the vascular flora of pinnacle peak park, scottsdale, arizona

Arizona-Nevada Academy of Science

The Vascular Flora of Pinnacle Peak Park, Scottsdale, ArizonaAuthor(s): Gerald A. Rosenthal, Gretchen Mills, David W. Mills, Tracy Weaver and DianeMcCoy-BerneySource: Journal of the Arizona-Nevada Academy of Science, Vol. 39, No. 1 (2007), pp. 33-45Published by: Arizona-Nevada Academy of ScienceStable URL: http://www.jstor.org/stable/27641759 .

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The Vascular Flora of Pinnacle Peak Park, Scottsdale, Arizona

Gerald A. Rosenthal, Gretchen Mills, David W. Mills, Tracy Weaver, and Diane McCoy

Berney, The Biodiversity Study Team of Pinnacle Peak Park, Scottsdale, AZ 85262

ABSTRACT Pinnacle Peak Park (PPP), located in the northern portion of Scottsdale, Arizona, is a 61 -hectare (ha), municipal park

established by the City of Scottsdale as part of its recreational and educational programs. All of the plants in 31,10-m x 10-m quadrats (0.5% of the total park area) were identified, tallied, and measured for their coverage to establish the

Importance Value for the 33 taxa constituting the dominant, perennial flora of PPP. An exhaustive inventory of the vas cular flora revealed the presence of 143 taxa of vascular plants representing 47 families. Analyses of the basic geology and chemical composition of the soil were conducted. A description of the park and its flora, as well as background infor

mation on the creation and patterns of use for PPP, is also provided.

Introduction Pinnacle Peak Park (PPP), located in north

Scottsdale, Arizona, is a 61-ha, municipal park that

consists of Pinnacle Peak, elevation 966 m, and a

companion mountain, officially unnamed, elevation 916 m. This small jewel in the crown of Scottsdale's

Open Space Program is open daily for public use, free of charge, from dawn to dusk.

Pinnacle Peak Park, opened April 20, 2002, receives an average of 200,000 out-of-town and

resident visitors annually. Visitor-use patterns vary

throughout the year, with the highest visitation occurring during the cooler months of November

through April, when tourism peaks. During this

time, a weekend day may receive up to 1,500 visi

tors, whereas a weekday range is between 500 to

800 visitors.

During the warmer months of May through Oct

ober, when daytime highs frequently exceed 35?C, usage drops dramatically. Heaviest visitation tends to occur in the early morning hours between dawn and 9 AM and in the early evening hours from 5 PM

to dusk. Weekends may draw up to 500 guests

daily; in contrast, weekdays attract up to 200 visi tors daily. During this time of year, most visitors are

year-round residents and live within a 16-km radius

of the park.

Beginning at an elevation of 783 m, a 2.8-km

(5.6 km out and back) park trail ascends a bajada from the east, gradually gains elevation, and then,

through a series of switchbacks, traverses about

half-way up Pinnacle Peak. The peak itself is an

imposing tower of granite boulders, weathered into

rounded, vertical blocks (see Fig. 1 and Geology). Rock climbing on Pinnacle Peak and two boulder strewn outcroppings located in the northern part of the park is permitted only by experienced climbers.

These outcroppings are known as Cactus Flower and Y-Crack, so named for the appearance of their

boulders (see Fig. 2). The park trail continues by wrapping part way

around Pinnacle Peak and ascending to a maximum

elevation of 881 m near trail marker 35. (The trail is marked at 100-ft [30-m] intervals with numerical

Figure L Pinnacle Peak.

Rosenthal, G., G. Mills, D. W. Mills, T. Weaver and D. McCoy-Berney. 2007. The vascular flora of Pinnacle Peak

Park, Scottsdale, Arizona. Journal of the Arizona-Nevada Academy of Science 39(1 ):33-45.



Vascular flora of Pinnacle Peak Park + Rosenthal etal

PINNACLE PEAK PARK, SCOTTSDALE, ARIZONA

TtaBDsta Sac3e,TSN,RSE

Park boundary viGtMTY MAP*

wtii?jMraftir CW7TUS FLOWER, FORMATION^

nsr^ur

TOANSECTB

THANSECTA

>ifr YCRACK FORMATION

Figure 2. Map of Pinnacle Peak Park, indicating location of sample quadrats (black squares).

markers.) The trail then descends, through a series

of additional switchbacks, to a saddle and natural

drainage area, near trail marker 56, that separates Pinnacle Peak from the mountain to the west. As the

trail climbs for a second time, it traverses part way

up the second mountain and crosses a landscape that

is visually distinct from the remainder of PPP. The rock outcroppings high on this mountain exhibit a

more angular shape, and the soil derived from its

bedrock has a far darker, reddish coloration. This

section ofthe park, designated in this paper as Red

Face (RF; Fig. 3), extends from marker 57 through 75. There are less pronounced boulder outcroppings in this portion ofthe park. One ofthe most striking features of RF is the sparseness of saguaros (Carneg iea gigantea). After marker 75, the trail drops further

until it reaches 721 m at the west boundary of PPP.

