Reconciling inconsistencies in precipitation–productivity relationships: implications for climate change

Abstract

Contents 41I.41II.42III.43IV.44V.45Acknowledgements46References46 Summary Precipitation (PPT) is a primary climatic determinant of plant growth and aboveground net primary production (ANPP) over much of the globe. Thus, PPT–ANPP relationships are important both ecologically and to land–atmosphere models that couple terrestrial vegetation to the global carbon cycle. Empirical PPT–ANPP relationships derived from long‐term site‐based data are almost always portrayed as linear, but recent evidence has accumulated that is inconsistent with an underlying linear relationship. We review, and then reconcile, these inconsistencies with a nonlinear model that incorporates observed asymmetries in PPT–ANPP relationships. Although data are currently lacking for parameterization, this new model highlights research needs that, when met, will improve our understanding of carbon cycle dynamics, as well as forecasts of ecosystem responses to climate change.

Asymmetric Responses of Primary Productivity to Altered Precipitation Simulated by Ecosystem Models across Three Longterm Grassland Sites

“…This emphasizes the finding that most models overestimate drought effects and/or underestimate wet year impacts on primary productivity of dry and mesic sites for current 5 precipitation variability. When moved to extreme conditions with modified precipitation, models were in line with the hypothesis and the data showing that ANPP saturates in very wet conditions but declines strongly in very dry conditions (Knapp et al, 2017b). Because there are still only few extreme precipitation manipulation experiments (Knapp et al, 2017a) and limited associated understanding and model development (Meir et al, 2015), we are currently unable to evaluate the point of ecosystem collapse and point of release from water limitations.…”

“…In temporal P-ANPP relationships, an important observation is the asymmetric responses of productivity in grasslands to altered precipitation (Knapp et al, 2017b;Wilcox et al, 2017). Compared to negative anomalies of ANPP from years with decreased precipitation, positive anomalies of ANPP during years with increased precipitation were usually found to have a larger absolute magnitude, suggesting a convex positive response (positive asymmetry) (Bai et al, 2008;Knapp and Smith, 2001;Yang et al, 2008).…”

“…4c), which suggested greater declines of ANPP in dry years than increases in wet years (Knapp and Smith, 2001). Knapp et al (2017b) proposed the following underlying mechanisms: (1) In dry years, the carry-over effects of soil moisture from previous years alleviate strong declines of ANPP (Sala et al, 2012), which is usually treated as a time-lag effect …”

“…Relationships between precipitation and grassland productivity have previously been studied with site observations (Hsu et al, 2012;Knapp et al, 2017b;Luo et al, 2017;Wilcox et al, 2017;Estiarte et al, 2016), but they remain to be quantified and characterized in ecosystem models used for diagnostic and future projections of the coupled carbon-water system in grasslands, in particular grid-based models used as the land surface component of Earth System Models. In this study, we aim to evaluate 5 the responses of simulated productivity to altered precipitation from fourteen ecosystem models at three sites representing dry, mesic, and moist rainfall regimes.…”

“…For projecting the effect of climate change on grassland productivity in the coming decades, inter-annual relationships are arguably more informative than spatial relationships because spatial relationships reflect long-term adaptation of ecosystems, and because P-ANPP relationships from spatial gradients are confounded by the co-variation of gradients in other environmental variables (e.g. temperature and radiation) and soil properties (Estiarte et al, 2016;Knapp et al, 2017b). …”