United States 

Department of 

Agriculture 

Forest Service 

An Old-Growth Definition for 

Southwestern Subtropical 

Upland Forests 

Southern David D. Diamond 

Research Station 

A Section of the Old-(irowlih Definition Serie 
The Author: 

David D. Diamond, Director, Missouri Resource 

Assessmnt Pamership, Environmental Technolog y 

Center, 4200 New Haven Road, Columbia, MO 65201. 

Preface 

Old growth is widely acknowledged today as an essential part of managed forests, particularly on public lands. However, this 

concept is relatively new, evolving since the 1970's when a grassroots movement in the Pacific Northwest began in earnest to 

define old growth. In response to changes in public attitude, the U.S. Department of Agriculture, Forest Service, began 

reevaluating its policy regarding old-growth forests in the 1980's. Indeed, the ecological significance of old growth and its 

contribution to biodiversity were apparent. It was also evident that definitions were needed to adequately assess and manage the 

old-growth resource. However, definitions of old growth varied widely among scientists. To address this discrepancy and other 

old-growth issues, the National Old-Growth Task Group was formed in 1988. At the recommendation of this committee, old 

growth was officially recognized as a distinct resource by the Forest Service, greatly enhancing its status in forest management 

planning. The committee devised "The Generic Definition and Description of Old-Growth Forests" to serve as a basis for 

further work and to ensure uniformity among Forest Service Stations and Regions. Emphasis was placed on the quantification 

of old-growth attributes. 

At the urging of the Chief of the Forest Service, all Forest Service Stations and Regions began developing old-growth 

definitions for specific forest types. Because the Southern and Eastern Regions share many forest communities (together they 

encompass the entire Eastern United States), their efforts were combined, and a cooperative agreement was established with 

The Nature Conservancy for technical support. The resulting project represents the first large-scale effort to define old growth 

for all forests in the Eastern United States. This project helped bring the old-growth issue to public attention in the East. 

Definitions will first be developed for broad forest types and based mainly on published information and so must be viewed 

accordingly. Refinements will be made by the Forest Service as new information becomes available. This document represents 

1 of 35 forest types for which old-growth definitions will be drafted. 

In preparing individual old-growth definitions, authors followed National Old-Growth Task Group guidelines, which differ 

from the standard General Technical Report format in two ways-the abstract (missing in this report) and the literature 

citations (listed in Southern Journal of Applied Forestry style). Allowing for these deviations will ensure consistency across 

organizational and geographic boundaries. 

September 1998 

Southern Research Station 

P.O. Box 2680 

Asheville, NC 28802 
An Old-Growth Definition for 

Southwestern Subtropica and Forests 

David D. Diamonrl 

Old-growth Southwestern subtropical upland forests, broadleaved and mostly evergreen, are restricted to the Lower Rio 

Grande Valley of south Texas, primrily Cameron County, 

southern and western Hidalgo County, and extreme 

southern Willacy and Starr Counties. This forest type, 

although restricted to a small geographic area, was 

apparently once the prevailing upland vegetation of the 

lower valley. It occupied moist uplands and resaca (ox-bow) 

terraces over heavy-textured soils (mostly silty clay loam 

and silty clay Mollisols and Alfisols), which are now almost 

entirely in row-crop production. This evergreen low forest 

formed landscape mosaics with related floodplain hardwood 

forests dominated by deciduous species such as sugarberry 

(Celtis laevigata Willd.), [nomenclature follows Correll and 

Johnston (1970)], cedar elm (Ulmus crassifolia Nutt.) , and 

Berlandier ash (Fraxinus bertadieri A. DC.) (Clover 1937; 

Inglis 1964; Gonzalez-Mekano 1972; Bush and Van Auken 

1984; Jahrsdoerfer and Leslie 1988; Wood and Wood 1988, 

1989; Vora 1990'). A more restricted Texas palmetto (Sabal 

mexicam Mart.)SLominated forest once occupied the Rio 

Grande floodplain downstream from Brownsville but 

probably never extended far upstream (Clover 1937, Davis 

1942, Williams and Allday-Bondy 1979). 

