Maintaining and increasing native grass cover is a common goal in California grassland restoration. When established, native communities can suppress invasion of the noxious weeds goatgrass (Aegilops triuncialis) and medusahead (Elymus caput-medusae), due to their phenological overlap of soil moisture use. However, native grasslands are composed of small-scale vegetation patches, and how the dynamics at the edges of patches influence suppression of invasives is not well understood. Competitive interactions differ between the center and edge, which may allow less-competitive species, such as goatgrass and medusahead, to persist at the edge and establish in the area. Precipitation is also a major driver of competitive outcomes. We investigated a) the spatial patterns of noxious weeds in native-planted communities, b) how those spatial patterns interact with temporal priority of 1 vs 2 years, and c) how they are affected by precipitation treatments. Ten year old experimental grassland plots, which were seeded with various mixtures of native, naturalized, and noxious invasive plants under normal, wet, and drought precipitation treatments, were allowed to be naturally invaded by plants included in the study design and measured with spatially explicit composition sampling of the plots (centers vs. edges).
Results/Conclusions
Noxious weed cover was similarly low in the edge as in the core of treatments originally seeded with native species across all precipitation treatments and regardless of the natives being given one or two years of priority. While all native seeded plots had low noxious weed cover ten years later, the plots in the dry precipitation treatments had significantly lower cover, possibly due to goatgrass and medusahead’s poor performance in dry soils. These results suggest that when given priority, even of just one year, native grasses can persist and suppress noxious weed invasion in the long term, across a variety of precipitation levels. Despite the potential of patch edges to serve as a refuge for invaders, native patches are resistant on their edges as well as their more dominant core.