1. Department of Plant Biology, University of Illinois at Urbana‐Champaign, Urbana, IL 61802, USA;
2. Current address of author: Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA
3. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA;
4. Department of Biology, Ithaca College, Ithaca NY, 14850, USA;