The prevailing climate of the lower valley is warm with a 

frost-free period averaging more than 330 days. Although 

the area has suffered at least two severe freezes since 

December 1983, native species are typically not severely 

damaged or killed (Lonard and Judd 1985, 199 1). Average 

precipitation ranges from 20 inches [50 centimeters (em)] in 

the west to 30 inches (75 cm) in the east with no 

pronounced sumer drought. However, high temperatures 

during the sumer months cause water stress on upland 

sites during most years (Clover 1937). The Lower Rio 

Grande Valley is underlain mostly by Pleistocene and some 

Quaternary deposits (Wynd 1944). To the north, these 

deposits are overlain by eolian sands that support tall 

"eck, R.W.; Riskind, D.H. Undated. Biotic communities of southern 

Texas and nmheastern Mexico--potential biotic communities and 

anthropogentc alterations. 42 p. Unpublished report. On file with: David D. 

Diamond, Director, Missouri Resource Assessment Partnership, 

Environmental Technology Center, 4200 New Haven Road, Columbia, MO 

6520 1. 

grasslands with live oak mottes (Johnston 1955, 1963; 

Dimond and Fulbright 1990), whereas, to the west and 

northwest, these deposits grade gradually into older, 

generally sandier deposits, higher and drier landscapes, and 

drier, cooler climates that support shrubland or deciduous 

thorn woodland vegetation (Huss 1959, McMahan and 

Inglis 1974, Drawe et al. 1978, O'Brien 1980, Archer et al. 

1988, McLendon 199 1'). Both to the north and to the west, 

rangeland is the dominant land use, whereas, the lower 

valley is dominated by cropland and urban development. 

Texas ebony [PithecellobiumJlexicaule (Benth.) Coult.] is 

always among the leading dominants of this forest type. 

Since few old-growth stands exist, and few or none have 

ken quantitatively sampled, the status of secon&ry species 

is difficult to assess. Anacua [Ehretia amcua (Tertin & 

Berland.) I.M. Johnst.] is probably most often the second 

most important overstory tree in high-quality stands. Oidgrowth shrub components include brasil (Coadalia hookeri 

M.C. Johnst.), colima [anthoxylumfagara (L.) Sarg. 1, 

snake-eyes (Phaulothamus spinescens Gray), and coma 

(Bumelia eelastrim H.B.K.). These species may reach 16 

feet [5 meters (rn)] in height, which may represent the upper 

canopy on dry sites. Granjeno (Celtis pallida Ton.), 

sugarberry, cedar elm, and mesquite (Prosopis glandulosa) 

may be important but are also important in younger forests 

or mottes (woody vegetation clusters within an herbdominated area). 

Periodic flooding and fluctuations in the Rio Grande delta's 

water regime, along with catastrophic drought, were 

probably the dominant influences on this forest type (Clover 

1937, Inglis 1964, Drawe et al. 1978). Fire may not have 

played an important role in shaping this rnesic c~munity.~ 

The importance of smaller disturbances has not been 

Everitt, J.H.; Gonzalez, C.L; Gerbermann, A.H. Undated. Botanical 

composition of eleven south Texas rangeland sltes. 27 p. Unpublished 

report. On file with: David D. Diamond, Director, Missouri Resource 

Assessment Pmnership, Environmental Technology Center, 4200 New 

Haven Road, Columbia, MO 65201. 

' Hanselka, C.W.; White, LD. Undated. Fire: history, effects, and use 

in the Tamaulipan Biotic Province. 17 p. Unpubltshed report. On file with: 

David D. Diamond, Director, Missouri Resource ?tssessment Partnership, 

Environmental Technology Center, 4200 New Haven Road, Columbia, 

MO 6520 1. 
Table 1 (English unitsbstandardized table of old-growth attributes for southwestern subtropical upland forests from 

Vora (1P90) for six stands (0.247-acre plots) on Santa Am National Wildfife Refuge (these are the only quantitative 

data on this forest type) 

Value 

Range Mean Notes 

Stand density (no-lacre) 

-trees >2 in. d.b.h. 

