Satoshi Kitaoka

Methane (CH₄) is the second most important greenhouse gas after carbon dioxide (CO₂), and its atmospheric concentration is on the rise. Soil CH₄ consumption (=absorption) capacity is declining due to reduced forests and green spaces, as well as other environmental factors and anaerobic stresses. Environmental and stand structure parameters were cross-referenced with publicly available international ecosystem databases, such as FLUXNET, ICOS, NEON, AmeriFlux, the TRY plant trait database and the Oak Ridge FACE site. Searches were conducted using keywords such as region, water level, and stand density. The data indicate that under high-CO₂ conditions, the increase of forest canopy density leads to increased litter accumulation on the forest floor and reduced sunlight penetration, creating anaerobic conditions. This can cause forests to shift from CH₄ consumption to CH₄ release. Based on these findings, we discussed methods to maintain and enhance the CH₄-absorbing capacity of forest soils. This can be achieved through management practices that improve environmental conditions and increase soil fauna’s activity, such as those associated with thinning operations in overmature forest stands across various regions. This ecological manipulation through thinning practices promotes ground-level temperature increases and the activities of soil fauna, as well as maintaining aerobic conditions near the soil surface.

Robinia pseudoacacia (black locust), an alien species in Japan, has been commonly planted to rehabilitate degraded land and as a resource for the production of honey. Although very few R. pseudoacacia adult trees are found in established mature forests, the management of R. pseudoacacia is a vital and urgent matter because of astounding growth potential, which negatively affects biodiversity and native vegetation. For this reason, we evaluated the seasonal shade‐acclimation capacity of black locusts from the viewpoint of photosynthetic nitrogen use (PNU) and their partitioning. We assessed in situ photosynthetic N use traits of R. pseudoacacia leaves at open sites (relative photosynthetic photon flux density: (rPPFD) > 90%) and shaded sites (rPPFD < 22%) on the forest floor in larch (Larix kaempferi Carr.) plantations. Leaf unfolding started in late May and leaves turned yellow by mid‐October just before leaf shedding. Shoot length at open sites was three times longer than at shaded sites. R. psudoacacia at open sites has more the leaflet number of each pinnate compound leaf than shaded sites from mid‐August to October, and the size of leaflets at shaded sites was about 20% smaller than it was at the open sites. Leaf mass per area showed (mean ± standard deviation) 40.00 ± 2.14 g m⁻² at the open sites and 22 ± 1.56 g m⁻² at the shaded sites from June to October. The light‐saturated photosynthetic rate peaked at 22 μmol m⁻² s⁻¹ at the open sites in July. At the shaded sites, it gradually increased to 7–10 μmol m⁻² s⁻¹ around August, and then decreased at both sites toward zero in mid‐October. The total N content during the growth period was 1.60 ± 0.17 g m⁻² at open sites and 1.16 ± 0.15 g m⁻² at shaded sites. Unlike other tree species, N partitioning to photosynthetic organs was not affected by light conditions. These findings suggested R. psudoacacia cannot acclimate to shady conditions.

Although seasonal differences in drought damage to Japanese cedar (Cryptomeria japonica D. Don) have been well studied, the mechanisms underlying these seasonal differences remain poorly understood. In this study, we measured seasonal changes in current-year shoot growth; leaf mass per area (LMA); water potential at turgor loss point (Ψ_tlp), predawn (Ψ_pre), and midday (Ψ_mid); osmotic potential (Ψ_s); maximum transpiration rates; and maximum stomatal conductance in a 43-year-old Japanese cedar stand, with particular focus on the relationship between the shoot growth period and seasonal variation in Ψ_tlp, and the turgor safety margin (Ψ_mid – Ψ_tlp) to identify the season in which these trees are most vulnerable to drought. Current-year shoot growth began in April, with high shoot growth continuing until August. Ψ_tlp varied significantly throughout the year and was particularly high during the shoot growth period. Increasing LMA and decreasing Ψ_s caused a reduction in Ψ_tlp from autumn to winter, and Ψ_mid dropped to approximately the level of Ψ_tlp during the summer due to high transpiration activity, and increased with decreasing transpiration rates during the winter. Thus, the turgor safety margin was smaller in the summer than in the winter. These results suggest that Japanese cedar is most vulnerable to drought in the summer due to high Ψ_tlp during the shoot growth period.

Larch species are widely distributed in the northern hemisphere where permafrost and seasonal frozen soil exist. This species with heterophyllous shoots has been intensively planted in northeast Asia as well as in northeast China as the principal afforestation species for restoring agricultural lands to forests from 1999. Although approximately 15 species exist in the northern hemisphere and they are easy to hybridize. Among them, Japanese larch grows the fastest and was exported to Europe as a breeding species from early 20s. Although Japanese larch is tolerant to cold, it suffered from various biological stresses. After nearly 40 years of vigorous breeding effort, hybrid larch F1 (Dahurian larch × Japanese one) was developed with simple propagation methods. With the use of free-air CO2 enriched (FACE) systems, we revealed growth responses of the F1 and its parent larches to environmental conditions. From experiments, F1 showed high responses to elevated CO2 and O3 but not so much to N loading. As future perspectives for larch plantations as an important eco-environmental resource, we expect to afforest F1 seedlings infected with ectomycorrhizae (e.g., Suillus sp.) for efficient afforestation at nutrient-poor sites and at the same time for the production of delicious mushrooms.

