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Spatiotemporal prediction of the response of planted forests to a changing climate is increasingly important for the sustainable management of forest ecosystems. In this study, we present a methodology for estimating spatially varying productivity in a planted forest and changes in productivity with a changing climate in Japan, with a focus on Japanese cedar (Cryptomeria japonica D.Don) as a representative tree species of this region. The process-based model Biome-BGC was parameterized using a plant trait database for Japanese cedar and a Bayesian optimization scheme. To compare productivity under historical (1996–2000) and future (2096–2100) climatic conditions, the climate scenarios of two representative concentration pathways (i.e., RCP2.6 and RCP8.5) were used in five global climate models (GCMs) with approximately 1-km resolution. The seasonality of modeled fluxes, namely gross primary production, ecosystem respiration, net ecosystem exchange, and soil respiration, improved after two steps of parameterization. The estimated net primary production (NPP) of stands aged 36–40 years under the historical climatic conditions of the five GCMs was 0.77 ± 0.10 kgC m-2year-1(mean ± standard deviation), in accordance with the geographical distribution of forest NPP estimated in previous studies. Under the RCP2.6 and RCP8.5 scenarios, the mean NPP of the five GCMs increased by 0.04 ± 0.07 and 0.14 ± 0.11 kgC m-2year-1, respectively. The increases in annual NPP were small in the southwestern region because of the decreases in summer NPP and the small increases in winter NPP under the RCP2.6 and RCP8.5 scenarios, respectively. Under the RCP2.6 scenario, Japanese cedar was at risk in the southwestern region, in accordance with previous studies, and monitoring and silvicultural practices should be modified accordingly.

As climate change progresses, it is becoming more crucial to understand how timber species respond to increased drought frequency and severity. Photosynthetic traits in a 40-year-old clonal Japanese cedar (Cryptomeria japonica) plantation were assessed under artificial drought stress using a roof to exclude rainfall and a control with no exclusion. C. japonica is a commercial tree that is native to Japan and has high growth on mesic sites. The maximum carboxylation rate (Vcmax), maximum electron transfer rate (Jmax), and dark respiration rate (Rd) in current-year shoots in the upper canopy were determined from spring to autumn over two growing seasons. In addition, the photosynthetic rate at light saturation (Pmax), stomatal conductance (gs), and intrinsic water use efficiency (WUEi) were measured in the morning and afternoon during the same period. Leaf mass per unit area (LMA) and nitrogen concentration (N) were also measured. The values of Vcmax, Jmax, Rd, N, and LMA did not differ between the two plots. By contrast, significantly lower Pmax and gs and higher WUEi were found in the drought plot, and the reduction in Pmax was accompanied by low gs values. Midday depressions in Pmax and gs were more pronounced in the drought plot relative to the control and were related to higher WUEi. Under drought conditions, mature Japanese cedar experienced little change in photosynthetic capacity, foliar N, or LMA, but they did tend to close the stomata to regulate transpiration, thus avoiding drought-induced damage to the photosynthetic machinery and improving WUEi.

s In an experimental watershed located around 120 km southwest of the Fukushima Daiichi Nuclear Power Plant with a drainage area of 59.9 ha, suspended solids (SS) and radioactive cesium discharge from a forested headwater catchment were monitored before and after line thinning. The lower slopes in the experimental watershed were covered with plantation conifer trees (Japanese cedar), while the upper slopes were covered with deciduous trees. In 2012, line thinning was carried out at a thinning rate of 35% across 17% of the northeastern part of the watershed and across the remaining part in 2013. Spur roads were constructed along all tributaries without water, and logged trees were dragged and grappled using forestry machinery and transported along these roads to timber yards using forwarder-type forestry vehicles. A V-notch weir and a water level gauge were installed at the watershed outlet and stream water was sampled twice a month during base flow, whereas during flood flow, stream water samples of 1 L were collected every hour using an automatic water sampler. These samples were filtered through 0.5 μm glass fiber filters to measure the SS concentration. SS concentration data was collected for 21 floods before thinning and for 37 floods after thinning. A time-integrated SS sampler was installed in the stream close to the weir and SS samples were collected every two or three months to measure their Cs-137 concentrations. SS concentrations before (from July 2010 to August 2012) and after thinning (from October 2013 to December 2018) were compared, where the maximum SS concentrations before and after thinning were 211 and 790 mg L−1, respectively. It was discovered that some SS concentrations during flood flow were higher after carrying out thinning than before. Some ΣLss values (specific cumulative load of SS in a flood event) also showed the same results as the SS concentrations. Thus, it was clear that SS discharge immediately increases after thinning, but as it increases Cs-137 export is limited. This is related to a change in SS source brought about by the process of thinning, a decrease with time in the Cs-137 concentration in organic solid expected from that in litter, and a regrowth of vegetation on spur roads, protecting them against soil erosion. Therefore, it was concluded that thinning does not drastically increase Cs-137 export from a forested watershed.