Loamy Bottomland 25-32 PZ

Description

The Hardwood Grassland State is the Reference State. The information is derived from professional judgment of experienced range management specialists and from the Range Site Description, PE 31-40, approved July 25, 1972.

This is a mid-grass site producing limited amounts of tall grass adjacent to the stream channel. Scattered live oak, elm, blackgum, hackberry, and pecan trees are a significant part of the historic vegetation. Dominant grasses are vine mesquite, sideoats grama, switchgrass, and Canada wildrye. A wide variety of forbs is present in limited amounts.

The reference plant community on the Loamy Bottomland site is a hardwood midgrass complex supporting trees, a mixture of midgrasses and tallgrasses with excellent forb and shrub plant diversity. Species such as pecan (Carya illinioensis), live oak (Quercus virginiana), Chinkapin oak (Quercus muehlenbergii) and bald cypress (Taxodium distichum) are found along the stream bank and first bottom terrace, where recurring fires have kept the woody species in check. Elm (Ulmus spp.), hackberry (Celtis spp.) and American sycamore (Platanus occidentalis) are also common. Typical shrubs found at the site are bumelia (Sideroxylon spp.), elbowbush (Forestiera pubescens), Brickellbush (Brickellia spp.), Mexican buckeye (Ungnadia speciosa). Many vines such as grape (Vitis spp.) and greenbriar (Smilax spp.) are common in the woodland areas. Woody species decrease in density and canopy cover as the distance from the stream bank increased, taking on a savannah structure as the distance from the watercourse increases. Grasses, shrubs and forbs thrive in the open grassland and in the interspaces and beneath the trees, creating a complex of grassland and woodland. This pattern varies depending on soil, grazing regimen, and fire frequency. Wildfires which are postulated to have occurred at 7 to 12 year intervals in this region (Frost 1998) are thought to have exerted the greatest influence in shaping the reference state(Smeins, et.al 1997).

Sideoats grama (Bouteloua curtipendula), little bluestem (Schizachyrium scoparium), Indiangrass (Sorghastrum nutans), switchgrass (Panicum virgatum), eastern gamagrass (Tripsacum dactyloides) and wildryes (Elymus spp.) dominate the grassland. Maximilian sunflower (Helianthus maximiliani), bundle flower (Desmanthus spp.) and Engelmann’s daisy (Engelmannia peristenia) are important forbs. Numerous perennial forbs, grasses, shrubs and woody vines contribute to the diversity of the understory vegetation. Switchgrass and eastern gamagrass, along with numerous sedges (Carex spp.), spike rushes (Eleocharis spp.), flatsedges (Cyperus spp.) and brush species occur immediately adjacent to the watercourse. These species have a positive effect on stream bank stabilization during flooding events. The deep fertile soils and runoff from adjacent uplands and occasional flooding cause this site to be more productive than the climate dictates, creating a stable vegetation type. The grassland component makes up 60 to 70 percent of the herbage production.

Continued overgrazing of this site with livestock will cause a vegetation transition (retrogression) from midgrasses/tallgrasses to woodland with lower seral species and lower forage production. The transition to the woodland state can be halted or reversed by applying moderately intensive management practices like prescribed grazing, brush management, and prescribed burning until the woody component reduces burning effectiveness. The threshold for this occurrence is generally between 20 and 30 percent woody cover. It occurs when there is not enough fine fuel produced by the grass component to control or suppress the invading species. Once that threshold is breached, this State transitions to the Hardwood Woodland State (2).

Submodel

Description

The information for this community comes from experienced NRCS field specialists.
Continued livestock overgrazing and a lack of brush management causes a shift from a Hardwood Grassland State (1) with 15 to 35 percent woody canopy cover to a Hardwood Woodland State (2) with over 35 percent cover. During this retrogression process, there is a decline in the grassland component both from the grazing pressure and also from the effect of shading. The grasses shift to more shade tolerant cool-season species. With decreasing amounts of grass, there is a corresponding decrease in ground cover, litter, mulch, and soil organic matter. Soil structure declines and the exposed soil surface is subject to crusting and erosion. Litter and soil losses occur during floods.

