Saguaro Life History written by Brian McGillSeeding and Germination1st year growth1st decadeGrowth beyond the 1st decade and deathReproductionPlant animal interactionsPopulation dynamicsPaleohistoryNicheBibliographySaguaro Life History written by Brian McGillGeneral sources are (Shreve 1951; Niering, Whittaker et al. 1963; Steenbergh and Lowe 1969; Steenbergh and Lowe 1983)Seeding and Germination1. A mature stand of 50 saguaros/hectare produces 5,000,000 viableseeds embedded in a pulpy fruit that ripens in late June and earlyJuly2. The fruit is usually eaten and undigested seeds (most of them) aredeposited on the ground3. 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.4. 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)5. Approximately 10,000 seeds/ha germinate (0.2%) or elsewhere estimate at <0.1%1st year growth1. 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)2. 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 19663. Within 20 hours of the first summer monsoon full turgor is reestablished and growth begins again4. About 1% of seedlings survive the 1st year (~100/ha)5. 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 decade1. 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 years2. 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.3. 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 opento 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 19894. There is considerable debate on whether the Saguaro hurts its nurse plant or merely outlives it (exploitation vs commensalism). McAuliffe 1984, Venable pers commGrowth beyond the 1st decade and death1. Later growth is highly variable. A saguaro 1 m high may be 20-50 years old. After reaching 2-3m, growth isabout 10cm /yr in the main trunk and slightly less in the branches2. 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.3. 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.4. 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 dueto a broken arm in the moist summer.5. 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.6. 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 itssouthern 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.7. 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.Reproduction1. 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.2. 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 interactionsIn addition to pollination, a number of animals depend on saguaros.1. 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)2. 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 saguaro3. Many native human populations depended on the saguaro.Population dynamicsFor discussion on the two papersPaleohistoryA packrat midden