In the arid southwestern United States and northern Mexico, riparian corridors have long served as key wildlife habitat. The wetlands and naturally sub-irrigated uplands adjacent to rivers and streams provide vital food, water, and cover for hundreds of species of breeding and migratory birds. However, these corridors were also the most attractive for development leading to significant loss of riparian habitat. New research coordinated through the Gulf Coast Prairie Landscape Conservation Cooperative (GCP LCC) evaluated efforts to restore riparian corridors and the implications for native wildlife populations.

European settlers were drawn to riparian corridors for their potential to support agriculture and grazing. The lower Rio Grande valley, one of the largest and most productive corridors has been particularly affected. Extensive areas of forest and brush lands were cleared to make way for row crops, pastures with non-native grasses, cities, and transportation infrastructure. In recent decades, rapid population growth, urban sprawl, and border security measures have added to the impacts on wildlife.

The US Fish and Wildlife Service began efforts to slow and reverse the impacts of habitat modification on the lower Rio Grande valley over 20 years ago. The Service purchased agricultural or other heavily disturbed lands within the corridor and re-vegetated/restored these lands by planting native species in an effort to recreate pre-development conditions. Woodlands with drastically different structure and species composition have developed over time, depending on species planted, initial treatments, soils, proximity of native forest, seed bank, and other variables.

In 2012, the GCP LCC identified the need to assess the results of efforts to date and the response of wildlife populations to re-vegetated/restored areas. This information was needed to determine what approaches were working best in order  to inform future restoration efforts. With support from WMI, the GCP LCC funded a study by Dr. Tim Brush and Dr. Patricia Feria of the University of Texas-Pan American located in the lower Rio Grande valley. Dr. Brush and Dr. Feria used several methods to contrast remnants of existing mature riparian corridor forest habitats with areas subject to past and ongoing re-vegetation/restoration treatments. The researchers also evaluated responses of select bird species to the differences between habitats to provide guidance for future re-vegetation efforts.

Dr. Brush and Dr. Feria worked over two field seasons to gather data on plant community structure in a variety of habitats representing different stages of re-vegetation/restoration. Given the level of human development in the lower Rio Grande, the researchers also included patches of forest within urban areas among the habitats examined. In some cases, these urban forests represented the most intact mature native plant communities over a large area. Plant community variables measured included foliage height profiles and canopy cover, leaf litter quality, number and size of woody plants, distance to non-wooded areas, open water or resaca edge, and distance to the nearest mature forest stand.

The researchers selected nine species of birds as indicators of mature riparian forests and 16 other species as indicators of various stages of plant community recovery. Seasonal point counts were conducted at 92 different locations over a study area that spanned 120 miles of the Rio Grande valley to monitor use of the various habitats throughout the year.

Results of the project demonstrate that re-vegetated sites have developed plant communities that are attractive to common breeding bird species. Many of the tracts that are 15-25 years old support similar bird communities to mature stands. The researchers did find that stands with higher ground cover in grasses and less in leaf litter were less favored by several of the key indicator bird species. In addition to limiting grass invasion of sites, results show that developing tall riparian forest stands will require significant water management except in low areas directly adjacent to the Rio Grande. The research also determined that declining bird species that are dependent on mature forest and thorn scrub remain rare in the lower Rio Grande valley. This may be due to the limited amount of re-vegetated habitat in the area. Finally, the study found that patches of mature forest in urban areas are readily used by several focal species, once thought to be restricted to non-urban areas. Efforts to restore mature forest in urban areas may be more beneficial than previously thought.

Dr. Brush and Dr. Feria will present a webinar on the study on May 6, 2015 and the final report is available on the GCP LCC website. (cs)