Identifying the underlying mechanisms and the sources of variation in plant productivity is critical to the development of accurate predictive models of ecosystem responses to environ-

Identifying the underlying mechanisms and the sources of variation in plant productivity is critical to the development of accurate predictive models of ecosystem responses to environmental change, including increased nitrogen (N) deposition and altered precipitation patterns. A great deal of variation in aboveground net primary production (ANPP) in tallgrass prairie may be accounted for by variation in tiller density, rather than differences in tiller size ( Hartnett and Fay, 1998 ). In addition, many tallgrass prairie plants maintain a substantial bud bank: a belowground population of meristems associated with rhizomes or other perennating organs (sensu Harper, 1977 ). The bud bank serves as the recruitment source for the majority of aboveground tillers in tallgrass prairie ( Benson and Hartnett, 2006 ; Dalgleish and Hartnett, 2006 ). Consequently, bud bank dynamics play a fundamental role in local plant population persistence, structure, and dynamics. If variation in ANPP in tallgrass prairie can be explained principally through changes in tiller density, population processes such as the demography of buds and tillers are important not only for understanding population dynamics of individual species, but for explaining temporal and spatial variability in ANPP as well. The study of bud bank demography represents a crucial link between organismal (formation and maintenance of the bud bank, rates of tiller emergence) and ecosystem level processes (ANPP). In tallgrass prairie, plant productivity may be limited by three key resources: N, water, and light ( Seastedt and Knapp, 1993 ; Seastedt et al., 1998 ). The relative importance of these resources for limiting plant growth varies considerably, both spatially and temporally. In addition, human alteration of the N cycle through burning fossil fuels and agricultural activities has led to at least a doubling of the rates of terrestrial N input ( Vitousek et al., 1997 ). Increased N deposition has many important impacts on plant communities ( Clark et al., 2007 ). In this research, we tested the role of the bud bank in plant responses to N. Grassland plants respond to available N as a key resource that can limit plant growth, but recent theory suggests that N may also play a role as a proximal cue for bud dormancy and emergence ( Tomlinson and O ’ Connor, 2004 ). The known importance of N for limiting growth in tallgrass prairie plants as well as its potential to cue bud emergence, may result in ecologically signifi cant effects of altered N availability on bud bank and tiller demography, rather than effects on tiller growth alone. Although previous studies have examined the effects of N on tiller demography in grasses ( Noble et al., 1979 ; Briske and Butler, 1989 ; Hartnett, 1993 ; Derner and Briske, 1999 ), none have examined the effects of resource availability on the ultimate source for tiller recruitment, the bud bank. Understanding bud bank dynamics in conjunction with aboveground tiller responses is important because changes in tiller recruitment or tiller size may not necessarily lead to similar changes in the bud bank. The maintenance of a belowground bud bank, like a seed bank, represents a reserve population of dormant propagules that may have important ecological consequences. We addressed the general question of how tallgrass prairie plants will respond to increased resource availability. We hypothesized that tallgrass prairie plant growth and production would respond to changing resource availability principally through changes in the demography of bud banks (changing 1 Manuscript received 23 August 2007; revision accepted 27 March 2008. The authors thank J. Sinn, J. Hill, Z. Gill, M. Williamson, J. Birmingham, and J. Jonas for fi eld assistance and Dr. E. G. Towne for help fi nding suitable populations of study plants. This work was supported by the Konza Prairie LTER, the Kansas State University Division of Biology, NSF grant DEB-0234159 and one REU supplement grant for DEB-0234159. 2 Author for correspondence (e-mail: [email protected]), present address: Department of Wildland Resources, Utah State University, 5230 Old Main Hill, Logan, UT 84322 USA 3 Present address: Department of Biology, Biology-Psychology Building, University of Maryland, College Park, MD 20742 USA