Introduction and Background
In 2002, the Chihuahuan Desert Network supported a two-year inventory of the vascular plants in Amistad National Recreation Area (NRA) by the Wildlife Diversity Program of the Texas Parks and Wildlife Department. Three Texas Ecoregions converge within Amistad NRA: South Texas Plains (mixed Tamaulipan shrublands), Edwards Plateau (oak-juniper woodlands), and Trans Pecos (sotol-lechuguilla Chihuahuan desert scrub). Plant communities are influenced by climate, geology, soils, topography, and hydrology. Amistad NRA is primarily an arid environment with springs, rivers, and a large reservoir, created by the Amistad Dam, that contributes to the presence of more mesic (moderately moist) vegetation than is typically found in this region.
Amistad NRA vegetation is impacted by previous and current land use in and around the area. Fire suppression may have led to an increase in woody vegetation and ranching was common throughout the area. “Improvements” to the range and disturbances associated with development have also shaped the land. Disturbances include removing woody, native vegetation and seeding nonnative grasses for forage and associated impacts. Some areas adjacent to the low-lying land lining the Rio Grande may have been used for irrigated row crops, but are now inundated by Amistad Reservoir. Areas along the changing edge of the reservoir are disturbed.
A 2002 list of plant communities proposed by park staff presents several vegetation alliances present in Amistad NRA: three woodland, three shrubland, two herbaceous, and two sparse (rock outcrop or butte and open cliff sparse alliances). Prior to 2002, only one inventory of plants was conducted in Amistad NRA and it was limited to the recreation area managed by Laughlin Air Force Base; no comprehensive inventory of vascular plants has been completed at Amistad NRA.
Sampling locations were chosen based on experience with Amistad NRA flora, topography and hydrology with the goal of locating the most taxa to create a comprehensive park list of vascular plants. In total, 106 potential sampling sites were identified and 69 were selected as high priority due to being very diverse botanical areas and/or were large areas of vegetation that were not inundated (flooded periodically) by the reservoir. In the end, 61 sites were visited, including 57 of the original 69 priority sites and four additional sites; twelve priority sites were not sampled due to inaccessibility or other issues.
Prior to sampling, the researcher compiled lists of plant species potentially occurring in Amistad NRA from park specimen collections created in the 1970s and in 2001, existing surveys in nearby state-managed areas, literature searches, and herbarium searches at University of Texas at Austin, Texas A&M University, and Sul Ross State University.
The site location and species frequency based on visual estimates were recorded for each species encountered. Voucher specimens were collected for species that had not been previously vouchered and when positive identification could not be made in the field. When possible, two vouchers were collected for inclusion in the University of Texas at Austin and Sul Ross State University herbariums. Vouchers were compared to the herbarium collections to check for accuracy and range.
A total of 582 vascular plant taxa were observed during the survey. Through the literature and herbarium searches of specimens previously collected in Amistad NRA, an additional 124 vascular plant taxa were located, though they were not observed during this inventory. A total of 123 species, including five nonnative species, were deemed highly likely to occur, but were not found during this survey. The researcher determined that 116 species listed on the 2002 potential species list either do not occur or are highly unlikely to occur in Amistad NRA; generally because of misidentifications or taxonomic changes or discrepancies in range.
Forty taxa found during thee survey were new to the park and 17 species of conservation concern were detected. Forty-two nonnative species were observed, and an additional seven nnonnative species were documented in Amistad NRA prior to this survey. Nonnatives account for 7% of the vascular species in the park.
Most of the plants observed in Amistad are encountered rarely (63%; found at less than 10 sites) or infrequently (17%; found at 10–25 sites). A total of 16% of species were considered common (found at 26–40 sites) and 2% were considered frequent (found at 41–50 sites) or abundant (found at more than 50 sites). The ten most common species were native woody plants. Nonnative grasses, such as King Ranch bluestem (Bothriochloa ischaemum) and Bermuda grass (Cynodon dactylon), are in the top 20 most common species and highly prevalent in the reservoir flooding zone.
A total of 76 plant communities were identified in Amistad NRA, 48 of which fall within the broad plant alliances proposed by park staff in 2002. Many of the 28 communities that did not fit within the plant alliances proposed were disturbance and/or wetland, aquatic, or intermittently flooded communities.
The Vachellia rigidula-Leucophyllum frutescens-Senegalia berlandeieri (blackbrush acacia-cenizo-Guajillo) shrubland plant alliance was the largest and most widely spread plant community observed in Amistad NRA, occurring at almost every upland site; 26 plant communities were observed in this alliance. The Hilaria belangeri-Bouteloua curtipendula (curly mesquite-sideoats grama) herbaceous vegetation alliance was not found, but eight other grassland communities were identified during the survey. Nearly all of these communities consisted of nonnative species or native species of poor forage quality, indicating the areas were severely overused. The Quercus gravesii-Pistacia mexicana (Graves oak-Texas pistachio) woodland was present at Amistad NRA, a truly unique community for the United States, though it is common in Mexico. .
Despite many areas that are altered by the reservoir, Amistad NRA has very high plant diversity that may be related to its large geographic range spanning three different ecoregions. Many plants are highly threatened by nonnative plant and animal species (e.g., large numbers of free-ranging livestock). Nonnative plants thrive on the disturbance areas where the reservoir level rises and recedes.
Several areas visited in Amistad NRA during this inventory are of interest. The Devils River area upstream of Indian Cliffs Canyon and the wetland and upland canyon areas of Satan Canyon are unique to the area and contain many rare species. The plateau liveoak grove areas of Satan Canyon, Lowry Springs, Live Oak Creek, Seminole Canyon, and Oak Motte in Hunt Area 4 represent communities at the edge of their range and should be monitored. The Graves oak-Texas pistachio woodland community in Pink Cave Cove is very rare and should also be monitored. The upper portion of the Pecos River above the reservoir and the uplands and cliff face of the Pecos River picnic and boat launch area are unique and should be restored.
The resulting vascular plant taxa and community list for Amistad NRA is largely based on single site visits. Future efforts will improve this list and allow for greater knowledge and protection of the park.
Poole, J.M. 2004. An inventory of the vascular plants at Amistad National Recreation Area. Wildlife Diversity Program, Texas Parks and Wildlife Department: Austin, Texas.
Jackie Poole, Wildlife Diversity Program, Texas Parks and Wildlife Department, Austin, TX, USA, firstname.lastname@example.org
Prepared by the Chihuahuan Desert Network Inventory and Monitoring Program, updated in 2018.
Last updated: November 25, 2018