Matt Ryan (Penn State University )
Weed community assembly in a long-term organic and conventional cropping systems trial: linking management-related filters and weed species traits

Community assembly theory states that communities assemble from pools of species (i.e. the seed bank), and that this process is constrained by abiotic (management, climate etc.) and biotic (pathogens, competition, etc.) filters that determine community composition and structure.  The objective of this study was to test for community assembly processes operating on weed seed banks in long-term organic and conventional cropping system trials, and to rank the importance of potential filters constraining assembly in these systems. Results suggest that (1) organic and conventional systems exert differential effects on the assembly of weed seed banks, (2) herbicides are stronger filters on the assembly of weed seed banks than mechanical or cultural management practices, (3) assembly of seed banks is strongly constrained by seed inputs occurring the previous season; however, over time, “legacy-effects” may reduce the importance recent seed inputs, and (4) weed emergence periodicity and responsiveness to soil fertility are the most important functional traits determining how communities assembled.