Stand basal area (ft2/acre) 

Age of large trees (years) 

Number of 4-in. size classes 

-starting at 4 in. d.b.h. 

D.b.h. of largest trees (in.) 

-all species 

Mesquite (Prosopis 

glandulosa Ilbrr.) 

Texas ebony 

[Pithecellobiurn 

flexicaule (Benth.) 

Coult.] 

Standing snags 

Decadent trees 

Number of canopy layers 

Percent canopy in gaps 

Other features 

Canopy height (ft) 

65-1 10 Not reported Values by stand not reported; qualitative 

observations indicate some stands with much 

higher stem density 

Not measured -- Overstory canopy cover varied from 30 to 100 

percent 

Not measured Not measured 

Not reported Not reported Largest trees were 21 inches d.b.h., indicating 

some stands had as many as five 

Not reported Values reported are mxhum size across all six 

stands 

Not reported Not reported 

Not measured Not measured 

2 Shrub canopy measured from 40 to 85 percent; 

"shrubs" often reach to the upper canopya 

Not measured Not measured Qualitative observation shows few gaps in 

mature standsb 

Cedar elm (Utrnus crassifolia Nutt.) was reported 

16-43 -- among the canopy dominants; important "shrubs" 

included Texas ebony, coma (Burnetla cehstrirta 

H.B.K.), guayacan [Guaiacum angustqoliurn 

(Engelm.) Gray], anacua [Ehretia anacua (Terrin 

& Berland.) I.M. Johnst.], brasil (Condalia 

hookeri M.C. Johnst.), and colima [Zanthoxylum 

fagara (L.) Sarg. ] 

a See under text headings "Narrative of Old-Growth Conditions" and "Forest Dynamics and Ecosystem Function." 

See under text heading "Forest Dynamics and Ecosystem Function." 
Table 1 (metric units)-Standardized table of old-growth attributes for southwestern subtropical upland forests from 

Vora (1991)) for six stands (0.10-hectare plots) on Santa Ana National Wildlife Refuge (these are the only quantitative 

data on this forest type) 

Value 

Quantitative attribute Range Mean Notes 

Stand density (no.lha) 

-trees >5 cm d.b.h. 

160-380 Not reported Vdues by stand not reported; qualitative 

observations indicate some stands with much 

higher stem density 

Stand basal area (m2/ha) Not measured -- Overstory canopy cover varied from 30 to 100 

percent 

Age of large trees (years) Not measured Not measured 

Number of 10-cm size classes Not reported Not reported Largest trees were 53 cm d.b.h., indicating some 

-starting at 5 em d.b.h. stands had as many as five 

D.b.h. of largest trees (cm) 

-all species 

Mesquite (Prosopis 

gtandulosa Ton.) 

Texas ebony 

[Pithecellobiurn 

JZexicaule (Benth.) 

Coult.] 

Standing snags (noha) 

Decadent trees 

Number of canopy layers 

Percent canopy in gaps 

Other fea~res 

Canopy height (m) 

Not reported Values reported are maximum size across all six 

stands 

Not reported Not reported 

Not measured Not measured 

2 Shrub canopy measured from 40 to 85 percent; 

"shrubs" may reach to the upper canopya 

Not measured Not measured Qualitative observation shows few gaps in 

mature standsb 

Cedar elm (Ulmus crassvolia Nutt.) was reported 

among the canopy dominants; important "shrubs" 

included Texas ebony, coma (Bumelk eekstrina 

H.B.K.), guayacan [Cuaiacum angustgolium 

(Engelm.) Cra y], macua [Ehretia anactka (Terdn 

& Berland.) I.M. Johnst.], brasil (Condalia 

hookeri M.C. Johnst.), and coiima [Zanthoxylum 

fagara (L.) Sarg.] 

