Riparian Vegetation of the Lower Rio Grande

The riparian vegetation of the lower reach of the Rio Grande was studied at 7 locations using 3 line intercepts at each location. There were no trees at the mouth of the river and the vegetation was similar to that found along the Laguna Madre shore of barrier islands. Mesquite (Prosopis glandulosa) was the dominant tree near the coast and in the western section of the river near Falcon Dam. Sugar hackberry (Celtis laevigata) was the dominant tree species at all other sites except at Santa Ana National Wildlife Refuge, where cedar elm (Ulmus crassifolia) and anacua (Ehretia anacua) were the dominant trees. Granjeno (Celtis pallida) was a dominant shrub throughout the riparian corridor. The dominant trees and shrubs appeared to be replacing themselves. Species similarity in the tree, shrub, and ground layers was greater among transects at a given site than between sites. The introduced Guinea grass (Panicum maximum) and buffel grass (Pennisetum ciliare) were the dominant species in the ground cover, displacing native species. - La vegetación riparia de la sección baja del Río Bravo fue estudiada en siete localidades utilizando tres líneas de intercepción en cada localidad. No hay árboles en la desembocadura del río y la vegetación es semejante a la de la orilla de las islas barrera de la Laguna Madre. Mesquite (Prosopis glandulosa) es el árbol dominante cerca de la costa y en la sección occidental del río cerca de la Presa Falcón. El palo blanco (Celtis laevigata) es la especie de árbol dominante en todos los demás sitios excepto en el Refugio Silvestre Nacional de Santa Ana donde el olmo (Ulmus crassifolia) y la anacua (Ehretia anacua) son los árboles dominantes. Granjeno (Celtis pallida) es el arbusto dominante en todo el corredor ripario. Parece que los árboles y arbustos dominantes se están reemplazando. La semejanza de especies entre árboles, arbustos y cobertura de tierra es más alta entre transectos en un sitio que entre sitios. Las hierbas exóticas, Panicum maximum y Pennisetum ciliare, son las especies dominantes de la superficie terrestre, desplazando a las especies nativas

Phenology of Acacia berlandieri, A. minuata, A. rigidula, A. schaffneri, and Chloroleuconebano in the Lower Rio Grande Valley of Texas during a Drought

Flowering and fruiting phenology of Acacia berlandieri, A. minuata, A. rigidula, A. schaffneri, and Chloroleucon ebano were studied at 3 sites in the Lower Rio Grande Valley of Texas from July 1998 through August 1999. Severe drought conditions prevailed for the 6 months preceding this study, and rainfall was 20% lower during the study than the long-term mean. Acacia berlandieri had the longest flowering period (5 months); each of the other 3 Acacia species flowered for 3 months. All of the Acacia species flowered in winter or spring (at relatively low temperatures and increasing photoperiod). Peak flowering occurred in February in A. berlandieri, A. rigidula, and A. schaffneri. Peak flowering occurred in March in Acacia minuata. There was significant variation in percent flowering among months within A. minuata, A. rigidula, and A. schaffneri. Significant variation in percent flowering occurred among species during February, March, and April. Chloroleucon ebano flowered in only 1 month (September) following heavy rain. All of the Acacia species dropped their fruit before new fruit were developed, but C. ebano had mature fruit from the previous year and developing fruit on the same individuals. Few shrubs or trees of any species had fruit from November through April. There were significant differences in percentage of individuals with mature fruit among species in most months, and there was significant variation in percentage of shrubs and trees with mature fruit among sites within species. Acacia minuata and A. schaffneri showed significant positive correlations between percent fruiting and photoperiod and temperature. We suggest C. ebano flowers at higher temperatures than the Acacia species and at peak or declining photoperiod rather than during increasing photoperiod. - La floración y fructificación de Acacia berlandieri, A. minuata, A. rigidula, A. schaffneri y Chloroleucon ebano se estudiaron en 3 sitios del Valle del Río Bravo de Texas durante julio de 1998 hasta agosto de 1999. Condiciones severas de sequía se presentaron 6 meses antes de la investigación y durante el estudio la lluvia fue 20% menos que el promedio de precipitación a largo plazo. Acacia berlandieri tuvo el período de floración más prolongado (5 meses); las otras 3 especies de Acacia florecieron durante 3 meses. Todas las especies de Acacia florecieron en el invierno y en la primavera (en temperaturas relativamente bajas y en fotoperíodos crecientes). La máxima floración se efectuó en febrero en A. berlandieri, A. rigidula y A. schaffneri. La máxima floración de A. minuata fue en marzo. Hubo una diferencia significativa en la proporción de floración entre meses en A. minuata, A. rigidula y A. schaffneri, y una diferencia significativa en la proporción de floración entre especies en febrero, marzo y abril. Chloroleucon ebano floreció solamente en septiembre después de una abundante lluvia. Todas las especies de Acacia desprendieron su fruta antes del desarrollo de nueva fruta, pero C. ebano tenía fruta madura del año anterior junto con fruta desarrollándose en los mismos individuos. Pocos arbustos o árboles de cualquiera de las especies fructificaron desde noviembre hasta abril. Hubo diferencias significativas en el porcentaje de plantas con frutas maduras entre especies en la mayoría de los meses y variación significativa del porcentaje de plantas con frutas maduras dentro de especies entre sitios. Acacia minuata y A. schaffneri enseñaron correlaciones positivas y significativas entre el porcentaje de fructificación, fotoperíodo y temperatura. Sugerimos que C. ebano florece a temperaturas más elevadas que las especies de Acacia y en el punto máximo del fotoperíodo o cuando decrece en lugar del fotoperíodo creciente

