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 yr of management-related
filtering in three different row-crop management systems in southeastern
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 (greater than one growing season) time scales.
Legacy effects of community-level filtering might be more important than
previously assumed, and should be incorporated into predictive models of
weed community assembly.