Dubrovsky (1996, 1998) found that seeds of the columnar cacti Carnegiea gigantea, Pachycereus pecten-aboriginum, Stenocereus gummosus, and S.
McAuliffe and Janzen (1986) documented the effect of intraspecific competition for limited water in dense aggregations of young Carnegiea gigantea plants associated with the canopy of perennial shrubs; in this case, the effects of competition were decreases in water uptake, water storage, apical growth, and reproductive potential (McAuliffe & Janzen, 1986).
In contrast, high summer and low winter temperatures, soil-moisture availability, and pre- and post-summer dry periods are among the main environmental factors that affect the growth rate of Carnegiea gigantea (Steenbergh & Lowe, 1977).
With regard to allometry, there is some variation in the annual growth rate of Carnegiea gigantea related to changes in plant form associated with ontogeny (Hastings & Alcorn, 1961; Steenbergh & Lowe, 1977).
thurberi grow through the production of new stems, whereas Carnegiea gigantea produces a single stem with several branches.
In the long-lived columnar cacti, plants reproduce for the first time when individuals are 33 years old in Carnegiea gigantea (Steenbergh & Lowe, 1977), 70 years old in Cephalocereus columna-trajani (Zavala-Hurtado & Diaz-Solis, 1995), and more than 90 years old in Neobuxbaumia macrocephala (Esparza-Olguin et al.
1992), and the columnar cacti Carnegiea gigantea, Escontria chiotilla, Lophocereus schottii, Neobuxbaumia macrocephala, N.
These favorable conditions appear to be related to particular precipitation and temperature combinations for species such as Carnegiea gigantea, Echinocactus horizonthalonius, Ferocactus cylindraceus, Neobuxbaumia macrocephala, and Opuntia echios (Reid et al.
For instance, the Carnegiea gigantea and the Mammillaria magnimamma 3 populations (a columnar long-lived species and a globose shorter-lived species, respectively) are located at the far-right comer of the triangle, almost in the vortex, while those populations located closer to the center of the triangle include Escontria chiotilla 2, Coryphantha robbinsorum 1, Opuntia rastrera 1, and Mammillaria magnimamma 2, representing three different life-forms.
Carnegiea gigantea) and some populations with a relatively high [lambda] show very low elasticity values for the seedling phase (Table IV; Fig.
uncertainty in the proposed aspect) Species Nurse plant Life-cycle stage Columnar cacti Carnegiea Ambrosia deltoidea S-J-A gigantea * Cercidium microphylum Encelia farinosa Laura tridentata Olneya tesota Prosopis juliflora Cephalocereus Caesalpinia S-J hoppenstedtii * melanadenia Escontria Acacia S-J-A chiotilla * cochliacantha Fouquieria formosa Mimosa luisana M.
