Ecohydrological controls on grass and shrub above‐ground net primary productivity in a seasonally dry climate

ECOHYDROLOGY Ecohydrol. 8, 1572–1583 (2015) Published online 11 February 2015 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1605 Ecohydrological controls on grass and shrub above-ground net primary productivity in a seasonally dry climate Anthony J. Parolari, 1,2 * Michael L. Goulden 3 and Rafael L. Bras 4,5 Nicholas School of the Environment, Duke University, Durham, NC, USA Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA Department of Earth System Science, University of California, Irvine, CA, USA School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, USA School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA ABSTRACT Seasonally dry, water-limited regions are often co-dominated by distinct herbaceous and woody plant communities with contrasting ecohydrological properties. We investigated the shape of the above-ground net primary productivity (ANPP) response to annual precipitation (P a ) for adjacent grassland and shrubland ecosystems in Southern California, with the goal of understanding the role of these ecohydrological properties on ecosystem function. Our synthesis of observations and modelling demonstrates grassland and shrubland exhibit distinct ANPP-P a responses that correspond with characteristics of the long-term P a distribution and mean water balance ﬂuxes. For annual grassland, no ANPP occurs below a ‘precipitation compensation point,’ where bare soil evaporation dominates the water balance, and ANPP saturates above the P a where deep percolation and runoff contribute to the modelled water balance. For shrubs, ANPP increases at a lower and relatively constant rate across the P a gradient, while deep percolation and runoff account for a smaller fraction of the modelled water balance. We identify precipitation seasonality, root depth, and water stress sensitivity as the main ecosystem properties controlling these responses. Observed ANPP-P a responses correspond to notably different patterns of rain-use efﬁciency (RUE). Grass RUE exceeds shrub RUE over a wide range of typical P a values, whereas grasses and shrubs achieve a similar RUE in particularly dry or wet years. Inter-annual precipitation variability, and the concomitant effect on ANPP, plays a critical role in maintaining the balance of grass and shrub cover and ecosystem-scale productivity across this landscape. Copyright © 2015 John Wiley & Sons, Ltd. KEY WORDS soil moisture dynamics; seasonal climates; primary production; rain-use efﬁciency Received 10 July 2014; Revised 10 January 2015; Accepted 15 January 2015 INTRODUCTION Semi-arid ecosystems are often characterized by a patchwork of plant communities, such as the herbaceous and woody vegetation mosaic found in regions with seasonally dry climate. Because of their socio-economic importance and vulnerability to change under future climate and land use conditions (Schlesinger et al., 1990; Rietkerk et al., 2004; Reynolds et al., 2007; Schneider et al., 2007), a mechanistic understanding of the structure and function of such water-limited ecosystems is essential to their proper management. Extensive research has highlighted both bottom-up (i.e. competition for soil moisture and nutrients) and top-down (i.e. ﬁre and herbivory) controls on the productivity of semi-arid woody and herbaceous vegetation (Scholes and Archer, 1997). *Correspondence to: Anthony J. Parolari, Nicholas School of the Environment, Duke University, Durham, NC 27708, USA. E-mail: anthony.parolari@duke.edu Copyright © 2015 John Wiley & Sons, Ltd. The link between primary productivity and precipitation is an important indicator of dryland function. At the scale of an individual site, the response of above-ground net primary productivity (ANPP) to annual precipitation (P a ) varies substantially with soil type, plant functional type (PFT) and climate. ANPP generally increases with P a , with evidence for both linear (Briggs and Knapp, 1995; Huxman et al., 2004; Hsu et al., 2012) and nonlinear (saturating) (Hsu et al., 2012) ANPP-P a responses. However, ANPP-P a correlations are often weak, and in some cases, ANPP is insensitive to P a (Sala et al., 2012). Previous analyses of the ANPP-P a relation have been relatively descriptive, and the ecohydrological basis for these patterns has yet to be explored in detail. Because of nonlinearity in the surface hydrological response to precipitation, interaction between the soil water balance and vegetation productivity may be one factor that controls the shape of the ANPP-P a response. Annual evapotrans- piration, the water ﬂux that facilitates productivity, tends to saturate with increasing annual precipitation (Budyko, 1974).