Leaf nutrient resorption is a critical process in plant nutrient conservation during leaf senescence. However, the ecological mechanisms underlying the large variability in nitrogen (NRE) and phosphorous (PRE) resorption efficiencies among trees remain poorly understood. We conducted a comprehensive study on NRE and PRE variability using 61 tree individuals of 10 temperate broad-leaved tree species. Three potentially interrelated intrinsic ecological mechanisms (i.e., leaf senescence phenology, leaf pigments, and energy residual) were verified. We found that a delayed leaf senescence date, increased degradation of chlorophylls and carotenoids, biosynthesis of anthocyanins, and reduced nonstructural carbohydrates were all positively correlated with NRE and PRE at the individual tree level. The intrinsic factors affecting resorption efficiency were ranked in decreasing order of importance: leaf pigments > energy residual > senescence phenology. These factors explained more variability in NRE than in PRE. Our findings highlight the significance of these three ecological mechanisms in leaf nutrient resorption and have important implications for understanding how nutrient resorption responds to climate change.

We explored the response of photosynthetic capacity to nitrogen (N) deposition among Larix gmelinii trees in different crown classes (e.g., suppressed, intermediate, and dominant trees) in a 12-year field experiment in a forest in the Greater Khingan Mountains in Northeast China. Four N-addition treatments were established: control (CK), low N (LN), medium N (MN), and high N (HN) (0, 25, 50, and 75 kg N·ha⁻¹·year⁻¹, respectively). Photosynthesis and its influencing factors were measured in 2023. Nitrogen addition significantly increased the maximum net photosynthetic rate (P_max), maximum carboxylation rate (V_cmax), and maximum electron transport rate (J_max) of suppressed and intermediate trees. The suppressed trees showed maximum P_max and V_cmax in MN and HN, and maximum J_max in HN. The intermediate trees showed maximum P_max, V_cmax, and J_max in MN. For dominant trees, P_max was increased in LN and MN and decreased in HN, and V_cmax was increased by N addition and peaked in MN. Nitrogen addition significantly increased the leaf N content (N_mass), chlorophyll content (Chl_m), the ratio of N to phosphorous (N:P), and photosynthetic enzyme activities in all crown classes. N_mass had significant nonlinear relationships with P_max, V_cmax, and J_max. Enzyme activity and Chl_m positively affected the photosynthetic capacity of suppressed and intermediate trees, and N:P negatively affected the photosynthetic capacity of dominant trees. The promoting effect of N addition on photosynthetic capacity was stronger in suppressed and intermediate trees than in dominant trees. Therefore, the crown class should be considered when studying the effect of N deposition on the boreal forests.

To investigate the effects of climate warming on photosynthesis, Dahurian larch (Larix gmelinii Rupr.) trees from four sites (spanning ~ 5.5° in latitude and ~4 °C of warming) within the geographic range in China were transplanted into a common garden close to the warmer border in 2004. Throughout the growing season of 2018, the CO₂- and temperature-response curves of the photosynthesis in the common garden and at the original sites were measured. It was discovered that warming treatment considerably increased the maximum net photosynthetic rate (A_max) by 23.4–35.3% depending on the sites, signifying that warming upregulated A_max with respect to the degree of warming. At 25 °C, warming enhanced the maximum Rubisco carboxylation rate (V_cmax), maximum electron transport rate (J_max), and mass-based leaf nitrogen concentration (N_mass). The climate warming effect (CWE) on A_max was positively associated with the CWEs on V_cmax, J_max and N_mass, which indicated that warming promoted A_max primarily via increasing carboxylation and photosynthetic electron transport rates and leaf nitrogen supply. The CWE in optimal photosynthetic temperature (T_opt) was significant for the trees from the northern sites rather than the southern sites; however, the effect vanished for the trees transplanted to the common garden; this implied that T_opt exhibited limited local thermal acclimation. Nevertheless, warming narrowed the temperature-response curve, the effect of which was positively associated with the warming magnitude. These findings implied that trees transplanted into warmer areas changed the photosynthetic optimum temperature and sensitivity. In summary, our results deepen the understanding of the underlying mechanisms of intraspecific responses of photosynthesis to temperature changes, including which of the modeling would improve the prediction of tree growth and forest carbon cycling under climate warming.

Nonstructural carbohydrates (NSCs) are key storage molecules that can be used for tree growth and metabolism. The trade-off between NSC storage and biomass production has been long reported on. However, the carbon source limitation (indicated by NSC storage) to biomass production remains poorly quantitively assessed. The seasonal whole-tree NSC pool dynamics of 12 temperate tree species were quantitatively evaluated across seven seasonal sampling points. The ratio of seasonal variation in whole-tree NSC pool to annual biomass increment (the ΔNSC/ABI ratio) and the linear relationship of annual biomass increment to NSC storage were used to assess the coupling of NSC storage to annual biomass production. Whole-tree NSC pools were consumed in early summer when structural growth peaked and recovered in the nongrowing season, indicating a short-term trade-off between storage and growth. The ΔNSC/ABI ratio was on average 0.59, with a large interspecific variation. Notably, there was a significant positive correlation between the storage of NSC and the 10 yr mean annual biomass increment, indicating a storage–growth coupling and the source limitation of growth in the long term. However, the storage cost of biomass production decreased along the slow-growth-to-fast-growth species continuum, mirroring the spectrum from conservative to acquisitive NSC use strategies. These findings highlight the critical role of time scale in understanding the relationship between storage and growth, which should be considered in the framework of simulation and conceptual models.