Lagging behind: have we overlooked previous-year rainfall effects in annual grasslands?

doi: 10.1111/1365-2745.12671 Journal of Ecology Lagging behind: have we overlooked previous-year rainfall effects in annual grasslands? Joan Dudney 1 *, Lauren M. Hallett 2,3 , Loralee Larios 4,5 , Emily C. Farrer 6 , Erica N. Spotswood 1 , Claudia Stein 7 and Katharine N. Suding 2 Department of Environmental Science Policy and Management, University of California at Berkeley, Berkeley, CA 94720, USA; 2 Department of Ecology and Evolutionary Biology and Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA; 3 Environmental Studies Program and Department of Biology, University of Oregon, Eugene, OR 97403, USA; 4 Department of Botany and Plant Sciences, University of California, Riverside, CA 92507, USA; 5 Division of Biological Sciences, University of Montana, Missoula, MT 59812, USA; 6 Department of Ecology and Evolutionary Biology, Tulane University, New Orleans, LA 70118, USA; and 7 Department of Biology & Tyson Research Center, Washington University in St. Louis, One Brooking Drive, St Louis, MO 63130, USA Summary 1. Rainfall is a key determinant of production and composition in arid and semi-arid systems. Long- term studies relating composition and water availability primarily focus on current-year precipitation patterns, though mounting evidence highlights the importance of previous-year rainfall particularly in grasslands dominated by perennial species. The extent to which lagged precipitation effects occur in annual grasslands, however, remains largely unexplored. 2. We pair a long-term study with two manipulative experiments to identify patterns and mecha- nisms of lagged precipitation effects in annual grasslands. The long-term study captured variation in functional group (exotic annual forbs and grasses) abundance and precipitation across 8 years at three northern California grassland sites. We then tested whether lagged rainfall effects were created through seed production and litter (residual dry matter, RDM) by manipulating rainfall and litter, respectively. 3. Rainfall from the previous-year growing season (both seasonal and total rainfall) shifted func- tional group abundance. High lagged rainfall was associated with increased grass and decreased forb abundance the following year. Current-year seasonal rainfall also inﬂuenced species composition, with winter rain increasing forb and decreasing grass abundance. Lagged precipitation effects were generally stronger for forbs than for grasses. Our experimental studies provided evidence for two mechanisms that contributed to lagged effects in annual grasslands. Higher rainfall increased seed production for grasses, which translated to more germinable seed the following year. Higher rainfall also increased biomass production and RDM, which beneﬁted grasses and reduced forb abundance. 4. Synthesis. Our results highlight the importance of previous-year precipitation in structuring annual community composition and suggest two important biotic pathways, seed rain and RDM, that regu- late lagged community responses to rainfall. Incorporating lagged effects into models of grassland diversity and productivity could improve predictions of climate change impacts in annual grasslands. Key-words: annual plant seed production, determinants of plant community diversity and struc- ture, functional groups, lagged effects, legacy effects, litter dynamics, plant population and commu- nity dynamics, plant–climate interactions, precipitation effects, species responses to weather Introduction Describing how resource availability modiﬁes ecosystem structure is a long-standing ecological challenge and increas- ingly more important given the uncertainties surrounding *Correspondence author. E-mail: jdudney@berkeley.edu climate change. Water availability is a strong driver of plant community structure in a number of systems (Noy-Meir 1973; Westoby, Walker & Noy-Meir 1989) and can lead to shifts in species diversity and composition over time (Cleland et al. 2013). Rainfall-driven ﬂuctuations in composition are particularly pronounced in annual grasslands. For example, wet years often promote recruitment of all species, while only © 2016 The Authors. Journal of Ecology © 2016 British Ecological Society