Seedlings of tree species Abies faxoniana and Betula albo-sinensis were grown under different competition regimes (single plant, monoculture or mixture) at ambient temperature (AT) and elevated temperature (ET) to evaluate the impact of inter- and intraspecific competition on photosynthesis capacity, N resorption efficiency, N uptake patterns, water use efficiency (foliar carbon isotope composition, δ¹³C) and root carbon reserve accumulation. Our results showed that ET did not have significant effects on relative height growth rate (RGR_H), relative diameter growth rate (RGR_D), chlorophyll fluorescence parameters (F_v/F_m, Φ_PSII), light-saturated photosynthetic rate (A₁₄₀₀) and nitrogen uptake in A. faxoniana, but it significantly reduced the values of these parameters in B. albo-sinensis. On the other hand, RGR_H of A. faxoniana was not influenced by intraspecific competition irrespective of temperature regimes, while in B. albo-sinensis intraspecific competition was severer under AT than under ET. Decreased photosynthetic capacity and reduced plant size detected in B. albo-sinensis under ET likely contribute to its low intraspecific competitive ability. However, we found positive effects of interspecific competition on A. faxoniana but negative effects on B. albo-sinensis under ET, as A. faxoniana exposed to interspecific competition showed higher RGR_H, RGR_D, A₁₄₀₀, chlorophyll fluorescence parameters, foliar δ¹³C, foliar N, and percentages of ¹⁵N-ammonium and ¹⁵N-nitrate recovery than did A. faxoniana exposed to intraspecific competition or no competition. The results indicate that A. faxoniana is a better competitor than B. albo-sinensis under ET. Further, A. faxoniana altered resource use patterns and had a high plasticity in its ammonium uptake in mixtures compared with monocultures under ET, while B. albo-sinensis did not change its N use pattern. In all, our results showed that high temperature stress tolerance (e.g., higher root carbon accumulation), high ability to keep carbon balance (e.g., higher water use efficiency, lower leaf respiration rate, higher F_v/F_m), as well as a better N acquisition ability make A. faxoniana a better competitor when compared to B. albo-sinensisg under ET.