Ecology, 68(6), 1987, pp. 2032-2045 ? 1987 by the Ecological Society of America CHIHUAHUAN DESERT ANNUALS: IMPORTANCE OF WATER AND NITROGEN' JULIO R. GUTIERREZ2 AND WALTER G. WHITFORD Biology Department, New Mexico State University, Las Cruces, New Mexico 88003 USA Abstract. We examined the effects of water supplementation and nitrogen amendment on biomass, cover, and density of annual plants on a termite-free and a termite-present area in the Chihuahuan Desert. Soil moisture was higher in the termite than in the termite- free plots, and in the watered than in the unwatered plots during the spring and summer. There were no differences in soil moisture among plots during the winter. Soil nitrogen was higher in the termite-free than in the termite plots. There were no differences in total plant biomass produced in termite and termite-free areas. There were significant differences in relative abundances of species among treatments. Natural rainfall was sufficient for maximum spring-annual biomass development on all plots except for the termite-free unfertilized, unwatered plots. These were the driest plots but had high soil nitrogen. Most of the herbaceous species responded to the water amendments by lengthening growing seasons, increasing density, or increasing biomass. When there was sufficient water for most of the spring annuals, high soil nitrogen levels favored increased densities and bio- masses of Descurainia pinata and Lepidium lasiocarpum. The absence of C4 summer annuals in the high-nitrogen plots suggests that relatively high soil nitrogen adversely affected the summer annuals. Termite-free watered plots had higher soil moisture than the termite-unwatered plots, but summer annuals were relatively abundant on the latter. Water amendments had a greater effect on the species abundances in the termite-free area than in the one with termites. In the area with termites, nitrogen amendments had a greater effect on species abundances. Species diversity and richness were affected by site fertility as was species composition. This study demonstrates that we must understand patterns of soil nitrogen availability and processes affecting nitrogen availability in addition to water availability, in order to understand productivity and species composition of Chihuahuan Desert annual plants. Key words: biomass production; Chihuahuan Desert, community composition, density; desert an- nual plants; irrigation; nitrogen fertilization; phonology; soil moisture; soil nitrogen; termites. INTRODUCTION are C4 species with a high photosynthetic efficiency at high temperatures when water is available (Mulroy and Water is generally considered to be the main limiting Rundel 1977). factor for germination, growth, and productivity of her- If water is a limiting factor, then additional water baceous plants and shrubs in desert ecosystems (Went should increase plant productivity in a desert ecosys- 1948, 1949, Went and Westergaard 1949, Juhren et al. tem. In the Chihuahuan Desert, however, there is some 1956, Tevis 1958a, b, Beatley 1967, 1974, Noy-Meir evidence that productivity does not vary directly with 1974). Annuals occur as live plants only during short water (Ludwig and Flavill 1979, Gutierrez and Whit- periods that are favorable for germination and growth. ford 1987). Nitrogen is generally considered to be lim- They survive as seeds during periods when soil mois- iting after water in North American deserts (West and ture and temperature are not suitable for growth. Des- ert annual plants are classified as summer annuals if Skujins 1978, Skujins 1981). Applications of nitrogen fertilizer had little effect on shrub production but great- they germinate after summer or early autumn rains ly increased production of annual herbs in the northern when ambient temperatures are above 20'C, and as Mojave Desert (Wallace et al. 1978). In the Chihua- winter annuals if they germinate in autumn or early huan Desert, nitrogen fertilization increased the bio- winter rains at temperatures below 200 (Beatley 1974). mass production of the shrub Larrea tridentata and Seasonal differences in temperature affect the growth rates of annual species. Summer annuals have a shorter the grass Erioneuron pulchellum (Ettershank et al. 1978). In the Mojave and Sonoran Deserts, growth of winter life cycle and a greater overlap of phenological phases annuals appears to be so dependent upon nutrient than winter annuals. However, most summer annuals availability that relatively nitrogen-rich "fertile is- lands" beneath shrubs support much larger plants with ' Manuscript received 20 November 1985; revised 2 Sep- higher production efficiency than the nitrogen-poor soil tember 1986; accepted 19 September 1986. 2 Present address: Departamento de Biologia, Universidad in open areas (Mott and McComb 1974, Halvorson de La Serena, La Serena, Chile. and Patten 1975, Patten 1978, Romney et al. 1978). This content downloaded from 128.123.176.43 on Mon, 18 Nov 2019 15:43:35 UTC All use subject to https://about.jstor.org/terms December 1987 CHIHUAHUAN DESERT ANNUALS 2033 Nevertheless, many subtle effects of both nitrogen and tion ?1 SD is 211 ? 