Plant biodiversity assessment through soil eDNA reflects temporal and local diversity

Several studies have shown the potential of eDNA‐based proxies for plant identification, but little is known about their spatial and temporal resolution. This limits its use for plant biodiversity assessments and monitoring of vegetation responses to environmental changes. Here we calibrate the temporal and spatial plant signals detected with soil eDNA surveys by comparing with a standard visual above‐ground vegetation survey. Our approach compares vegetation in an old‐growth boreal forest in southern Norway, surveyed in 100 permanent 1‐m2 plots seven times over a 30‐year period, with a single soil eDNA metabarcoding‐based survey from soil samples collected at the same 100 plots in the year of the last vegetation survey. On average, 60% and 10% of the vascular plants and bryophytes recorded across all vegetation surveys were detected by soil eDNA. Taxa detected by soil eDNA were more representative for the local taxa pool than for the specific plot, and corresponded to those surveyed over the 30‐year period although most closely matched the current taxa composition. Soil eDNA detected abundant taxa better than rare ones although both rare taxa and taxa unrecorded by the visual survey were detected. Our study highlights the potential of soil eDNA assessments for monitoring of vegetation responses over broad spatial and temporal scales. The method's ability to detect abundant taxa makes it suitable for assessment of vegetation composition in a specific area and for broad‐scale plant diversity assessments.