Few multiyear precipitation–reduction experiments find a shift in the productivity–precipitation relationship

Abstract

; Janssens, Ivan A. 2016. Few multiyear precipitationreduction experiments find a shift in the productivity-precipitation relationship. Global Change Biology, 22 (7). 2570-2581. 10.1111/gcb.13269 Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. AbstractWell defined productivity-precipitation relationships of ecosystems are needed as benchmarks for the validation of land-models used for future projections. The productivityprecipitation relationship may be studied in two ways: the spatial approach relates differences in productivity to those in precipitation among sites along a precipitation gradient (the spatial fit, with a steeper slope); the temporal approach relates inter-annual productivity changes to variation in precipitation within sites (the temporal fits, with flatter slopes). Precipitation- Accepted ArticleThis article is protected by copyright. All rights reserved.reduction experiments in natural ecosystems represent a complement to the fits, because they can reduce precipitation below the natural range and are thus well suited to study potential effects of climate drying. Here, we analyze the effects of dry treatments in eleven multi-year precipitation-manipulation experiments, focusing on changes in the temporal fit. We expected that structural changes in the dry treatments would occur in some experiments, thereby reducing the intercept of the temporal fit and displacing the productivity-precipitation relationship downward the spatial fit. The majority of experiments (72%) showed that dry treatments did not alter the temporal fit. This implies that current temporal fits are to be preferred over the spatial fit to benchmark land-model projections of productivity under future climate within the precipitation ranges covered by the experiments. Moreover, in two experiments, the intercept of the temporal fit unexpectedly increased due to mechanisms that reduced either water-or nutrient losses. The expected decrease of the intercept was observed in only one experiment, and only when distinguishing between the late and the early phases of the experiment. This implies that we currently do not know at which precipitationreduction level or at which experimental duration structural changes will start to alter ecosystem productivity. Our study highlights the need for experiments with multiple, including more extreme, dry treatments, to identify the precipitation boundaries within which the current temporal fits remain valid.

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

“…The P-ANPP sensitivities obtained from spatial relationships are usually higher than those obtained by temporal relationships (Estiarte et al, 2016;Fatichi and Ivanov, 2014;Sala et al, 2012). Possible mechanisms behind the steeper spatial relationship may be (1) a 'vegetation constraint' reflecting the adaptation of plant communities over long time scales in such a way that grasslands make the best use of the water received from rainfall for growth, and (2) the spatial variation in structural and functional traits of ecosystems (soil properties, nutrient pools, plant and 15 microbial community composition) that constrain local P-ANPP sensitivities (Lauenroth and Sala, 1992;Smith et al, 2009;Wilcox et al, 2016).…”

“…Steeper spatial than temporal slopes of ANPP to precipitation are usually explained by two hypotheses: (1) 'vegetation constraint' effects on ANPP responses to precipitation play a more important role in the temporal as compared to the spatial domain (Knapp et al, 2017b;Estiarte et al, 2016); (2) biogeochemistry (mainly resource limitations) and confounding factors (e.g. temperature and radiation), rather than species attributes, constrain community level ANPP in response to precipitation (Huxman et al, 2004).…”

“…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). …”

“…The sign of the asymmetric response of grassland productivity to altered rainfall thus depends on 30 the magnitude of rainfall anomalies, the size-distribution of rainfall events, and ecosystem mean state (Gherardi and Sala, 2015; Biogeosciences Discuss. Parolari et al, 2015;Peng et al, 2013).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.…”

“…NPP responses to precipitation have been observed using multi-year, multi-site observations (Hsu et al, 2012;Estiarte et al, 2016;Knapp and Smith, 2001;Wilcox et al, 2015).…”