photo by M.F.Davis

Larrea tridentata UofAz 1253

Larrea tridentata UofAz

Larrea tridentata:
Creosote bush is the most abundant and characteristic shrub of the hot deserts of North America. Of the Zygophyllaceae Fagonia, Guajacum, Larrea, and Zygophyllum, are tricolporate and reticulate, but Kallstroemia and Tribulus are periporate. As shown below, grains of Fagonia are noticably larger, more prolate, and have a larger pore area than Larrea.

Larrea tridentata col. Huspeth Co., TX

Fagonia californica col. Yuma, AZ

Shrubs 1 (- 3) m high with thin, gray-white twigs. Leaves dark green, resinous, alternate, composite (2 leaflets) 13-25 mm long. Flowers yellow, regular, 5/5/10. Fruits are hairy 5-celled capsules. Larrea has a very interesting amphi-tropical distribution. The North American species are autopolyploids of one another, but all are considered one species. Four species are recognized in southern South America (Hunziker et al., 1977).

Pollen light micrograph:
Larrea pollen is very small, and its reticulations are often not visible with light microscopes at 40X. The furrows are broad and distinct, and the pore area of the furrow is thin and protruding. The inner wall of the inter-copal area is thinned (transverse endopore).

Pollen scanning electron micrograph (SEM)
The lumina of the reticulation is small and shallow. The reticulation is less distinct in the polar area. "Blobs" on the SEM image above are presumable an artefact of processing.

Production and Dispersal:
Creosote bush flowers year-round after rains. It is pollinated by a variety of insects. In surface samples (Davis, 1995) Larrea pollen percentages are 1-3% (up to 10%) where creosote bush is present. It is sporadically abundant (> 30%) in packrat midden samples. Production and dispersal are low.

The thin equatorial wall may make the pollen grain susceptable to breakage. However, ancient grains have been recovered. Preservation good.

Fossil Occurrence:
Larrea pollen is present in small numbers in packrat middens and lake sediments during the late Quaterary of the American Southwest. The oldest Larrea pollen in Owens Lake core (Woolfenden, 1996) is 47,000 years in the Badwater core of Death Valley the oldest is 109,000 years (Bader, 1999). Larrea pollen is present after 11,500 yr in the packrat middens of the Waterman Mountains (Anderson and Van Devender, 1991) and is abundant in the oldest (20,000 yr) packrat midden of the Tinajas Altas series (Davis, 1990).

    Anderson, R.S. and Van Devender, T.R. 1991. Comparison of pollen and macrofossils in packrat (Neotoma) middens: A chronological sequence from the Waterman Mountains of southern Arizona, U.S.A. Review Palaeobotany Palynology 68: 1-28.

    Bader, N. E. 1999. Pollen analysis of Death Valley Core DV93-1: A closeup of Marine Oxygen Isotope stage 6 and Glacial Termination II. M.S. Pre-publication Manuscript , University of Arizona.

    Bader, N. E., 2000. Pollen analysis of Death Valley sediments deposited between 166 and 114 Ka. Palynology, 24: 49-62.

    Davis, O.K. 1990. Caves as sources of biotic remains in arid Western North America. In: Davis, O.K. (Editor), Paleoenvironments of Arid Regions. Palaeo. Palaeo. Palaeo. 76:331-348.

    Davis, O.K. 1995.
    Climate and vegetation patterns in surface samples from arid western U.S.A.: Application to Holocene climatic reconstructions. Palynology. 19:97-120.

    J.H. Hunziker, R.A. Palacios, L. Poggio, C.A. Naranjo, and T.W. Yang. 1977.
    Geographic distribution, morphology, hybridization, cytogenetics, and evolution. p. 10 - 47 IN T.J. Mabry, J.H. Hunziker, and D.R. DiFeo, Jr. Creosote bush biology and chemistry of Larrea in the new world deserts. Dowden, Hutchinson & Ross, Inc. Stroudsburg, Penn.

    Woolfenden, W.S. 1996. Late-Quaternary vegetation hsitory of the southern Owens Valley region, Inyo County, California. Ph.D. dissertation, University of Arizona.

Owen Davis 5/01