Saguaro Life History written by Brian McGill
General sources are (Shreve 1951; Niering, Whittaker et al. 1963; Steenbergh and Lowe 1969; Steenbergh
and Lowe 1983)
Seeding and Germination
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A mature stand of 50 saguaros/hectare produces 5,000,000
viable seeds embedded in a pulpy fruit that ripens in late
June and early July
The fruit is usually eaten and undigested seeds (most of
them) are deposited on the ground
Ants, rodents, and birds (curve-billed thrasher, Gambel’s quail, cactus wren, brown towhee) eat
the vast majority of seeds and many more land in unsuitable sites.
As the summer monsoons start, germination begins and is heavily dependent on the rainfall
(almost always occurring after two rainfall events in a 5 day period)
Approximately 10,000 seeds/ha germinate (0.2%) or elsewhere estimate at <0.1%
1st year growth
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At 2 months stem size is 3-5 mm diameter. Plants add diameter but not height in Nov-Jan. Apical
growth starts again as soon as adequate warm temperatures and rains occur (often as early as
February)
The most critical survival period is the dry, hot period just before summer rains. Shading was
necessary for survival. An interaction between soil darkness and moisture further determined
survival (plants could survive on light soils with ambient rain or on dark soils with added
moisture). Turner et al 1966
Within 20 hours of the first summer monsoon full turgor is reestablished and growth begins again
About 1% of seedlings survive the 1st year (~100/ha)
First year mortality was 42% due to biotic factors (21% due to rodents – digging up but not eating,
19% due to insects, 2% miscellaneous) and 58% due to abiotic factors (46% due to drought mostly
in rocky bajadas, 7% due to frost mostly at lower elevations, 5% due to erosion in lower elevations
on slopes)
1st decade
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About 10% survive the 2nd year (~10/ha). About 30% survive the next 5 years (~2/ha). At this
point (year 7) mortality levels off to near zero (0.7%/year) for the next 70 years
Height in cm for the 1st 8 years can be predicted by log ht(cm)=-0.7+0.15*age(yrs), reaching
about 2cm in height at 8 years.
Nurse plants are crucial to survival in the early years. They minimize the effects of drought and
frost, and increase nutrient availability. Saguaros mostly nurse (89% of all seedlings) under
Cercidium (Palo Verde) and Ambrosia (Bursage). Peak summer soil temperatures (occurring about
2PM) drop from 60 in the open to 45 under Ambrosia and 40 under Cercidium (the peak lasts
for a shorter period under Cercidium as well). Ambrosia reduces PAR (photosynthetically active
radiation) by 77%, leading to 90% less CO2 uptake. There is possibly a water competition as well
(although Saguaro roots lie above Ambrosia and Cercidium). Thus there is a growth/survival
trade-off. Franco & Nobel 1989
There is considerable debate on whether the Saguaro hurts its nurse plant or merely outlives it
(exploitation vs commensalism). McAuliffe 1984, Venable pers comm
Growth beyond the 1st decade and death
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Later growth is highly variable. A saguaro 1 m high may be 20-50 years old. After reaching 2-3m,
growth is about 10cm /yr in the main trunk and slightly less in the branches
At roughly 50-70 years (and roughly 2-3 m) branching begins. This increases vulnerability to
abiotic factors such as freezing (changes aerodynamics), wind blowover, and lightning and hence
mortality begins again.
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These abiotic factors cause a roughly linear die-off with most saguaros most dead by about 150
and a few lasting into their 3rd century.
Biotic factors rarely cause death in mature saguaros. An old saguaro may well have 40
woodpecker holes that it has successfully callused off. Bacterial infections can kill if they occur in
frost-damaged tissue or due to a broken arm in the moist summer.
It has been shown that the northern geographic limit is set by the limit at which infrequent freezing
temperatures persist for more than 24 hours. As you move north within the saguaro range,
diameter increases and # of flutings decreases (increasing surface/volume ratio). Nobel (1980,
1982) showed that Saguaro has the same tissue resistance to freezing that Stenocereus has (which
is significantly lower than some Opuntias), but the larger diameter and greater number of spines
allows it to resist freezing better and extend its range further north.
Overall, saguaros are limited in range to the north by the increasing freezing temperatures as you
go up the Mogollon rim to the Colorado plateau, to the east again by temperatures as you rise into
the Rockies and its southern extensions, to the west by the Colorado river due to increasing aridity
(a few sites just across the river in California occur). Saguaros extend to the south in mainland
Mexico (but not the baja) down to about Guaymas, and presumably are limited by competition
from more tropical species.
In an attempt to find human causes for decline in Saguaros Kolberg & Lathja (1997 2 papers)
showed that while soil and tissue Cu levels are higher near smelters, it is not high enough to cause
toxicity. Moreover, browning is uncorrelated with Cu but appears to be a result of tissue scorching
from the sun.
Reproduction
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You all know what a saguaro flower looks like. It is white, many-petalled with a hypanthium
(corolla fused onto ovary). Flowers come out in the evening and last into the early morning the
next day. They occur at the apex of the plant.
Pollination is a mixture of nocturnal (nectar feeding bats) and diurnal (bees and doves) animals.
Bees are relatively ineffective pollinators (Alcorn, McGregor et al. 1961; McGregor, Alcorn et al.
1962; Fleming, Tuttle et al. 1996; Fleming 2000)
Plant animal interactions
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In addition to pollination, a number of animals depend on saguaros.
It has recently been shown that white-winged doves depend heavily on saguaros for water and
nutrition in the driest/hottest part of the year (just before monsoons when saguaros flower and
fruit)
There is an entire ecosystem on decaying saguaros including drosophila, beetles, and (I believe) a
lizard that lives on the bugs that live on the dying saguaro
Many native human populations depended on the saguaro.
Population dynamics
For discussion on the two papers
Paleohistory
A packrat midden site (Eagle Mountain) just south of the Mogollon rim and at an elevation/precipitation
level intermediate between Phoenix and Tucson was studied. It found that:
22000-~14000 ya
pine/sagebrush with oak/juniper
14000-10000 ya
oak/juniper
10000-present
Ephedra, Acacia, Carnegia, Ferocactus
7000-present
above + Larrea, Encelia, Cercidium, Opuntia
The proffered explanation for the present day community disjunction is that Acacia, Carnegia, and Ferrocactus are
bird dispersed, while Larrea, Encelia and Cercidium are all rodent dispersed and hence dispersal limitations drove
this pattern (MacAuliffe and Van Devender 1998)
A 120 year study of an untouched area (MacDougal Crater) shows that (Turner 1990)
 Larrea & Cercidium declined ~60% first half this century, no recruitment since
 Carnegia increase 4X (and had three major recruitment episodes in 1790-1960)
 Prosopis increased 200X, Encelia increased greatly
This is proported to be due to drought 1936-1964 and that establishment is related to heavy preciptation?
Niche
Cactus in general:
 excel at handling water stress due to stem succulence and CAM but lose ability
to compete for light for these same reasons (and hence are found in dry open
areas)
 wide, shallow root systems and can idle without water for extended periods and
hence excel at the temporal niche of using briefly available water
 Slopes and areas facing southwest dry faster and hence are where Saguaro
dominate
Saguaro in particular relative to other columnar cacti tend to:
 Have a larger stem and more spines making it more cold tolerant (hence found
further north and further up slopes and even in valleys when soil appropriate)
 Have shallower roots than some (e.g. vs senita) and hence prefers coarse, rocky
soils that drain quickly and that provide structural support despite shallow roots
Yeaton & Cody 1979; Parker 1988
Bibliography
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