AVIAN SPECIES RICHNESS IN DIFFERENT-AGED STANDS OF RIPARIAN FOREST ALONG THE MIDDLE RIO-GRANDE, NEW-MEXICO

Riparian forests are important for maintaining vertebrate species rich ness in the southwestern United States, but they have become restricte d in distribution due to both historical and current management practi ces. In order to counteract continued loss of this habitat, several mi tigation programs were developed in the middle Rio Grande Valley of Ne w Mexico. Three areas ranging from 50 to 140 ha were revegated with na tive trees using pole planting and cattle exclosures, and changes in v egetation structure were quantified after 2, 3, and 5 years of growth. As expected, the older site contained the most heterogeneous mix of p lant species and the greatest structural diversity. We compared year-r ound avian use of the revegetated sites with a mature cottonwood fores t site of approximately 30 years of age. As the revegetated sites matu red and salient habitat features changed, the population dynamics of i ndividual avian species and patterns of guild structure varied. The ol der revegetated sites showed a greater similarity to the mature cotton wood site, suggesting that reclamation efforts established quality rip arian habitats for birds in as little as 5 years. The revegetated site s appeared especially important for neotropical-migrant birds. We sugg est that a mosaic of riparian woodlands containing mixtures of native tree and shrub species of different size classes is necessary to maint ain avian species richness in the middle Rio Grande drainage, and prob ably throughout the southwestern United States. .