The saguaro cactus (Carnegiea gigantea) is a Sonoran Desert (southwestern USA-northwest Mexico) keystone species that can live for over 150 years, but only establishes intermittently when conditions are favorable (including mild winters, cool summers) forming cohorts. The late 1800s to early 1900s was an extended period of successful establishment in several documented populations, a period which also experienced many volcanic eruptions such as Krakatau (1883), Mt. Pelée, Soufriere, Santa Maria (1902), Ksudach (1907), Katmai (1912), and others. Saguaros experience near 100% mortality during the first few years of life, with mortality declining dramatically after. This time scale (2-3 years) coincides with the climatic changes (e.g. milder summers and winters) that follow volcanic eruptions. The Weighted Historical Dust Veil Index (WHDVI) is an indicator of climatically relevant volcanism. Two datasets representing more than a century each of saguaro regeneration were created, one from a marginal population, the other an amalgamated dataset from 30 populations spanning 50,000 km2 of the Sonoran Desert. Individual plant ages for sampled plants were calculated, and annual establishment variables were generated. Statistical analyses tested the relationships between year of establishment in the two databases and the WHDVI.
This study demonstrates the interplay of geology, climatology, and ecology at global and local scales. The local eruption of Krakatau in 1883 (for example) altered global climate enough to create suitable conditions for regeneration on the other side of the world in Arizona, and the large cohort that established during that period persists today, representing a large fraction of the saguaro’s current populations. The peaks in saguaro establishment are significantly (P<0.001) related to global-scale volcanism (WHDVI) both in the marginal and regional datasets. The relationship between WHDVI and regeneration is particularly stunning in the marginal population that is closer to the species’ environmental limits. Assessing volcanism-climate relationships locally in the Sonoran Desert shows that winter precipitation, winter soil moisture, annual precipitation, and the crucial rainfall in the pre-summer rain period, are higher during periods with a high WHDVI (P<0.05 for all). The implications extend beyond saguaros; hundreds of species rely on this keystone species, and fluctuations in regeneration have broad implications for the whole ecosystem. It is likely that other species in other ecosystems are also affected by global scale, large geological events, but have not yet been identified.