Spatiotemporal patterns of rising annual plant abundance in grasslands of the western Pacific Northwest, USA

Abstract Context: Plant communities are undergoing compositional changes that affect ecosystem function. Invasions are not always uniform across the landscape due to heterogenous topographic and edaphic conditions. To predict areas most at risk of invasion, it is necessary to identify the landscape drivers affecting plant abundance. Objectives Non-native annuals have been increasing across the western USA. Here, we quantified the change in annual plant abundance and identified landscape sensitivities contributing to that change over the past 35 years. Methods We focused on Willamette Valley (Oregon) grasslands because they represent a new horizon in this phenomenon. To understand the spatiotemporal patterns of annual plant abundances between 1986–2020, we combined a remote-sensing vegetation cover dataset from the Rangeland Analysis Platform with gridded soils data and topographic variables. We determined the rate of change in percent cover for each pixel and regressed cover against heat load, soil depth, and sand content to determine landscape sensitivity through time for > 5,975 hectares. Results We found a tendency toward increasing annual plant cover, with a median gain of + 15% cover among pixels experiencing a significant increase. However, change was uneven across the landscape, with annual plant cover increasing markedly in areas with high heat load and shallower soils. Conclusions We identified steep, south-facing slopes as being particularly sensitive to rising annual plant cover. Non-native annual invasions may be lagging in this region compared to what has already occurred in California and the Great Basin, but trends here suggest it may just be a matter of time.