The overall appearance and floristic composition of

the western side of PPP exhibits little difference from the remainder ofthe park, with the exception of RF.

The marked degree to which RF, both from the

viewpoint of its flora and overall physical appear

Figure 3. Red Face from marker #35.



35 Vascular flora of Pinnacle Peak Park + Rosenthal et al

anee, differs from the rest of PPP forces one to view RF as an entity distinct within PPP.

The southern boundary of PPP is N33?43'36"

and the irregularly shaped park boundary extends

north to N33 ?44'14". The east and west extremities

ofPPP are Wlll051'31"andWlH052'25", respec

tively (see Fig. 2).

History of the Park History of human activity in the Pinnacle Peak

area and the surrounding region spans from Archaic time to modern day. A general consensus drawn

from archaeological studies support that early cul

tural groups in the area were Archaic hunters and

gatherers. They roamed the area from the Middle

Archaic period (5000-3000 BC) to the Late Archaic

period (600 AD). After this point, it is believed that the Upland Hohokam resided in the region until

approximately 1200 AD. Archaeological findings rich in artifacts and sites in the McDowell Moun

tains support this conclusion. "While the number of

formally recorded sites is relatively low, it is esti

mated that hundreds of sites are present" (Woodall

1999). Pinnacle Peak Village at Boulder Pass is a

formally studied and recorded site located 2.4 km

southeast of PPP. Unfortunately, many other sites were not studied before they were lost to develop ment. Acquisition of private and State land, how

ever, through efforts of the McDowell Mountain

Conservancy, will help preserve the integrity of

many remaining archaeological sites. It is not clear why the Hohokam vanished from

the area. The Yavapai Culture moved into the

McDowell Mountains around 1400 AD and was a

dominant presence until approximately 100 years

ago. In the late 1800s, during the later part ofthe

Yavapai dominance in the McDowells, ranchers

began to move into the area. Ranching activity con

tinued for several decades (but has waned in recent

years due to development). In the early 1930's, homesteaders j oined ranchers in the region when the Homestead Act and Desert-Land Entries Act initi

ated an interest in homesteading in the area that

included the Pinnacle Peak foothills. The home

steaders were drawn to the unique landmark of Pin

nacle Peak Mountain and its lush Sonoran Desert

beauty. As roads improved, it became easier for families to make the saguaro-studded area their new

home. They enjoyed the beauty ofthe natural desert and the western lifestyle it encouraged while the

lights of Phoenix flickered 40 km away. In addition to the Homestead Act and Desert

Land Entries Act that affected the region surround

ing Pinnacle Peak, it is important to mention that

"Pinnacle Peak and a majority ofthe surrounding

land were once part of Arizona State Trust Land, held by the State and managed for the benefit of the State's educational trust" (Scottsdale Community Services Department 1999). During this time, Pin

nacle Peak Mountain and its surrounding area was

not public open space or a park. However, for many

years, the Peak and surrounding area were popular with outdoor enthusiasts who enjoyed horseback

riding, rock climbing, hiking, and cycling. In 1982, a large area of land including Pinnacle Peak was

annexed into Scottsdale. This land was still in mixed

ownership of Arizona State Trust Lands and private

holdings. In 1985, Scottsdale's City Council

approved the development of the "Troon North"

community, including the preservation of Pinnacle

Peak as a 72-ha City of Scottsdale Park that would

allow the use of the land for public recreation. In

1994, an amendment to the Master Plan, for the

community now known as Estancia, reduced the

future park by 12 ha. At this time, the developer

agreed to provide funds for building Pinnacle Peak's

main trail. The park was closed to public access for

the next eight years, during which time the follow

ing occurred: public hearings, trail building, land

appraisals, and the purchase of 1.3 ha of State Land on the east side of Pinnacle Peak for a trailhead.

Once the 1.3-ha parcel of land was acquired, consul

tants drew up plans and a contractor was hired to

complete the trailhead office and parking lot.

Pinnacle Peak Trail was constructed with mini

mal impact on the environment with careful con

sideration given to conservation of the flora, fauna

and topography being of primary importance. In

April 2002, the trail opened to accommodate a vari

ety of visitors including naturalists, hikers, eques trians and technical rock climbers.

Geology The soils in and around PPP are derived from

local rocks that make up Pinnacle Peak in the east

ern section of the park, and the unnamed second

mountain in the west. The soils are primarily grus

(bits and pieces from the granitic bedrock) of vari

able thickness mantling the granitic bedrock.

