Long-term changes in flowering synchrony reflect climatic changes across an elevational gradient

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Figure 1. Some flowering species found in Finger Rock Canyon. (a) Allium macropetalum, (b) Uropappus lindleyi, (c) Portulaca umbraticola subsp. lanceolata, (d) Claytonia perfoliata, (e) Mammillaria grahamii, (f) Carnegiea gigantea, (g) Echinocereus coccineus, (h) Passiflora mexicana, (i) Epilobium canum.


By Alessandro Fisogni, Natasha de Manincor, C. David Bertelsen, Nicole E. Rafferty


The idea of writing this paper came after a major disruption related to the Covid-19 pandemic. Both of us (Natasha and Alessandro) had just arrived as postdocs in the Rafferty Lab at UCR in February 2020, with plans for new research and experiments to start soon after. A few weeks later, all the facilities on campus – as in virtually any university – closed down for several months, hampering our plans for the year. We had then to think about something that didn’t require an empirical approach to avoid an important gap in our research. Thanks to a long-time collaboration between Nicole E. Rafferty and C. David Bertelsen, we were given the opportunity to work on an amazing dataset that engaged us for well over a year.

Since 1981, Dave Bertelsen has hiked up and down the Finger Rock Canyon in the Catalina Mountains near Tucson, Arizona (USA). These mountains host an important and extremely diverse array of species, both for plants (Fig. 1) and animals (Fig. 2). Fascinated by the changes in diversity observed throughout the canyon, and fueled by a unique passion, curiosity, and willpower, he saw an opportunity to learn more about spatial and temporal changes in plant communities. The outcome of such an effort is an unparalleled collection of phenological data including the entire flora found along a 5-mile trail encompassing a 1267m elevational gradient and five major plant communities (Fig. 3), from desert scrub to pine forest (Bertelsen 2018). To date, this record spans five decades (1980s – 2020s) and includes thousands of datapoints reporting flowering presence for the 599 plant species found in the canyon.

Figure 2. Some animal species found along the Finger Rock Canyon. (a) ring-necked snake, (b) Mexican spotted owl, (c) reticulate gila monster.

In a recent paper led by Nicole Rafferty, the authors explored this dataset to show that flowering time was diverging among communities across the gradient in a non-linear fashion, and these changes were related to elevation-specific changes in climatic variables (Rafferty et al. 2020). How such variation is reflected in the flowering overlap within and between communities, however, remained an open question.


Figure 3. Views from the trail. (a) Looking up Finger Rock Canyon from the trailhead. The foreground vegetation is desert scrub while that on top of the distant ridge is oak-pine woodland, (b) Tucson from Finger Rock Canyon, looking SW from 6300 feet elevation. Vegetation is oak-pine woodland.

In the present article published in Ecography, we used circular statistics (Staggemeier et al. 2020) to estimate temporal changes in flowering synchrony within and between communities in a 33-year period (within 1984-2019), and relate them to changes in temperature and precipitation recorded across the gradient. We found that flowering synchrony significantly decreased in all plant communities, especially at the lowest and more arid elevations (28.1% synchrony lost), and such decreases were negatively correlated with increasing temperatures. Reduced precipitation interacted with warmer temperatures accelerating the decrease in synchrony, again especially at the lowest desert scrub communities. At the same time, flowering synchrony within plant species occurring in multiple communities increased between most community pairs at accelerated rates in recent years, except between the two highest-elevation communities, indicating a generalized homogenization of flowering phenology across the gradient.

Because temperature increase and precipitation decrease are predicted to continue in the future in the study area (Vose et al. 2017, Dannenberg et al. 2019), we might expect an intensification of the patterns observed in our study, with potentially negative effects on the plant communities and on the entire ecosystem. This is particularly true if the North American monsoon, which provides about half the annual precipitation, continues to weaken (Pascale et al. 2017). Moreover, another variable that is certainly playing an important role in defining flower availability is the increasing frequency of wildfires (Abatzoglou et al. 2021), which are likely to interact with other climatic changes in reshaping the flowering synchrony in the years to come.

Figure 4. Sunset from Finger Rock Canyon.

All photos by C. David Bertelsen.




Abatzoglou, J. T. et al. 2021. Projected increases in western US forest fire despite growing fuel constraints. – Commun. Earth Environ. 2: 227.

Bertelsen, C. D. 2018. Thirty-seven years on a mountain trail: vascular flora and flowering phenology of the Finger Rock Canyon watershed, Santa Catalina Mountains, Arizona. – Desert Plants 34: 3–268.

Dannenberg, M. P. et al. 2019. Reduced tree growth in the semiarid United States due to asymmetric responses to intensifying precipitation extremes. – Sci. Adv. 5: eaaw0667.

Pascale S. et al. 2017. Weakening of the North American monsoon with global warming. – Nat. Clim. Change 7: 806–812.

Rafferty, N. E. et al. 2020. Changing climate drives divergent and nonlinear shifts in flowering phenology across elevations. – Curr. Biol. 30: 432–441.

Staggemeier, V. G. et al. 2020. The circular nature of recurrent life cycle events: a test comparing tropical and temperate phenology. – J. Ecol. 108: 393–404.

Vose, R. et al. 2017. Temperature changes in the United States. – In: Wuebbles, D. et al. (eds), Climate science special report: fourth national climate assessment, Vol. I.U.S. Global Change Research Program, Washington, DC, USA, pp. 185–206.