Analyzing a phenological anomaly in Yucca of the southwestern United States

Laura Brenskelle,Lucas C Majure,Vijay Barve,Rob P Guralnick,Daijiang Li

doi:10.1038/s41598-021-00265-y

Laura Brenskelle, Lucas C Majure + Show 3 more

Open Access

https://doi.org/10.1038/s41598-021-00265-y

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Abstract

Yucca in the American desert Southwest typically flowers in early spring, but a well-documented anomalous bloom event occurred during an unusually cold and wet late fall and early winter 2018–2019. We used community science photographs to generate flowering presence and absence data. We fit phenoclimatic models to determine which climate variables are explanatory for normal flowering, and then we tested if the same conditions that drive normal blooming also drove the anomalous blooming event. Flowering for Yucca brevifolia (Joshua tree) and Yucca schidigera (Mojave yucca) is driven by complex, nonlinear interactions between daylength, temperature, and precipitation. To our surprise, early-season flowering odds are highest in colder and drier conditions, especially for Joshua trees, but increase with precipitation late-season. However, the models used to fit normal blooming overpredicted the number of anomalous blooms compared to what was actually observed. Thus, predicting anomalous flowering events remains a challenge for quantitative phenological models. Because our model overpredicted the number of anomalous blooms, there are likely other factors, such as biotic interactions or other seasonal factors, which may be especially important in controlling what is presumed to be rare, out-of-season flowering in desert-adapted Yucca.

Highlights

In the winter of 2018–2019, park rangers and visitors to Joshua Tree National Park were surprised to see Joshua tree (Yucca brevifolia Engelm.) and Mojave yucca (Yucca schidigera Roezl ex Ortgies) in bloom
We scored a total of 1375 presence/absence observation records for Y. brevifolia and 1637 for Y. schidigera ranging from 2009 to 2020
We found similar results for the 120-day accumulation model for Y. brevifolia; all top models had a three-way interaction and second-order polynomial daylength term, with marginally better model validation statistics compared to the 30-day models for both training and testing

Summary

Introduction

In the winter of 2018–2019, park rangers and visitors to Joshua Tree National Park were surprised to see Joshua tree (Yucca brevifolia Engelm.) and Mojave yucca (Yucca schidigera Roezl ex Ortgies) in bloom. Yucca elata (Engelm.) and Y. baccata (Torr.) have been shown to have increased reproductive output with increased winter and spring precipitation, with the slowest growth in early to mid-summer during what is often the driest s eason[10,11] These studies did not focus on how environmental cues determine bloom timing. Formal modeling approaches that can test the relative importance of temperature, precipitation, and photoperiod as drivers of timing of Yucca phenology are critically missing Such models are important, because species of Yucca have highly specific, co-evolved obligate pollinators and herbivores—the yucca moths in Scientific Reports | (2021) 11:20819. If anomalous blooms become more common due to changing environmental conditions or more extreme climate variation, this could have negative consequences for Yucca, given its specialized pollination system

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