N source preferences in generalist and specialist species
It is generally believed that generalist species show higher plasticity than unique species (Liang, et al. 2020). Denelle et al. (2020) found that at the regional and community levels, generalist species show high interspecific functional similarity. In this study, the generalist species in the two meadows were E. nutans , P. annua, andP. kansuensis , which grew together in the 0.25 m2 sampling area. Among these species, E. nutans and the P. annua are grasses and P. kansuensis is a forb. These generalist species distributed broadly in the undegraded and degraded alpine meadows showed different N preference strategies, with inconsistent directions of change (Fig. 2b). It is possible that plant preferences are species-specific and do not depend on the classification of generalist or specialist (Zhang et al. 2019) or that the preferences were influenced by neighboring species competition (Hong et al. 2017). Moreover, two additional explanations cannot be ignored. The difference among generalists may be related to whether the trait evaluated is a response trait or an effector trait (Suding et al. 2008); in a given environment, response traits (response to environmental changes, such as N preference) are subjected to diversifying selection, while effect traits (effects on ecosystem function, such as N concentrations) are directional (Roquer-Beni et al. 2021). It may also be related to the study scale; at a larger scale, generalist species may show greater plasticity under environmental variation, while at the microlocal level, divergent responses may explain the complementary effects of generalists and thus a broad distribution. However, the three generalist species in this study still showed interspecific complementarity in N source absorption and reduced interspecific competition.
The four unique species that were evenly distributed in the undegraded alpine meadow almost preferred NO3-, while the unique species that were unevenly distributed preferred NH4+ (Fig. 2c). In the degraded alpine meadow, a significant N preference was not observed for one unique species with an even distribution, while a preference for glycine was observed for the other unique species with uneven distributions (Fig. 2c), which can directly affect the total N preference in the degraded alpine meadow owing to their large biomass (Fig. 2a). However, there is likely no need for further differentiation in N preference among these unique species because their uneven distribution at the scale of 0.25 m2 resulted in spatial separation; based on the distribution of plant roots in soil, competition for glycine among unique species can be avoided. This ”micro” landscape heterogeneity in the plant distribution undoubtedly contributes to a stable community (Li et al. 2021). These results provide a reasonable explanation for the observed N absorption by unique species in the patchy landscapes of the degraded alpine meadow and emphasize the importance of considering heterogeneity in studies of species coexistence.