Plant traits and soil fertility mediate productivity losses under extreme drought in C3 grasslands

Extreme drought decreases aboveground net primary production (ANPP) in
most grasslands, but the magnitude of ANPP reductions varies especially in
C3-dominated grasslands. Because the mechanisms underlying such
differential ecosystem responses to drought are not well-resolved, we
experimentally imposed an extreme 4-year drought (2015-2018) in two C3
grasslands that differed in aridity. These sites had similar annual
precipitation and dominant grass species (Leymus chinensis) but different
annual temperatures and thus water availability. Drought treatments
differentially affected these two semiarid grasslands, with ANPP of the
drier site reduced more than at the wetter site. Structural equation
modeling revealed that community-weighted means for some traits modified
relationships between soil moisture and ANPP, often due to intraspecific
variation. Specifically, drought reduced community mean plant height at
both sites, resulting in a reduction in ANPP beyond that attributable to
reduced soil moisture alone. Higher community mean leaf carbon content
enhanced the negative effects of drought on ANPP at the drier site, and
ANPP-soil moisture relationships were influenced by soil C:N ratio at the
wetter site. Importantly, neither species richness nor functional
dispersion were significantly correlated with ANPP at either site.
Overall, as expected, soil moisture was a dominant, direct driver of ANPP
response to drought, but differential sensitivity to drought in these two
grasslands was also related to soil fertility and plant traits.