Management Filters and Species Traits: Weed Community Assembly in Long-Term Organic and Conventional Systems

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 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.

Impacts of Dryland Farm Management Systems on Weeds and Ground Beetles (Carabidae) in the Northern Great Plains

“…Compared to no-till conventional fields where crop residues may impede carabid movement; tillage in organic fields may affect the carabid activity positively by reducing crop residues and hence their capture rates could be higher [52]. Bengtsson et al [9] and Ryan et al [53] also found a greater abundance of weed seeds, insect pests and, concomitantly, more carabid beetles in organic systems compared with conventional systems. By contrast, previous studies have suggested that increased soil disturbance due to the fact that tillage in organic fields may reduce ground beetle habitat suitability [47] and bury weed seeds, thereby excluding seed-preying beetles [54], and increasing the beetle larvae and adult mortality [55].…”

Impacts of Organic Zero Tillage Systems on Crops, Weeds, and Soil Quality

“…Research regarding the effects of ZT on weeds in conventional systems might be used to anticipate consequences of ZT in organic systems for processes such as weed seed distribution within the soil profile. However, other observations from ZT studies that include herbicide applications may need reconsideration when applied to organic systems since herbicide use is such a strong filter on weed community assembly processes [59][60][61]. Likewise, research results from conventional vs. organic tillage studies oftentimes will differ because organic fields tend to have greater weed species density and diversity than fields managed conventionally [6,[62][63][64][65].…”

“…Conservation tillage also favors the spread of perennial weeds [69,70]. Previous research has demonstrated increases in winter annual weed species in organic systems [60], and increases in troublesome perennial species such as dandelion (Taraxacum officinale Weber) and Canada thistle (Cirsium arvense L.) in organic conservation tillage systems [53,68]. For these reasons, organic ZT systems may be particularly prone to winter annual and perennial weed problems.…”

“…Even within an organic system, the choice of N source may have impacts on weed communities. For instance, Ryan [60] showed that weed seed bank species in organic systems that included manure but not hairy vetch were more similar to conventional system seed bank species (indicative of higher N) than organic systems that excluded manure but included hairy vetch. These differences could have been due to a filtering effect of the hairy vetch, weed seed contamination of manure, or N differences.…”

Timing of Tillage as a Driver of Weed Communities

“…Hence, viewed within a community assembly framework (sensu Booth and Swanton 2002), tillage is a strong assembly "filter" that can either constrain or advance the membership of species within the subsequent weed community (Ryan et al 2010;Smith 2006).…”