" See under text headings "Narrative of Old-Growrh Conditions" and "Forest Dynamics and Ecosystem Function." 

'See under text beading Forest Dynamics and Ecosystem Function.'" 
documented, althou* Archer et al. (1988) have show the 

hpmnce of the interaction of fire reduction and mesquite 

seed dispersal by domestic cattle. This interntion has 

briggered rapid change in the modern landscape from 

myam& to thorn wocrdland in relaied c 

north. The processes that cone01 suc~ession on floodplains 

have been discussed by Bush and Van A&en (1986% 

1986b, 1987). Southwestern subtropical upland forests are 

easily distinguished from other forests in the southeast. 

They have broad-leaved, subtropical evergreens mong the 

dominant species, and they occur only in south Texas 

(table I). 

Associated Society of American Foresters Cover Types: 

None 

Physiographic Provinces: 

West-Gulf Coastal Plain 

Narrative of Old-Growth Conditions 

Diamond et al. (1987) and Diamond ( 1992) broke upland 

subtropical evergreen forests into two series for 

classification based on prevailing dominant species and 

differences in physiognomy: the Texas ebony-anacua 

series; and the Texas ebony-snake-eyes series. The former 

is described as a well-developed forest, whereas, the latter is 

a low forest grading into shrubland. These series represent a 

continuum of related comunities that occupy uplands of 

the Lower Rio Grande Valley-the former on well-watered 

sites and the latter on drier sites. Both axe listed by Diamond 

et al. ( 1987) as among the most endangered cornunity 

types of Texas. The Texas Natural Heritage Program's 

Element Occwence data base lists six examples of the 

Texas ebony-snaire-eyes series; and eight occurrences of 

Texas ebony----macua series in fair or better condition. 

Many of the observations made here and elsewhere are 

based on personal visits to these sites along with re\lrews of 

the literature. 

Upland evergreen forests of the Rio Grande Valley grow on 

a range of sites: some only slightly better drained than 

floodplain hardwoods and others almost too dry to support a 

forest. Texas ebony appears to be the leading dominant on 

all the best sites. 

Canopy height reaches to over 50 feet ( 15 m) on moist sites, 

with ebony reaching more than 24 inches (60 cm) in 

diameter at breast height (d,b.h.). Remant ebony trees in 

residential sections of Brownsville and a few individuals in 

Santa Ana National Wildlife Refuge 

30 inches (80 en) d.b.h. On drier up1 

reach heights of more than 26 feet (8 m). Even on the driest 

sites that suppfl this type, ebony can form a nearly 

monmulture canopy at about 13 feet (4 m). These low 

forests have flat, interloeking upper canopies and essentially 

no middle story or ground cover and are usually open 

gh to crawl or walk kough, dthough stem density can 

e wabg difficult. "Shrub" species may be present but 

often grow from single stems and reach the upper canopy at 

13 to 16 feet (4 to 5 m). The edges of these low forests are 

sornethes nearly impssible to penetrate due to vigorous 

growth of shrubs between forests and cropland or roads. 

Taler, moister forests grow in mosaics with deciduous 

floodplain forests, as at Santa ha NWR (Vora 1990). 

Although this type is mapped on about one-third of Santa 

ha refuge (Vora 1990), I am not convinced that any 

undisturbed old-growth stands as large as an acre exist. 

Large Texas ebony and anacua trees are present but are 

often intermixed with early successional patches of 

sugarberry, cedar elm, soapberry (Sapidus drummondii 

Hook. & Am.), and tepeguaje [Leucaena pulverulenta 

(Schlecht.) Benth.]. The existing patches are rather open, 

and various shrubs grow along the network of trails that 

interlace the refuge. The ecology of the shrub species is 

poorly known, but perhaps species such as brasil, snakeeyes, and Texas persimmon (Diospyros texana Scheele) 

would be represented in a true understory of old-growth 

forest on these moist sites. Certainly, species of the upper 

canopy would be represented in the understory. Unlike most 

retired cropland, old-growth examples of this forest type do 

not contain mesquite, huisache (Acacia smllii Isel y), or 

sugarberry among the leading dominants. 

