Causes of Synchronous Flowering in Astragalus Scaphoides, an Iteroparous Perennial Plant

Abstract

Temperature and precipitation determine the conditions where plant species can occur. Despite their significance, to date, surprisingly few demographic field studies have considered the effects of abiotic drivers. This is problematic because anticipating the effect of global climate change on plant population viability requires understanding how weather variables affect population dynamics. One possible reason for omitting the effect of weather variables in demographic studies is the difficulty in detecting tight associations between vital rates and environmental drivers. In this paper, we applied Functional Linear Models (FLMs) to long-term demographic data of the perennial wild-flower, Astragalus scaphoides, and explored sensitivity of the results to reduced amounts of data. We compared models of the effect of average temperature, total precipitation, or an integrated measure of drought intensity (standardized precipitation evapotranspiration index, SPEI), on plant vital rates. We found that transitions to flowering and recruitment in year t were highest if winter/spring of year t was wet (positive effect of SPEI). Counterintuitively, if the preceding spring of year t À 1 was wet, flowering probabilities were decreased (negative effect of SPEI). Survival of vegetative plants from t À 1 to t was also negatively affected by wet weather in the spring of year t À 1 and, for large plants, even wet weather in the spring of t À 2 had a negative effect. We assessed the integrated effect of all vital rates on life history performance by fitting FLMs to the asymptotic growth rate, log(k t). Log(k t) was highest if dry conditions in year t À 1 were followed by wet conditions in the year t. Overall, the positive effects of wet years exceeded their negative effects, suggesting that increasing frequency of drought conditions would reduce population viability of A. scaphoides. The drought signal weakened when reducing the number of monitoring years. Substituting space for time did not recover the weather signal , probably because the weather variables varied little between sites. We detected the SPEI signal when the analysis included data from two sites monitored over 20 yr (2 9 20 observations), but not when analyzing data from four sites monitored over 10 yr (4 9 10 observations).

Desynchronization and re‐synchronization of reproduction by Astragalus scaphoides, a plant that flowers in alternate years

“…1). Flowering by individuals was synchronous, both within and among sites, with only a slight tendency toward stronger synchrony within than among sites (Crone and Lesica 2004).…”

“…Over these 14 years, plants tended to flower in alternate years, as evidenced by negative lag-1 autocorrelations, and a bimodal distribution of the number of flowering plants through time (Crone and Lesica 2004;see Fig. 1).…”

“…We (Crone and Lesica 2004) evaluated spatiotemporal patterns of synchrony, and potential drivers of synchrony, using monitoring data collected at Haynes Creek, McDevitt Creek, and Sheep Corral Gulch between 1986 and 1999. Over these 14 years, plants tended to flower in alternate years, as evidenced by negative lag-1 autocorrelations, and a bimodal distribution of the number of flowering plants through time (Crone and Lesica 2004;see Fig.…”

“…For the past two decades, my colleagues and I have studied causes of synchronous reproduction by Astragalus scaphoides, a perennial herb that flowers in alternate years (Lesica 1995;Crone and Lesica 2004). Initially, we believed that reproduction might be synchronized by exogenous factors, particularly precipitation during bud initiation, in this semi-arid environment.…”

“…I also use these follow-up studies to test whether longer-term monitoring revealed stronger signals of exogenous environmental drivers. The first of these studies is an analysis of monitoring at three sites (Crone and Lesica 2004;Crone et al 2005), in which we previously analyzed patterns from 1986 to 1999, and for which monitoring is ongoing. The second is an experiment in which we attempted to desynchronize plants in these plots by adding supplemental water from 2000 to 2002 (Crone and Lesica 2006).…”