Phytogeography of South Padre Island, Texas

The phytogeographic affinities of the native flora of South Padre Island, Texas, are analyzed and the probable dispersal agent(s) for each species identified. The native flora consists of 99 species and 44% of the species also occur on the adjacent Texas mainland; 28% of the native species have tropical affinities. Only three species have probably dispersed from the island to the mainland. Two species may be endemic to Padre Island and the Tamaulipan barrier islands. Most of the tropical species have reached the island by oceanic drift, but birds are the most important dispersal agent for the native species. Man is responsible for the introduction of 117 species

Effects of a Severe Freeze on Native Woody Plants in the Lower Rio Grande Valley, Texas

The effects of a severe freeze in December, 1983 on the native plants of the lower Rio Grande Valley, Texas were determined for 75 species. Damage was initially assessed by examining leaves, apical meristems, and cambial tissues of stems. Initial assessments were confirmed by surveys in the spring when new leaves were present. Forty-three species were either undamaged or had only minor leaf damage. Twenty-two species with damaged leaves had intact apical meristems or showed evidence of damage for less than 10 cm below the apical meristem. Three species, Leucaena pulverulenta, Cordia boissieri, and Karwinskia humboldtiana, had more extensive damage, i.e., 10 cm - 40 cm below the apex, but showed evidence of recovery by the presence of undamaged cambial and phloem tissues below the apex. Eight species, Avicennia germinans, Capsicum annuum var. glabriusculum, Cereus pentagonus, Chiococca alba, Citharexylum berlandieri, Erythrina herbacea, Iresine palmeri (male plants), and Petiveria alliacea, were seriously damaged. Temperatures lower than those occurring in December, 1983, or similar temperatures for much longer periods of time, would be necessary to kill most of the native species in the lower Rio Grande Valley, Texas

Comparison of the Effects of the Severe Freezes of 1983 and 1989 on Native Woody Plants in the Lower Rio Grande Valley, Texas

Freeze damage to native woody plants was assessed at the same four sites in the lower Rio Grande Valley, Texas, in 1983 and 1989. The number of severely damaged species was greater in 1989 possibly because temperatures in 1989 were lower and longer in duration below freezing versus 1983. Thirty0five species (48.6%) had the same damage index value for the two freezes, and 24 (33.3%) species showed no damage in either year. More than half of the 72 species analyzed (55.5% in 1983 and 51.3% in 1989) showed no damage or only leaf damage, indicating most of the native woody species are well adapted to survive periodic freezes. In 1989, Avicennia germinans, Cordia boissieri, Leucaena pulverulenta, Karwinskia humboldtiana, and Pithecellobium pallens included individuals killed to their bases. Heights of some of the L. pulverulenta exceeded 10 m. All species showed evidence of recovery by spring 1990

Annotated Checklist of the Flowering Plants of South Padre Island, Texas

This study catalogues the flowering plants of South Padre Island, Texas. A total of 207 species in 47 families is listed. The abundance and ecological distribution of each taxon are noted. Comparisons are made with the flora of Mustang Island, Texas. Ninety-three species, 96 genera and 36 families are common to both islands. Three families, Compositae, Gramineae and Leguminosae, contain large shares of the total number of species on both islands. Apparently, much of the floral diversity is due to the presence of a large number of species that are represented by small populations

