Spatial Distribution of Blackbrush (Coleogyne ramosissima Torr.) Populations in the Mojave Desert.
The pattern of spatial distribution of a plant population is a fundamental characteristic of that population, but it is a feature that is extremely difficult to describe in precise and meaningful terms (Clark and Evans 1954). At the local level, even in a relatively homogeneous environment, individuals in a single plant population can be distributed randomly, clumped together, or in highly regular patterns (Cunningham and Saigo 1999).
Blackbrush (Coleogyne ramosissima Torr.) shrubs are established primarily along the Colorado River drainage and several adjacent enclosed basins of the Great Basin-Mojave Desert transition (Bowns and West 1976). Coleogyne ramosissima shrubs are a common vegetation type in mid-elevations of the Mojave Desert, with creosote bush-white bursage (Larrea tridentata-Ambrosia dumosa) shrublands occurring on valley floors and lower desert mountain slopes and with pinyon pine-Utah juniper (Pinus monophylla-Juniperus osteosperma) woodlands occurring at higher mountain slopes (Lei 1995).
Some ecological aspects of C. ramosissima and its shrublands have been documented (Beatley 1974 and 1976; Bowns 1973; Bowns and West 1976; Bradley 1964; Brown 1982; Callison and Brotherson 1985; Korthuis 1988; Lei 1995, 1999a, b; Lei and Walker 1995, 1997a, b; and Pendleton et al. 1995 and 1999). However, C. ramosissima was selected as a representative species for such a study because the spatial distribution of individuals and populations within the C. ramosissima shrublands in isolated mountain ranges in the Mojave Desert has not been quantatively investigated. From casual observations, the spatial distribution of C. ramosissima individuals and populations appears to be clumped throughout the Mojave Desert. This hypothesis was tested by field measurements with appropriate statistical analysis.
Field studies were conducted at fully developed C. ramosissima shrublands throughout the Mojave Desert during July 2000. The five study sites were located in the Spring and Newberry Mountains of southern Nevada, the Clark Mountain of southeastern California, the Virgin Mountains of northwestern Arizona, and the Mormon Range of southwestern Utah (Table 1). Elevation varied considerably, ranging from 1,250 m in the Mormon Range to 1,450 m in the Spring Mountains. These five sites were fairly representative of C. ramosissima populations, and formed nearly monospecific blackbrush vegetation zones in the Mojave Desert. Each of the five sites represented an isolated mountain range for C. ramosissima population.
The weather pattern of the Mojave Desert is characterized by wide-ranging daily air temperatures, high year-round air temperatures, extremely high potential evaporation, little cloud cover, low relatively humidity, low annual precipitation (Lei 1999b). The Mojave Desert resembles the Mediterranean-type climate characterized by hot, dry summers and cool, wet winters. Episodic monsoonal winds and thunderstorms occur during summer seasons. Winter rainfalls, although varying considerably from year to year, contribute to most of the total annual precipitation (Lei 1999b). The terrain consists of rocky slopes and alluvial fans dissected by dry wash channels. Soil profiles are poorly developed, and soils are composed primarily of weathered granite and limestone bedrock. Organic decomposition and soil formation are slow due to the arid nature of the region (Lei 1999b).
Within each site, the spatial distribution of C. ramosissima population was examined on a 10-ha plot. The nearest neighbor method was used to determine whether shrubs of the same species are distributed at random, are clumped, or are regular (Barbour et al. 1987). At each site, 100 random points were distributed evenly among the four parallel transects located on the 10-ha plot. Intervals between two parallel transects are 20 m apart. However, intervals between two points within a transect were randomly selected to avoid biased field measurements. The distance measured was from the center of individual C. ramosissima located closest to the random point to the center of its nearest C. ramosissima neighbor.
One-way Analysis of Variance (ANOVA), followed by Tukey's Multiple Comparison Test (Analytical Software 1994) was used to detect differences among distances between one C. ramosissima to its nearest C. ramosissima neighbor, and to compare site means when a significant distance effect was detected, respectively. Mean distance values were presented with standard errors, and statistical significance was determined at P [less than or equal to] 0.05.
