ABSTRACT

    The Dead Horse Mountains area of Big Bend National Park is one of the least explored areas in Texas. This is the first collective body of work to document the area's vascular flora through a major new collection of vouchered specimens and a survey of existing herbarium specimens. Field work was performed between August 2003 and September 2006. The study area covered 176,800 acres (71,548 hectares; 276 mi², 716 km²) with a 4100 ft (1250 m) range in elevation. Non-vascular specimens (bryophytes, hepatophytes, and lichens) were also collected but are not a part of the current report. A total of 1584 vascular specimens were collected during the current study, and 2212 previously collected specimens were found in various herbaria.

    The total of the Dead Horse flora documented to date consists of 671 taxa at or below the species level (663 species) from 91 families and 344 genera. The current effort increased vouchered taxa by 12% and documented 55 novel species for the Dead Horse, a 10% increase over historical records. The best-represented families are the Asteraceae (12.0 % of total flora, 80 species), Poaceae (11.3 %, 75), Fabaceae (7.8 %, 52), Euphorbiaceae (5.1 %, 34), Cactaceae (5.1 %, 34), Pteridaceae (3.0 %, 20), and Boraginaceae (3.0%, 20). Seymeria falcata var. falcata was collected for the first time in the United States, in addition to seven new species records for Brewster County. One additional taxon may be undescribed or new to the United States. Twenty four non-native species have been collected in the Dead Horse; three are state-listed noxious weeds, and six are considered invasive. Eleven species endemic to Texas are found [p. iv] within the study area, and 17 species are considered rare. The Dead Horse flora differs from other southwestern floras, mainly in terms of the relative importance of monocots and the co-dominance of Asteraceae and Poaceae in the overall floristic composition of the Dead Horse. This and other factors suggest connections to several peripheral floristic areas, with influences ranging from the Mogollon Rim and the Great Plains, to South Texas and southern Mexico. Using similarity index calculations to compare regional floras should become an integral part of the future of defining floristic relationships of Trans-Pecos Texas and the Chihuahuan Desert Region.

ACKNOWLEDGMENTS

    Mike Powell is offered my greatest appreciation and thanks for his vision for this project, which enabled my eventual involvement; for his generous assistance, helpful guidance, and consistent support; and for the collegial respect, companionship, and friendship we have enjoyed throughout this project. I wish to also recognize and thank Billie Turner, for providing such an enjoyable work environment, and for providing so much invaluable assistance towards this final product. It is because of botanists like these that current-day floristic work can occur with the degree of ease and facility we find possible. Botanists of the next generation will remain indebted to their publications and perhaps may never see their equal in their incredible command of regional floristics, from simple identifications to an intimate understanding of wide-ranging, field-level botanical relationships. They have also done an outstanding job at curating and maintaining the Sul Ross University herbarium; the exceptional organization made my work much easier by a significant degree.
    I am thankful for the support given to me by Martin Terry, in addition to his excellent command of language and his perennial good-natured outlook on life. Joe Sirotnak has been an important friend and presence in my life, and he always reminds me of the joy to be found in field work. I appreciate James Zech's encouragements to attain the highest standards possible in my work. Sul Ross State University should be recognized for continuing to support and house a vascular plant herbarium.

p. vii

TABLE OF CONTENTS

		Page
Abstract		iii
Acknowledgements		v
Chapter
I.	Introduction	1
	Geography and Study Area Description	5
	Climate	12
	Geology	21
	Soils	33
	Human History	43
	Disturbance Regimes	49
	Paleoenvironment	58
	General Vegetation Description	62
	Previous Work, Notable Species	69
II.	Methods	73
	Herbarium search	73
	Field Collections	75
	Identifications and Nomenclature	77
III.	Results	79
	Herbarium Search	79

p. ix

Chapter		Page
	Vegetation Patterns	80
	Mixed Desert Scrub	81
	Desert Arroyo	85
	Desert Canyon	87
	Rio Grande Riparian Corridor	89
	Sotol-Yucca Grassland	91
	Chaparral	95
	Field Collections	94
IV.	Discussion	121
	Herbarium Search	121
	Field Collections	123
	Floristic Analysis and Comparison: Local level	129
	Floristic Analysis and Comparison: Regional level	138
	Phytogeographic Context	144
V.	Literature Cited	151
Appendix A:	Annotated Species List of the Dead Horse Mountains Flora	166
Appendix B:	Potential Species of the Dead Horse Mountains	263
Appendix C:	Species Unlikely to Occur in the Dead Horse Mountains	268
Vita		270