The pinnacle of granite that gives the park its name is middle Proterozoic, unfoliated, coarse

grained granite with abundant large grains of pink

to-light-gray crystals (that evolved from the mag

ma). The ground mass is composed of light-gray

plagioclase feldspar (2-8 mm diameter), clear-gray

quartz (2-6 mm diameter), and black biotite. This

unit tends to weather into spheroidal boulders and

erodes easily to form smaller pieces. This granite is

probably of comparable age to other undeformed

granitic plutons approximately 1.4 billion yr old



(Leighty et al. 1997). The weathered granite is tan,

moderately sorted, and unconsolidated. X-ray dif fraction analysis ofthe silt and clay-sized fraction of

decomposed granite samples taken near Big Brownies Hill, approximately 3.2 km north of Pin

nacle Peak, indicate the occurrence of quartz,

potassium feldspar, plagioclase feldspar, smectite, and illite (Doom and Pewe 1991).

The medium- to fine-grained granite which

comprises the second mountain is composed of

light-tan, potassium feldspar, plagioclase feldspar, clear-gray quartz, and biotite. This granite tends to

weather slightly darker, and with greater red color, than does the coarse-grained granite, and forms ang ular blocks more than spheroidal boulders. The con

tact (where two rock types come together) with the

coarse-grained granite to the east and to the west is,

typically, sharp (Leighty et al. 1997).

Climate Temperatures at PPP range from a maximum of

40?C or more during the summer to an occasional

overnight low in winter that falls below freezing

(Fig. 4). High summer temperatures can last for

weeks and this continued heat places considerable stress on the flora of the Park, particularly if the summer monsoon season is weak. Freezing night time lows do not last long; the vegetation does not

suffer from sustained exposure to frigid conditions. In the Arizona Upland Series of the Sonoran

Desert scrub, virtually all rainfall is delivered either in the winter months (December-March) or as part of summer monsoons (July-September). When they occur, supplemental rains in October or November can contribute significantly to spectacular floral dis

plays the following spring. Monsoonal rains typic

ally form from moist, tropical air masses that can

create violent, thunderstorms with potentially des

tructive winds. Summer rains, typically deposited

by powerful storm cells, are localized-often in nar

rowly delineated areas, and account for 30-60% of

the annual precipitation. Pinnacle Peak Park adheres to this generalized

climatic picture with a bimodal annual rainfall (Fig.

5). The fall and winter seasons of 2004-2005 were

particularly wet with more than 100 mm falling in

January and over 160 mm in February. Additional

rainfall in March and April culminated in a truly

spectacular 2005 spring flora display. Palo verde

trees (Parkinsonia microphylla) were awash in

color; the west side of Pinnacle Peak was a sea of

golden-yellow poppies (Eschscholtzia californica var. mexicana) and blue lupines (Lupinus sparsi

florus). Seed and fruit production were extraordi

nary. Summer monsoons were also strong, the last

occurring in early August. The subsequent two

months were not unusual, but after 18 October, no

measurable rain fell for 144 days until 11 March of

2006. This established a modern record for a sus

tained period without measurable precipitation fall

ing at Sky Harbor International Airport (official Phoenix weather station). Thus, conditions had been

unusually dry for more than four consecutive months

when we conducted our vegetational analyses of PPP

from the end of 2005 through early 2006. No signif icant display of spring wildflowers was observed in

2006.

Description of the Flora A diverse mixture of leguminous trees; shrubs,

cacti, and other perennials; and ephemerals cover

4$

\ ?O- Minimum ;""^M?ximum ]

Figure 4. Temperature extremes at Pinnacle Peak Park.




Mm Apr M* Am Jtf A? S?

?02002 ?2003 mtocxwtom ?*K6 j

Figure 5. Monthly rainfall at Pinnacle Peak Park.

about one-third ofthe landscape of PPP, the remain

der being granitic boulder outcroppings and grus, the accumulation of angular, coarse-grained frag

ments created by the granular disintegration ofthe boulders. The vegetation community falls within the Sonoran Desert Scrub: Paloverde-Mixed Cacti Series as designated by Brown et al. (1979); it is also known as Arizona Upland Series.

The trees are predominantly Parkinsonia micro

phylla (foothill palo verde) and, to a lesser extent, Olneya tesota ironwood). Four Prosopis velutina

(velvet mesquite) reside within the park. Much ofthe park is covered with a mixture of

variously sized shrubs, most notably: Acacia greggii (catclaw acacia), Ambrosia deltoidea (triangle-leaf bursage), Eriogonum fasciculatum (flat-topped buckwheat), Hyptis emoryi (desert lavender), Justicia californica (chuparosa), Lycium ssp. (desert thorn), Simmondsia chinensis (jojoba), Viguiera

parishii (Parish's goldeneye), and a massive popula tion ofEncelia farinosa (brittlebush).

