Harsh winters are a characteristic element of Arctic ecosystems. Yet, winter conditions are often neglected in climate change impact studies, and analyses comparing the importance of winter vs. summer conditions on Arctic plant communities based on field-quantified microclimate measurements are lacking. Here, we use fine-scale microclimate data with plot-scale records of vascular plants, lichens and bryophytes from three Arctic areas, and show that topographically heterogeneous tundra holds marked spatial variation especially in winter temperatures. Winter conditions are the strongest environmental variable related to the fine-scale patterns in tundra vegetation, whereas summer temperatures mainly explain the coarse-scale differences among the Arctic areas. However, the three species groups, and also individual species, show often contrasting and complex responses along the local environmental gradients. Our results highlight the importance of local conditions and heterogeneity for tundra plants and call for joint research efforts to incorporate these complex relationships into more realistic simulations of tundra vegetation under climate change.