Mauz, K. 2004. Spatial Variability in Saguaro Growth from a Serendipitous Timeline in the Sonoran Desert. Journal

Cactus in four populations of saguaros (Carnegiea gigantea) that were damaged in a 1987 hailstorm were measured, and the heights of individuals in 1987 and 2003 were obtained trigonometrically. Net growth in this 16-year interval ranged from 0.15-3.00 m across the 300 saguaros measured, and averaged 1.03-1.48 m across the four sites. Average cactus height in both years, as well as net growth, differed significantly between sites. Mean net growth was strongly correlated with mean site elevation, despite that neither 1987 nor 2003 mean saguaro height were. Bivariate relationships between height in 2003 and height in 1987 were strong and positive at each site. Corresponding relationships between net growth and 1987 height were each negative – an outcome consistent with other studies which indicate declining saguaro growth rate with age – but these associations were weak at all sites. For both height in 2003 and change in height, the effect of 1987 height as an explanatory variable was dependent upon site. Log-transformed percent-change-inheight was strongly, negatively correlated with 1987 height, and this relationship was not site-dependent. INTRODUCTION AND OBJECTIVES During a week of late-season storms in October 1987, a violent hailstorm struck a portion of the Sonoran Desert north of Gila Bend, Arizona, causing widespread damage to infrastructure and wildland vegetation alike (R. Hanson, BLM, pers. comm.). The likely date of this event was October 29th, when a storm bearing “ping-pong ball and baseball size” hail formed southwest of Phoenix and moved to the east-northeast, affecting a region from at least Gila Bend to Interstate 10 west of Phoenix (Arizona Storms Database 2002). Across a swath of several square miles in the intervening region, the southwest face of nearly every saguaro (Carnegiea gigantea) exhibits evidence of hail-related tissue damage in the form of black-scarred holes and scabs in the epidermis and cortex of its stem (anatomical terms of Steelink et al.1967). Although the density and severity of these injuries varies across the storm-affected area, on a typical cactus the damaged portion of the stem extends uniformly from the base to a definable level, above which the surface is normal (Fig.1). As damage was incurred relatively instantaneously across this population, essentially forming a time line on the cacti, this pattern presents a unique opportunity to document spatial variability in saguaro growth. Variability in net growth over the 16-year interval is described at two scales: within study sites across a spatially contiguous population of individuals, and between study sites across an elevation gradient. STUDY AREA Location The study area was in the north-central Sonoran Desert, northeast of Gila Bend, Maricopa County, southwest Arizona (Fig. 2), in the northwestern portion of the saguaro's range (see Turner et al. 1995). Study sites were within the northern Maricopa Mountains on the Sonoran Desert National Monument. Fieldwork was conducted with permission from the Bureau of Land Management Phoenix Field Office, which administers the Monument. Physical Geography Both uplands and piedmont surfaces in this area comprise outcrops and sediment derived from rocks of granitic composition (Reynolds and Skotnicki 1993, Cunningham et al. 1987). Weathered porphyritic granite, in particular, has contributed to the angular and coarse, light-colored pavements on alluvial surfaces in the study area. Shallow bedrock surfaces of low relief are common in the piedmont areas, more so than existing geologic maps suggest. In the northern Maricopa Mountains, the majority of the piedmont area is occupied by alluvium of Middle to Late Pleistocene age (250-10 ka B.P.). Remnants of older (Middle Pleistocene age, 790250 ka B.P.) surfaces are present within the range, particularly immediately adjacent to and adjoining the mountain front. Isolated fan remnants of this age are also present on the west side of the range between the mountain front and the Gila River. 96 SPATIAL VARIABILITY OF SAGUARO GROWTH g MAUZ Figure 2. Study area location in Arizona (inset). Map shows geographic features referred to in the text: Sonoran Desert National Monument (SDNM), North Maricopa Mountains Wilderness Area (NMM), and the towns of Gila Bend (G), Buckeye (B), and Maricopa (M), relative to major interstate highways. Bold square is the location of the hail-damaged saguaro area illustrated in Figure 3. Figure 1. An example of a hail-damaged saguaro in the study area, showing the typical black-scarred, dimpled stem. Arrow indicates the top of the damaged portion, above which the surface – growth since 1987 – is normal in appearance. Photo by the author, Dec 2003. Holocene age and active alluvial deposits occur in the Rainbow Valley to the northeast, on the Gila Plain to the west, and along large washes within the study area (Demsey 1989). The granite-gravel surfaces in this area are consistently covered with lichens and other manifestations of cryptobiotic organisms. Precipitation in this part of the Sonoran Desert is generally low and bimodal in its distribution, with about half of the annual total falling during the summer monsoon and half falling during the winter months (Comrie and Broyles 2002). Drought is most pronounced during the late spring. Average annual precipitation in the study area is 201 mm. Interannual variability is high, and annual rainfall has ranged between extremes of 86 mm and 417 mm over a 72-year record for stations at Gila Bend, Buckeye, and Maricopa. Average monthly temperature ranges from 5oC in January to 35oC in July (NOAA 2002). METHODS The area over which the hail-damaged cacti occur was delineated on aerial photographs and topographic maps from direct observation and, in some locations, by remote observation using binoculars (10×) and the theodolite (25×) as a telescope. Coordinates of cactus in the hail-affected area and of measured cacti were collected using a hand-held Garmin 12 GPS receiver. Reconnaissance and mapping occurred on several occasions during 20022003, and all measurements at these study sites were made in October and December 2003. Potential study sites within this area were identified based upon aerial photographs and initial reconnaissance in the field. Four sites, each occupying a relatively uniform and extensive alluvial surface that also supported a substantial number of haildamaged saguaros, were selected. Study sites 1, 2, and 3 were located on Middle-Late Pleistocene age surfaces, and Site 4 was on a Middle Pleistocene age surface, based upon the mapping of Demsey (1989). The sites were spaced at intervals of approximately 2 km, roughly parallel to the direction of hailstorm movement. Elevation, slope, and prevailing aspect of the four study sites were derived from 10-m and 30-m National Elevation Dataset digital elevation models (USGS NED SPATIAL VARIABILITY OF SAGUARO GROWTH g MAUZ 97 Table 1. Physical attributes of study sites where saguaros were measured.