The tree and brush canopy of the Hardwood Woodland State (2) exceeds 35 percent. With time and no tree or shrub control, the canopy can approach 100 percent canopy cover. Bald cypress, pecan, oaks, hackberry and elms dominate the overstory. Unless removed for lumber, bald cypress generally dominates along the stream edge. Pecan groves with inclusions of oaks, hackberry and elm are typical on second and third bottom areas. Most sites have had trees removed for lumber. Ashe juniper and sometimes mesquite form dense thickets. Texas persimmon is found along the outer. Common understory shrubs are yucca (Yucca spp.), elbowbush, Texas kidneywood (Eysenhardtia texana), ampelopsis (Ampelopsis spp.), grape (Vitis spp.) and Mexican buckeye. Mid and shortgrasses and low quality annual and perennial forbs and grasses replace the palatable midgrasses. Grasses that are common for this plant community include Texas wintergrass, Arizona cottontop (Digitaria californica), broadleaf woodoats (Chasmanthium latifolium) and buffalograss (Bouteloua dactyloides). The grasses and forbs in this plant community make up less than 25 percent of the annual biomass production. Common forbs include Aster (Aster spp.) white crown-beard (Verbesina virginica), giant ragweed (Ambrosia trifida) Ruellia (Ruellia spp.), orange zexmenia (Wedelia hispida), verbena (Verbena spp.), Western ragweed (Ambrosia psilostachya), prairie coneflower (Ratibida columnifera) and western indigo (Indigofera spp.).

Given enough time and reduction of the mid-story shrubs and trees, either with grazing or browsing animals, fire, or selective brush management, the overstory canopy will get high enough to allow light penetration. At this time, there will be some recovery of herbage production and warm season grasses.

Annual primary production of the Hardwood Woodland State (2) can range from as low as 2000 pounds per acre in dry years to over 5000 pounds per acre in good moisture years. Grasses and forbs provide less than 35 percent of this production. If the Hardwood Woodland State is the result of long term overgrazing by livestock, the remaining grass, forb and shrub species are generally low-quality forage plants, further reducing the value of this plant community as rangeland.

The tree and shrub canopy acts to intercept rainfall and increase evapotranspiration losses creating a more xeric microclimate and reducing soil moisture and infiltration. Soil fauna and litter are reduced exposing more soil surface to erosion in interstitial spaces until the canopy closes. Without major brush management and grazing management inputs, the Hardwood State cannot be reversed into the Hardwood Grassland State. It will continue to become dense woodland until it stabilizes with the climate and soil. Although this state provides good habitat cover for wildlife, only limited preferred forage or browse is available for livestock or wildlife.

Description

The information for this community comes from experienced field specialists.

Abandoned cropland areas, or cleared areas, are often seeded to historic native species or exotic naturalized grass species such as bermudagrass (Cynodon spp.) or Kleingrass (Panicum coloratum). Herbage production on those seeded to adapted introduced grasses or native grasses reach peak production within a few years if a full stand is established. In this case, herbage production will equal the reference state if species such as big bluestem or switchgrass are seeded. Adapted introduced species plantings may surpass reference production. The practice of including adapted legumes or other forbs will enhance productivity and usefulness, especially for wildlife.

Invasion of the seeded fields by brush species such as mesquite, pricklypear (Opuntia spp.), condalia (Condalia spp.), willow baccharis (Baccharis spp.), Texas persimmon and juniper is common. Drought and reduced soil cover due to cropping and grazing and a nearby seed source trigger the invasions. The shrub ‘seedlings’ that appear in seeded or abandoned fields are true seedlings established by seeds brought in by animals, water or wind. The invading brush must be controlled with grazing management, prescribed burning or other brush management methods or the woody invaders will again dominate.

Many fields, however, have been abandoned and let ‘go back’ to native range or to invading exotic grasses for pasture.