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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
carbonwater 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.