This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Spacing Patterns in Mojave Desert Trees and Shrubs

Matthew Fidelibus Raymond Franson David Bainbridge

Abstract-Planting shrubs at an excessive density or planting may have poor moisture holding capacity (Fidelibus, unpub­ incompatible species together may reduce desert revegetation suc­ lished data). As a result, belowground interactions may be cess. As part of the revegetation program at a Mojave Desert mine, a different in revegetation sites than in native systems. At study was initiated to detennine spacing patterns among the woody Viceroy Gold Corporation's Castle Mountain Mine, reveg­ perennial species in areas not disturbed by the current mining etation success will be determined by comparing plant activity. Shrub density ranged from approximately 9,000-14,000 diversity and density of revegetated lands to undisturbed individualslha. Most shrubs were found to be randomly dispersed; sites 10 years after revegetation is completed. Iftransplants however, larger shrubs such as creosote bush (Larrea tridentata), and are placed too close together or in antagonistic relationships, Joshua tree (Yucca brevifolia), were regularly dispersed and widely revegetation goals may not be met. spaced. Plants were found to often associate with con specific neigh­ To facilitate selection and placement of transplants at the bors. Data on species composition, density, dispersion, and neighbor mine site, a study was initiated to determine inter- and preferences will be used in mine site revegetation. intra-specific shrub spacing patterns in areas not disturbed by current mining. Point to plant and nearest neighbor methods were used to determine species density and disper­ sion patterns, and examine inter- and intra-specific neigh­ Climatic conditions favorable for germination and estab­ bor preferences. This data will be used to determine species lishment of native perennials are infrequent and unpredict­ composition, juxtaposition and distancing when replanting able in the Mojave Desert (Barbour 1968), limiting the value disturbed areas. of direct seeding for desert revegetation (Bainbridge and Virginia 1990; Lippitt and others 1994). Fortunately, nurs­ ery stock can be successfully transplanted in hot deserts Methods (Bainbridge and Virginia 1990; Fidelibus and Bainbridge ------1994; Romney and others 1989), facilitating recovery and Current mining operations will disturb approximately speeding visual relief. Factors such as poor substrate qual­ 343 ha (at an average elevation of 1,300 m). Vegetation was ity, lack of plant protection and drought have been found to classified into three major communities by Everett (1991), limit initial survival oftransplants on bare areas (Bainbridge based upon the descriptions of Holland (1986). The commu­ and Virginia 1990; Bainbridge and Fidelibus 1994). Plant nities in order of decreasing acreage are: Joshua tree (Yucca interactions occurring several years after planting, how­ brevifolia) woodland (JTW), blackbrush (Coleogyne ever, may ultimately determine transplant success. ramosissima) scrub (BBS),andMojavemixedsteppe(MMS). Previous studies have shown that competition in the An on-going study is determining substrate qualities, eleva­ desert is limited to belowground interactions (Cody 1986; tion, slope, and aspect of undisturbed communities in order Rundel and Nobel 1991). Fonteyn and Mahall (1981) have to determine which assemblages of native plants will be best documented competition for soil moisture between creosote suited for the modified substrates and new topography bush (Larrea tridentata) and bursage (Ambrosia dumosa). created by mining operations. It is expected that each Chew and Chew (1965) found that creosote bush density is community type will be recreated within the mine site. Sites correlated with mean annual rainfall suggesting intra-spe­ for vegetation sampling were chosen to include each major cific competition. Furthermore, Cody (1986) has shown that community and were coordinated with the soil study. some Mojave Desert shrub species may "prefer" or "avoid" Seventeen 50 m by 50 m quadrats were established within specific species as neighbors, possibly because of competitive areas which were judged to be representative ofthe commu­ interactions between plants with similar root structure. nity sampled; 11 quadrats in JTW, 4 quadrats in BBS, and Mine spoils and other disturbed substrates are often defi­ 2 quadrats in MMS. The JTW and BBS quadrats were cient in nutrients, organic matter, and microsymbionts, and established on relatively flat substrates of similar (within community type) elevation and substrate to insure that slope, aspect, elevation, or substrate differences were not In: Barrow, Jerry R; McArthur, E. Durant; Sosebee, Ronald E.; Tausch, Robin J., comps. 1996. Proceedings: shrubland ecosystem dynamics in a responsible for observed vegetation patterns. The MMS changing environment; 1995 May 23-25; Las Cruces, NM. Gen. Tech. Rep. community occurs on steep, rocky, slopes. For these sites, INT-GTR-338. Ogden, UT: U.S. Department of Agriculture, Forest Service, quadrats were established on slopes of similar aspect, eleva­ Intermountain Research Station. Matthew Fidelibus is Biologist, San Diego State University Foundation, tion, and grade. 5500 Campanile Dr., San Diego, CA 92182. Raymond Franson is Ecologist, In each quadrat, twenty sampling points were selected Viceroy Gold Corporation, Castle Mountain Mine, P.O. Box 68, Searchlight, using stratified random coordinates. At each point, the NV 89046. David Bainbridge is Restoration Ecologist, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182. distance to the nearest perennial shrub was measured, and