p. x

LIST OF TABLES

Table		Page
1	Outline of plant communities of the Chihuahuan Desert Region	65
2	Vegetation types of Big Bend National Park	67
3	Percent composition of vegetation types in the BBNP portion of the study area	68
4	Top 20 plant families and top 10 genera of the Dead Horse Mountains	96
5	Taxonomic composition of the flora of the Dead Horse Mountains	98
6	Summary of the Dead Horse Mountain flora by life form	104
7	Non-native species collected in the Dead Horse Mountains	105
8	Species endemic to Texas occurring in the Dead Horse Mountains	107
9	Rare species collected in the Dead Horse Mountains	109
10	Top 10 families of the Dead Horse Mountains and the Solitario	131
11	Genera with the highest number of species occuring in Texas	134
12	Comparison of top ten families in terms of proportion of flora, between four	135
13	Comparison of family percentages between the southern Big Bend of Texas and the average of 50 southwestern U.S. floras	140
14	Taxonomic summary of southwestern floristic areas	142
15	Symbols used in association with the species entries in Appendix A	169
16	Abundance categories used to describe species occurrences	171
17	Acronyms for study area vegetation types	172

p. xi

18	Herbarium acronyms	174
19	Index to workers who have collected in the study area	175

p. xii

LIST OF FIGURES

Figure		Page
1	Vegetational areas of Texas	2
2	Map of Big Bend National Park showing study area location	6
3	Dead Horse Mountains study area locations	8
4	The extent of the Chihuahuan Desert Region	14
5	Average annual high and low temperatures of the Dead Horse Mountains	15
6	High and low temperatures of weather stations of Big Bend National Park	17
7	Average annual precipitation by month for Big Bend National Park and Black Gap Wildlife Management Area weather stations	18
8	Average annual precipitation totals for Big Bend National Park and Black Gap Wildlife Management Area weather stations	19
9	Low-lying clouds over higher Dead Horse peaks	22
10	Historical annual precipitation levels for Big Bend National Park weather stations and for the Black Gap Wildlife Management Area	23
11	Yearly precipitation patterns during study period, 2003-2006	24
12	Geologic periods, typical life forms, and a general Big Bend geologic record	26
13	Stratigraphic layers in Big Bend National Park	27
14	Diagram of upthrust fault blocks	31
15	Schematic drawing of high elevation soil units	35
16	Schematic drawings of lower elevation soil units	40

p. xiii

Figure		Page
17	Erosion of Tornillo soils into badlands formations	41
18	Tobosa grassland on the western side of Ernst Basin	42
19	Recorded fire events in the Dead Horse Mountains	52
20	Non-native Phragmites australis growing densely along the Rio Grande	54
21	Impacts from trespass livestock	56
22	Schematic map of Trans-Pecos mountain ranges	71
23	Mixed Desert Scrub vegetation type	82
24	Potentially less-appreciated members of the Dead Horse flora	86
25	Desert Canyon vegetation type	88
26	Riparian Corridor vegetation type	90
27	Sotol-Yucca Grassland vegetation type	93
28	Chaparral vegetation type	96
29	Study area collection locations	102
30	Rare species of the Dead Horse Mountains	111
31	Reconfirmed and newly collected rare species of the Dead Horse Mountains	112
32	Unusual species of the Dead Horse Mountains	114
33	Environment of the high saddle northwest of Sue Peaks	117
34	Views from Stuarts Peak	118
35	Views of Sue Peaks	120
36	Species/effort trend after 100 years of collecting effort in the Dead Horse	124
37	Species/effort trend for collections made 2003–2006 in the Dead Horse	125
38	Species/effort trend after three visits to the Passionflower Canyon area	126

p. xiv

Figure		Page
39	Species/effort trend after two visits to the Sue Peaks area	128

"Away from the railroad, the Big Bend – sometimes called the Bloody Bend – is a hardy country, that is, one in which, through lack of water, civilization finds it difficult to gain a foothold. Although abundantly supplied with waterworks, such as scarped and canyoned streamways, it possesses a minimum of water. These great arroyos are mocking travesties which suggest that nature becomes tired of making this country before turning on the water."