The ratio of the observed mean distance to the expected mean distance (R-value) served as a measure of departure from randomness (Clark and Evans 1954). The nearest neighbor method (Clark and Evans 1954) was used to determine whether the C. ramosissima populations were randomly distributed within the C. ramosissima shrublands. The significant difference in the value of R for C. ramosissima populations was tested with the c value of 1.96, representing the 5% level of significance for a two-tailed test (Clark and Evans 1954; McClave and Dietrich 1991).
Mean distances between C. ramosissima and its nearest C. ramosissima neighbor were significantly different (P [less than or equal to] 0.05; Table 2) among the five isolated mountain ranges in the Mojave Desert. The mean distances ranged from 210 cm in the Mormon Range in southwestern Utah to 275 cm in the Clark Mountains of southeastern California.
Coleogyne ramosissima populations among the five isolated mountains ranges were strongly aggregated, ranging from 0.13 in the Mormon Range to 0.17 in the Clark Mountains (Table 2). When examining the spatial distribution of C. ramosissima populations, there was a tightly clustering of mean values of distance from one C. ramosissima plant to another and for R-values. For the spatial distribution in this study, R [much less than] 1.0, indicating a significant departure from random expectation in the direction of aggregated spacing by the c test. According to the distance to nearest neighbor as a measure of spatial distribution in plant populations, R = 0 in a maximum aggregation, since all of the individuals occupy the same locus and the distance to nearest neighbor is therefore 0. In contrast, R = 2.1491 in a maximum uniformity, since individuals will be distributed as evenly and widely as possible in a hexagonal pattern (Clark and Evans 1954). The ratio of observed to expected mean distance to nearest neighb or provides a measure of the degree to which the distributional pattern of the observed population deviates from random expectation (Clark and Evans 1954).
The distance from one individual plant to another provides a variable for the measurement of spacing that obviates the use of quadrats, and, therefore, eliminates the effect of quadrat size (Goodall 1953). The measure of spacing in this study is a measure of the degree to which the distribution of individuals in C. ramosissima populations at given areas departs from that of a random distribution. Thus, randomness is a spatial concept, intimately dependent upon the boundaries of the space chosen by the investigator (Clark and Evans 1954). In this study, all five C. ramosissima populations on isolated mountains ranges clearly departed from random expectation with a high degree of significance as the distance to nearest neighbor is concerned. Although significant differences in mean distances were detected, only a slight difference in the degree of aggregation was found among the five C. ramosissima populations in the Mojave Desert.
I gratefully acknowledge Steven Lei, David Valenzuela, and Shevaun Valenzuela for valuable field assistance. Steven Lei assisted with statistical analysis and provided helpful comments on earlier versions of this manuscript.
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Accepted for publication 28 January 2001.
Table 1. Geographical characteristics of the five C. ramosissima study sites. Location, as well as approximate latitude (N), longitude (W), and elevation (m) are shown. Mountain ranges are arranged alphabetically in the Mojave Desert. Location County, State Latitude Clark Mountain San Bernadinom, CA 35[degrees]05' Mormon Range Washington, UT 37[degrees]20' Newberry Mountains Clark, NV 35[degrees]20' Spring Mountains Clark, NV 35[degrees]50' Virgin Mountains Mohave, AZ 36[degrees]50' Location Longitude Elevation 2Clark Mountain 115[degrees]50' 1300 Mormon Range 114[degrees]00' 1250 Newberry Mountains 114[degrees]50' 1300 Spring Mountains 115[degrees]35' 1450 Virgin Mountains 114[degrees]00' 1300 Table 2 Distance (mean [+ or -] SE) and R-values from one individual C. ramosissima to its nearest C. ramosissima neighbor in five isolated mountain ranges of the Mojave Desert (N = 100 per mountain range). Mean values followed by different letters indicate significant differences at P [less than or equal to] 0.05 using Tukey's Multiple Comparison Test. Location Mean distance (cm) R-value Clark Mountain 275 [+ or -] 24.7 a 0.17 Mormon Range 210 [+ or -] 19.4 b 0.13 Newberry Mountains 251 [+ or -] 22.3 ab 0.16 Spring Mountains 239 [+ or -] 28.4 ab 0.15 Virgin Mountains 247 [+ or -] 11.2 ab 0.16
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|Author:||Lei, Simon A.|
|Publication:||Bulletin (Southern California Academy of Sciences)|
|Date:||Aug 1, 2001|
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