CE in LP. • ANC and ALK of TF and SF were significantly higher compared to BP. Atmospheric deposition of base cations in contact with the canopy alters their chemical characteristics and plays a vital role in offsetting soil acidification and nutrient imbalance by replenishing the base cationic pool in forest ecosystems. However, the effects of site characteristics on deliveries of base cations fluxes via throughfall (TF) and stemflow (SF) across topographic gradients within a watershed remain less understood. Here, we examined the concentrations and fluxes of base cations (potassium (K+), calcium (Ca2+), magnesium (Mg2+) and sodium (Na+)), acid neutralizing capacities (ANC) and alkalinity (ALK) in bulk precipitation (BP), TF and SF in an upper slope plot (UP) and a lower slope plot (LP) in a subtropical broadleaved montane forest, Southeast China. In addition, a canopy budget model was applied to distinguish the relative contributions of dry deposition (DD) and canopy exchange (CE) to net TF + SF (NTF) flux. Results showed that annual volume weighted mean concentrations of base cations in TF and SF were significantly higher than those in BP in both UP and LP. Annual fluxes of TF or SF base cations had no significant differences between UP and LP, resulting in a comparable of the NTF fluxes. However, the relative contributions of DD and CE to NTF were different at the two slope plots. NTF K+ flux was dominated by CE regardless of slope position. Unlike K+, NTF Ca2+ and Mg2+ fluxes were dominated by DD in UP, while CE in LP. In addition, ANC and ALK of TF and SF were significantly higher compared to BP , suggesting rainwater beneath the canopy became less acidic and may mitigate in part the risk of soil acidification, especially in the dormant season. This study can provide insights into the spatial variability of chemical characteristics of base cations, and improve our understanding of the quantities and controls of external and internal nutrient inputs occurring within the canopy in forest watershed. [ABSTRACT FROM AUTHOR]