Carnegiea Gigantea, Cactaceae) Growth Rate Over Its American Range and the Link to Summer Precipitation
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THE SOUTHWESTERN NATURALIST 50(1):65±106 MARCH 2005 NOTES SAGUARO (CARNEGIEA GIGANTEA, CACTACEAE) GROWTH RATE OVER ITS AMERICAN RANGE AND THE LINK TO SUMMER PRECIPITATION TALY DAWN DREZNER* Department of Geography, Bolton 410, P.O. Box 413, University of Wisconsin±Milwaukee, Milwaukee, WI 53201-0413 *Correspondent: [email protected] ABSTRACT Using an index of saguaro (Carnegiea gigantea) growth rate, relative growth was com- pared among multiple saguaro populations and mapped across the American range of the species. Relative growth at 10 sites was strongly linked to summer precipitation. This is the ®rst study of relative growth and the ®rst to link growth rate to environmental variables in multiple populations. RESUMEN Usando unÂndice õ de la tasa de crecimiento del saguaro (Carnegiea gigantea), se com- paro el crecimiento relativo entre poblaciones muÂltiples de saguaro y se ilustro a traveÂs del rango americano de la especie. El crecimiento relativo en 10 sitios fue fuertemente ligado a la precipi- tacioÂn de verano. Este es el primer estudio de crecimiento relativo y el primero en ligar la tasa de crecimiento a variables ambientales en poblaciones muÂltiples. Saguaro (Carnegiea gigantea) growth rates are The ®rst objective of this study was to com- generally consistent relative to height (e.g., pare saguaro growth rates across multiple lo- slow growth at youth, highest growth rates be- cations that are geographically distant. In 2000, tween 3 and 4 m in height, and declining I used a telescoping leveling rod to collect data growth rates following) (Steenbergh and on saguaro trunk height for 16 to 21 saguaros Lowe, 1983; Turner, 1990; Niklas and Buch- of varying heights and number of branches at man, 1994). Actual growth, however, varies by several locales, all in ¯at areas (growth rate locale (Steenbergh and Lowe, 1983). Drezner varies by slope and aspect; Pierson and Turner, (2003a) recently developed a new method to 1998); washes were excluded (McAuliffe and estimate saguaro growth rate and age in any Janzen, 1986; Drezner, 2003b). I revisited 2 of population. By assuming a ®xed growth pat- these locales in 2002 (see also Drezner and tern by height, a local growth factor can be Balling, 2002), and I revisited 5 others in 2003. computed from repeat sampling of individuals. I calculated the factor for the 7 resampled This value serves as an index, and growth rates locales by the procedure outlined in Drezner can then be compared between populations (2003a). The Drezner (2003a) technique is (Drezner, 2003a). The value of the factor at based on a single general formula that models Saguaro National Park, East was set to 1.0, by saguaro growth over its life, with a multiplica- de®nition, and values were calculated for Sa- tive adjustment factor for relative growth at guaro National Park, West (factor 5 0.743) each site. This local adjustment factor (or in- and Organ Pipe Cactus National Monument dex value) can be calculated through repeat (factor 5 0.617) (Drezner, 2003a). Thus, for sampling of individuals and ®tting height and example, a saguaro that grows 10 cm per year growth at that height to the general growth at a given height at Saguaro National Park, curve. Height and growth are ®tted to the East will only grow 7.43 cm at that same height growth curve by minimizing the sum of at Saguaro National Park, West and 6.17 cm at squares, the same principle used for ®tting a Organ Pipe Cactus National Monument. regression line (Drezner, 2003a). 66 The Southwestern Naturalist vol. 50, no. 1 For the ®rst time, saguaro growth in differ- able, stepwise regression yielded one variable, ent populations (with unique population struc- mean July precipitation (P , 0.001; r2 5 0.84; tures) can be quanti®ed and compared across Fig. 2). The resulting formula based on these the range of the species. A preliminary map of 10 locales is: growth factor 5 0.2705 1 0.2713 saguaro growth rates over its Arizona range is (mean July precipitation). shown in Fig. 1. Saguaros generally exhibit rel- One could indirectly estimate the growth atively fast growth rates in the southeast, and factor if mean July precipitation is available. It growth rates decline northward and westward is important to note, however, that the growth across their American range. factor is an estimate, and relying on summer A second objective of this study was to ex- precipitation data to deduce growth rate might plore the relationship between saguaro growth compound the error and should be discour- rate and climate over its geographic range in aged. The growth factor should be calculated Arizona. I calculated 62 monthly climate vari- from direct ®eld observation. It is clear that ables. These were based on 30 years of data where July rain is high, growth