1. Treeline Ecological Research, Sherwood Park, AB T8E 1E3, Canada.
2. School of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA.
3. Department of Civil and Environmental Engineering, Tufts University, Medford, MA 02155, USA.
4. BC Hydro, Burnaby, B.C. V3N 4X8, Canada.