1. Variation in plant functional traits and demographic behaviour in populations of widely distributed species on geographic gradients may be key to understanding their geographic range limits. Oryza rufipogon, a wild progenitor of rice, occurs in discrete wetland populations over a wide latitudinal range in China. 

2. We examined correlations between plant functional traits, latitude, longitude and climatic variables in 34 populations of O. rufipogon across its northern range limit. We also investigated the responses of individuals from all populations after transplantation to two experimental gardens: one north of its range and the other in the extreme south. 

3. Seed mass in the field was positively correlated with latitude and longitude, whereas flag-leaf area was negatively correlated with latitude; both effects could be explained largely as a response to temperature. Transplantation revealed that both plasticity and population differentiation contributed to geographic variation. All populations reproduced and overwintered in the southern garden, but in the northern garden, only populations from the northern range were able to reproduce (65%) and survive the winter (58%). 

4. Synthesis: Environmentally determined variation in plant functional traits is complex, with both plastic trade-offs between traits and genetic differentiation between populations contributing to the location of the northern limit of Oryza rufipogon.