Western North American Naturalist Volume 61 Number 1 Article 4 1-29-2001 Experimental manipulations of fertile islands and nurse plant effects in the Mojave Desert, USA Lawrence R. Walker University of Nevada, Las Vegas Daniel B. Thompson University of Nevada, Las Vegas Fredrick H. Landau University of Nevada, Las Vegas Follow this and additional works at: https://scholarsarchive.byu.edu/wnan Recommended Citation Walker, Lawrence R.; Thompson, Daniel B.; and Landau, Fredrick H. (2001) "Experimental manipulations of fertile islands and nurse plant effects in the Mojave Desert, USA," Western North American Naturalist: Vol. 61 : No. 1 , Article 4. Available at: https://scholarsarchive.byu.edu/wnan/vol61/iss1/4 This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Western North American Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Western North American Naturalist 61(1), © 2001, pp. 25–35 EXPERIMENTAL MANIPULATIONS OF FERTILE ISLANDS AND NURSE PLANT EFFECTS IN THE MOJAVE DESERT, USA Lawrence R. Walker1, Daniel B. Thompson1, and Frederick H. Landau1 ABSTRACT.—In a mixed desert shrub community we removed and added shrub canopies to examine above- and belowground influences of 3 species of shrubs on islands of soil fertility and the survival of transplanted Ambrosia dumosa seedlings. Soils sampled under shrubs in the wet season had higher pH, water content, organic matter, and both total and mineralizable nitrogen than soils in adjacent open areas, confirming a widely established pattern in arid lands. However, we also found species differences in soil parameters. Soils under Coleogyne ramosissima had highest pH, soils under A. dumosa had highest water content and nitrogen mineralization rates, and soils under Larrea tridentata had low- est water content. Soils sampled under shrubs in the dry season, 7 months after experimental shrub removal, maintained higher organic matter and total and mineralizable nitrogen content than adjacent open soils, but pH and water were altered by shrub manipulations. Species differences persisted only in soil water levels (A. dumosa soils were driest). Over a 1-year period, transplanted A. dumosa seedlings had highest survivorship in shrub removal and open treatments and died most rapidly under control shrubs of all 3 species, suggesting that shrubs had a strong negative effect on seedling survival, even in the presence of higher organic matter, nutrients, and (initially) higher water content of fertile islands. Our results suggest that nurse plants and islands of soil fertility have the potential to facilitate growth of other species by nutrient additions, but that the net effect of nurse plants can be negative due to shading and/or root competition. Key words: competition, facilitation, aridland soils, Ambrosia dumosa, Coleogyne ramosissima, Larrea tridentata. The pattern of higher soil fertility under shrub roots may also compete with seedlings desert shrubs than in adjacent open areas is for available nutrients and water, and canopies well documented for arid lands (Charley and of established shrubs may reduce light levels West 1975, Virginia 1986, Franco and Nobel enough to limit primary productivity of under- 1989, Garner and Steinberger 1989, Schlesinger story seedlings (Charley and West 1975, Franco et al. 1996). Colonization of open areas (e.g., and Nobel 1989, Hunter 1989, Callaway et al. by species in the genus Ambrosia; McAuliffe 1996). The net effect of these potentially posi- 1988) apparently begins a cycle of habitat tive and negative attributes of desert shrubs amelioration in deserts in which organic mat- on recruitment of new seedlings can best be ter is built up from plant litter and entrapment examined with experimental manipulations of of wind-blown debris, and further enhanced fertile islands and associated shrubs. by secondary colonization of other shrubs and Fertile islands have important effects on the annuals. Animals presumably contribute to soil dynamics of desert ecosystems. Experimental nutrients, and their burrowing under shrubs studies of desert ecology have demonstrated increases soil aeration and water infiltration to that establishment and growth of dominant deeper soil layers (Garner and Steinberger shrub species are limited by nitrogen and 1989). These “fertile islands” under desert water (Fonteyn and Mahall 1981, Lajtha 1987, shrubs are therefore distinguished from adja- Smith et al. 1987, Fisher et al. 1988, Lajtha cent open areas between shrub canopies by and Whitford 1989). Thus, it is not surprising higher levels of organic matter and soil nutri- that there is strong evidence from arid lands of ents, higher rates of nutrient turnover, higher nurse plants that facilitate establishment of soil water content, higher productivity of other species (McAuliffe 1984, 1988, Franco annual plants, and increased soil microbe and and Nobel 1989). As the facilitated plant grows, vertebrate animal activity. Although these it may eventually out-compete the nurse plant. properties of fertile islands may enhance The fertile island can thus become the center recruitment of new seedlings under shrubs, of a centuries-long process of colonization and 1Department of Biological Sciences, Box 454004, University of Nevada–Las Vegas, Las Vegas, Nevada 89154-4004. 