182 distances from this shrub to its first and second nearest Results neighbor and its nearest conspecific (if the nearest neighbor ------was a different species) were also recorded. For each species, Density the average point to plant distance (m) was calculated, and this value was used to estimate species density (# individu­ Shrub density varied among community type (table 1). alslha) following the methods of Cottam and Curtis (1956). The BBS community had the highest shrub density (14,800 The observed mean distance between conspecifics was shrubslha), but the lowest diversity (9 species). The JTW compared to the expected mean distance (generated by community had the lowest shrub density (8,900 shrubslha) assuming a random distribution of each species at their but the highest species diversity (26 species). The MMS had measured density). Departure from randomness was then intermediate shrub density (12,500 shrubslha) and diver­ determined by the ratio ofthe observed mean distance to the sity values (12 specie~). Relative shrub densities often dif­ expected mean distance as described by Clark and Evans fered dramatically among communities, and some species (1954). were characteristic of, or limited to, specific communities. To determine if a specific species was a nearest or second The density of blackbrush, for instance, was nearly 9,000 nearest neighbor more or less frequently than would be individualslha in the BBS community; a density greater expected based upon their density, chi-square analyses were than the overall shrub density in the JTW community (table 1). performed (Cody 1986). Species are described as "preferred" By contrast, blackbrush occurred only infrequently in JTW, neighbors if they occur as neighbors more often than would and was absent from the MMS sites. be expected by chance (p < 0.05), and "avoided" if they occur Few species (other than blackbrush) were found in their as neighbors less often than expected by chance. If species highest relative densities in the BBS community (table 1); were found to be neighbors as often as would be expected by however, Mormon tea (Ephedra nevadensis), Pima rhatany chance alone (p > 0.05), then their relationship is described (Krameria erecta) and Anderson's thornbush (Lycium as "neutral." The mechanisms behind preferred, avoided or andersonii) were equally abundant in BBS and MMS, occur­ neutral relations were not investigated. ring in JTW at lower relative densities (table 1). The arbores­ Revegetation of Joshua trees is an important goal at the cent monocots, Mojave yucca (Yucca schidigera) and Joshua mine, and because the species is present in lower densities tree, were found to occur at identical densities across the than other perennials, the distance from each random point BBS andJTW communities. Cooper's goldenbush (Ericameria to the nearest Joshua tree, and the distance to the nearest cooperi), a very abundant species in JTW (2,952lha), was neighbor Joshua tree, were also recorded (if Joshua trees much less common in BBS (561lha). were within 10 m of a point) to insure an adequate data set for this species.

Table 1-The density (# of individualslha) of selected shrub species in three different plant communities surrounding Castle Mountain Mine, in the east Mojave Desert.

Common name Species name BBS1 JTW2 MMS3 Anderson's thornbush Lycium andersonii 1,684 1,011 1,562 Beavertail cactus Opuntia basilaris 0 202 0 Blackbrush Coleogyne ramosissima 8,984 647 0 Buckhorn cholla Opuntia acanthocarpa 0 121 0 California barrel cactus Ferocactus cylindraceus 0 40 1,562 California buckwheat Eriogonum fasciculatum 187 40 2,500 Cooper's goldenbush Ericameria cooperi 561 2,952 625 Creosote bush Larrea tridentata 936 1,051 1,875 Joshua tree Yucca brevifolia 374 364 0 Mojave prickly pear Opuntia erinacea 0 243 0 Mojave yucca Yucca schidigera 374 364 625 Mormon tea Ephedra nevadensis 1,123 647 1,250 Pima rhatany Krameria erecta 749 566 0 Total shrub densitt 14,800 8,900 12,500 lBBS = Blackbrush Scrub. 2JTW = Joshua Tree Woodland. 3MMS = Mojave Mixed Steppe. 4The calculation of total shrub density is based upon all observed species, including several less common shrubs not displayed in this table.

