Germination Adjustment to Local Temperatures in a Patagonian Invader

ABSTRACT Understanding how invasive species adjust to temperature variation is critical to predicting their responses to climate change. We tested for population‐level adjustment in germination responses of the invasive species Hieracium pilosella L. (Asteraceae, syn. Pilosella officinarum Vaill.) along a 120‐km climatic gradient in Tierra del Fuego Island, Argentina. We collected seeds from six populations varying in mean growing season temperature and exposed them to five constant temperatures (5°C, 10°C, 15°C, 20°C, and 25°C) in growth chambers. We analysed the interaction between population temperature (PT) and chamber temperature (CT) as a test of local adjustment, hypothesizing that populations from warmer sites would germinate best at warmer experimental temperatures. Germination percentage, our focal demographic rate, was strongly influenced by a positive PT × CT interaction (standardised coefficient = 0.27), supporting the hypothesis of local adjustment. This clinal pattern likely reflects a combination of mechanisms, with rapid local adaptation and maternal environmental effects being the most plausible contributors. Notably, our observed germination rates (up to 80%) greatly exceeded those reported in earlier studies, possibly reflecting apomictic reproduction, ploidy variation, or hybrid origin in Fuegian populations. These traits may facilitate rapid germination responses and enhance invasion potential under warming scenarios. Given projected warming and drying in the region, H. pilosella is likely to benefit from climate change, especially if native species cannot match its rapid germination adjustment. More broadly, our analytical approach yields a continuous measure of population adjustment strength that is easily comparable across gradients and species, unlike traditional analyses based on the population‐identity × CT interaction. Quantifying interactions between field collection conditions and experimental environments offers a valuable framework to assess environmental adjustment and potential evolutionary responses in invasive species.