The most impressive cacti are the Carnegiea gigantea (saguaro), a signature plant ofthe Sonoran Desert that is visible from some distance as it rises 10 m or more. Cylindropuntia bigelovii (teddybear cholla) is also striking in appearance as specimens often grow in large colonies; their white barbed

spines reflect the sunlight, contrasting with the more

muted pallet of many desert plants. Less conspic uous cacti include Mammillaria grahamii var.

grahamii (fishhook pincushion) and Cylindropuntia acanthocarpa (buckhorn cholla).

Also eye-catching, especially when flowering, is Fouquieria splendens (ocotillo). Its terminal pani cles of blood-red tubular flowers appear in April at the top oftall "coach-whip" canes that bear bright green leaves episodically throughout the year fol

lowing sufficient rainfall. An array of yellow blooms projecting above the

dome-shaped assemblage of gray-green leaves of

Encelia farinosa visually dominates much of the

park in spring. A wide array of often colorful

ephemerals appears also after sufficient winter rain

fall. Some of the more spectacular displays include

Lupinus sparsiflorus, Eschscholtzia californica var.

mexicana, and Phacelia crenulata and R distans.

Perennials with showy blooms, even in years of

little rainfall, in addition to the above mentioned

Encelia and Fouquieria, include Justicia californ ica, Parkinsoniamicrophylla, Olneyatesota, Cross

osoma bigelovii and cacti: Carnegiea gigantea, Echinocereus englemannii and Cylindropuntia

acanthocarpa. While the number of gymnosperms in the park is

limited, the presence of three Juniperus coahuilensis

(red-berry jumper) specimens is noted (Godec 1999). These are modern remnants of former pinyon-juniper (Pinus-Juniperus) forests of the cooler and wetter

Pleistocene Epoch, still persisting in this present-day desert scrub community (Bowers 1993).



Vascular flora of Pinnacle Peak Park + Rosenthal et al 38

Inventory of the Known Vascular Plants of Pinnacle Peak Park

Acanthaceae Carlowrightia arizonica Gray Justicia californica (Benth) Gibson

Agavaceae Yucca baccata Torrey

Amaranthaceae Amaranthus fimbriatus (Torrey) Benth.

Tidestromia lanuginosa (Nutt.) Standlley

Apiaceae Bowlesia incana Ruiz Lopez & Pavon Daucus pusillus Michaux

Asclepiadaceae Matelea parvifolia Torrey

Asteraceae Acourtia wrightii (Gray) Reveal & King Adenophyllum porophylloides (Gray) Strother Ambrosia ambrosioides (Cav.) Payne A. deltoidea (Torrey) Payne Artemisia ludoviciana Nutt.

Baccharis salicifolia (Ruiz & Pav.) Pers. B. sarothroides Gray

Bailey a multiradiata Harv. & Gray Bebbia j?ncea (Benth.) Greene

Brickellia coulteri Gray Cirsium neomexicanum Gray Encelia farinosa Torrey & Gray Ericameria laricifolia (Gray) Shinn.

Eriophyllum lanosum (Gray) Gray

Gnaphalium arizonicum Gray Gutierrezia sarothrae (Purch) Britton & Rusby

Heliomeris longifolia (Robins & Greenm.) Cocker ell

Heterotheca psammophila Wagenkn.

Hymenothrix wislizeni Gray Isocoma acradenia (Greene) Greene

Machaeranthera pinnatifida (Hook.) Shinn.

Porophyllum gracile Benth.

Salsola tragus L.

Senecio flaccidus Less.

S. lemmoni Gray

Stephanomeria pauciflora (Torrey) Nelson Tanacetum camphoratum Less. Trixis californica Kellogg

Uropappus lindleyi (DC.) Nun.

Viguiera parishii Gray

Berberidaceae Mahonia haematocarpa (Woot.) Fedde.

Boraginaceae Amsinckia menziesii var. intermedia (Lehm.)

Nelson & Macbr.

Cryptantha angustifolia (Torrey) Greene

Pectocarya recurvata I. M. Johnson

Brassicaceae Arabisperennans S. Wats.

Brassica tournefortii Gouan

Caulanthus lasiophyllus (H. & A.) Payson Descurainia incana (Fischer & C. Meyer) Dorn

Lepidium lasiocarpum Torrey & Gray L. virginicum L.

Sisymbrium irio L.

S. orientale L.

Thysanocarpus curvipes Hook.