183 Table 2-Average distance (m) and standard error between the nearest neighbor (NN) and the nearest conspecific (NCSP) of several species of shrubs surrounding Castle Mountain Mine.

BBS JTW MMS Species NN NCSP NN NCSP NN NCSP Anderson's thornbush 0.40 ± 0.06 1.24 ± 0.46 0.79±0.13 1.79 ± 0.29 0.50 ± 0.11 1.04 ± 0.31 Beavertail cactus 0.50 ± 0.22 5.39 ± 1.88 Blackbrush 0.61 ± 0.03 1.39 ± 0.60 0.62 ± 0.07 1.05 ± 0.17 Buckhorn cholla 1.83 ± 1.00 6.48 ± 2.79 California barrel cactus 0.72 ± 0.17 2.01 ± 0.73 California buckwheat 0.53 ± 0.08 0.59 ± 0.09 Cooper's goldenbush 0.77 ± 0.09 1.1 ± 0.3 0.70 ± 0.06 1.05 ± 0.16 1.02 ± 0.78 2.17 ± 0.61 Creosote bush 1.02 ± 0.30 3.40 ± 0.98 1.07 ± 0.13 1.83 ± 0.29 0.69 ± 0.10 1.64 ±0.29 Joshua tree 6.97 ± 0.69 6.97 ± 0.69 Mojave prickly pear 0.74±0.19 3.55 ± 0.77 Mojave yucca 0.47 ± 0.22 5.67 ± 0.03 0.45 ± 0.17 10.31 ± 1.96 1.10±0.76 7.27 ±2.13 Mormon tea 0.72 ± 0.30 1.03 ± 0.49 0.79 ± 0.16 2.72 ± 0.70 0.62 ± 0.29 1.15 ± 0.55 Pima rhatany 0.38 ± 0.13 0.98 ± 0.31 0.51 ± 0.11 1.50 ± 0.34

The density data indicates that Joshua tree woodland Neighbor Preference or Avoidance is primarily composed of a variety of small shrub species (table 1), with Joshua trees occurring at relatively low Neighbor preference or avoidance was only observed with densities (374lha, nearly the same density as in BBS). a few species, and positive interactions (neighbor prefer­ Cooper's goldenbush was the most abundant species, occur­ ences) were more common than negative interactions (neigh­ ring at a density nearly three times that ofthe two next most bor avoidances). Conspecific preferences were observed for abundant species in JTW; Anderson's thornbush and creo­ Anderson's thornbush, blackbrush, California buckwheat, sote bush (Larrea tridentata). Cooper's goldenbush, creosote bush, Mormon tea, and pima In the MMS community Joshua trees were absent, while rhatany. Avoidance was observed between Anderson's thorn­ Mojave yucca occurred at a density nearly equal to the bush and creosote bush, Cooper's goldenbush, and pima combined density of both Yucca species in BBS or JTW. rhatany. The significant neighbor preferences and avoid­ There were few species of cacti in MMS, aside from the ances for Cooper's goldenbush are displayed as an example California barrel cactus, (Ferocactu8 cylindraceus), a domi­ (table 4). nant plant on rocky slopes surrounding the mine (table 1).

