1. Rangeland Resources and Systems Research Unit USDA‐ARS Fort Collins CO 80526 USA
2. Agronomy College Qingdao Agricultural Univ. Qingdao 266109 China
3. Dep. of Civil and Environmental Engineering Princeton Univ. Princeton NJ 08544 USA
4. Plant Science Research Unit USDA‐ARS Raleigh NC 27695 USA
5. Center for Agricultural Resources Research USDA‐ARS Fort Collins CO 80526 USA