Forest Dynamics and Ecosystem Function 

Archer et al. ( 1988) described the conversion of open 

grassland to thorn woodland in comunities to the north of 

the Lower Rio Grande Valley. Mesquite becornes 

established due to the reduction of fie and rapid 

dissentjnation of seeds by cattle. Other shbs then become 

established under mesquite in a predictable sequence, and 

these mottes may grow together to form diverse thorn 

wacrdlands on some sites. Van Auken and Bush (Bush and 

Van Auken 1984, 1986a, 1986b, 1987; Van Auken and 

Bush 1985) have shown how differences in light and 

nutrient requirements may control secondary succession on 

floodplains of the San Antonio River. Wuisache, an early 

successional species with higher light and lower nutrient 

requirements than sugarbeny, would come fist in the 

succession. Establishment processes and invasion of 
gsassland described by Archer, and lightlnukent processes 

descriw by Van Auken and Bush, my both play a role in 

cs of upland subtropical forests, but these 

processes have not been investigated in t Graing 

by large ungulates was apparently not co the valley 

before Ewopan settlement, as there are few or no records 

of bison in the area. 

Revegetation of retired cropland is ongoing in the hwer 

Rio Grande Valley. More than 5,000 acres [2024 hectares 

(ha)] have been replanted, but only dubious records on 

planting methods and rates of "success" have been kept. In 

"naf~ral" old fields, huisache, sugarberry, and cedar elm are 

comon on wet sites, and mesquite is comon on all sites; 

however, sugarbey and cedar elm are absent on dry sites. 

Granjeno is also a comon, early successional species on 

almost all sites. Anacua grows in early successional stands, 

but Texas ebony is generally absent. 

Unfortunately, little is known about processes in old-growth 

Texas ebony forests, nor about presettlernent disturbances. 

Apparently, periodic flooding and severe drought were once 

controllhg factors. These disturbances would certainly have 

caused patchiness in the presettlement landscape. The Rio 

Grande is now almost entirely "controlled" as it reaches the 

lower valley, and essentially all the fresh water in the river 

is allocated to human use before it gets to the Gulf of 

Mexico. Overflow in some areas that were once periodically 

flooded is now totally controlled. At the same time, some 

areas may be wetter than ever due to the collection of 

irrigation water in resacas. No record exists of the effects of 

the droughts of the 1930's and 1950's on the vegetation of 

the Rio Grande Valley. Workers who visited the area 

between 1937 and the 1960's did not note drought effects as 

they did in other areas of Texas and North America. Nor do 

standing dead trees provide residual evidence of the 1950's 

drought. 

Gap-phase succession is not clearly apparent in old-growth 

upland forests of the Rio Grande Valley. There is often 

almost no mesquite in the interior of the oldest stands I have 

viewed. Texas ebony or a mixture of ebony and anaeua 

general1 y form a near1 y monoculture canopy; sometimes 

these species emerge slightly from a Iower canopy of mixed 

shrubs and low trees. These stands have little or no 

understory, but the stems of species that reach near the 

canopy ( 16 to 26 feet or 5 to 8 m) may be so dense that 

walking is difficult. Paradoxically, the view is most often 

nearly clear under the canopy, since little green foliage 

grows below 15 feet (5 m). In other stands, a few individual 

mesquite trees along with Texas ebony emerge to about 26 

feet (8 m) from a lower, mixed, small tree and shrub canopy 

(16 feet or 5 m). Since individual trees have small crowns 

that often interlwk with other individuals in the canopy, 

light gaps formed by dead or dying canopy trees are 

hconspicuous and fill in quickly. In this situation, shade 

intolerant mesquite is nearly absent, and Texas etwjny and 

macua eventually dominate, especially in the least dis~bed 

stands. 

