PREMISE: Dominant in many ecosystems around the world, clonal plants can reach considerable ages and sizes. Due to their modular growth patterns, individual clonal plants (genets) can consist of many subunits (ramets). Since single ramets do not reflect the actual age of genets, the ratio between genet size (radius) and longitudinal annual growth rate (LAGR) of living ramets is often used to approximate the age of clonal plants. However, information on how the LAGR changes along ramets and how LAGR variability may affect age estimates of genets is still limited. METHODS: We assessed the variability of LAGR based on wood-section position along the ramets and on the duration of the growing season on three genetically distinct genets of Salix herbacea growing in the Northern Apennines (Italy). We compared genet ages estimated by dividing genet radius by the LAGRs of its ramets. RESULTS: LAGR increased significantly from the stem apex to the root collar; indicating that ramet growth rate decreased with time. Furthermore, a difference of ca. 2 weeks in the onset of the growing period did not impact LAGR. Considering the high LAGR variability, we estimated that the three genets started to grow between ~2100 and ~7000 years ago, which makes them the oldest known clones of S. herbacea even considering the most conservative age estimate. CONCLUSIONS: Our findings indicate that analyzing ramets at the root collar provides an integrative measurement of their overall LAGR, which is crucial for estimating the age of genets.

Department of Agricultural and Forest Sciences and Engineering University of Lleida Lleida Spain

Department of Chemistry Life Sciences and Environmental Sustainability University of Parma Parma Italy

Department of Geography Masaryk University Brno Czech Republic

Department of Geography University of Cambridge Cambridge UK

Forest Biometrics Laboratory Faculty of Forestry Stefan cel Mare University of Suceava Suceava Romania

Global Change Research Centre Brno Czech Republic

Institute of Biosciences and BioResources Firenze Italy

Joint Research Unit CTFC AGROTECNIO CERCA Solsona Spain

Lancaster University Lancaster Environment Centre Lancaster UK

Swiss Federal Research Institute WSL Birmensdorf Switzerland

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