Cactaceae Carnegiea gigantea (Engelm.) Britton & Rose

Cylindropuntia acanthocarpa Kauth

C. bigelovii Kauth C. leptocaulis Kauth

Echinocereus engelmannii (Engelm.) Lemaire

Ferocactus cylindraceus (Engelm.) Ore.

Mammillaria grahamii Engelm. Opuntia engelmannii Engelm.

Caryophyllaceae Silene antirrhina L.

Celastraceae Canotia holocantha Torrey

Crossosomataceae Crossosoma bigelovii Watson

Cupressaceae Juniperus coahuilensis (Martinez) Gaussen ex

Adams

Ephedraceae Ephedra fasciculata Nutt.

Euphorbiaceae Argythamnia lanceolata (Benth.) Pax & Hoffin.

A. neomexicana (Muell. Arg.) Heller

A. serrata (Torr.) Muell.-Arg.

Chamaecyse albomarginata Torrey & Gray C. polycarpa Benth.




Fabaceae Acacia greggii Gray Calliandra eriophylla Benth.

Dalea mollis Benth.

Lotus rigidus (Benth.) Greene

Lupinus concinnus Agardh L. sparsiflorus Benth.

Olneya tesota Gray Parkinsonia florida Gray P. microphylla (Torrey) Rose & I.M.

Prosopis velutina (Woot.) Senna covesii (Gray) Irwin & Barneby

Fouquieriaceae Fouquieria splendens Engelm.

Geraniaceae Erodium cicutarium (L.) L'Her.

Hydrophyllaceae Emmenanthe penduliflora Benth.

Phacelia crenulata Torrey P. distans Benth.

Pholistoma auritum (Lindl.) Lilja

Krameriaceae Krameria grayi Rose & Painter

Lamiaceae Hyptis emoryi Torrey Solazarla mexicana Torrey Salvia columbariae Benth.

Liliaceae Dichelostemma capitatum (Benth.) Wood

Malpighiaceae Janusia gracilis Gray

Malvaceae Abutil?n incanum (Link) Sweet A. palmeri Gray Herissantia crispa (L.) Brizicky Sphaeralcea ambigua Gray

Nyctaginaceae Boerhaavia coulteri (Hook, f.) Watson

Mirabilis bigelovii Gray

Oleaceae Menodora scabra Engelm. ex Gray

Onagraceae Camissonia californica (Nutt. ex Torrey & Gray)

Raven

C. micrantha (H?rnern, ex Spring.) Raven

Orobanchaceae Orobanche cooperi (Gray) Heller

Papaveraceae Eschscholtzia californica Cham. ssp. mexicana

(Greene) Clark

Plantaginaceae Plantago patag?nica Jacq.

Poaceae Aristida purpurea Nuttall var. purpurea Bromus carinatus Hooker & Arnott B. rubens L.

Muhlenbergia porteri Scrubker ex Beal

Pennisetum setaceum (Forsskal) Chiovenda Schismus spp.

Polemoniaceae Eriastrum diffusum (Gray) Mason

Giliaflavocincta Nelson

Polygonaceae Eriogonum deflexum Torrey var. turbinatum

(Small) Reveal E.fasciculatum Benth. E. wrightii Torr. ex. Benth.

Portulaceae Calandrinia ciliata (Ruiz & Pav.) DC.

Portulaca oler?cea L.

Pteridaceae Notholaena californica Eaton

Ranunculaceae Anemone tuberosa Rydb.

Delphinium parishii Gray

Rhamnaceae Zizyphus obtusifolia (Hook ex T. & G.) Gray

Rubiaceae Galium stellatum Kellogg

Selaginellaceae Selagenella lepidophylla Maxon



Vascular flora of Pinnacle Peak Park + Rosenthal et al 40

Scrophulariaceae Antirrhinum nuttallianum Benth.

Maurandya antirrhiniflora (Humb. & Bonpl. ex

Willd) Rothm. Castillejo exserta (Heller) Chuang & Heckard

Penstemon eatonii Gray P. parryi (Gray) Gray P. palmeri Palmer

Simmondsiaceae Simmondsia chinensis (Link) Schneid.

Solanaceae Lycium andersonii Gray L. berlandleri Dunal

L. exsertum Gray Nicotiana obtusifolia Martens & Galeotti

Physalis crassifolia Benth. P. hederaefolia (Gray) Waterf.

Ulmaceae Celtis pallida Torrey

Verbenaceae Aloysia wrightii (Gray) Heller

Viscaceae Phoradendron californicum Nutt.

Zygophyllaceae Larrea divaricata Cav. ssp. tridentata (Ses. & Moc.

ex DC.) F. & L.