Distance Between Plants

The average distance between shrubs and their nearest Table 3-Dispersion pattern of common shrubs in three plant neighbors was found to be similar among communities (table 2). communities near Castle Mountain Mine, in the East Most shrub species were found to grow within 0.5-1.0 m of Mojave Desert (p < 0.05). another shrub. The average distance between conspecifics Species BBS JTW MMS was typically much higher than the average distance to a nearest neighbor. Anderson's thornbush random random random Beavertail cactus random Blackbrush regular random Dispersion Buckhorn cholla random California barrel cactus regular Most shrubs were found to be randomly dispersed (table 3). California buckwheat clumped A regular dispersion pattern was observed for some large Cooper's golden bush random shrubs for example, blackbrush, California barrel cactus, creo­ Creosote bush regular random random sote bush, Joshua tree, and Mojave yucca; table 3). California Joshua tree regular regular Mojave prickly pear random buckwheat (Eriogonum fasciculatum) was the only species Mojave yucca regular tested which showed a clumped distribution. Mormon tea random random random Pima rhatany random random

184 Table 4-Selected results of chi square analysis of first and second nearest neighbor data for Cooper's goldenbush. Results with a Chi square value greater than 3.88 indicate significance at p < 0.05. Insignificant results are not displayed.

Cooper's goldenbush NN1 NN2 Neighbors Expected Observed Expected Observed Cooper's goldenbush 24.88 42 11.77 27.54 49 16.72 Creosote bush 8.86 3 3.88 9.8 a 9.8

2 Discussion expected based upon mean density (approximately 1 plantim ). ------We also observed that the nearest conspecific was usually Density much further away (1.0-7.0 m) than the nearest neighbor (of any species). These findings are consistent with the observa­ High shrub densities were found in BBS and MMS com­ tions that shrubs may form small groups or "islands" where munities (12,500-14,800 individualslha;table 1). These find­ plants are more densely packed, and that these islands may ings agree with Lei and Walker (1994) who reported that often be composed of several shrub species. Shrub islands blackbrush were "extremely abundant" within BBS commu­ are common in the deserts of the Southwest, and they play nities (in southern Nevada), and Holland (1986) who charac­ an important role in the ecology of desert shrubs (Goodall terized MMS as being a "fairly dense" plant assemblage. and Perry 1979). Thus, planting inter-specific assemblages Everett (1991) also noted that density and diversity increase of shrubs may bean effective revegetation strategy. in MMS sites with increased grade and rockiness, and these sites were rocky and relatively steep grade. Cody (1986) noted that species diversity and density is related to sam­ Dispersion and Neighbor Preference pling area. Consequently, more species may have been noted The results of the dispersion and neighbor preference data in BBS and MMS communities if sampling intensity were analysis indicates that shrubs are generally randomly dis­ higher. persed (in relation to other individuals of the same species), Although less abundant than BBS or MMS, plant density with a few larger shrub species being regularly dispersed in JTW (table 1) was similar to, but higher, than values (table 3). A random dispersion may simplify revegetation at reported by Cody (1986); 8,900 plants/ha vs. 7,000 to 8,000 the mine provided that shrubs are not planted near incom­ plants/ha. However, much of the desert surrounding the patible neighbors. The mechanism(s) creating a regular mine is free range, and JTW suffers the most intense grazing dispersion of trees, such as Joshua trees, is unknown but pressure. Evidence of grazing is expressed as heavily browsed using the wide spacing observed in nature may be important grasses and shrubs and well marked cattle trails. Thus, for revegetation success. The clumped dispersion of Califor­ plant density may be reduced in JTW because of herbivory. nia buckwheat is probably the result of the rocky terrain If the 900 acres of disturbance were revegetated at a where sites for plant establishment are not uniformly dis­ density similar to that of an undisturbed community (ap­ 2 tributed across the quadrat. proximately 1 plantim ), more than 3.6 million shrubs would The results of the nearest neighbor analysis are less clear. be required. Producing and planting this many shrubs Conspecific preferences were noted for several species in­ would be impossible; however, plant spacing and neighbor cluding blackbrush, and Cooper's goldenbush (table 4), al­ interactions are still important. though these interactions are less common for second near­ est neighbors than they are for first nearest neighbors. It is Diversity possible that observed conspecific preferences are the result of clonal growth forms, however, this hypothesis is not The low species diversity found in BBS was also noted by supported by distances to nearest conspecifics (which are Lei and Walker (1995) who attributed the low diversity in typically longer than distances to the nearest neighbor). BBS to blackbrush abundance. Species diversity in JTW (26 species) was similar to Cody's (1986) findings (35 species); although Cody's study, unlike ours, included perennial grass Conclusions species. Much of the diversity in JTW comes from small ------shrubs. Larger shrubs such as creosote bush, Mojave yucca, Species composition and patterns in vegetation are often and Joshua tree are visually important, however, they were difficult to distinguish without performing systematic sam­ found to grow in low densities (table 1). The revegetation pling. Vegetation sampling should be conducted to improve program's emphasis on species diversity appears to be an the chance of revegetation success by detecting spacing ecologically sound management criteria. patterns and neighbor interactions, and to avoid focusing on charismatic mega-flora. Natural plant assemblages in desert systems may be slope, aspect, or substrate dependent, and Distance belowground relations may be altered on disturbed sub­ strates. However, belowground competition has been docu­ Distance to nearest neighbor data (table 2) show that mented and should be considered when planning revegeta­ many shrubs grow closer together (0.5-1.0 m) than might be tion projects.

