Cedar Elm Ulmus crassifolia (Cedar elm) is a drought tolerant Texas native,with numerous dark green leaves which rank among the smallest of the native Elms. The tree is a deciduous perennial and sheds the bulk in the leaves on schedule in the fall. This leaf drop is signaled by environmental cues that predict the onslaught of winter. Other changes in the environment such as lack of water, or shading of leaves in the interior of the canopy can also trigger leaf drop through out the year. The dropping of leaves is termed abscission and it is preceded by leaf death which is termed senescence. This link explains how these natural processes work in the life of the plant.