rates are high, (1968 through 1997) from 47 ®rst-order me- and areas that receive less July rainfall support teorological stations and the cooperative ob- slower growing saguaro populations. The pop- serving network (National Climate Data Cen- ulations sampled in this study exhibited sub- ter, Summary of the Day database) ranging stantial variations in growth rate (Fig. 1). For 8 from the USA±Mexico border to 34.5 N lati- example, a 5-m tall saguaro at the westernmost tude and from the Arizona±California border site (Fig. 1) with a factor of 0.41 is estimated 8 to 110 W longitude and 4 stations in eastern- to be 135 years old (e.g., Drezner and Balling, most California. Although summer precipita- 2002), while at Saguaro National Park, East, a tion is highly variable over the short term (e.g., 5-m tall saguaro is estimated to be only 59 years 1 rain event), the data for precipitation are re- old (Steenbergh and Lowe, 1983). Thus, if no liable when calculated for a 30-year period. ®eld data are available, mean July precipitation The 62 variables used were lowest maximum will likely provide a better estimate than the monthly temperature and lowest minimum data of Steenbergh and Lowe (1983) from Sa- temperature in December, in January, in Feb- guaro National Park (which receives large ruary, and in all 3 months combined (8 vari- quantities of summer rain); this is most notably ables); mean minimum and mean maximum true for sites far from Saguaro National Park monthly temperature for these same months and the high summer rainfall belt. Nonethe- (8 variables); mean maximum monthly tem- less, extreme caution should be used in inter- perature in May through September (5 vari- preting any results based on mean July precip- ables); mean minimum monthly temperature in May through September (5 variables); high- itation data rather than direct ®eld observa- est maximum monthly temperature in May tion of populations. through September (5 variables); highest min- Others have observed, through intensive di- imum monthly temperature in May through rect measurement over a summer season, that September (5 variables); highest maximum saguaros grow in response to rainfall and dur- temperature observed from June to August (1 ing the summer (Hastings, 1959±1960; Has- variable); maximum monthly precipitation and tings and Alcorn, 1961; Steenbergh and Lowe, average monthly precipitation (24 variables); 1983). These observations have all been made and mean annual precipitation (1 variable). at Saguaro National Park. The present study These were interpolated to the 10 sampled lo- quanti®ed growth rate at multiple locales over cales based on the methods described in Drez- the northern Sonoran Desert. For the ®rst ner (2003b), with adjustments from Steen- time, a quantitative comparison of growth rate bergh and Lowe (1983). A complete list of the in different populations in different parts of meteorological stations and their locations is the range of the species has been conducted, provided in Drezner and Garrity (2003). Using and a preliminary trend map is shown that de- the Kolmogorov-Smirnov normality test, all var- picts saguaro growth patterns over the north- iables were normally distributed at P . 0.01, ern portion of their range. Finally, after an and most were normal at P . 0.05. analysis that included many possible environ- With growth factor as the dependent vari- mental variables that might be linked with March 2005 Notes 67 FIG.1 The location and growth factor of the 10 populations of saguaro (Carnegiea gigantea) sampled, including Saguaro National Park East (SNP-E), Saguaro National Park West (SNP-W), and Organ Pipe Cactus National Monument (OPCNM). The growth factor values for SNP-W, SNP-E, and OPCNM were derived using the data of Steenbergh and Lowe (1983) (Drezner, 2003a). The saguaro range is based on Turner et al. (1995). 68 The Southwestern Naturalist vol. 50, no. 1 FIG.2. The regression line for the growth factor of saguaros (Carnegiea gigantea) in 10 northern Sonoran Desert plots and mean July precipitation (cm). growth rate, I linked summer precipitation HASTINGS, J. R. 1959±1960. Precipitation and saguaro with growth over Arizona. growth. University of Arizona Arid Lands Collo- quia 1959±1960/1960±1961:30±38. This project was supported in part by the National HASTINGS,J.R.,AND S. M. ALCORN. 1961. Physical Science Foundation (NSF#9987612). I thank A. Ellis, determinations of growth and age in the giant Arizona State Climatologist, for climate data, R. cactus. Journal of the Arizona Academy of Sci- Turner for helpful suggestions, M. Weesner, Saguaro ence 2:32±39. National Park Science and Resource Management MCAULIFFE,J.R.,AND F. J. JANZEN. 1986. Effects of Chief for information about climate data, and B. intraspeci®c crowding on water uptake, water Trapido-Lurie for help in the creation of the map. I storage, apical growth, and reproductive poten- thank R.