Distribution, morphology, and population structure of saguaro (Carnegiea gigantea (Engelm.) Britt. & Rose): A Sonoran Desert icon
The saguaro is probably the most iconic symbol of the southwestern US and northwestern Mexico. It ranges from the limits of the tropics in the southern Sonoran Desert—close to its origin—to the edge of the desert in Arizona and California—where it is a newcomer. We studied 16 populations of saguaro throughout its range mapping and measuring 100 individuals in each population to describe and model the phenotypic variation in terms of allometry and adult traits, and correlate that variation with the physical and biological environment; and to assess and model the spatial variation among populations in ecological structure, recruitment and growth. Also, we model the niche extent in terms of the physical environment using GARP.
Using a GARP, we found that the potential distribution of saguaro at least partly controlled by climatic factors including winter minimum temperatures, summer precipitation, and irradiance. There is great variation among populations in morphological traits like plant size, shape, spine number and shape, and features of ribs. Differences among populations were statistically significant, not only on the characteristics of reproductive adults but also in the patterns of ontogenetic development. With the exception of the number and shape of the ribs, no consistent geographical or climatic patterns for morphological traits were found. The variation in the number and shape of the ribs seems to follow the association between surface-volume ratios and the economy of water. Populations in more mesic environments grow faster than those in more xeric environments. Although the criterion for the selection of populations was high density and prominence, there is a huge variation in population densities in the 16 studied populations: from 5 to over 200 ind ha-1. The distribution of size classes and / or age in any population does not follow a stable structure. Recruitment has a clear episodic component with numerous peaks. The frequency of recruitment is highly variable, ranging from less than 6 to more than 12 years on average recruitment events. It shows high correlation with strong ENSO events. It is likely that periods of high and constant rainfall during summertime, or during successive rainy summers during very strong ENSO events control recruitment. We conclude with projections of future research, including the genomics of saguaro.