Climate-driven in-situ trait variation in an annual ruderal grass across Europe

•Background and Aims: Plant functional traits link environmental conditions to plant performance and adaptation. Growing evidence suggests that intraspecific trait variation can be as important as differences between species, yet large intraspecific studies of in-situ variation remain rare. While most studies have focused on plant morphological traits, the concentrations of elemental nutrients in seeds have received much less attention so far.
•Methods: We conducted a large-scale in-situ study of the widespread annual ruderal grass Hordeum murinum. We sampled 2070 individuals from 207 populations across a large part of its native range in Europe and North Africa. We measured seed ripening phenology and growth-related traits in-situ and analyzed concentrations of elemental nutrients in the seeds.
•Key results: We found that Hordeum murinum grew larger, produced seeds later, and had heavier seeds in colder and wetter regions. Plants growing in denser vegetation were taller and produced heavier seeds but formed fewer spikes. Concentrations of elemental nutrients in the seed generally declined with seed weight and were primarily driven by climatic variables, whereas soil conditions had only minor effects on plant traits and seed nutrients. Population identity explained a substantial proportion of trait variation, indicating a possible genetic component.
•Conclusions: Our findings provide a comprehensive view of how Hordeum murinum responds to environmental gradients across its European distribution. Climatic variables, particularly temperature, are key drivers of reproductive timing and concentrations of elemental nutrients in the seed, whereas local environmental conditions, such as biotic pressures, are more critical for growth-related traits. Together, these patterns indicate that Hordeum murinum modulates its growth and reproductive investment along environmental gradients, balancing phenology, stress tolerance, and limited competitive capacity.