Impacts of competition and nitrogen addition on plant stoichiometry and non-structural carbohydrates in two larch species

Previous research has shown that competition between plants can have differential effects on leaf stoichiometry and non-structural carbohydrate (NSC) in different environments. However, little attention has been given to understanding these effects on non-photosynthetic organs, particularly of deciduous tree species. Here we assess the impact of competition on below and aboveground biomass, stoichiometry, nutrient composition and NSC in pure and mixed forests of two Larch species, Larix kaempferi and L. olgensis under nitrogen (N) addition. Nitrogen enrichment did not result in stronger intraspecific competition for both species and L. olgensis benefited from the presence of L. kaempferi under different N levels. Stems kept relatively stable C/N compared to roots and branches in response to competition, while N addition imposed stronger impacts on N/P of different organs rather than competition. In contrast to stable C concentrations, starch and soluble sugar concentrations were more easily impacted by competition and the addition of nitrogen. Competition forced L. kaempferi and L. olgensis to allocate more carbon into storage by increasing their starch concentration and starch/soluble sugar of stems under competition. However, no significant differences in stoichiometry and NSC concentration between intra- and interspecific competition were found. NSC and nutrient pools of L. kaempferi stems, branches and coarse roots consistently declined due to competition regardless of N addition. Coarse and fine roots of L. kaempferi accumulated more N when in competition with L. olgensis than with a conspecific neighbor under N addition. Our results show that NSC was more sensitive to competition relative to stoichiometric traits (N and P) of non-photosynthetic organs.