Population structure and growth patterns of Opuntia echios var. gigantea along an elevational gradient in the Galapagos Islands

Cacti growing in forests potentially experience growth limitation due to re duced light availability. To test: this hypothesis, we studied the populati on structure of Opuntia echios var, gigantea at 15 sites on the south side of Isla Santa Cruz, Galapagos Islands, Ecuador. Populations were located in communities ranging from arid scrub at low elevations to closed-canopy tro pical dry forest at higher elevations. Ordination confirmed the existence o f a strong elevation-vegetation gradient. Opuntia abundance peaked at lower elevations (<ca 30 m), with lower densities in dosed-canopy sites. For pop ulations in scrub vegetation, density declined fairly regularly with plant height. Populations in forested sites had few plants of intermediate height , suggesting periodic recruitment. Scrub populations had random dispersion, while those in forests were aggregated. The change in spatial pattern may be related to a change in primary reproductive mode from asexual propagatio n via fallen fruits to propagation via fallen cladodes. Height was signific antly correlated with stem diameter. Intercepts of these relationships incr eased toward higher elevations, probably in response to the increasing heig ht of the surrounding canopy.