Weed community assembly in cover crops: What is the relative importance of niche processes?
Cover crops are increasingly being used to provide weed suppression services to agroecosystems, however, the relative role that niche-based processes play in suppressing weeds is unclear. If niche processes mediate the weed-suppressive effects of cover crops, then the potential exists to “guide” the assembly of weed communities through careful selection of cover crops or cover crop mixtures. We examined evidence for niche-based weed community assembly in a field experiment conducted in Durham, NH. This experiment was conducted over two growing seasons in two sites (four site-years total), included eight cover crop species representing a variety of plant functional groups each grown in monoculture in a randomized complete block design (n=4). In each treatment we measured cover crop biomass and the composition and abundance of the weed community that assembled from the weed seed bank in the late fall and the subsequent spring. We used several multivariate approaches, including non-metric multidimensional scaling (NMDS), PerMANOVA, and indicator species analysis (ISA) to examine the “fall” and “spring” weed communities for evidence of cover crop species- or functional group-specific effects on assembled weed communities.
We found little evidence to support the hypothesis that individual cover crop species suppress weeds via niche-based processes under realistic agronomic situations. Only one cover crop species, tillage radish, was associated with a fall-assembled weed community that was distinct from the other cover crop treatments (PerMANOVA: P < 0.05). Spring-assembled weed communities exhibited more differentiation based on cover crop treatment (PerMANOVA: P < 0.05); however, ISA revealed only weak cover crop-weed species associations for 11 weed species (i.e. no IV > 25). These results suggest that cover crops do not necessarily act as particularly strong or predictable biotic filters during the assembly of weed communities in annual cropping systems and that stochastic processes may play a larger role in determining weed community composition at the plot and field scale than has previously been assumed.