Late Pleistocene and Early Holocene Plant Remains from Hueco Tanks State Historical Park: The Development of a Refugium

Abstract

Plant macrofossils from packrat middens from the Hueco Moun- tains, El Paso and Hudspeth Counties, Texas, allow comparison of the present and past vegetations on igneous and limestone substrates. Middens document a pinyon- juniper woodland at 13,500 + 250 B.P. in the Hueco Tanks State Historical Park and at 12,030 + 210 B.P. in the adjacent limestone Hueco Mountains. A more xeric juniper-oak woodland was present in the Hueco Tanks at 9380 ± 270 B.P. and at 9370 ± 130 and 8150 + 170 B.P. in the limestone Hueco Mountains. About 8000 years ago, the onset of middle Holocene climates differentially reduced wood- land elements on the nearby limestone massifs while the igneous, syenite-derived soils of Hueco Tanks supported a comparatively mesic, low-elevation oak-juniper woodland refugium. Favorable edaphic conditions and topography allowed wood- land species to persist at the Hueco Tanks until the present time. Hueco Tanks is an unusual area where middle Tertiary syenite and monzonite have intruded into older sediments in the Hueco Bolson about 40 km NE of El Paso, El Paso County, Texas (Wise 1977). The igneous rocks weather into large rounded boulders that often have de- pressions called huecos, tinajas, or tanks where water collects. Erosion and weathering of the syenite has resulted in a myriad of talus, fissure, and shelter caves, short steep header canyons, and in the formation of fairly extensive mesic microenvironments throughout the Tanks. Coarse neutral to acidic soils with high moisture-holding capacity are formed from the weathered syenite. Permanent water in the Tanks has attracted human visitors for thousands of years, and there are many Indian pictographs on the rocks. Hueco Tanks in a low-elevation (1370-1470 m) island containing numerous mesic plants in the vegetation surrounded by xeric, mostly 127