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The successful utilization of stable carbon isotope approaches in investigating forest carbon dynamics has relied on the assumption that the carbon isotope compositions (δ13C) therein have detectable temporal variations. However, interpreting the δ13C signal transfer can be challenging, given the complexities involved in disentangling the effect of a single environmental factor, the isotopic dilution effect from background CO2 and the lack of high-resolution δ13C measurements. In this study, we conducted continuous in situ monitoring of atmospheric CO2 (δ13Ca) across a canopy profile in an old-growth temperate forest in northeast China during the normal year 2020 and the wet year 2021. Both years exhibited similar temperature conditions in terms of both seasonal variations and annual averages. We tracked the natural carbon isotope composition from δ13Ca to photosynthate (δ13Cp) and to ecosystem respiration (δ13CReco). We observed significant differences in δ13Ca between the two years. Contrary to in 2020, in 2021 there was a δ13Ca valley in the middle of the growing season, attributed to surges in soil CO2 efflux induced by precipitation, while in 2020 values peaked during that period. Despite substantial and similar seasonal variations in canopy photosynthetic discrimination (Δ13Ccanopy) in the two years, the variability of δ13Cp in 2021 was significantly lower than in 2020, due to corresponding differences in δ13Ca. Furthermore, unlike in 2020, we found almost no changes in δ13CReco in 2021, which we ascribed to the imprint of the δ13Cp signal on above-ground respiration and, more importantly, to the contribution of stable δ13C signals from soil heterotrophic respired CO2. Our findings suggest that extreme precipitation can impede the detectability of recent photosynthetic δ13C signals in ecosystem respiration in forests, thus complicating the interpretation of above- and below-ground carbon linkage using δ13CReco. This study provides new insights for unravelling precipitation-related variations in forest carbon dynamics using stable isotope techniques., (© The Author(s) 2024. Published by Oxford University Press.)

Previous studies with different experimental methods have demonstrated that photosynthesis significantly influences soil respiration (RS). To compare the experimental results of different methods, RS after girdling and defoliation was measured in five-year-old seedlings of Fraxinus mandshurica from June to September. Girdling and defoliation significantly reduced RS by 33% and 25% within 4 days, and 40% and 32% within the entire treatment period, respectively. The differential response of RS to girdling and defoliation was a result of the over-compensation for RS after girdling and redistribution of stored carbon after defoliation. No significant effect on RS was observed between girdling and defoliation treatment, while the soluble sugar content in fine roots was higher in defoliation than in girdling treatment, indicating that defoliation had less compensation effect for RS after interrupting photosynthates supply. We confirm the close coupling of RS with photosynthesis and recommend defoliation for further studies to estimate the effect of photosynthesis on RS.

In forest ecosystems, the onset of spring photosynthesis may have an important influence on the annual carbon balance. However, triggers for the onset of photosynthesis have yet to be clearly identified, especially for temperate evergreen conifers. The effects of climatic factors on recovery of photosynthetic capacity in a Korean pine forest were investigated in the field. No photosynthesis was detectable when the soil temperature was below 0 °C even if the air temperature was far beyond 15 °C. The onset of photosynthesis and sap flow was coincident with the time of soil thawing. The rates of recovery of photosynthetic capacity highly fluctuated with air temperature after onset of photosynthesis, and intermittent frost events remarkably inhibited the photosynthetic capacity of the needles. The results suggest that earlier soil thawing is more important than air temperature increases in triggering the onset of photosynthesis in Korean pine in temperate zones under global warming scenarios.

Based on the measurements of leaf photosynthesis, the light response curves of Pinus koraiensis, Tilia amurensis, Quercus mongolica and Fraxinus mandshurica photosynthesis in broadleaved Korean pine forest of Changbai Mountains were fitted with rectangle and non-rectangle hyperbolae. The results showed that rectangle hyperbola was simpler for fitting, while non-rectangle hyperbola was more reasomable because of its fitted results better matched physiological meanings. The values of intrinsic quantum efficiency for CO2 uptake (alpha), light-saturated net photosynthetic rate (Pmax), and dark respiration rate (Rd) of the four tree species were higher when fitted with rectangle hyperbola than with non-rectangle hyperbola, while that of light compensation point (Lcp) varied with tree species. The values of alpha and Rd obtained from the two hyperholae had the same sequences, i.e., F. mandshurica > T. amurensis > Q. mongolica > P. koraiensis for alpha, and F. mandshurica > Q. mongolica > T. amurensis > P. koraiensis for Rd, but for Pmax and Lcp, the sequences were not the same.

Detrending correction and sonic anemometer tilt correction were made to modify the raw time series measured from eddy covariance system in broad-leaved Korean pine forest of Changbai Mountains during the growing season of 2003, and the impact of different correction methods on CO2 flux was analyzed quantificationally. The results showed that the forest CO2 flux during growing season was overestimated when calculated from raw time series. The ratio of correction to origin flux (Fc(raw)) was 1.6% and 1.8% for linear and nonlinear detrend, respectively, which suggested that there was little difference between these two detrending methods. It was 3.7% and 4.7% for the planar fit coordinate transforming (PF) correction and the streamline coordinate system transforming (ST) correction, respectively, suggesting that there was a clear difference between these two sonic anemometer tilt correction methods. When detrended time series used, it was 5.5% and 4.6% for ST correction and PF correction, respectively. It was recommended that raw time series should be corrected synthetically with linear detrend method and PF method.

Leaf respiration is an important component of carbon exchange in terrestrial ecosystems, and estimates of leaf respiration directly affect the accuracy of ecosystem carbon budgets. Leaf respiration is inhibited by light; therefore, gross primary production (GPP) will be overestimated if the reduction in leaf respiration by light is ignored. However, few studies have quantified GPP overestimation with respect to the degree of light inhibition in forest ecosystems. To determine the effect of light inhibition of leaf respiration on GPP estimation, we assessed the variation in leaf respiration of seedlings of the dominant tree species in an old mixed temperate forest with different photosynthetically active radiation levels using the Laisk method. Canopy respiration was estimated by combining the effect of light inhibition on leaf respiration of these species with within-canopy radiation. Leaf respiration decreased exponentially with an increase in light intensity. Canopy respiration and GPP were overestimated by approximately 20.4% and 4.6%, respectively, when leaf respiration reduction in light was ignored compared with the values obtained when light inhibition of leaf respiration was considered. This study indicates that accurate estimates of daytime ecosystem respiration are needed for the accurate evaluation of carbon budgets in temperate forests. In addition, this study provides a valuable approach to accurately estimate GPP by considering leaf respiration reduction in light in other ecosystems. [ABSTRACT FROM AUTHOR]