Plant defense trait responses to grazing along an aridity gradient in Inner Mongolian grasslands

Silicon (Si) and phenolics are vital players in the plant kingdom,
enabling plants to combat both biotic stresses, like herbivory, and
abiotic challenges, such as drought. Yet, the interplay between
community-level silica and phenolic defenses in response to livestock
grazing and climatic factors across large spatial scales remains
underexplored. We analyzed the Si and phenolic concentrations of plant
communities from 15 widely distributed sites, strategically selected along
a 1700 km aridity gradient in the temperate grasslands of northern China,
with livestock grazing manipulated (grazed vs. ungrazed) at each location.
Our findings revealed that grazing significantly enhanced community-level
silica-based defenses in less arid grasslands, mainly due to increased
intraspecific Si concentration, while its impact on phenol-based defenses
was minimal. Notably, as aridity increased, Si concentration surged while
phenolic concentration dwindled, indicating a shift from phenol- to
silica-based defenses in drier environments. In ungrazed grasslands,
silica and phenolic defenses were negatively correlated, a relationship
that vanished in grazed treatment. Further analysis revealed that Si
concentration was primarily influenced by grazing management, while
phenolic concentration was mainly associated with aridity. Our study
provides the community-level and regional-scale assessment of how grazing
management impacts plant silica- and phenol-based defenses along an
aridity gradient in the temperate grasslands. Together, our results showed
that plants accumulated Si but not phenolics as a defense mechanism in
response to decades of livestock grazing. Importantly, our findings
suggested that grazing disrupted the trade-off between the two plant
defense strategies along the aridity gradient. These insights emphasize
the critical role of grazing management and aridity in shaping silica and
phenolic defenses, revealing a complex interplay between environmental
conditions and plant defense strategies in grassland ecosystems.