77 mm, with most rainfall oc- water have not been considered in the interaction of curring during late summer from convectional storms. limiting factors in determining whether or not response Maximum summer temperatures reach 40'C and freez- to either will be obtained (Romney et al. 1978). ing temperatures are recorded from October to mid- In the northern Chihuahuan Desert, several studies March (Houghton 1972). showed that subterranean termites affect soil properties (Parker et al. 1982, Brown 1983, and Elkins et al. 1986). METHODS AND MATERIALS Parker et al. (1982), in a study comparing annual plants Original study design and data on a series of plots from which termites had been elim- inated with plots with termites present, found that al- In October and November 1977, a factorial exper- though total annual-plant density and biomass were iment was set up to study the effects of termite removal. not different, dominant species on termite-free plots The experimental design consisted of four blocks with ranked considerably lower than where termites were five completely randomized 30 x 40 m treatment plots present. Parker et al. (1982) attributed these changes per block. Two plots in each block were treated with in species importance to the increased nitrogen levels chlordane, at an application rate of 10.3 kg/ha, to elim- in soils from which termites had been eliminated. In inate termites. Chlordane is a recalcitrant chlorinated a subsequent study Elkins et al. (1986) reported that hydrocarbon that binds to clay particles in the soil, water infiltration was significantly lower on plots where thereby losing its contact toxicity. Subterranean ter- termites had been removed. Thus plots without ter- mites ingest soil and the chlordane apparently is re- mites are characterized by higher soil nitrogen and low- leased in their guts. The only soil animals that did not er water availability. survive the chlordane treatment or recolonize the treat- These relationships led us to re-evaluate the findings ed areas were subterranean termites. Chlordane treat- of Parker et al. (1982). We hypothesized that the species ment had no apparent effect on numbers of ant colo- composition and productivity of Chihuahuan Desert nies. The densities of ants are very low on this annual plant communities vary as a function of the creosotebush-dominatedbajada(Whitford 1977). There interaction between water and nitrogen availability. were six ant colonies on the termite-free plot and five We hypothesized that some species, such as Descu- ant colonies on the termite plot at the time this ex- rainia pinnata and Lepidium lasiocarpum, that in- periment was conducted. The termite-free plot had two creased in importance on termite-free plots (Parker et Pogonomyrmex desertorum colonies, two colonies of al. 1982) are nitrogen-limited species and that other Pheidole spp., and one colony each of Novemessor species, such as Astragalus tephrodes are water-limited cockerelli and Myrmecocystus desertorum. The other species. We hypothesized that productivity would be area had three Pogonomyrmex desertorum colonies and highest and species assemblages most similar on ni- two Pheidole spp. colonies. No active microarthropods trogen-rich sites over a broad range of soil moisture were found on or in treated soils to a depth of 20 cm availability based on the data in Gutierrez and Whit- through the spring of 1978, but in spring 1979 there ford (1987). By imposing irrigation and nitrogen fer- were no detectable differences in soil microarthropods tilization on small plots on termite-free and termite- and nematodes in chlordane-treated and untreated soils. present areas, we were able to obtain a wide range of At the time of this study the populations of soil mi- nitrogen and water contents in the plots to test these croarthropods were higher on termite-free plots (2300 hypotheses. The results of these studies also provide individuals/M2) than on plots with termites present an answer to the question of whether the primary in- (1100 individuals/) (Silva et al. 1985). The taxo- direct influence of termites on annual-plant commu- nomic composition of the soil microarthropod assem- nities is by their effects on nitrogen or on water. blages was not different. Parker et al. (1982) sampled all termite-free and ter- STUDY AREA mite-present plots in their study of changes in nitrogen The study area is located at the Jornada Long-Term and annual plants following termite removal. Elkins et Ecological Research Site on the New Mexico State Uni- al. (1986) utilized plots on two of the original four versity Experimental Ranch, 40 km north northeast of blocks in their study of infiltration and sediment yield. Las Cruces, Dona Ana County, New Mexico. This site Based on those studies, we were confident that differ- is a desert watershed that empties into an ephemeral ences in infiltration, soil water storage, and soil nitro- lake. The watershed varies in elevation from - 1200- gen were attributable to removal of termites.