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 (Vc_max), maximum electron transfer rate (J_max), and dark respiration rate (R_d) 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 (P_max), stomatal conductance (g_s), and intrinsic water use efficiency (WUE_i) 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 Vc_max, J_max, R_d, N, and LMA did not differ between the two plots. By contrast, significantly lower P_max and g_s and higher WUE_i were found in the drought plot, and the reduction in P_max was accompanied by low g_s values. Midday depressions in P_max and g_s were more pronounced in the drought plot relative to the control and were related to higher WUE_i. 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 WUE_i.

To elucidate changes in the defensive traits of tree seedlings under global environmental changes, we evaluated foliar defensive traits of the seedlings of successional trees, such as beech, oak, and magnolia grown in a natural-light phytotron. Potted seedlings were grown under the combination of two CO₂ concentrations (360 vs. 720 ppm) and two nitrogen (N) treatments (4 vs. 15 kg N ha⁻¹ yr⁻¹) for two growing seasons using quantitative chemical analyses and anatomical method. We hypothesized that the effects of CO₂ and N depend on the successional type, with late successional species providing greater defense of their leaves against herbivores, as this species exhibits determinate growth. Beech, a late successional species, responded the most to both elevated CO₂ concentration (eCO₂) and high N treatment. eCO₂ and low N supply enhanced the defensive traits, such as the high leaf mass per area (LMA), high carbon to N ratio (C/N ratio), and increase in the concentrations of total phenolic and condensed tannin in agreement with the carbon–nutrient balance (CNB) hypothesis. High N supply decreased the C/N ratio due to the high N uptake in beech leaves. Oak, a mid–late successional species, exhibited different responses from beech: eCO₂ enhanced the LMA, C/N ratio, and concentration of total phenolics of oak leaves, but only condensed tannin increased under high N supply. Magnolia did not respond to all treatments. No interactive effects were observed between CO₂ and N supply in all species, except for the concentration of total phenolics in oak. Although the amounts of phenolic compounds in beech and oak varied under eCO₂ and high N treatments, the distribution of these compounds did not change. Our results indicate that the changes in the defensive traits of forest tree species under eCO₂ with N loading are related to the successional type.

Plant growth is not solely determined by the net photosynthetic rate (A), but also influenced by the amount of leaves as a photosynthetic apparatus. To evaluate growth responses to CO₂ and O₃, we investigated the effects of elevated CO₂ (550–560 µmol mol⁻¹) and O₃ (52 nmol mol⁻¹; 1.7 × ambient O₃) on photosynthesis and biomass allocation in seedlings of Japanese white birch (Betula platyphylla var. japonica) grown in a free-air CO₂ and O₃ exposure system without any limitation of root growth. Total biomass was enhanced by elevated CO₂ but decreased by elevated O₃. The ratio of root to shoot (R:S ratio) showed no difference among the treatment combinations, suggesting that neither elevated CO₂ nor elevated O₃ affected biomass allocation in the leaf. Accordingly, photosynthetic responses to CO₂ and O₃ might be more important for the growth response of Japanese white birch. Based on A measured under respective growth CO₂ conditions, light-saturated A at a light intensity of 1500 µmol m⁻² s⁻¹ (A₁₅₀₀) in young leaves (ca. 30 days old) exhibited no enhancement by elevated CO₂ in August, suggesting photosynthetic acclimation to elevated CO₂. However, lower A₁₅₀₀ was observed in old leaves (ca. 60 days old) of plants grown under elevated O₃ (regulated to be twice ambient O₃). Conversely, light-limited A measured under a light intensity of 200 µmol m⁻² s⁻¹ (A₂₀₀) was significantly enhanced by elevated CO₂ in young leaves, but suppressed by elevated O₃ in old leaves. Decreases in total biomass under elevated O₃ might be attributed to accelerated leaf senescence by O_3, indicated by the reduced A₁₅₀₀ and A₂₀₀ in old leaves. Increases in total biomass under elevated CO₂ might be attributed to enhanced A under high light intensities, which possibly occurred before the photosynthetic acclimation observed in August, and/or enhanced A under limiting light intensities.

Japanese larch (Larix kaempferi = L. leptolepis) is often characterized by its high growth rate with heterophyllous shoots, but the functional differences of heterophyllous shoots still remain unclear. Recently, abrupt high temperature and drought during spring induced high photosynthetic rate via change in leaf morphology of the deciduous habit. In order to reveal the photosynthetic characteristics of both short and long-shoot needles of sunny canopy of the larch trees using a canopy tower, we calculated the seasonal change of gas exchange characters and leaf mass per area (LMA) and foliar nitrogen content (N) of heterophyllous needles: short and long-shoot needles over 3 years. No marked difference in light-saturated photosynthetic rates (P_sat) was observed between short and long shoots after leaf maturation to yellowing, although the difference was obvious in a specific year, which only shows that seasonal change in temperature and soil moisture determines the in situ photosynthetic capacity of needles. The large annual and seasonal variations in P_sat in both shoots were found to be mainly determined by climatic variations, while shoot types determined the strategy of their photosynthetic N utilization as well as the stomatal regulation.