Vegetation Structure at Punta Cirio, Sonora, Mexico

Abstract

Contiguous north facing slopes, south facing slopes, arroyos, and shoreline at Punta Cirio, Sonora, Mexico support distinct plant communities. Arroyos and north slopes are the habitats that are most similar in vegetation structure. The shoreline, dominated by Frankenia palmeri and Lycium californicum, has the most distinctive vegetation type. Several species are exclusive to one habitat. Of the 13 major species, only Opuntia bigelovii and Jatropha cuneata, the most important species, occur in all four habitats. The shoreline habitat is the least diverse and has the greatest cover and tallest vegetation. South slopes are also low in diversity, but have the least cover. Cover and plant height, but not density, increase with descending elevation on north and south slopes. In arroyos, vegetation height increases with distance from the Gulf. Species differ in response to the gradients. For example, height of Euphorbia misera increases inland, while Ambrosia dumosa decreases in size. Punta Cirio is located in the mainland Central Gulf Coast Desert, which is considered to be the driest of Shreve's six Sonoran Desert sections; mean annual rainfall is ca. 120 mm and the mean annual temperature is ca. 26?C (Shreve and Wiggins, 1964; Humphrey, 1974). Sonoran Desert vegetation in the USA has been the focus of many studies, and several investigators have shown that water availability influences community structure (Balding et al., 1974; Phillips et al., 1978). However, little has been published on vegeta- tion structure in the Central Gulf Coast Desert. Humphrey (1974) described the vegetation on north and south slopes at Punta Cirio, and attributed dif- ferences in plant composition to insolation-induced aridity differences. This study compares the vegetation on contiguous north and south slopes, arroyos, and shoreline. Changes in community and species dimensions are described along an elevation gradient on the slopes and with distance from the Gulf in arroyos.