Methodology Quadrat Sampling

Two transects (A & B), following the park trail, were established within the boundaries of PPP (Fig. 2). Markers were located along each transect at

30-m intervals; 25 markers along transect A and 6 markers along transect B were selected with a

random-number generator. Due to the steepness of the terrain, dangerous footing, and other concerns

for field crew safety, certain quadrats could not be

sampled. In these few instances, the nearest quadrat, taken from a supplemental list of randomly gener ated quadrats, was substituted. At each of the selected markers, a line was projected, 10 or 20 m,

perpendicularly from the angle of the transect, to

establish the center point of the quadrat. Four corner poles were positioned to create a

10-m x 10-m square quadrat, that was enclosed with

orange rope to mark its perimeter. This boundary delineation aided greatly in determining whether a

given border plant fell within the sampling area. All

ofthe perennial plants within the quadrat were iden

tified and measured for their area. Plant area was

approximated by measuring both a long and short

axis ofthe plant; their average value was taken as

the plant diameter. For saguaros, trees, and other

plants with an open growth form, their area at breast

height was recorded. All plant data were recorded and processed em

ploying Excel spreadsheets that provided informa

tion on coverage, % coverage, frequency, % fre

quency, density, and % density (Cottam and Curtis

1956). The longitude, latitude, and elevation ofthe

quadrat central point was recorded and marked for

future use with a labeled rebar pounded into the

earth. A general description ofthe physical features ofthe quadrat was also recorded.

From appropriate values, an Importance Value was calculated for every species found within all of

the sampled quadrats. Thus, approximately 0.3 ha of

the park, or 0.53% of its total area was analyzed. The quadrats were segregated into two groups:

those 25 quadrats occurring throughout the park, except for RF, a subdivision of PPP, and 6 quadrats on RF. Importance Values were determined for RF

and the remainder of PPP.

Identification The plants of PPP were identified with standard

reference sources including: Epple (1995), Spellen

berg (2003), Kearney and Peebles (1960), Shreve and Wiggins (1964), and Baldwin (2002). Voucher samples of many of the taxa were deposited at

Academic Insights of Scottsdale, Arizona.

Soil Sampling Soil analyses were conducted to obtain basic

soil nutrient data from these locations within PPP: east side of Pinnacle Peak, Y-Crack and Cactus

Flower, west side of Pinnacle Peak, the first half of

the RF transect, the second half of the RF transect, and the west side ofthe second mountain. These six

sample zones were selected to represent the princi

pal regions of PPP, and to emphasize comparison of

the soil of RF with the remainder of PPP. Sampling was conducted within the previously established

plant-analysis quadrats except for RF where some

supplemental areas were also analyzed. After clearing litter from the area to be sampled,

a V-shaped hole approximately 15-cm deep was

excavated; duplicate 125 mL samples were taken

from each quadrat and combined with other samples from the same zone in a clean, plastic bucket. After

removing rocks and any debris, a 0.5-L sample from

the combined, mixed samples was placed in a sam

ple bag, sealed, and sent for analysis. Nutrient soil




testing was performed by the Soil, Water and For

age Testing Laboratory, Texas Agricultural Exten

sion Service, Texas A&M University System.

Results Plant Analysis

Forty seven perennial plants were identified in

the 31 sample quadrats. Inspection of Figure 6

reveals that the occurrence of additional species

appeared to stabilize after evaluating the quadrats on

the east side of Pinnacle Peak (PP01-PP23) and its associated areas: Cactus Flower and "Y" Crack

(CF02-YC01). Additional taxa were found after Transect A reached the trail summit (just past PP36) and then, on the northwest exposure of Pinnacle

Peak, descended to the saddle that separates Pin

nacle Peak from the RF section. The final burst of new taxa represents the flora unique to the RF

section (PP61-PP75). The similarity in vegetation between Pinnacle Peak and the western side of the

park was demonstrated by the lack of new inventory in the final sampled quadrats

The data of Table 1 disclose the perennial taxa of Pinnacle Peak Park whose Importance Value (IV) is in excess of 1%. Only 33 of the 143 taxa, or 23%, known to inhabit PPP achieved this minimum level of importance. If the IV is raised to 5%, about one

half of these taxa would be excluded. These IVs dis

close that only about 10% of the established taxa of PPP are part of the assemblage of dominant plants.

Of all the arborescent taxa that are commonly

part of the Arizona Upland Zone of the Sonoran Desert, only Parkinsonia microphylla contributes to

this elite grouping. Olneya tesota is spartan through out much of PPP, except for the base of the western

side of the park, where it is relatively abundant.

