Rio Grande Delta Thorn Woodland and Shrubland

Geology

Quaternary alluvium.

Landform

Sites within the historic floodplain of the Rio Grande delta, typically on slight rises such as old natural levees or resaca banks.

Soils

Often on Clayey or Loamy Bottomland Ecological Sites, but occasionally on Clay Loam or Gray Sandy Loam types.

Parent Description

This diverse, usually broad-leaved evergreen, woodland is found on resaca banks and old natural levees on the Rio Grande delta. Sites are well-watered, somewhat elevated relative to the surrounding landscape, and tend to occupy loamy or clayey bottomland soils. Occasionally occurrences can be found on clay loams (such as Raymondville or Racombes soils) or gray sandy loams (such as Hidalgo sandy clay loam). The sometimes patchy canopy of these woodlands often contains species such as Ebenopsis ebano (Texas ebony), Ehretia anacua (anacua), Celtis laevigata (sugar hackberry), Ulmus crassifolia (cedar elm), and Celtis ehrenbergiana (granjeno), and may reach heights of 15 m. Species such as Phaulothamnus spinescens (snake-eyes), Amyris madrensis (Sierra Madre torchwood), Amyris texana (Texas torchwood), Diospyros texana (Texas persimmon), Leucaena pulverulenta (tepeguaje), Guaiacum angustifolium (guayacan), Malpighia glabra (Barbados cherry), Adelia vaseyi (Vasey's adelia), Bernardia myricifolia (oreja de raton), Sideroxylon celastrinum (la coma), Condalia hookeri (brasil), Forestiera angustifolia (desert olive), Havardia pallens (tenaza), Iresine palmeri (Palmer's bloodleaf), Trixis inula (tropical trixis), Xylosma flexuosa (brush- holly), and Randia rhagocarpa (crucillo) may occur as shrubs or in the sub-canopy, and some individuals of a few of these species may reach heights of 4 to 5 meters. This shrub or understory layer can be extremely dense, almost impenetrable. Woody cover, including the patchier overstory canopy and the almost continuous shrub/understory layer, often reaches greater than 90%. This system is sometimes referred to as a tall shrubland, since shrubs are often the dominant lifeform, but frequently reach heights resembling the stature of woodland. Prosopis glandulosa (honey mesquite) may occasionally be absent or uncommon in the canopy, and is generally not dominant except in disturbed situations. The herbaceous layer is generally represented by a only a few species and is relatively sparse, with species such as Rivina humilis (pigeonberry), Plumbago scandens (climbing plumbago), Celosia nitida (West Indian cock's comb), Chromolaena odorata (crucita), Leersia monandra (bunch cutgrass), Digitaria californica (Arizona cottontop), Setaria spp. (bristlegrasses), Salvia coccinea (tropical sage), Petiveria alliacea (hierba de las gallinitas), Malvastrum americanum (Rio Grande false-mallow), Urtica chamaedryoides (slim stinging nettle), Verbesina microptera (southern frostweed), Calyptocarpus vialis (straggler daisy), and Justicia pilosella (hairy tubetongue) sometimes present. Vines such as Serjania brachycarpa (littlefruit slipplejack), Urvillea ulmacea (apaac), Cocculus diversifolius (orientvine), Mikania scandens (climbing hemp-weed), Cardiospermum spp. (balloon-vines), Chiococca alba (David's milkberry), Cissus trifoliata (ivy treebine), and Passiflora spp. (passionflowers) may also be commonly encountered. The rather rare epiphyte Tillandsia baileyi (Bailey's ballmoss) may be found in these woodlands, along with the more common Tillandsia recurvata (ballmoss) and Tillandsia usneoides (Spanish moss). Younger occurrences, especially those occupying drier sites, tend to present as shrublands, often dominated by similar, though shorter, canopy species. These occurrences also tend to be less diverse, lack the layered structure, and usually support fewer epiphytes. This system differs from the related Tamaulipan Floodplain system in that it has higher diversity, a significant evergreen component to the canopy, a higher subtropical component to the species assemblage, is restricted in range to the Rio Grande delta and vicinity, and often occurs as slight rises in the otherwise relatively level landscape.