Oecologia (Berl.) 34, 353-362 (1978) Oecologia 9 by Springer-Verlag 1978 Influence of Summer Rainfall on Root and Shoot Growth of a Cold-Winter Desert Shrub, A triplex confertifolia * Ken C. Hodgkinson**, Pat S. Johnson, and Brien E. Norton Department of Range Science, Utah State University, Logan, UT 84322, USA Summary. The influence of irrigation and nitrogen fertilization in early sum- mer on root and shoot growth of AtripIex confertifoIia, a C4 shrub species, was examined in a cold-winter desert community in northern Utah. Soil water and xylem pressure potentials were monitored during the summer period. At the time of watering the surface soil (0-30 cm) was dry but there were turgid fine roots in this horizon. Watering of the soil reduced plant water stress from -30 to -15 bars (dawn values) indicating that roots near the surface were capable of absorbing water, and induced root growth in the 0-30 cm zone. The addition of N to the water treatment did not further increase root production. However, watering and watering + N fertili- zer failed to stimulate shoot elongation or any dry weight increase of shoots. This shoot dormancy during summer is not typical of C4 plants and is probably associated with adaptation to the cool arid environment. Introduction Aridlands are characterized by low and discontinuous precipitation (Noy-Meir, 1973). The effectiveness of precipitation events in activating biological processes and in plant biomass build-up, would be expected to vary, and would depend on such factors as prevailing temperature, and the intensity and duration of the precipitation event. This stochastic nature of precipitation in desserts is important in the timing of growth responses in plants and is justifiably included in simulation models of desert ecosystems (Goodall, 1967). In warm arid environments where temperatures in the winter are not low enough to cause soil water to freeze, it would appear that the perennial plants make growth during the summer months as long as rainfall events exceed a threshold amount (Went, 1949; Adams and Strain, 1969). All or the majority of the perennial plants in these warm arid zone communities possess the C4 * This work was carried out while the senior author was on study leave from CSIRO ** Permanent address and addressfor offprint requests." CSIRO Division of Land Resources Manage- ment, Riverina Laboratory, Private Bag, P.O., Deniliquin, N.S.W. 2710, Australia 0029-8549/78/0034/0353/$02.00