Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

Creosotebush, Larrea tridentata, is the dominant plant species in desert tortoise habitat. Creosotebush is a C3 evergreen shrub with a flexible rooting pattern and variable morphology. The C3 photosynthetic physiology results in cool season growth, and stem-leaf orientation that maximizes light interception in the early morning hours when cool temperatures and high relative humidity allow open stomata and efficient water use during photosynthesis (Neufeld et al. 1988). Creosotebush frequently have deep roots (> 3m) plus fine shallow roots emanating from the root crown. The inverted cone morphology of young plants and plants growing in extremely dry environments maximizes stem flow and channelization of stem flow water by roots to deep storage in the soil. This deep soil water can be used by creosotebush to produce foliage, flowers and fruits during droughts (Martinez-Meza and Whitford 1996, Whitford et al. 1995).

In relatively high rainfall areas (> 100 mm . yr-1) productivity of creosotebush is limited by nitrogen availability. The canopy morphology of creosotebush shrubs is a determinant of soil organic matter content and nitrogen content of the sub-canopy soils. Shrubs with inverted cone morphologies do not accumulate litter layers under the canopy while those with a spheroid morphology develop litter layers and elevated soil nitrogen under the canopy (deSoyza et al. 1997). Stemflow water is nutrient enriched from the black, crustose, microbial layer on the stems of the shrub (Whitford et al. 1997). Shrub canopy morphology affects the abundance and diversity of annual plants that grow under the shrubs.

There are important feedbacks between canopy morphology and animals that burrow under creosotebushes. In the Mojave, shrubs with kangaroo rat burrows exhibited higher stem growth increments and higher biomass, abundance and diversity of below canopy annuals. Below canopy burrows of animals affect infiltration of throughfall water, soil water storage, and nitrogen mineralization rates. While desert tortoises may not eat creosotebush foliage, the functional ecology of this shrub is an important determinant of habitat suitability for tortoises.

References

deSoyza, A.G., W. G. Whitford, E. Martinez-Meza, J. W. Van Zee. 1997. Variation in creosotebush (Larrea tridentata) canopy morphology in relation to habitat, soil fertility, and associated annual plant communities. Amer. Midl. Nat. 137:13-26.

Neufled, H. S., F. C. Meinzer, C. S. Wisdom, M.S. Sharifi, P. W. Rundel, M.S. Neufeld, Y. Goldring, G. L. Cunningham. 1988. Canopy architecture of Larrea tridentata (D.C.) Cov., a desert shrub: foliage orientation and direct beam radiation interception. Oecologia 75:54-60.

Whitford, W. G., G. Martinez-Turanzas, E. Martinez-Meza. 1995. Persistence of desertified ecosystems: explanations and implications. Environ. Monitor. Assessment 37:319-332.

Whitford, W. G., J. Anderson, P.M. Rice . 1997. Stemflow contribution to the 'fertile island' effect in creosotebush, Larrea tridentata. J. Arid Environ. 35:451-457.