Agricultural and Forest Meteorology 152 (2012) 189–200 Contents lists available at SciVerse ScienceDirect Agricultural and Forest Meteorology jou rn al h om epa g e: www.elsevier.com/locate/agrformet Ecological controls on net ecosystem productivity of a seasonally dry annual grassland under current and future climates: Modelling with ecosys R.F. Grant a,∗ , D.D. Baldocchi b , S. Ma b a Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada T6G 2E3 b Department of Environmental Science, Policy and Management & Berkeley Atmospheric Science Center, University of California, Berkeley, CA, United States a r t i c l e i n f o Article history: Received 14 January 2011 Received in revised form 26 August 2011 Accepted 10 September 2011 Keywords: CO2 fluxes Net ecosystem productivity Phenology Seasonally dry grasslands Climate change Ecosys a b s t r a c t Net ecosystem productivity (NEP) of seasonally dry grasslands in Mediterranean climate zones is deter- mined by the duration and intensity of rainy vs. dry seasons. Precipitation in these zones is expected to decline with climate change during the next century, possibly reducing NEP. Ecosystem models used to study climate change impacts on grasslands in these zones need first to simulate effects of soil wetting and drying on the duration and intensity of net C uptake and emission during rainy and dry seasons under current climate. Continuous eddy covariance (EC) measurements of CO 2 and energy exchange provide well constrained tests of such models. In this study, hourly CO 2 and energy exchange from the ecosystem model ecosys were tested against EC measurements recorded over an annual grassland at Vaira Ranch, CA in a Mediterranean climate zone during eight years (2001–2008) with variable rainy seasons. Variation in measured CO 2 and latent heat fluxes was sufficiently well simulated during each year of the study (0.7 < R 2 < 0.9) that most of the variation unexplained by the model could be attributed to uncertainty in the measurements. Interannual variation in NEP from the model was also correlated with that from EC measurements (R 2 = 0.75). Annual NEP from both the model and EC were correlated with the dura- tion of net C uptake, but not with the amount of precipitation, during the rainy seasons. Average annual NEP of the grassland modelled from 2001 to 2008 was 29 g C m -2 y -1 with an interannual variation of ±110 g C m -2 y -1 caused by that in the duration of net C uptake. During climate change (SRES A1fi and B1 under HadCM3), changes in modelled NEP were determined by changes in duration and intensity of net C uptake in rainy seasons vs. net C emission in dry seasons. In years with briefer rainy seasons, modelled NEP rose because rates of net C uptake increased with higher temperature and CO 2 concentration, while the duration of net C uptake remained limited by that of the rainy season. However in years with longer rainy seasons, modelled NEP declined because the duration of net C uptake was reduced when warming hastened phenological development and caused maturity of annual plants to be reached before the end of the rainy season. As climate change progressed, declines in annual NEP gradually exceeded rises, causing the small C sink modelled under current climate to be almost completely lost after 90 years under SRES A1fi (2 ± 103 g C m -2 y -1 ) and B1 (6 ± 95 g C m -2 y -1 ). © 2011 Elsevier B.V. All rights reserved. 1. Introduction Seasonally dry annual grasslands are an important ecosys- tem type in Mediterranean climate zones. Those in California alone occupy more than 4.4 Mha or 20% of the area of the state (Huntsinger et al., 2007). Net ecosystem productivity (NEP) of these grasslands is thought to be determined by the duration and inten- sity of precipitation in the rainy season during which soils are wet enough to sustain net C uptake by plants (Ma et al., 2007). This net C uptake is offset by slow net C emission during the dry season when grasslands have senesced, by rapid, rainfall-induced pulses of net ∗ Corresponding author. Tel.: +1 780 492 6609; fax: +1 780 492 1767. E-mail address: robert.grant@ales.ualberta.ca (R.F. Grant). C emission at the start of the next rainy season before grasslands regrow, and sometimes by rapid net C emission at the end of the rainy season if grassland growth terminates before the start of the following dry season. At an annual time scale, net C uptake during the mid rainy season may or may not exceed net C emission during the dry season and during the start and end of the rainy season, so that these grasslands may be either sinks or sources of C. These ecological controls on net C uptake vs. emission in season- ally dry grasslands have been demonstrated with eddy covariance (EC) measurements. Xu and Baldocchi (2004) attributed annual NEP of a northern California grassland to the timing of rain events that determined the duration of net C uptake in the rainy season, rather than to the total amount of rainfall received during the year. Ma et al. (2007) found that changes in the duration of net C uptake with precipitation caused this grassland to alternate between a sink 0168-1923/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2011.09.012