Incorporating clonal growth form clarifies the role of plant height in response to nitrogen addition
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Nutrient addition to grasslands consistently causes species richness declines and productivity increases. Competition, particularly for light, is often assumed to produce this result. Using a long-term dataset from North American herbaceous plant communities, we tested whether height and clonal growth form together predict responses to fertilization because neither trait alone predicted species loss in a previous analysis. Species with a tall-runner growth form commonly increased in relative abundance in response to added nitrogen, while short species and those with a tall-clumped clonal growth form often decreased. The ability to increase in size via vegetative spread across space, while simultaneously occupying the canopy, conferred competitive advantage, although typically only the abundance of a single species within each height-clonal growth form significantly responded to fertilization in each experiment. Classifying species on the basis of two traits (height and clonal growth form) increases our ability to predict species responses to fertilization compared to either trait alone in predominantly herbaceous plant communities.
KeywordsClonal growth Competition Grassland Nitrogen addition Productivity
We are grateful to the many researchers and technicians who originally collected these data, maintained the experiments, or contributed to the trait compilations, including: Gus Shaver (ARC); Chris Field, Hal Mooney, and Erika Zavaleta (JRG); Carol Baker (KBS); John Blair (KNZ); Terry Theodose (NWT); and Karen Wetherill (SEV). Significant funding for the collection of these data was provided by multiple grants from the National Science Foundation (NSF) to the LTER Network office and individual LTER sites and investigators including: DEB-9810222, DEB-1026843 and OPP-0909507 (ARC); DEB-0080382 (CDR); OCE-0620959 (CAR and GCE); DEB-0423627 and DEB-9810220 (KBS); DEB-0423662, DEB-1027341 and DEB-9810218 (NWT); DEB-0080529, DEB-8811906 and DEB-0620482 (SEV); and DEB-0217631 (SGS). Support for data collection in the Jasper Ridge Global Change Experiment was provided by NSF, the David and Lucile Packard Foundation, the Morgan Family Foundation, and the Jasper Ridge Biological Preserve. Collaboration among the authors has been supported by cross-site synthesis grants from the LTER Network office and the National Center for Ecological Analysis and Synthesis (NCEAS), a Center funded by NSF (DEB-0553768), the University of California, Santa Barbara, and the State of California. K.L.G. was supported on a sabbatical fellowship from NCEAS during the final stages of preparing this manuscript; as part of that fellowship, NCEAS also provided funding for one group meeting of the co-authors.
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