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Title: Management filters and species traits: Weed community assembly in long-term organic and conventional systems

Author
item RYAN, MATT - Pennsylvania State University
item SMITH, RICHARD - Pennsylvania State University
item Mirsky, Steven
item MORTENSEN, DAVID - Pennsylvania State University
item SEIDEL, RITA - Rodale Institute

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2010
Publication Date: 4/5/2010
Citation: Ryan, M.R., Smith, R., Mirsky, S.B., Mortensen, D.A., Seidel, R. 2010. Management filters and species traits: Weed community assembly in long-term organic and conventional systems. Weed Science. 58(3):265-277.

Interpretive Summary: Weed species vary in their competitive effects on crops. The composition of a weed community in agroecosystems is constrained by both climate and crop/weed/soil management practices. These constraints essentially act as a filter, augmenting the resulting weed community. Community assembly theory is one useful framework for predicting the outcomes of climate and management related filters, thus informing the grower which weeds may become problematic on their farm. We used an assembly approach to investigate the response of weed seed banks to 25 years of management-related filtering in three different row-crop management systems in SE Pennsylvania; organic manure-based, organic legume-based, and conventional. Organic cropping systems had higher weed seed banks and greater species diversity. Tillage intensity, chemical vs. mechanical weed control, fertility management and crop planting date are the primary factors that influenced the differences in weed seed bank density and diversity. This is likely due to differences in weed life history, timing of emergence, seed size, and nitrogen use efficiency. Another important factor influencing the weed community assembly in this long-term experiment is the legacy of good and bad weed control years. A better understanding of how management practices influence weed community dynamics will inform model predictions by weed ecologists and help practitioners make long-term weed management decisions.

Technical Abstract: Community assembly theory provides a useful framework to assess the response of weed communities to agricultural management systems and to improve the predictive power of weed science. Under this framework, weed community assembly is constrained by abiotic and biotic "filters" that act on species traits to determine community composition. We used an assembly approach to investigate the response of weed seed banks to 25 years of management-related filtering in three different row-crop management systems in SE Pennsylvania; organic manure-based, organic legume-based, and conventional. Weed seed banks were sampled in April of 2005 and 2006 and quantified by direct germination in a greenhouse. We also assessed the filtering effects of weed management practices and relationships between assembled seed bank and emergent weed communities by allowing or excluding weed control practices within each management system and measuring emergent weed community response. Germinable weed seed bank densities and species richness in the final year of the study were over 40% and 15% higher, respectively, in the organic systems relative to the conventional system. Seed bank community structure in the organic systems was different from the conventional system, and the relationships between assembled seed banks and the emergent flora varied. Primary tillage, weed control, timing of planting, and fertility management appeared to be the main filters that differentiated weed seed banks in the three systems. Weed life history, emergence periodicity, seed size, and responsiveness to soil fertility and hydrology appeared to be the most important functional traits determining how weed species responded to management-related filters. Our results suggest that management systems can exert strong filtering effects that can persist over relatively long (> one growing season) time scales. Legacy effects of community-level filtering may be more important than previously assumed, and should be incorporated into predictive models of weed community assembly.