Robinia pseudoacacia L. has been widely planted worldwide for a variety of purposes, but it is a nonindigenous species currently invading the central part of Japanese river terraces. To understand and control this invasion, we investigated how this species invests nitrogen resources in different functions depending on the leaf location, and how these resources are used in physiological reactions such as photosynthesis. The Tama river terrace was examined in Tokyo, Japan. The leaf nitrogen (N) concentration, chlorophyll (Chl) concentration, Chl a/b ratio, leaf mass per unit area (LMA) and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) concentration were all significantly lower in shade leaves than in leaves exposed to the sun. Conversely, the net photosynthetic rate in saturated light conditions (P_max), the net photosynthetic rate under enhanced CO₂ concentration and light saturation (A_max), the maximum carboxylation rate of RuBisCo (V_cmax) and the maximum rate of electron transport driving RUBP regeneration (J_max) were all significantly lower in shade leaves than in leaves exposed to the sun. We also found that RuBisCo/N and Chl/N were significantly less in shade leaves, and values of J_max/N, V_cmax/N less in shade leaves than in sun leaves, but not significantly. Allocation of nitrogen in leaves to photosynthetic proteins, RuBisCo (N_R) was broadly less in shade leaves, and N_L (light-harvesting complex: LHC, photosystem I and II: PSI and PSII) and N_E (electron transport) were also lower. The N remaining was much greater in shade leaves than in sun leaves. We suggest that N remobilization from RuBisCo is more efficient than remobilization from proteins of N_E, and from N_L. This study shows that R. pseudoacacia has an enhanced ability to adapt to environmental changes via characteristic changes in N allocation trade-offs and physiological traits in its sun and shade leaves.

Aims: To propose a comprehensive classification framework for Japanese forest vegetation, an expert system based on a traditional system was developed, using a large‐scale data set covering the whole of Japan.Location: The entire Japanese archipelago.Methods: A data set of 12,720 vegetation plots from the National Vegetation Survey database was established. Then, an expert system for automatic hierarchical classification was developed, based on the traditional system. The classification effectiveness was verified using fidelity measures, the occurrence of traditional characteristic species, consistency with semi‐supervised K‐means and modified TWINSPAN, and the similarity between each unit. To investigate correspondence to the environmental variables, a detrended correspondence analysis with the “envfit” function was used. Additionally, the occurring taxa were collated and compared with the flora lists of areas surrounding Japan.Results: The Japanese forest data set was classified into 34 alliances by the expert system, and 14 orders and six classes were distinguished. Most of these units had diagnostic taxa including existing characteristic species and the heatmap of the Bray–Curtis index showed the independence of each unit, and the similarity of the plot groups assigned to the same unit. Out of 34 alliances, clustering with semi‐supervised K‐means supported 21 alliances, and modified TWINSPAN supported 16 alliances. The four classes correspond to the thermal conditions and support the existing concept of vegetation zones. The lower‐level vegetation units correspond to a narrower range of environmental variables than higher‐level units, and species composition varied with limited environmental variables. A floristic composition comparison emphasized the complexity of the species composition of Japanese forests.Conclusions: This study proposes the first formal Japanese forest classification at the class, order, and alliance levels, reflecting traditional phytosociology systems and based on large‐scale vegetation plot data. Our classification provides an important foundation for the future revision of formal phytosociological systems throughout East Asia.

The range of food resources consumed by opportunistic carnivores is shaped by habitat-specific availability and productivity. However, baseline data on the diet of the Japanese weasel (Mustela itatsi) in managed wetland habitats remain scarce. Firstly, we conducted a diet study examining Japanese weasel scats from the Watarase-yusuichi wetland (WYW) (September 2024–August 2025). Based on 103 fecal samples, we calculated seasonal frequency of occurrence (FO) for 16 food categories and estimated their relative volume (RV) using the point-frame method. Surprisingly, plant material and fruit seeds were eaten frequently across seasons, and fruit seeds constituted a major component of scat volume, particularly in summer. Secondly, we compared baseline WYW data with published datasets from two contrasting aquatic habitats (a suburban river area and a rice paddy landscape) using harmonized categories and multivariate analyses. Diet composition differed markedly among study sites, and the PERMANOVA analysis indicated a significant effect of sampling site (R² = 0.643, p = 0.001) with a weaker seasonal effect (R² = 0.208, p = 0.038), with no significant pairwise differences between seasons (all p ≥ 0.300). Although cross-site comparisons should be interpreted cautiously because datasets were collected in different decades (1998, 2018, and 2024–2025), these findings support habitat-associated trophic plasticity in the Japanese weasel, with dietary shifts across habitats and seasons apparently reflecting variation in available food resources. In managed floodplain wetlands such as WYW, maintaining a mosaic of reed beds, ponds, and channels may help sustain dietary flexibility and population persistence in this species.