Truncated Saguaro Spear Survey, McDowell Sonoran Preserve, Scottsdale, Arizona
Note: Tables, photographs, and charts called out in this journal entry are available upon request.
A Photographic Essay of Resilient Saguaros
Introduction
Saguaros in the McDowell Sonoran Preserve (Preserve) in north Scottsdale, Arizona are under stress from an ongoing drought. The impact of the drought on the saguaro population in the Preserve is evident from the increased mortality of mature plants and the loss of arms and/or central spear. A sample census of saguaros along judgmentally selected trails and repeat photography of saguaros with a truncated central spear is an on-going project to document the health status of the saguaro community in the Preserve (Figure 1). Resilient saguaros are defined as a living plant whose central spear has been truncated or severely damaged.
Selected saguaros along 11 established trails are used as a proxy to measure the drought’s impact on the saguaro community by using a qualitative examination of plants in the Preserve. Photographs of saguaros whose central spear has been truncated are previously stressed plants. Following the plant’s health and continued growth through time is expected to measure the rate of mortality for the most vulnerable plants. The current health status of these truncated saguaros is documented from photographs that generally show the number of arms, their lengths and diameter, and angle of arm(s) relative to the main trunk. Repeat photography of the plants will allow assessment of health status and mortality.
Methods
An estimate of the Preserve’s saguaro population is created through a census of saguaros that are situated within 20-30 m on either side (approximately 200 feet total width) of the 11 trails in this study. Size of the sample survey is calculated as: Length of trail x 200 ft ÷ 43,560 sq ft = acres. Saguaros in the census are classified by height (<6 feet tall, >6 feet tall but no arms, plants with arms, and dead saguaros), plus a separate class for a truncated or severely damaged central spear. A saguaro was not counted if the base of the plant could not be seen from the trail because of obscuring brush or a ridge/elevation. This requirement to see the base of the plant prevents duplicate enumeration of plants where trails may loop back upon themselves as they traverse both sides of ridge tops. The initial census was conducted in July 2023 when weeds were high, and the survey could have missed small saguaros that are shorter than 1 ft.
Repeat photographs of saguaros with damaged or missing central spears are collected during pedestrian surveys within the Preserve. The photo stand for each plant’s location is documented with a hand-held GPS unit using WAAS-enabled software that displays the UTM (NAD83).
A qualitative assessment of the environmental setting along the 11 surveyed trails is based on presence/absence of boulders or bedrock and topography (level, rolling hills, steep slopes, or mixed). Slope face exposure is noted for the slopes. The landscape surrounding the trails are classified as having recent evidence for catastrophic wildfire damage (none, some, or severe) on the basis of scorched saguaros and charred plant remains.
Purpose and Expected Results
Documentation of the truncated saguaro’s condition in July 2023 is compared through repeat photographs to assess whether they remain static, thrive, or die. Repeat photographs will provide a record of the condition of individual plants and document trends in growth and/or death over time. Expected changes to truncated saguaros that survive are: 1) arms will grow close to the point of truncation, 2) existing arms will have enhanced buttress growth near the trunk, 3) the number of arms will increase, 4) arms may start growing close to the base of plant, and 5) the plant’s diameter below the truncation will become more robust. Severely stressed saguaros may also exhibit higher death rates than undamaged plants.
Trails as Sample Transects
Eleven trails selected for the judgmental sample of the study area are north of Dynamite Boulevard/Rio Verde Drive in the Preserve (Table 1). Lengths of the trail examined for the census are based on distances provided in the map “Scottsdale’s McDowell Sonoran Preserve Northern & Central Region Spring 2023.” Beginning and ending points of the sample transect are based on the waypoint designations on trail signs (see Table 1).
The trail’s environmental settings are described by three characteristics: 1) recent evidence for wildfire damage (none, some, or severe) on the basis of scorched saguaros and other charred plant remains in the vicinity of the trail, 2) presence/absence of boulders or bedrock, and 3) overall topography (level, rolling hills, steep slopes, or mixed). The direction of slope face exposure is noted for cardinal directions.
