Spatiotemporal patterns of rising annual plant abundance in grasslands of the Willamette Valley, Oregon (USA)

Abstract

Abstract Context Plant communities are undergoing compositional changes that affect ecosystem function. These changes are not always uniform across the landscape due to heterogenous topographic and edaphic conditions. To predict areas most at risk of change, it is necessary to identify the landscape drivers affecting plant abundance. Objectives Annual plants are increasing across the western USA, largely driven by non-native annual invasions. Here, we quantified change in annual plant abundance and identified landscape factors contributing to that change over the past 35 years. Methods We focused on Willamette Valley (Oregon) grasslands because they represent a new example in this phenomenon. To understand the spatiotemporal patterns of annual plant abundances between 1986 and 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 30 × 30 m pixel and regressed cover against heat load, soil depth and sand content for > 5975 hectares to determine areas most sensitive to rising annual cover. Results We found a tendency toward increasing annual cover, with a median gain of + 15% cover among significantly increasing pixels. However, change was uneven across the landscape, with annual 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 cover. Annual plant invasions may be lagging in this region compared to elsewhere in the western USA, but trends here suggest it may just be a matter of time.