Date of Award

5-2015

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

Abstract

Shrub encroachment into grasslands is a phenomenon facilitated by fire suppression, climate warming, and overgrazing of grasses by cattle, and if left unmanaged, can alter ecosystem structure and function. In this study, I compare the effectiveness and cost/benefit of various restoration strategies (prescribed fire, mechanical, and chemical) used singly and in different combinations at decreasing two shrubs, honey mesquite (Prosopis glandulosa Torr.) and huisache (Acacia farnesiana (L.) Willd.), and promoting growth and survival of gulf cordgrass (Spartina spartinae (Trin.) Merr. ex Hitchc.), the dominant coastal prairie grass within the Laguna Atascosa National Wildlife Refuge-Bahia Grande Unit in deep south Texas. The U.S. Fish and Wildlife Service (USFWS) currently manages this unit for the Federally–endangered aplomado falcon (Falco femoralis septentrionalis), a grassland-dependent species. This study is unique as it evaluates shrub reduction, shrub resprouting, and grass recovery in response to eight treatment regimes using up to three restoration strategies in various orders. These responses were assessed at 4-mo intervals over a 12-mo post-treatment period and then ranked using a dimensionless scale to create a ‘success index’ comparable across treatments. By 12-mo post-treatment, all treatments caused a similar reduction in original stem basal area and density. This response was immediate when treatments used rollerchopping, but more gradual following a single prescribed fire. Treatments where herbicide followed roller-chopping most effectively killed adult shrubs and limited resprouting. Gulf cordgrass recovery was slowest when roller-chopping was the only or last treatment, likely because roller-chopping created leaf litter and woody debris that hindered grass seed germination. Overall, index results suggest that roller-chopping followed by herbicide then fire was the most effective treatment for reducing woody shrubs while facilitating gulf cordgrass recovery, while a single prescribed fire was least effective. Though more costly than other treatments, roller-chopping followed by herbicide then fire may provide land managers with a restoration strategy for both removing woody shrubs and restoring gulf cordgrass cover in coastal prairie ecosystems.

Granting Institution

University of Texas Brownsville

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