Aim: Widespread species encounter a range of variable climates that can lead to intraspecific trait clines. Such clines can be the result of phenotypic plasticity, genetic differences, or both. Although latitude often explains a large part of trait variation, it is crucial to investigate the underlying environmental variables to understand current and future trait responses. Cross-continental comparisons of species that are native on multiple continents provide a rarely used approach that can help identify the environmental drivers of intraspecific trait clines.

Location: Europe and North America

Time period: 2021-2023

Major taxa studied: Milium effusum L. (Poaceae)

Methods: To quantify the influence of environmental gradients on functional traits across geographical regions that vary in climate, we sampled M. effusum seeds from 23 European and 14 North American populations and transplanted them in a common garden. We measured ten vegetative, reproductive and phenological traits. We used 30-year averages of nineteen bioclimatic variables, while accounting for the latitudinal and elevational position of the population origins, to compare the trait-environment relationships between continents.

Results: Our results showed that European populations occupy a broader climatic range than North American populations. Differences between continents were found in most of the traits as well as in the multivariate trait space. The traits were affected more by bioclimatic variables than by latitude or elevation. While flowering, leaf thickness, specific leaf area, and reproductive height showed parallel clines to the environment between continents, vegetative height and biomass showed contrasting clines.

Main conclusions: Environmental influences from population origins revealed parallel clines between the continents for functional traits, suggesting shared selective pressures, while contrasting clines for plant size indicated different evolutionary trajectories, potential bottlenecks, or interactions with unknown ecological factors. This study highlights the complex interplay of genetic, environmental, and evolutionary factors in shaping phenotypic variation in native species across continents.

Milium effusum (Poaceae) is a forest understorey grass, reaching heights of 50–150 cm, and flowering during June and July depending on its latitudinal and elevational location across its geographical range. Seeds have no particular dispersal adaptations, but dispersal over longer distances occurs occasionally by means of epi- and endozoochory. Stems, as well as the flat, smooth, hairless leaf blades and sheaths, ascend from decumbent bases. Re-sprouting through short stolons plays a role in perennation and in the case of M. effusum, which is clonal to a certain degree, vegetative growth can be viewed as a fitness proxy, since larger trait averages (i.e., greater vegetative proliferation) will typically be beneficial. Milium effusum is a shade-tolerant species, growing mostly in temperate deciduous woodlands, but can also occur in meadows, along railways and roads, and in rocky habitats. Milium effusum is native to large parts of the Northern Hemisphere, including most of Europe (subspp. effusum and alpicola), parts of North America (subsp. cisatlanticum), and in parts of Asia. Furthermore, both di-(2n = 14) and tetraploid (2n = 28) populations occur in Europe, while ploidy levels are unknown for North American populations. Milium effusum is classified as Least Concern (LC) globally.