Densities of plants are compared between trails are expressed as an average number of saguaros per acre. The number of acres surveyed is calculated by multiplying the length of each trail segment times the approximate width (200 ft [60 m]) examined and dividing by 43,560 sq ft. Previous portions of the Preserve have burned, and evidence of fire’s impacts is noted to partially explain differences in plant densities.
Data Presentation and Results
Trails that are close to each other in the Preserve are grouped together into five areas. The study area of the Preserve is north of Dynamite Boulevard/Rio Verde Drive and divided into northeast, southeast, south-central, southwest, and central portions.
Southwest Study Area in the Preserve
Three trails (Axle Grease, Rock Tank, and Power Line Road #1) are close to each other and have similar environmental characteristics with little evidence of wildfire, mixed topography of rolling hills with exposed boulders, and crossing/near, a major watercourse (Rawhide Wash). The 2.0-mile distance of Axle Grease Trail from Pima/Dynamite Trailhead to AG14 has 181 saguaros (Table 2). Of the 181 saguaros, only nine saguaros (4.9% of the trail’s total) have a truncated central spear. Likewise, the 0.7-mile distance of Rock Tank Trail from (AG14 to RC5) has 64 saguaros (Table 2) and only four saguaros (6.2% of the trail’s total) have a truncated central spear. The densities of saguaros along the Axle Grease and Rock Tank trails are roughly comparable, as is the number of truncated plants. The 1.0-mile distance of Power Line Road #1 Trail (RC5 to Pima/Dynamite Trailhead) has few saguaros (n=38) and a high percentage have a truncated central spear (n=6; 15.8 %). This number of truncated saguaros is substantially greater than the other two trail segments in the Southwest Portion of the Preserve. This difference may be related to construction and maintenance of the APS power line that has affected the conditions within the right-of-way.
South-central Study Area in the Preserve
One segment of Latigo Trail (LG1 to LG5) has an environmental setting with little evidence of wildfires among rolling hills that have numerous boulders. The 1.8-mile distance of Latigo Trail has 242 saguaros (Table 3). Twenty-nine saguaros (12.0% of the trail’s total) have a truncated central spear.
The mean number of living and dead saguaros along the Latigo Trail census tract (5.5 per acre [242 from 43.63 acres]) is the highest in the study area and only slightly higher when compared to the southwest study area in the Preserve (3.1 per acre [283 from 89.68 acres]). Latigo Trail traverses an area with more boulders and fewer level areas than the southwest portion of the study area in the Preserve, which could account for the different population densities.
Southeast Study Area in the Preserve
Portions of three trails (Whiskey Bottle, Turpentine, and Black Hills) comprise the Southeast study area in the Preserve (Table 4). The environmental setting of the three trails includes the northern slope of Fraesfield Mountain (Whiskey Bottle, WB4 to Fraesfield Trailhead) and level areas traversed by the Turpentine (WB3 to BH2) and Black Hills (BH2 to Fraesfield Trailhead) trails. Wildfires have severely affected the northern slope of Fraesfield Mountain and Whiskey Bottle Trail, which may account for the low number of saguaros in this portion of the survey. Bedrock outcrops are present along the upper elevations of Whiskey Bottle Trail and minimal evidence for wildfire is present along Turpentine and Black Hills trails where isolated boulders are rare. The 111 saguaros in this portion of the study area represent the lowest population density of the study areas with an average of just 1.01 saguaros per acre (111 for 109.49 acres). The intense wildfire and northern slope exposure along Whiskey Bottle Trail may contribute to this low density. Saguaros with truncated central spears have the highest frequency of the study area along the Turpentine Trail (22.7 %).
