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Long Live Methuselah! DESERT PLANT LONGEVITY


KIRK ANDERSON, GARDENS COLLECTION MANAGER One of the most frequently asked questions regarding


plants at T e Living Desert, next to ‘how much do I water it?’ and ‘how fast does it grow?’, is ‘how old is it?’.


T e answer can be simple if the planting date and age at planting are known. Plants that become part of the collection at T e Living Desert are accessioned. T at is, they are given a number and all relevant information is recorded. So we know, for instance, that the umbrella thorns (Acacia tortilis) with the Accession # 87-0-179, were grown from seed acquired from the National Herbarium in Zimbabwe in 1987, sown in 1990, and planted in the East African Garden in 1993.


Determining ages of plants of wild origin can be a bit trickier. In the case of long-lived woody species, a core sample can be taken and the annual growth rings counted. T e problem with this method is that plants may produce no rings in a given year due to environmental stresses or injury or they may generate more than one ring in response to weather patterns that create multiple growing seasons within a calendar year. Radiocarbon dating may also be used when a core sample from the center of the plant, essentially dead wood, is tested for its carbon-14 value. But since the atmospheric presence of this isotope can vary through time this test must be calibrated against a cross-dated tree ring dataset to ensure accuracy. T e problem with both of these techniques is that some plants, such as palms, cycads and succulents, do not have woody centers marked by growth rings.


Gauging a plant’s age by its size may also be, at best, educated guesswork. Many variables go into determining a plant’s growth rate and ultimate size such as genetics, precipitation patterns, soil structure and fertility, slope orientation, predation, frosts and droughts and competition or protection from neighboring plants. Plant size in relation to age will inevitably vary among plants from location to location and even within the same population at a given locale.


One of the most frequently asked about plants is the saguaro


(Carnegia gigantea). Since succulents produce no annual growth rings, their ages can only be surmised by recording growth rates and extrapolating how many years it took to achieve certain sizes. A long-term study near Tucson has set the average life span of saguaros at 125-175 years with a maximum of 300 years. Although the following statistics can vary widely, here are some numbers to consider: a saguaro may be ¼" tall at one


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