Robert I. Lonard

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.

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. A series of barrier islands comprise the predominant land form along the Gulf Coast of Texas. Padre Island is the longest of these barrier islands. It extends approximately 182 km from Corpus Christi on the north to the BrazosSantiago Pass at Port Isabel on the south. The Mansfield Channel separates the northern two-thirds of the island from South Padre Island. The flora and vegetation of South Padre Island is relatively well known compared to other areas of the barrier island chain (Dahl et al. 1974, Judd et al. 1977, Lonard et al. 1978). Lonard et al. (1978) reported that the vascular flora consisted of 209 species in 47 families and suggested that much of the floral diversity is due to the presence of a large number of species that are represented by small populations. Judd et al. (1977) provided information on the pattern of vegetation in relation to topography and reported that the vegetation occurs in distinct zones that correspond closely to topographic facets. Information on the geographic affinities of the flora is currently lacking. Although the mechanism(s) involved in the origin of the barrier islands is a subject of considerable controversy, workers agree that Padre Island began forming about 4,500 years ago (Dahl et al. 1974). Thus, the flora is post Wisconsin glaciation. Because of the proximity of Padre Island to the Texas mainland (maximum width of the Laguna Madre is about 16 km) and because of the length of the island, one might reasonably expect that many of the species present have been derived from the mainland. However, similar islands occur in the Gulf of Mexico south of Padre Island to Altimira, Tamaulipas, Mexico, 402 km south of Brownsville, Texas. These islands provide an avenue of northward dispersal for numerous organisms. Also, the longshore current is from south to north along South Padre Island and the Mexican barrier islands. Thus, species dispersed by drift would likely have tropical 313 NOVEMBER 14, 1980 This content downloaded from 207.46.13.57 on Thu, 08 Sep 2016 05:01:42 UTC All use subject to http://about.jstor.org/terms affinities. Furthermore, South Padre Island is situated in the flyway of many migratory birds that winter in Mexico and Central America. The purpose of this paper is to analyze the phytogeographic affinities of the flora of South Padre Island and to identify the major dispersal agents. This information should facilitate understanding the floristic relationships of Gulf Coast barrier islands in particular and contribute to knowledge of barrier island ecology in general. SOURCE MATERIAL. Description of the area, climate, and soil is provided in Dahl et al. (1974) and Judd et al. (1977). The flora of South Padre Island catalogued by Lonard et al. (1978) formed the basic list of taxa. Seven additional species have been collected subsequent to publication of that paper and have been included. Designations of topographic zones follow Judd et al. (1977). Much of the information on the distibution of the species found on South Padre Island was obtained from Correll and Johnstons Manual of the Vascular Plants of Texas (1970). Goulds (1975) Grasses of Texas provided information on the distribution of grasses. Ranges of continental taxa were taken from several sources, but McGregor et al.s (1977) Atlas of the Flora of the Great Plains was used extensively. Distributions of Gulf and Atlantic Coast species were primarily obtained from Duncans (1974) review of coastal halophytes. Britton and Millspaughs (1920) treatment of the Bahaman flora was consulted for taxa of Caribbean occurrence. Checklists of Taylor (1950) and Chippendale (1972) were consulted to determine the occurrence of pantropical or subcosmopolitan distributions. Thorne (1972) emphasized the importance of consulting recent monographic revisions and range extension reports when phytogeographic affinities are considered. Therefore, the following monographic treatments were examined for changes in nomenclature and for species ranges: Kral (1971), Sauer (1972), Lewis and Oliver (1974), Gillis and Steam (1974), Graham (1975), Palmer (1975), Rodman (1976), Smith (1976), Hopkins and Blackwell (1977), and Semple and Semple (1977). Nomenclature, in general, follows Correll and Johnston (1970) and Gould (1975). RESULTS AND DISCUSSION. Seven species have been added to the list of vascular species occurring on South Padre Island subsequent to publication of Lonard et al.s (1978) flora. These species are: Atriplex pentandra, Suaeda tampicensis, S. torreyana, Malva parviflora, Orobanche multiflora, Cooperia drummondii, and Dichanthium aristatum. Thus, 216 species of flowering plants are known to occur on South Padre Island. Judd et al. (1977) recognized six topographic zones on South Padre Island: Foreshore (FS), Backshore (BS), Primary dunes (PD), Secondary dunes and vegetated flats (SDVF), Tidal flats (TF), and Washovers (WO). Lonard et al. (1978) added Disturbed sites (DS) and noted the distribution in these zones for each species. One hundred seventeen species comprising 54.2% of the total flora are found in disturbed sites (Appendix). These areas are limited to the southernmost 8 km of the island and are associated with buildings, residences, and roads. Because we are primarily interested in the geographic affinities and the dispersal agents of the naturally occurring flora, the species introduced by man are not considered further. It should be emphasized, however, that man has been the single most important dispersal agent. Our survey indicates that 99 species comprise the naturally occurring flora of South Padre Island. Table 1 provides an inventory of the geographic affini-

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.

The biological flora of coastal dunes and wetlands: Spartina patens (W. Aiton) G.H. Muhlenberg.