Northeast Study Area in the Preserve
The Northeast study area is the 136th Street Express Trail from Granite Mountain Trailhead to BA1 (Table 5). The southern half of this trail is a middle bajada having few rock outcrops and dissected by numerous wash channels that presents as rolling ridges. Vegetation along the southern portion is mainly yucca, palo verde, and acacia on ridge tops and the shallow, incised wash channels. The northern half of the trail has boulder outcrops and a more substantial dissected appearance with mesquite trees growing along entrenched washes. None of the trail segment has evidence for wildfires. The average number of saguaros along the trail is 2.53 per acre (178 for 70.3 acres), which is roughly half the density recognized along Latigo Trail. Less than 10% of the saguaros are truncated.
Central Study Area in the Preserve
The Central study area is comprised of three trail segments including Brown’s Ranch Road (Brown’s Ranch Trailhead to BT1), Brown’s Mountain Trail (BT1 to UR7), and Upper Ranch Trail (UR7 to UR1). These trails include a mix of level ground, rolling hills, and mountain slopes that have evidence of wildfire. Truncated central spears are uncommon along the trails and account for as little as 0.02% along the Upper Ranch Trail.
Discussion
Five study areas are situated in the Preserve, north of Dynamite Boulevard/Rio Verde Drive in Scottsdale, Maricopa County, Arizona. The landscape in these five study areas is predominately granitic boulder fields, alluvial fans, and low hills. The five study areas range in size from 43.63 acres in the South-central study area to 111.49 acres in the Southeast study area (Figure 2). Eleven trail segments in the five study areas encompass a total surveyed area of 409.63 acres with 1,187 saguaros.
Saguaro densities are dissimilar within the study areas. The smallest survey area (the South-central study area; 43.63 acres) has 242 saguaros for an average density of 5.5 per acre (Table 7). All other study areas have fewer saguaros per acre. Environmental and landscape characteristics and the impacts of wildfires likely affected the population density of saguaros in the study areas.
The 1,187 saguaros were classified by height, the presence of arms, and whether the plant had a truncated central spear; dead plants were counted from observed ribs (Figure 3). For the five study areas, approximately 8.9% (n=106) of the saguaros were dead. Plants with a truncated spear, regardless of height or number of arms comprise 13.9% (n=165) of the sample. Plants that were less than 6 ft tall comprise 22.1% (n=263) of the sample and plants that were more than 6 ft tall but had no arms were 16.6% (n=198) of the sample. Plants with 1 or more arms account for 38.3% (n=455) of the saguaros.
Saguaros with truncated spears are not distributed equally within the five study areas (Figure 4). The Central study area includes the trail system around Brown’s Mountain, which had almost twice as many truncated saguaros (21.4%) as three other study areas (6.7% in the Southwest, 9.5% in the Northwest, and 11.9% in the South-central). Only the Southeast study area has a nearly comparable percentage (18.0%) of truncated saguaros. These low- to moderate-frequencies suggest truncated spears are not a common outcome during the plant’s growth cycle and instead represent random events that affected the plants.
Random factors that could contribute to truncation of a saguaro’s central spear include insect/bird/wind damage, disease, or other natural causes. The impact of such damage to the long-term survivability of individual plants can be measured in two ways: 1) repeat photography that showcases diachronic changes, and 2) comparison of plant mortality rates for all saguaros within the study areas and the truncated plants as a subgroup. The damage to a plant when the central spear is truncated may negatively affect its overall survivability. Therefore, a higher mortality rate can be predicted for the truncated plants when compared to the non-damaged plants in the survey area. The current survey and repeat photographs also may be able to identify the rate at which plants lose their central spears.
Mortality Record for January 2024
The initial baseline survey was conducted in July 2023 with the first resurvey in January 2024. Observed changes to the saguaros in the initial six months of the study include the death of 11 saguaros, regardless of age/size, plus 1 death of a plant with a previously truncated spear. The 12 newly dead saguaros from the study areas represents a 1.0% loss of plants.
Epilogue
Future repeat photography of the study areas will be completed in July 2024.
Photographs of the truncated saguaros are available upon request.
