It can be hot and dry in summer in New Mexico. Not great for leaves.
"The loss of numerous leaves from trees is not uncommon in New Mexico in June." -- http://aces.nmsu.edu/ces/yard/1997/063097.html
"The loss of numerous leaves from trees is not uncommon in New Mexico in June." -- http://aces.nmsu.edu/ces/yard/1997/063097.html