Recovery of vegetation following a wild fire on the margins of tidal flats, Padre Island National Seashore, Texas

Recovery of halophytic vegetation on the margin of tidal flats following a wildfire at Padre Island National Seashore, Texas, was studied for 26 months. Species richness, species diversity, species importance, evenness and vegetation abundance were evaluated. Species richness and diversity were restored within 108 days after the fire. Sporobolus virginicus was the dominant species on both burned and non-burned transects. Vegetative cover was slow to recover. It took 19 months for cover of burned areas to equal or exceed cover of non-burned areas. Biomass also was slow to recover and it did not equal biomass of non-burned transects in 26 months. Species colonizing the burned transects were mostly species in nearby non-burned areas. Only one exotic species (Polypogon monspeliensis) appeared in the burned transects. The principal way that fire affected the tidal flats community was in the reduction of standing dead plants and surface litter

Use of Archive Aerial Photography for Monitoring Black Mangrove Populations

A study was conducted on the South Texas Gulf Coast to evaluate archive aerial color-infrared (CIR) photography combined with supervised image analysis techniques to quantify changes in black mangrove [Avicennia germinans (L.) L.] populations over a 26-year period. Archive CIR film from two study sites (sites 1 and 2) was studied. Photographs of site 1 from 1976,1988, and 2002 showed that black mangrove populations made up 16.2%, 21.1%, and 29.4% of the study site, respectively. Photographs of site 2 from 1976 and 2002 showed that black mangrove populations made up 0.4% and 2.7% of the study site, respectively. Over the 26-year period, black mangrove had increases in cover of 77% and 467% on sites 1 and 2, respectively. These results indicate that aerial photographs coupled with image analysis techniques can be useful tools to monitor and quantify black mangrove populations over time

Dune and Vegetation Stability at South Padre Island, Texas, United States of America

Padre Island is the longest of five barrier islands occurring along the Texas Gulf Coast. South Padre Island is separated from the northern two-thirds of the island by the Mansfield Channel. The composition and pattern of vegetation on South Padre Island are relatively well known, but data on the interrelationship of dune and vegetation stability are lacking. We hypothesized that (1) there should be an inverse relationship between elevation change and percent cover on transects across the nearshore dunes of South Padre Island; and (2) percent cover, species composition, and species importance should be most stable where elevation change was least. We tested these hypotheses using three study sites differing in vegetation abundance. Elevation measurements were taken at 1.0-m intervals along three transects at each site using survey-grade Global Positioning System equipment. Vegetation abundance was determined in 10-m intervals along each transect. No sites or topographic zones were devoid of elevation change. Even a site that had a mean percent cover of 65.1% had a mean elevation change of 15.6 cm. However, cover does not have to be great to provide considerable stability. There was no significant difference in elevation change at Site 1, where cover was 57%, and Site 2, where cover was only 12.5%. As hypothesized, there was a significant inverse correlation between elevation change and percent cover when analyzed over all transects and sites, but the relationship did not hold for all sites or topographic zones when these were examined separately. Lack of correlation may be due to differences among sites and zones in the number of different perturbations and their intensities and frequencies. Only Site 2 showed a significant difference in percent cover between the initial and final samples. Species composition and importance were more stable where elevation changes were low

Survivorship and Growth Among Three Developmental Stages of Black Mangrove (Avicennia germinans) Seedlings in Southernmost Texas

Black mangrove is useful for shoreline stabilization in Texas and Mexico but there is insufficient information regarding propagule survivorship and seedling growth. We compared survivorship and growth of three develop-mental groups of black mangrove seedlings planted on three dredge spoil islands in the Lower Laguna Madre of Texas, to better understand optimum planting strategy. Each spoil island had 100 seedlings without emergent radicles, 100 seedlings with emergent radicles, and 75 head-started seedlings. Survivorship of seedlings with and with-out emergent radicles was low, (0-16%). Survivorship of head-started seedlings was higher, (60 – 62.7%). Varia-tion in survivorship among developmental categories was significant but variation among islands was not. The 16 surviving seedlings with emergent radicles censused 346 days after planting were similar in mean height (32.9 cm) and mean number of pairs of leaves (13.2) to head-started seedlings grown for 376 days (105 days in nursery, 271 days on Island 1). Growth rates for height ranged from 0.041 cm/day to 0.058 cm/day. Growth in mean pairs of leaves ranged from 0.042 pairs/day to 0.051 pairs/day. Comparison of the person-days required to have 300 seed-lings alive one year after field planting shows that head-starting is clearly the preferable planting strategy