Local temporal trajectories explain population‐level responses to climate change in saguaro (Carnegiea gigantea)

Abstract

Population demography is typically assumed to be strongly influenced by climatic factors, particularly with succulent plants and cacti. The saguaro cactus (Carnegiea gigantea) is a long-lived columnar cactus of the Sonoran Desert that experiences episodic recruitment and mortality. Previous studies have attributed long-term changes in saguaro populations to climatic factors, including increased germination and establishment during wet periods and mortality and reduced establishment during droughts and extreme freezes. We used a 48-yr data set of marked individuals at the Desert Laboratory in Tucson, Arizona, to test the hypothesis that local, temporal population trajectories are mediated by topographic heterogeneity that interacts with fluctuating climatic conditions. We tested the influence of local slope and aspect vs. climatic variability on a population of saguaro using >5800 marked individuals that have been measured since 1964. We examined the relationship between demography and climatic variables (drought, precipitation, and extreme temperatures) and found significant differences in growth and survival among aspects and among census periods. Saguaro population growth was higher during wet and cool periods (e.g., 1964-1970), and changes in age structures suggest that topographic differences interact with climatic fluctuations to produce unexpected demographic patterns including large recruitment events during periods of relatively unfavorable climate conditions. Our results highlight the importance of long-term data to detect demographic responses to climate that could not be predicted from short-term studies of plant physiology and population demography.

Episodic recruitment in the saguaro cactus is driven by multidecadal periodicities

“…2018, Rodríguez‐Buriticá et al. 2019), or even large numbers of small samples within the same regional locality (Drezner 2006 a , Drezner and Balling 2008). These long‐term data sets provide highly valuable, very fine temporal resolution, but little spatial variation.…”

“…2018, Rodríguez‐Buriticá et al. 2019). Seedling recruitment is highly correlated with the presence of nurse plants or rocks providing adequate conditions for germination and establishment (Turner et al.…”

“…The PDSI has been used in previous saguaro studies to explain recruitment or establishment events (Pierson and Turner 1998, Drezner and Balling 2002, 2008, Rodríguez‐Buriticá et al. 2019). For each population, we extracted a self‐calibrated PDSI from 1901 to 2017 using the KNMI Climate Explorer data (Cook et al.…”

“…Rodríguez‐Buriticá et al. (2019), studying the long‐term populations that Spalding and Shreve established at Tumamoc Hill, found that recruitment in saguaro was likely to occur in favorable wet periods, but was also influenced by local topographical features. Orum et al.…”

Temporal and Habitat-Based Growth Rate Variability in the Saguaro Cactus (Carnegiea gigantea)

“…Fluctuations in growth rate over time caused by any number of factors may result in less accurate estimations of saguaro age–height distributions. For example, the error when calculating age from height increased from ±2 y (Pierson and Turner 1998) to ±6 y (Rodríguez-Buriticá et al 2019) over 20 y at Tumamoc Hill. Given observed changes to the climatic drivers and rate of growth, and the instability of age estimates over time, the Steenbergh and Lowe (1983) model should be revisited.…”