Climatic effects on regeneration trends for two columnar cacti in the northern Sonoran Desert

• Published in: Annals of the Association of American Geographers Vol. 83; no. 3; pp. 452 - 474
• Main Author: Parker, Kathleen C.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Taylor & Francis Group 01-09-1993; Blackwell Publishers; Blackwell Publishing Ltd; Association of American Geographers
• Subjects: arizona; Bgi / Prodig; Biogeography; cactaceae; Cactus; Carnegiea gigantea; clima; climat; climate; Climate models; climatic fluctuation; columnar cacti; conservacion de la naturaleza; conservation de la nature; Deserts; Drought; environmental factors; evolucion de la poblacion; evolution de la population; facteur du milieu; factores ambientales; grazing; natural regeneration; nature conservation; Organ pipes; pastoreo; paturage; Physical geography; plant demography; Plant geography; Plants; population dynamics; regeneracion natural; regeneration naturelle; Seedlings; Sonoran Desert; stenocereus; Stenocereus thurberi; Vegetation; Winter; xerofitas; xerophyte; xerophytes; United States; Arizona; Human impact; Climatic variation; Vegetation; Biogeography; Arid area; Population dynamics; Desert; Holocene
• ISSN: 0004-5608; 1467-8306
• DOI: 10.1111/j.1467-8306.1993.tb01945.x

A traditional perception of desert vegetation is one of relatively slow change in a consistently harsh environment. More recent studies of population dynamics and paleoenvironmental reconstructions in arid environments have revealed the more dynamic nature of desert vegetation, in both a short-term and long-term perspective. This paper examines relationships of population dynamics to climatic fluctuation and anthropogenic factors in two columnar cactus species in the northern Sonoran Desert. The results underscore the responsiveness of desert plant populations to temporal variability in environmental stresses as well as the influence of temporal scale on our views of population dynamics.
I use allometrically determined ages for 327 organ pipe cacti on hillslopes and 473 saguaros on flat sites in Organ Pipe Cactus National Monument, Arizona, to relate regeneration trends for each species to climatic variation in the last century. Residuals representing the deviation of the actual age distribution from a hypothetical distribution, based on constant age-specific survivorship rates, serve as the dependent variable in regression models that quantify regeneration trends. Until the 1960s the trends for both species were similar; episodes of establishment corresponded to relatively wet periods with mild winters, and poor regeneration coincided with prolonged drought. Regeneration steadily declined through the 1960s in saguaro and remained relatively high in organ pipe cactus. The occurrence of several severe freezes, intense grazing, or a combination of these two factors during this decade may have affected flat sites and hillslopes differentially. Flats supporting saguaro were more intensely grazed and often experience lower winter minimum temperatures than hillslopes supporting organ pipe cactus. At the time scale of centuries, the present-day mature stands of both species in the northern Sonoran Desert may be anomalously dense remnants from the late 1910s and 1920s, which was one of the wettest periods in the last four centuries.