Many of the few remining stands of this forest type are 

already in public ownership due to efforts of both the 

Federal (Lower Rio Grande Valley WR) and State (Los 

Palornas State Wildlife Management Area) governments. 

Management of these tracts, however, may not always favor 

old-growth forests, The valley is a rapidly urbanizing area, 

in addition to being intensively developed for crop 

production. Reportedly, more than 500,000 visitors spent 

the winter in the valley, and more than 200,000 visited 

Santa Ana WR, one of the better examples of this forest 

type. Thus, plans for park development, wildlife 

management, and visitor access often directly conflict with 

conservation. Conservation biologists have not 

systematically identified core ecological reserves nor made 

careful plans for expansion into restoration areas, even 

though thousands of acres of cropland in the Rio Grande 

Valley have been purchased for revegetation. Since as few 

as 15 fair or better-quality stands remain, according to the 

Texas Natural Heritage Program, and since most cover 

fewer than 20 acres (8 ha), such plans need to be developed 

and implemented soon in order to save this forest type from 

extinction. 

Representative Old-Growth Stands 

Consult Texas Natural Heritage Program data base for exact 

locations of these stands: 

* Camp Perry, Boy Scouts of America, Cameron County 

* Kelly Unit, Los Palomas State Wildlife Management 

Area, Hidalgo County 

* Iaguna Atascosa National Wildlife Refuge, Cameron and 

Willacy Counties 

* Longoria Unit, Los Palomas State Wildlife Management 

Area, Cameron County 

* Madero and Gabrielson Units, Lower Rio Grande Valley 

National Wildlife Refuge, Hidalgo County 

* Methodist Camp, Hidalgo County 

* Monte Meta Cemetery, Cameron County 

* Santa ha National Wildlife Refuge, Cameron County 

* Resaca de la Palmas State Park, Cameron County 

* Rio Grande-Bentson State Park, Hidalgo County 
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South Texas. Tex. J. Sci. 41 : 107- 108. 

Vora, R.S., and 2. Labus. 1988. An investigation of Texas ebony seed 

provenances in the Iawer Rio Grande Valley, Texas. Tex. J. Sci. 

40:452-454. 

Vora, R.S., and J.F. Messerly. 1990. Changes in native vegetation 

following different disturbances in the Lower Rio Grande Valley, Texas. 

Tex. J. Sci. 42: 15 1 - 158. 

Whittaker, R.H., and LE. Gilbert. Analysis of a two-phase pattern in a 

mesquite grassland, Texas. J. Ecol. 67:935-952. 

Lehmann, V.W. 1969. Forgotten legions. Sheep in the Rio Grande Plain 

of Texas. Tex. West. Press, El Paso. 
Diamond, David D. 1998. An old-growth definition for southwestern subtropical upland 

forests. Gen. Tech. Rep. SRS-21. Asheville, NC: U.S. Department of Agriculture, Forest 

Service, Southern Research Station. 7 p. 

Mainly evergreen, broad-leaved forests in the Southwestern United States are restricted to the 

Lower Rio Crande Valley of Texas. The soils and long growing season make this region 

valuable cropland, and thus almost all of the area once occupied by this forest type has been 

converted to row crops. Remaining old-growth forests are usually dominated by some 

combination of the broad-leaved evergreen Texas ebony and a host of other species. Few 

quantitative studies have described the composition of this forest type, and, likewise, IittIe is 

known of the dynamics. Droughts, flooding regime, and fire were large-scale disturbance 

factors. Now, the Rio Grande is used extensively for irrigation, and flooding is controlled. 

Therefore, the presettlement water regime has been greatly altered, and vegetation of the 

remaining forest fragments is also adjusting to the new moisture regime. Some areas are wetter 

and some drier than in historical times. Many of the fragments that remain have already been 

incorporated into public ownership by the U.S. Fish and Wildlife Service or the Texas Parks 

and Wildlife Department. 

Keywords: Anacua; evergreen, broad-leaved forests; old growth; Rio Grande Valley; Texas 

ebony.