Ecological Applications, 20(5), 2010, pp. 1320–1335 Ó 2010 by the Ecological Society of America Risk-based determination of critical nitrogen deposition loads for fire spread in southern California deserts LEELA E. RAO, 1,4 EDITH B. ALLEN, 1,2 AND THOMAS MEIXNER 3 1 Center for Conservation Biology, University of California–Riverside, 900 University Avenue, Riverside, California 92521 USA 2 Department of Botany and Plant Sciences, University of California–Riverside, 900 University Avenue, Riverside, California 92521 USA 3 Department of Hydrology and Water Resources, University of Arizona, 1133 East James E. Rogers Way, Tucson, Arizona 85721 USA Abstract. Fire risk in deserts is increased by high production of annual forbs and invasive grasses that create a continuous fine fuel bed in the interspaces between shrubs. Interspace production is influenced by water, nitrogen (N) availability, and soil texture, and in some areas N availability is increasing due to anthropogenic N deposition. The DayCent model was used to investigate how production of herbaceous annuals changes along gradients of precipitation, N availability, and soil texture, and to develop risk-based critical N loads. DayCent was parameterized for two vegetation types within Joshua Tree National Park, California, USA: creosote bush (CB) and pi ˜ non–juniper (PJ). The model was successfully calibrated in both vegetation types, but validation showed that the model is sensitive to soil clay content. Despite this fact, DayCent (the daily version of the biogeochemical model CENTURY) performed well in predicting the relative response of production to N fertilization and was used to determine estimates of fire risk for these ecosystems. Fire risk, the probability that annual biomass exceeds the fire threshold of 1000 kg/ha, was determined for each vegetation type and began to increase when N deposition increased 0.05 g/m 2 above background levels (0.1 g/m 2 ). Critical loads were calculated as the amount of N deposition at the point when fire risk began to increase exponentially. Mean critical loads for all soil types and precipitation ,21 cm/yr, representing the majority of our study region, were 0.32 6 0.07 and 0.39 6 0.09 g N/m 2 for CB and PJ, respectively. Critical loads decreased with increasing soil clay content and increasing precipitation, such that the wettest areas with clay contents of 6–14% may have critical loads as low as 0.15 g N/m 2 . Mean fire risks approached their maximum at 0.93 6 0.21 and 0.87 6 0.17 g N/m 2 in CB and PJ, indicating that precipitation is the driver of fire above these N deposition levels, which are currently observed in some areas of the Sonoran and Mojave Deserts. Overall, this analysis demonstrates the importance of considering both N deposition and precipitation when evaluating fire risk across arid landscapes. Key words: arid; biogeochemical model; Bromus; CENTURY; climate change; desert; exotic grasses; fire risk; fuel load; Joshua Tree National Park, California, USA; Larrea tridentate; Schismus. INTRODUCTION In the United States there is growing interest in managing atmospheric nitrogen (N) deposition using critical loads, which are defined as the amount of one or more pollutants below which there are no adverse ecological effects (Nilsson and Grennfelt 1988, Fisher et al. 2007, Burns et al. 2008). The European Union regulates N deposition using critical loads based on both acidification and fertilization effects (Nilsson and Grennfelt 1988). In the United States, recent research has improved our understanding of ecosystem thresh- olds for effects of N deposition, resulting in increased support for implementing critical loads on federal lands (Porter et al. 2005, Porter and Johnson 2007, Burns et al. 2008). In arid regions, acidification effects are limited due to low rainfall, lack of surface water bodies, and high soil buffering capacity. Instead, the primary effect of N deposition is chronic fertilization. Nitrogen fertilization can alter soil microbial processes, promote exotic species growth that would otherwise be N limited, and increase the growth of rapidly responding annual species above the threshold for carrying fire (Minnich and Dezzani 1998, Brooks 2003, Fenn et al. 2003a). In this paper, we applied a biogeochemical process model to determine critical loads for N deposition for its effects on increasing fire risk in the arid lands of southern California, USA. Fires in the arid regions of southern California have increased in frequency during the last several decades due in large part to a period of increased numbers of above-normal precipitation years and increased cover of Manuscript received 9 March 2009; revised 10 September 2009; accepted 15 September 2009. Corresponding Editor: J. P. Kaye. 4 Present address: CA Air Resources Board, Mobile Source Control Division, 9480 Telstar Avenue, No. 4, El Monte, California 91731 USA. E-mail: lrao@arb.ca.gov 1320