Abstract. Field measurements of aboveground net primary productivity (ANPP) in
temperate grasslands suggest that both positive and negative asymmetric
responses to changes in precipitation (P) may occur. Under normal range of
precipitation variability, wet years typically result in ANPP gains being
larger than ANPP declines in dry years (positive asymmetry), whereas
increases in ANPP are lower in magnitude in extreme wet years compared to
reductions during extreme drought (negative asymmetry). Whether the current
generation of ecosystem models with a coupled carbon–water system in
grasslands are capable of simulating these asymmetric ANPP responses is an
unresolved question. In this study, we evaluated the simulated responses of
temperate grassland primary productivity to scenarios of altered
precipitation with 14 ecosystem models at three sites: Shortgrass
steppe (SGS), Konza Prairie (KNZ) and Stubai Valley meadow (STU), spanning a
rainfall gradient from dry to moist. We found that (1) the spatial slopes
derived from modeled primary productivity and precipitation across sites were
steeper than the temporal slopes obtained from inter-annual variations, which
was consistent with empirical data; (2) the asymmetry of the responses of
modeled primary productivity under normal inter-annual precipitation
variability differed among models, and the mean of the model ensemble
suggested a negative asymmetry across the three sites, which was contrary to
empirical evidence based on filed observations; (3) the mean sensitivity of
modeled productivity to rainfall suggested greater negative response with
reduced precipitation than positive response to an increased precipitation
under extreme conditions at the three sites; and (4) gross primary productivity
(GPP), net primary productivity (NPP), aboveground NPP (ANPP) and belowground
NPP (BNPP) all showed concave-down nonlinear responses to altered
precipitation in all the models, but with different curvatures and mean
values. Our results indicated that most models overestimate the negative
drought effects and/or underestimate the positive effects of increased
precipitation on primary productivity under normal climate conditions,
highlighting the need for improving eco-hydrological processes in those
models in the future.
Primary Productivity and Precipitation-Use Efficiency in Mixed-Grass Prairie: A Comparison of Northern and Southern US Sites