A decade of flowering phenology of the keystone saguaro cactus (Carnegiea gigantea)

Abstract

Premise of the Study Phenology is the study of biological life cycle events, such as flowering and migration. Climate patterns can alter these life history events, having ecosystem‐wide ramifications. For example, warmer springs are associated with earlier leaf‐out for many species, impacting species interactions and growing‐season carbon dynamics. While phenological research has been conducted extensively in temperate regions, relatively little is known about the phenological responses in arid and semi‐arid regions. Methods In this study we looked at the flowering phenology of a keystone species in the Sonoran Desert, the saguaro cactus (Carnegiea gigantea). The timing and abundance of flowering was observed on 151 individuals for 10 years at a site near Tucson, Arizona, USA. Using six phenological traits, we explored the relationship between saguaro size and flowering and the climatic drivers of flowering. Key Results Our analyses demonstrated how the calculation of phenological traits at the individual versus the population level can yield differing responses to climate variability, suggesting that not all studies examining the same trait (e.g., first day of bloom) are directly comparable. We found that larger cacti began flowering earlier, flowered for longer, and produced more flowers. Warmer temperatures were correlated with advanced onset and higher bloom yields, while increased precipitation appeared to delay onset and reduce bloom yields. Conclusions Given that climate models predict that the Southwestern USA will become increasingly warmer with more variable precipitation, saguaros may begin flowering earlier in the season and flower more intensely, which could impact pollen availability and population dynamics.

Estimating transition dates from status-based phenology observations: a test of methods

“…Different interpretations of phenological metrics can yield different results (Renzi et al 2019), thus with the E. angustifolia dataset I used strict definitions in calculating the true values used in the analysis. For each year I calculated the following population-level metrics: 1) the start of flowering as defined by the day of year (DOY) of the first observed flower, 2) peak flower defined as the DOY when the most flowers were observed in a given year, 3) the end of flowering as defined by the last DOY a flower was observed.…”

“…While the Midway 7-Day method was the best for estimating flowering for individual plants, using the mean of those estimates from a population (Mean Midway 7-Day) did not provide the best population level estimates even with a large sample size. The onset of flowering for individuals is staggered over time and the mean of these start times is not equivalent to the population onset date (Ison and Wagenius 2014, Keyzer et al 2017, Renzi et al 2019).…”

“…Thus, unlike the skewed population flowering over time, estimators for the onset and end of individuals perform equally. This may not be the case when the study species are larger in size and/or contain numerous flowers which can be counted (Renzi et al 2019). In these cases the phenology over time may be more similar to a population, with a flowering peak and potentially skewed distribution (while E. angustifolia, being in the family Asteraceae, can have one or more flowering heads each with numerous florets, here I treated each individual plant as a single unit).…”

Peer Review #2 of "Estimating flowering transition dates from status-based phenological observations: a test of methods (v0.2)"

“…Manuscript to be reviewed not equivalent to the population onset date (Ison and Wagenius 2014, Keyzer et al 2017, Renzi et al 2019. 2010also did not evaluate the Weibull estimator, which outperformed the GAM method in many scenerios in the current study.…”

Nonlinear modeling of saguaro growth rates reveals the importance of temperature for size‐dependent growth

“…Britton & Rose] is a prominent keystone species of the Sonoran Desert (Drezner, ). Studies over the last century on individual growth rates and population structure have included response to climate (Steenbergh and Lowe, ; Pierson and Turner, ; Drezner, 2006b; Pierson et al., ; Hultine et al., ), flowering phenology (Schmidt and Buchmann, ; Renzi et al., ), recruitment (Steenbergh and Lowe, ; Drezner, , 2006a; Orum et al., ; Conver et al., ; Winkler et al., ), mortality (Pierson and Turner, ; Pierson et al., ) and individual growth (Hastings, ; Steenbergh and Lowe, ; Pierson and Turner, ; Drezner, ; Pierson et al., ) to understand its ecology and episodic recruitment, which have led to a peculiar, nonstable age structure (Steenbergh and Lowe, , ; Pierson and Turner, ; Pierson et al., ; Conver et al., ). In terms of individual growth, saguaro growth rates are strongly influenced by summer rainfall and affected by between‐year summer precipitation differences (Hastings, ; Drezner, ).…”