25 26 WESTERN NORTH AMERICAN NATURALIST [Volume 61 succession where nutrient cycling, plant pro- ductivity, and animal activities are all higher than in intershrub spaces. In this study we experimentally examined similarities and differences between above- and belowground influences of 3 species of desert shrubs on properties of fertile islands and transplanted seedlings by removing and adding shrub canopies. Species differences in fertile island characteristics were recognized Fig. 1. Diagram of natural pattern of islands of high soil by Charley and West (1975) but rarely have fertility under shrubs and less fertile spaces between been examined experimentally. We examined shrubs (top) and 5 treatments into which Ambrosia the effect of shrub canopy removal treatments dumosa seedlings were transplanted (bottom): S = control shrub, O = control open area, SR = removal of above- on survival of transplanted seedlings of ground shrub, OC = addition of cut shrub from SR to an Ambrosia dumosa, a species that may be criti- open area, SRC = removal followed by replacement of cal for establishing new fertile islands in the cut shrub in situ. Mojave Desert (cf. McAuliffe 1988). Our manip- ulations (Fig. 1) compared 5 treatments with various combinations of aboveground shade plots distributed along an elevational gradient effects, belowground nutrient effects, and be- from 1030 m to 1850 m in Lucky Strike Can- lowground root competition effects of desert yon. The study site is located on a 13-km-long shrubs. by 2-km-wide bajada with an average vertical slope of <5°. The slope lacks a central drainage METHODS channel but has multiple ephemeral channels scattered across the entire bajada that average Study Area 1–2 m deep; our study sites were only on the This study was conducted on a bajada (slope) broad, raised benches between washes. Soils in Lucky Strike Canyon (36°23′N, 115°28′W; of the canyon are limestone-derived with 1127 m elevation) on the east side of the cemented petrocalcic horizons (caliche) begin- Spring Mountain Range, 50 km northwest of ning at depths of 15–80 cm. Desert pavement Las Vegas, Nevada, in the Mojave Desert of and cryptobiotic crusts (cf. Evans and Belnap the southwestern United States. The study 1999) are moderately well developed at the area includes vegetation typical of regional site. Lucky Strike Canyon is characterized by desert bajadas and is located at the lower ele- hot summers above 35°C and cold winters vational boundary of the Coleogyne ramosis- below –10°C. Summer rainfall usually occurs sima Torr. (blackbrush) plant community (found during thunderstorms in July and August, but between 1100 and 2000 m elevation in the most precipitation comes as winter rains that Mojave Desert) and the upper boundary of a are widespread and may last several days. plant community dominated by Ambrosia du- Snow is occasional and not long lasting, and mosa (Gray) Payne (white bursage; Lei and mean annual precipitation at the site is <200 Walker 1997a, 1997b). Larrea tridentata Cov. mm. Precipitation increases with elevation and (creosote bush) is a co-dominant in the A. temperature decreases with elevation (Row- dumosa community. Shrub diversity is higher lands et al. 1982). During the study period at this ecotone than at any other elevation (March 1995–May 1996) total precipitation and on the bajada. Other common species include maximum and minimum temperatures were Krascheninnikovia lanata, Krameria erecta, 780 mm, 32°C, and –20°C at Kyle Canyon Ephedra nevadensis, E. viridis, and Acamp- (2135 m elevation) above Lucky Strike Canyon topappus shockleyi. For further descriptions of and 69 mm, 40°C, and –7°C at Corn Creek regional vegetation see Beatley (1975) or Run- (880 m elevation) at the base of Lucky Strike del and Gibson (1996). Nomenclature follows Canyon (National Climatic Data Center 1995, Hickman (1993). 1996). Relative humidity was typically low Experiments described here are part of a (<20%) and evaporation high during summer larger study of shrub dynamics on permanent months. 2001] EXPERIMENTS WITH FERTILE ISLANDS 27 Plot Designs, Treatments, at 550°C (Black 1965). We took care to avoid and Measurements higher temperatures that would cause loss of We examined responses of soil parameters carbonates. After oven-drying, a 0.4-g sample and of transplanted A. dumosa seedlings to of each soil was digested in sulfuric acid with experimental removals and additions of A. a mercuric oxide catalyst and then analyzed dumosa, L. tridentata, and C. ramosissima shrub colorimetrically for total Kjeldahl nitrogen canopies. In March 1995 we randomly selected using an automated salicylate procedure (Envi- 24 locations (8 sets of shrubs for each of 3 ronmental Protection Agency 1984).