Abstract Spartina patens (W. Aiton) G.H. Muhlenberg is a New World tropical-, subtropical-, and temperate-zone rhizomatous grass that often is a dominant species in coastal environments. It occurs in habitats ranging from foredunes to saturated soil conditions in high marshes and swales. Herein, we present a review of the extensive literature of this important species. Spartina patens occurs where salinity ranges from 1 to 27 ppt, but it will tolerate salinities up to 40 ppt for brief periods. It tolerates moderate tidal inundation, low nutrient levels, and low to moderate sand coverage. Also known as marshhay cordgrass, it may account for almost all net productivity in high marshes on the Gulf of Mexico coast of Louisiana. High marshes dominated by S. patens are affected by flooding depths associated with global warming and rising sea levels. It is a valuable species for coastal restoration projects for it quickly invades dredged material and storm washover sites. Spartina patens provides cover for wildlife and forage for livestock.

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

Abstract 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.

Biological Flora of Coastal Dunes and Wetlands: Uniola paniculata L

Abstract Uniola paniculata C. Linnaeus is a New World, tropical-, subtropical-, and temperate-zone, rhizomatous grass that is often a dominant species on foredunes and on the primary dune complex. Uniola paniculata L. has low freshwater-moisture requirements, tolerates aerosol salt spray, has low nutrient level requirements, tolerates high sand temperatures, and provides extensive sand coverage. Also known as sea oats, this grass inhabits mobile and semistable dune systems and plays a significant role in trapping sand and building primary dunes. Caryopsis production is low. However, germination is enhanced by cutting into the endosperm and leaching the caryopses. Northern populations require cold treatment before germination, but southern populations are not dependent on cold treatment before exposure to a germination protocol with a 35/25°C thermoperiod.

The Biological Flora of Coastal Dunes and Wetlands: Distichlis spicata (C. Linnaeus) E. Greene

ABSTRACT Lonard, R.I.; Judd, F.W., and Stalter, R., 2013. The biological flora of coastal dunes and wetlands: Distichlis spicata (C. Linnaeus) E. Greene. Distichlis spicata (C. Linnaeus) E. Greene is a New World temperate-, subtropical-, and tropical-zone rhizomatous, dioecious grass. It is an important species in coastal salt marshes, brackish marshes, and tidal wetlands, where it is often a dominant species. Inland ecotypes of this taxon occur in wet alkaline sites throughout temperate regions of the United States, Mexico, and South America. Distichlis spicata often occurs in wetlands, where total soluble salts seasonally range from about 20 to 43 ppt and where the pH varies from 5 to 7. Higher salinities in the rhizome–root matrix reduce shoot growth more than root growth. Also known as salt grass, D. spicata is a clonal stress tolerator that spreads laterally into disturbed sites in the upper marsh, and it recovers quickly from burial by wrack debris after storms. A typical water level range for salt grass is from 10 to 15 cm below to 5 cm above the marsh surface, but it can tolerate tidal amplitudes ranging from 1.36 to 1.74 m for 1 hour on some coastlines. Pistillate plants typically dominate lower elevations in tidal marshes, whereas higher elevations in marshes support populations dominated by male plants. Herein, we present a review of the biology of this important species.

The Biological Flora of Coastal Dunes and Wetlands: Batis maritima C. Linnaeus

Abstract Batis maritima C. Linnaeus (maritime saltwort) is a New World subtropical and tropical trailing subshrub that forms dense colonies in salt marshes, brackish marshes, and mangrove swamps and frequently is found on the margins of salt pans and wind-tidal flats. It typically occurs at elevations less than 1.0 m above mean sea level and at sites where salinity ranges from 18 to 50 ppt. Leaf succulence increases significantly in the dry season and leaves are shed, thereby reducing salt-induced stress. Batis maritima occurs in sites normally subject to minimal sand coverage. However, wrack deposits stimulate growth. Maritime saltwort provides cover and nesting sites for some species of birds, but glucosinolate compounds in shoots make the plants unpalatable to most large vertebrates, with the exception of marine iguanas in the Galapagos Islands. Recently, B. maritima is proposed to have a major role in reducing ozone levels in the stratosphere.

The Biological Flora of Coastal Dunes and Wetlands: Panicum amarum S. Elliott and Panicum amarum S. Elliott var. amarulum (A.S. Hitchcock and M.A. Chase) P. Palmer

Abstract Panicum amarum S. Elliott var. amarum and P. amarum var. amarulum (A.S. Hitchcock & M.A. Chase) P. Palmer are New World tropical-, subtropical-, and temperate-zone rhizomatous varieties of a grass species that is important on coastal foredunes and primary dune topographic facets. Herein, we present a review of the biology of these important taxa. Panicum amarum has low freshwater moisture requirements, and it tolerates low nutrient levels, high sand temperature, and extensive sand coverage. Also known as bitter panic grass, it inhabits mobile and semistable dune systems, and it plays a significant role in trapping sand and building primary dunes. Panicum amarum quickly recolonizes coastlines where all vegetation has been removed by storm surges.

Biological Flora of Coastal Dunes and Wetlands: Heterotheca subaxillaris (J. de Lamarck) N. Britton & H. Rusby

Abstract Heterotheca subaxillaris (J. de Lamarck) N. Britton & H. Rusby is a New World aromatic, pioneer species that rapidly colonizes disturbed, xeric habitats on temperate and subtropical coastal dunes. Also known as camphorweed, H. subaxillaris has low moisture requirements, and it tolerates low nutrient levels and high sand temperatures. Dimorphic achenes have a dual germination strategy. Disc achenes are nondormant, and seeds germinate over a wide range of temperature conditions, whereas ray achenes are dormant at maturity and require after-ripening before seed germination. Heterotheca subaxillaris competes well with other species in disturbed sites, but growth and reproduction are inhibited when it is in competition with dominant dune grasses.