Prosopis velutina is the only member of this genus in PPP; it is limited to four known individuals. No

Acacia except Acacia greggii is represented in the

flora of this park. Larrea divaricata, with an Importance Value of

only 4, demonstrates its relative scarcity. This find

ing may be because the steep, boulder-strewn terrain

discouraged cattle grazing that could disturb and compact the soil creating a favorable habitat for this

fiercely competitive shrub. Our quadrat sampling did not reveal the sporadic abundance of Senna

covesii, a leguminous plant, that forms extensive

patches in several places within the park; providing a valuable food resource for animal residents. Much

the same can be said of Solazar?a mexicana, that can be locally abundant on RF, and Chamaecyse

albomarginata, that clusters throughout the park. Five members ofthe Cactaceae had a combined

IV of 13 in this Palo Verde-Mixed Cactus commun

ity: Carnegiea gigantea, Cylindropuntia acantho

carpa, C. bigelovii, Echinocereus engelmannii, and

Ferocactus cylindraceus. Male jojoba plants normally occur in the

Sonoran Desert in excess relative to their female

counterparts (4:1), as reported by Phillips and Comus (1999). Our analysis of this taxon in PPP indicates that the sex ratio is reversed among the

104 plants identified (58% female to 42% male). The gender on about 20% of the specimens could

not be determined because no floral structures were

found on them.

The Importance Value for the 13 dominant, per ennial plants of PPP are shown in Figure 7. This

assemblage of plants is typical of xeric habitats in the Arizona Upland Zone ofthe Sonoran Desert at

m. > j

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x . , , . j , i , |

, , . , j?i i i ? i i i

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jr-i-f-??;-: ? i i M 1 t-M?

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i i i m fV ? MM *M MjMMj Ha* HM- PNH pM -*** pM? ?M tall ?M MM ?MM ?M* M*! mmm ? MM^IM _ |M ̂ M? pM ̂ kM ?H MM

Figure 6. Total number of species as a function of number of quadrats.



Vascular flora of Pinnacle Peak Park + Rosenthal et al

Table L The Importance Value for the dominant, perennial flora of Pinnacle

Peak Park. Importance Values of perennial flora of <1% are excluded.

Relative Relative Relative Importance Taxa Frequency Density Cover Value

Acacia greggii 6 2 8 16

Adenophyllum porophylloides 2 10 3

Ambrosia deltoidea 3 6 3 12

Argythamnia lanceolata 2 10 4

Brickellia coulteri 2 10 3

Calliandra eriophylla 12 14

Carnegiea gigantea 2 10 2

Chamaecyse albomarginata 2 2 0 5

Crossosoma bigelovii 2 113

Cylindropuntia acanthocarpa 2 0 0 2

C. bigelovii 13 15

Echinocereus engelmannii 2 10 2

Encelia farinosa 7 36 12 54

Ericameria laricifolia 10 0 2

Eriogonum fasciculatum 6 8 3 17

Ferocactus cylindraceus 10 0 2

Fouquierria splendens 2 1 3 10

Galium stellatum 5 2 3 6

Hyptis emoryi 5 2 3 10

Janusia gracilis 2 2 14

Justicia californica 4 4 7 15

Krameria grayi 10 0 2

Larrea divaricata 10 2 4

Lycium spp. 6 3 7 16

Olneya tesota 10 4 5

Parkinsonia microphylla 5 2 24 32

Salazaria mexicana 110 3

Simmondsia chinensis 3 6 10 19

Sphaeralcea ambigua 3 2 16

Stephanomeria pauciflora 10 0 2

Tra/s californica 10 0 2

Viguiera parishii 7 7 5 20

Yucca baccata 1113




i

Figure 7. Importance Value for the dominant, perennial flora of Pinnacle Peak Park.

elevations below 1,000 m. A mixture of trees,

shrubs, and sufifrutescent herbs dominate this assem

blage; Ambrosia deltoidea is usually a far more

important member of this floristic community. Encelia farinosa and Parkinsonia microphylla

typically are prominent components of this com

munity (Brown et al. 1979), but the importance of

Viguiera parishii and Eriogonumfasciculatum was

not expected. The pronounced IV of Encelia farinosa, relative

to its other cohorts, demonstrates its ability to

propagate efficiently, especially in the wet winter of

2004-2005, and to compete so effectively for the resources of new habitats. This ability reflects its

massive production of achenes, numerous adapta tions to sustained drought conditions, and ability to

employ allelochemicals that aid in its ability to exclude other, competitive taxa (Rosenthal and Janzen 1982).

Parkinsonia microphylla was the only dominant taxa whose IV was enhanced overwhelmingly by its relative cover. These are massive legumes with a

spreading, branching growth form (shrubby trees) that covers significant area. Relative cover also

factored significantly in the IV of Simmondsia chinensis, Justicia californica and Lycium spp. These also are shrubby plants whose growth form

encompasses a wide area.