185 Fonteyn, P. J. and B. E. Mahall. 1981. An experimental analysis of Acknowledgments structure in a desert plant community. Journal of Ecology. ------69:883-896. We thank John Tiszler, Robert MacAller, and Debbie Goodall, D. W. and R. A Perry, eds. 1979. Arid Land Ecosystems: Waldecker for field assistance and helpful editorial suggestions. Structure, Functioning, and Management, Vol. 1, (International Biological Programme 16). Cambridge University Press. 881 pp. Holland, R. F. 1986. Preliminary Descriptions of the Terrestrial References Natural Communities of California. State of California. The ------Resources Agency. Department ofFish and Game. Sacramento, Bainbridge, D. A and M. W. Fidelibus. 1994. Treeshelters improve CA 156 pp. woody transplant survival on arid lands (California). Restoration Lei, S. A and L. R. Walker. 1995. Composition and distribution of and Management Notes. 12(1): 86. (Coleogyne ramosissima) communities in southern Nevada. In: Bainbridge, D. A and R. A Virginia. 1990. Restoration in the Roundy, Bruce A; McArthur, E. Durant; Haley, Jennifer S.; Sonoran Desert. Restoration and Management Notes. 8(1):3-14. Mann, David K., comps. 1993. Proceedings: wildland shrub and Barbour, M. G. 1968. Germination requirements for the desert arid land restoration symposium; 1993 October 19-21; Las Vegas, shrub Larrea tridentata. Ecology. 50: 679-685. NY. Gen. Tech. Rep. INT-GTR-315. Ogden, UT; U.S. Department Chew, R. W. and A E. Chew. 1965. The primary productivity of a of Agriculture, Forest Service, Intermountain Research Station. desert-shrub (Larrea tridentata) community. Ecological mono­ pp:192-195. graphs. 35:355-375. Lippitt, L., M. W. Fidelibus, and D. A Bainbridge. 1994. Native seed Clark, P. J. and F. C. Evans. 1954. Distance to nearest neighbor as collection, processing, and storage for revegetation projects in the a measure of spatial relationships in populations. Ecology. western United States. Restoration Ecology. 2(2):120-131. 35(4 ):445-453. Romney, E. M., A Wallace, and R. B. Hunter. 1989. Transplanting Cody, M. L. 1986. Spacing patterns in Mojave Desert plant commu­ of native shrubs on disturbed land in the Mojave Desert. In: nities: near-neighbor analyses. Journal of Arid Environments. Wallace, A, E. D. McArthur, and M. R. Haferkamp, comps. 11:199-217. Proceedings-Symposium on Shrub Ecophysiology and Biotech­ Cottam, G. and J. T. Curtis. 1956. Use of distance measures in nology, Logan, Utah, June 30-July 2, 1987. pp: 50-53. phytosociological sampling. Ecology. 37:451-460. Rundel, P. W. and P. S. Nobel. 1991. Structure and function Everett, R. G. 1991. Castle Mountain Mine Project, San Bernadino in desert root systems. In: Plant Root Growth: An Ecological County, CA, Vegetation Analysis. California Desert Studies Con­ Perspective. D. Atkinson, ed. Blackwell Scientific Publications. sortium, Fullerton, CA London. p. 349-378. Fidelibus, M. W. and D. A Bainbridge. 1994. The effect of containerless transportation on desert shrubs. Tree Planters' Notes. 45(3):82-86.

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