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.