Becoming Bosque - Testing the benefits of mesquite restoration

Abstract

This study evaluated ecological conditions of three types of mesquite-dominated habitats on a site along the San Pedro River in Arizona. Naturally-occurring vegetation at the site includes forests of large mesquite trees near the river; young mesquite thickets that re-colonized a retired agriculture field on a nearby river terrace; and upland desert scrub habitats on hillsides away from the river. Riparian and upland mesquite habitats have been studied before, though seldom at the same site; habitat values of young terrace thickets have been mostly undocumented. Comparisons of vegetation, birds, and ecohydrology across these three stand types – mature mesquite bosque, second-growth riparian mesquite thickets, and upland mesquite scrub — were designed to better understand natural habitat gradients, and to evaluate the success of restoration at re-establishing mesquite bosques on disturbed river terraces. The study confirmed dramatic differences between mature riparian bosque and upland mesquite scrub stands, as has been documented elsewhere. Findings in young thickets were novel. In areas with appropriate growing conditions, we found that restoration was effective at recreating bosque-like habitat in as little as 20 years. Most of the site’s young thicket stands (last plowed in 2002) already have vegetation structure and breeding bird assemblages that are more similar to mature bosques than to upland scrub stands. We attribute this rapid recovery to shallow groundwater that is within the reach of deep mesquite roots, a plentiful supply of seeds on adjacent lands, and management designed to encourage natural recruitment of native plants. We also tested some targeted thinning practices to see whether this type of intervention could accelerate the full return to bosque form and function. Results suggest that selective removal of relatively small and densely-packed mesquite trees, aimed at replicating tree densities of mature bosques, can be compatible with managing for bosque wildlife. However, thinning was expensive, and its effects may be short lived. Where natural regeneration is successful, intensive treatments such as thinning may not be needed.