Melilotus officinalis (yellow sweetclover) causes large changes in community and ecosystem processes in both the presence and absence of a cover crop

Abstract

Non-native species are hypothesized to decrease native species establishment and cover crops are hypothesized to decrease non-native species abundance. Although many studies have compared invaded to non-invaded habitats, relatively few studies have experimentally added non-native species to directly examine their effects. In a greenhouse mesocosm experiment, we tested the effects of non-native forbs (Melilotus officinalis, Verbascum thapsus, and Lespedeza cuneata), a proposed C3 grass cover crop (Pascopyrum smithii), and a commonly seeded non-native C3 grass (Bromus inermis) on the establishment of target native C4 prairie grass species. All treatments contained the same seed density of target C4 species and were begun on bare soil collected from the field. The legume M. officinalis strongly decreased the abundance of all other species, species diversity, and light and soil moisture levels. Surprisingly, M. officinalis took up relatively large amounts of labeled nitrogen (15N) from the soil early in its development, but M. officinalis fixed nitrogen, thus increasing nitrogen in biomass nearly fivefold by the end of the study. We found few effects of either C3 grass species on non-native forbs or C4 target species, but seeded P. smithii did increase species diversity. Nonnative plants therefore impeded native C4 grass establishment through long-lasting effects of target species seedbank depletion (death of most target seedlings) and altered nutrient availability. The effects of M. officinalis were not reduced by the presence of a cover crop. Abstract Non-native species are hypothesized to decrease native species establishment and cover crops are hypothesized to decrease non-native species abundance. Although many studies have compared invaded to non-invaded habitats, relatively few studies have experimentally added non-native species to directly examine their effects. In a greenhouse mesocosm experiment, we tested the effects of non-native forbs (Melilotus officinalis, Verbascum thapsus, and Lespedeza cuneata), a proposed C 3 grass cover crop (Pascopyrum smithii), and a commonly seeded nonnative C 3 grass (Bromus inermis) on the establishment of target native C 4 prairie grass species. All treatments contained the same seed density of target C 4 species and were begun on bare soil collected from the field. The legume M. officinalis strongly decreased the abundance of all other species, species diversity, and light and soil moisture levels. Surprisingly, M. officinalis took up relatively large amounts of labeled nitrogen ( 15 N) from the soil early in its development, but M. officinalis fixed nitrogen, thus increasing nitrogen in biomass nearly fivefold by the end of the study. We found few effects of either C 3 grass species on non-native forbs or C 4 target species, but seeded P. smithii did increase species diversity. Non-native plants therefore impeded native C 4 grass establishment through long-lasting effects of target species seedbank depletion (death of most target seedlings) and altered nutrient a...

Assembly history alters alpha and beta diversity, exotic–native proportions and functioning of restored prairie plant communities

“…Overall, the percentage of C 4 species was much higher in spring timing plots when the prairie mix was added simultaneously compared to all other treatments. Our results are consistent with Dickson et al (2010), who found that an exotic legume was able to out-compete native perennial C 4 prairie grasses because it established earlier. If exotic species green up earlier, have stronger priority effects and have higher aboveground growth rates than natives (Leishman et al 2007;Wilsey, Daneshgar & Polley 2011;Dickson, Hopwood & Wilsey 2012), then exotics may have large and underappreciated consequences for predicting community assembly history outcomes (Wolkovich & Cleland 2011). Variation in abiotic conditions between spring and summer timing treatments suggests that early invaders can modify their environments and contribute to community divergence.…”

“…Above-ground productivity was higher in the more diverse plots containing a greater proportion of native species that resulted from the assembly history treatments. Our results support the few existing studies (Foster & Dickson 2004;Ko¨rner et al 2008;Fukami et al 2010;Dickson et al 2010) that found that community assembly history has ecosystem-level ramifications.…”

“…Our results provide stronger support for assembly history than for competition–colonization mechanisms in structuring restored tallgrass prairie communities in areas of high exotic propagule pressure (Dickson et al . ). Competition–colonization theory predicts that a single equilibrium will develop as long as propagules of a competitive species are available.…”

“…Our results are consistent with Dickson et al . (), who found that an exotic legume was able to out‐compete native perennial C 4 prairie grasses because it established earlier. If exotic species green up earlier, have stronger priority effects and have higher above‐ground growth rates than natives (Leishman et al .…”

Biodiversity, phenology and temporal niche differences between native- and novel exotic-dominated grasslands

“…We observed that species diversity was greater in nativedominated than exotic-dominated fields in central Texas, a trend similar to that reported elsewhere for invasions by Agropyron cristatum (Christian and Wilson, 1999), Imperata cylindrica (Brewer, 2008), 13 invasive species in Europe (Hejda et al, 2009), Melilotus officionale (Dickson et al, 2010) and exotic C 3 grasses (Miles and Knops, 2010). Our observational data provide some support for the passenger model in that exotic species did not invade and drive down species diversity at all sites.…”