Relative density contributed to the IV value of Ambrosia deltoidea and Eriogonum fasciculatum, while relative frequency was important in this context to Galium stellatum, Hyptis emoryi, Lycium spp., and Sphaeralcea ambigua. This finding re

flects the fact that these plants were able to occupy sites throughout the park.

The dry conditions that existed at the time of our sampling resulted in the absence of flowers and

foliage, making identification of Lycium to the

specific level not advisable. Much ofthe dominance

of these taxa can be attributed to their drought resistance and tolerance, and overall hardiness and

competitive success.

The data of Figure 8 provide an insightful examination ofthe fundamental differences between

the flora of Pinnacle Peak, and the RF section, by

calculating the difference in each taxa's IV. The

relative paucity of Encelia farinosa, the most domi nant member of the flora of PPP, and Ambrosia

deltoidea is a striking reminder ofthe fundamental

uniqueness of the RF. These taxa typically are

important members ofthe Arizona Upland Zone of

the Sonoran Desert, particularly below 1,000 m.

Another important member of this community is

Parkinsonia microphylla whose IV declined on RF.

Ambrosia deltoidea was present in only small

numbers.

Eriogonum fasciculatum, Gallium stellatum,

Viguiera parishii and Sphaeralcea ambigua were

far more dominant on RF than in other sections of

PPP. These plants typically assume a far less impor tant role in the flora ofthe Arizona Upland Zone.

The uniqueness of RF is also demonstrated by its

annual flora. Taxa such as Amaranthus fimbriatus, Boerhaavia coulteri, Artemisia ludoviciana, and

Gnaphalium arizonicum made their summer appear ance on RF, but nowhere else in PPP. In the spring, this section was the only site in which Sisymbrium irio and S orientale, were found; nearly all ofthe

observed Cirsium neomexicanum and Delphinium

parishii were on RF.



Vascular flora of Pinnacle Peak Park +Rosenthal etal 44

Figure 8. Significant perennial taxa differences: Pinnacle Peak vs. Red Face.

Soil Analysis Nutrient analyses were conducted on soil sam

ples taken to represent PPP as a whole (quadrats 01-23, CF-YC, 45-55,84-90), as well as RF (61-64, 65-68, 64-70). These determinations revealed that RF has a 63% higher level of soil nitrate than the rest of PPP. If one examines the first three samples only, the average value is <4 ppm, about one-fourth the level of RF. Nitrate values are of supreme

importance in any floral community since it is uni

versally recognized as the nutrient most rate

limiting to plant productivity. With the exception of phosphorus, an important soil nutrient, RF enjoys substantially higher levels of essential plant nutri ents. The pH ofthe soil of RF at 7.0 is more acidic than the pooled soil samples for the remainder ofthe

park (pH 7.7); this is 8.6 times more alkaline. The organic matter content for the soils of PPP

as a whole was 1.1%; this compares with 1.8% for RF - a substantial difference. Most desert soils have

organic matter content on the order of 0.6% (Texas Soil, Water, and Forage Testing Laboratory, pers. comm., May 2006). This soil component is essential for sequestering soil nutrients and preventing their

loss from the soil. Thus, PPP as a whole enjoys a

high organic matter content, but the soils of RF are

significantly more fertile in this regard. The sodium content of all the soils of PPP is

low and this is of positive value to the plants as this

cation can replace essential nutrients such as cal cium and magnesium from the soil organic matter. In conclusion, all ofthe soils of PPP contain sub

stantial amounts of such macronutrients as potas sium, calcium, magnesium and sulfur, and such

micronutrients as iron, zinc, manganese, and copper. However, RF, especially in its abundance of critical

nitrate and high organic matter content, is the most

potentially fertile region of the park.

Conclusion Pinnacle Peak Park is a particularly valuable

asset to the city of Scottsdale, its residents and

visitors, accommodating naturalists, hikers, eques trians and technical climbers. At the same time, it is a showcase of the Sonoran Desert-Scrub: Paloverde

Mixed Cacti Series which is "best developed away from valley floors on bajadas and mountain sides"

(Brown 2002). The rich and diversified collection of 143 taxa

representing 47 families is displayed on a 61-ha landscape of inspiring beauty and striking vistas.

The arrangement of subassociations within the plant communities, discussed in this paper, and their dis tribution throughout the park invites further study.

Acknowledgments The Biodiversity Team of Pinnacle Peak Park is

grateful to Gay Christensen-Dean and John Loleit, Pin nacle Peak Park Coordinator, for their assistance in our field studies, and to Steve Jones for his efforts in helping to build the plant inventory at PPP. A special thanks also to John Loleit and Les Landrum, Arizona State Univer

sity Herbarium, for their assistance in gaining approval to conduct this study.

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