Our primary concern in this report is with young saguaros so small and inconspicuous during the first years of life that they live and die virtually unnoticed in their desert microhabitats. While there have been several studies on saguaro growth, they have all quite naturally centered on relatively large, easy to find, and easy to measure individuals. While further contributing to such information, for a decade we have been studying the seedlings and juveniles in their natural habitats, and the data provided here on growth were designed to fill in the gap in the growth curve for the first critical years of the saguaro's life. In addition to analyzing the effects of environment and time on saguaro growth and growth-rate, the data reported here also permit correct age determination for young saguaros at Saguaro National Monument.
Environmental factors that influence the germination and establishment of young saguaros discussed in earlier chapters also play important roles in saguaro growth. Short of becoming limitingi.e., short of acting as final limiting factors in the species distributiontemperature, precipitation, and soil characteristics are among the important direct and indirect environmental modifiers of growth within the species range. Accordingly, as discussed under Geographic Variation in Growth, growth rates differ in different saguaro populations in different parts of its distribution.
In the presence of continuously available moisture following germination, seedling development and growth are rapid and continuous. Within 24 hr after rupture of the seed coat, the slender, pale yellow-green seedling emerges upright. The base swells, cotyledons expand and spread to a horizontal position, and the saguaro seedling develops a light-green coloror in strong light, a pale red colorationwithin one day following emergence. Stem development, with distinct areoles and spines, is evident within one week after the start of germination.
By the end of the first summer growth period, the green or reddish green seedling has 8, 12, or 16 areoles bearing well-developed spines. The total stem height is approximately 5.5 mm (0.22 inch), the diameter about 4.5 mm (0.18 inch), and the "taproot" is 1-2 cm (0.4-0.8 inch) in length. First-year stem height is ordinarily 1-2 mm (0.04-0.08 inch) greater in seedlings in flatland soils not associated with rock out cropping.
The principal growth occurs during June, July, August, and September, the period of warm summer rains, and ceases shortly after the end of those rains. Water uptakereplacement of moisture lost during the arid fall monthsbut no growth accompanies early winter rainfall and lower temperatures during late November, December, and January.
In years when late winter rains provide adequate soil moisture and warm temperatures prevail, a secondary and minor period of apical growth occurs as early as February and March.
In the presence of adequate moisture, some apical stem growth of seedlings (and larger saguaros as well) occurs during the arid fore-summer period. On seedlings this is evidenced by the growth of new, greenish-colored apical spines and on older juvenile plants, by the lengthening and red coloration of apical spines. However, because there is water stress and stem shrinkage during this driest portion of the year, an actual decrease in stem height and diameter of saguaros of all sizes can and usually does occur.
April, May, and June are the pre-summer drought months of increasingly severe moisture stress, with increasingly warmer temperatures and scanty if any rainfall; May and June are especially hot. During that period, seedlings commonly shrink to half their original volume, and exposed small and weak individuals die.
The second year of life, the post-seedling juvenile stage, begins abruptly with the first significant summer rain (0.20 inch; 5.1 mm or more) ordinarily in early July in southern Arizona and in late June in southern Sonora. Observable water uptake can occur within 20 hr after the wetting of the soil. With adequate moisture, full turgor is quickly attained and new apical growth is evident within 8 days.
There have been no quantitative data available for natural saguaro growth during the seedling stagethe first year of lifeand directly following early juvenile years. During these first critical years, saguaros are nearly invisible on the surface of the desert. Discovery requires work on hands and knees on the desert floor. As most one-year old saguaros are barely 5-6 mm (ca 0.25 inch) tall, and a large part of the tiny globular "stem" is underground, about all that the observant searcher can see is a small, concealing tuft of tan-colored spines.
After intensive searching, tiny saguaro seedlings and juveniles were discovered in the winter of 1965 in natural habitats under paloverde, mesquite, and bursage nurse-plants at Saguaro National Monument (see Chap. 6). The fate of these and other seedlings and young juvenile saguaros is reported by Steenbergh and Lowe (1969).
Naturally growing young saguaros at Saguaro National Monument (east) were measured to obtain data on stem form and growth rate characteristics for that population. Heights and diameters of individual, naturally established seedlings and young juvenile saguaros of known age (6-66 months) were measured in the field, the plants then removed and remeasured in the laboratory with precision calipers.
Data on the natural growth of larger juvenile plants were determined by yearly field measurements on a representative sample of the young saguaros at Saguaro National Monument (east) using permanent benchmarks and a specially designed gauge for stem height (Fig. 37) and machinist's calipers for diameter. The height and diameter of each individual was measured at one-year intervals. The difference between these consecutive annual measurements provides a series of one-year growth increments. These data provide a basis for estimating height, growth rate, and age relationships for young saguaros in this environment (Table 28; Figs. 39-41).
The stem growth of young saguaros increases exponentially ("geometrically") with age and size. Yearly apical growth of young saguaros at Saguaro National Monument (east) is graphed in Fig. 40. The regression of one-year growth increments on stem height for 62 naturally growing seedlings and juvenile saguaros (Fig. 40) is described by equation (2) below.
The regression of yearly apical growth for wild saguaros during the first growth-year of life (seedling) and subsequent juvenile growth-yearsfor the first 6 years of life in flat and rolling-hill (nonrocky) habitats at Saguaro National Monument (east)is described by
log Y = 0.741 + 0.012 X (1)
where Y is stem height in millimeters and X is age in months,1 for the germinating Class of 1963 to and including the Class of 1968. The associated product-moment correlation coefficient (r) is +0.936, N = 30. The years involved are free of the catastrophic-kill winter freezes as recently experienced in January 1962 and January 1971 in southeastern Arizona (see Niering et al. 1963; Steenbergh and Lowe 1976).
For the seedling year (Class of 1968) of this 6-year series, the measured mean stem height of the seedlings is 6.0 mm ± 0.37 (4.8-7.6), N = 8. Using equation (1), the least squares calculated first-year stem height (Y) is 6.5 (6.486) mm (N = 30), i.e., based on the growth data for all of the 6 successive years of saguaro classes (1963-68) in the study.
Accordingly, this derived best estimate for saguaro stem height attained during the first year of life at this specific locality is 0.6486 cm (0.26 inch). As it is the only natural first-year growth data in existence for the saguaro (or any other species of cactus), it might be assumed that it is applicable to saguaros elsewhere. We caution that that assumption is valid only in a highly restricted sense, for saguaro growth can and does vary significantly and widely from population to population in different environments, as well as from individual to individual in the same immediate environment. Compare, for example, the data in Tables 28-38.
Last Updated: 21-Oct-2005