SAGUARO Ecology of the Saguaro: II NPS Scientific Monograph No. 8

Summary and Conclusions

The saguaro is well adapted in its growth responses to the highly variable amounts and temporal distribution of summer precipitation that are characteristic of the climate of the Sonoran Desert. The shallow, widespreading root system enables the plant to utilize moisture from relatively small amounts of rainfall. It responds quickly with new growth to renewed soil moisture after a period of drought. Thus, the saguaro efficiently can utilize intermittently and briefly available moisture from in frequent summer rains.

Growth of the saguaro is distinctly seasonal and takes place during coincident periods of relatively high temperatures and plant-available soil moisture. Growth is limited by lack of moisture during the dry after-summer and arid fore-summer seasons, and by cold during the winter wet season.

The principal stem growth of the saguaro takes place during and immediately following monsoon rains in July, August, and September. Water uptake occurs and apical growth is resumed within a few hours after the start of the first saturating summer rain.

Moisture lost by transpiration during the dry after-summer is usually replaced during the early winter months. In the colder northern portions of the species distribution, water uptake without growth takes place during the period of winter rains and low temperatures in late November, December, January, and early February.

A second period of growth beginning as early as February—depending upon concurrently warm temperatures and available moisture—may occur during the late winter and early spring months. The potential increase in stem volume from such growth is offset by transpiration losses during the hot months of the arid fore-summer.

In the saguaro and other columnar cacti, there is a series of critical ontogenetic growth-form changes. In young saguaros, the first of these changes takes place when the young plant reaches a height of approximately 5.5 cm (2.2 inches), and the second change occurs when the plant reaches a height of approximately 200 cm (78.7 inches).

The annual growth increment of juvenile saguaros increases exponentially with age and increased size up to the age of first flowering. As much as 50% of the potential stem growth of adult saguaros is diverted into yearly reproductive growth.

The total summer growth of the saguaro is dependent upon the cumulative duration of periods of plant-available soil moisture. The annual increment of stem growth, therefore, is determined not only by the amount of precipitation, but in large measure, by its temporal distribution.

Growth rates of the saguaro are highly variable. Genetic variation and/or microenvironmental differences commonly result in a twofold (or larger) difference in annual height growth between individuals growing in the same immediate locality and topographic habitat. Similarly large variations in annual growth increments occur and are associated with year-to-year variation in summer precipitation and winter temperatures.

Significant differences in saguaro growth rates occur in different topographic habitats under the same general climate. These differences are directly related to the influence of slope and exposure upon temperature and plant-available moisture, and to different moisture relationships that result from differing physical characteristics of the as sociated soils.

The rate of saguaro growth differs widely from population to population in response to the broad range of climatic environments present within the extensive geographic range of the species. Generally, rates of saguaro growth increase along a gradient (1) of increasing summer precipitation from west to east, and (2) higher winter minimum temperatures that occur from high to low elevations, and from north to south latitudes.

Freezing depresses the growth rate of both juvenile and adult saguaros. Severe freeze-caused injury to the crowns of juvenile saguaros reduces growth for a period of 2 or more years following the freeze. Freeze-caused reduction in the growth rate of adult saguaros can continue for several years after injury.

For adult saguaros situated near the cold-limited northeastern boundary of its distribution, the growth-limiting effect of recurring catastrophic freezes offsets the advantage of relatively favorable moisture conditions. In these winter-cold environments the average growth of large saguaros is less than in some more arid but warmer environments.

Height-age relationships of saguaros growing in different portions of the species range vary not only with differences in precipitation and related availability of moisture but also are controlled by the frequency and intensity of subfreezing temperatures. Established height-age relationships for a saguaro population growing in one locality cannot be applied to populations growing in other locations without consideration of both of these controlling climatic variables.

Over a major portion of the saguaro species distribution, winter cold determines not only the growth rate but also quantitative aspects of its reproductive success, local and absolute limits of distribution, population density, and age-class structure, as well as the timing of the critical events in the life cycle of the saguaro.