Methods for broad-scale plant phenology assessments using citizen scientists' photographs

Abstract

Premise: Citizen science platforms for sharing photographed digital vouchers, such as iNaturalist, are a promising source of phenology data, but methods and best practices for use have not been developed. Here we introduce methods using Yucca flowering phenology as a case study, because drivers of Yucca phenology are not well understood despite the need to synchronize flowering with obligate pollinators. There is also evidence of recent anomalous winter flowering events, but with unknown spatiotemporal extents.

Methods: We collaboratively developed a rigorous, consensus-based approach for annotating and sharing whole plant and flower presence data from iNaturalist and applied it to Yucca records. We compared spatiotemporal flowering coverage from our annotations with other broad-scale monitoring networks (e.g., the National Phenology Network) in order to determine the unique value of photograph-based citizen science resources.

Results: Annotations from iNaturalist were uniquely able to delineate extents of unusual flowering events in Yucca. These events, which occurred in two different regions of the Desert Southwest, did not appear to disrupt the typical-period flowering.

Discussion: Our work demonstrates that best practice approaches to scoring iNaturalist records provide fine-scale delimitation of phenological events. This approach can be applied to other plant groups to better understand how phenology responds to changing climate.

Keywords: Yucca; anomalous flowering; citizen science; data integration; iNaturalist; plant phenology.

Figure 1

Flowering of the six focal Yucca species is shown with colored boxes for each week of the year from 2010–2019. Flowering absences are shown in different intensities of the color gray for different data resources (darkest for iNaturalist, lightest for National Phenology Network [NPN]). Flowering presences are indicated with different colors to identify whether these were documented by iNaturalist (red), NEON (light green), or NPN (blue). The coloring intensity indicates the number of reports from a given source during a specific time period. The colors are mixed when there are flowering presences reported from multiple sources for a single week.

Figure 2

The spatial distribution of flowering of three yucca species in fall–winter 2018 and spring 2019. Typical flowering time (yellow dots) in spring 2019 is defined broadly here to capture potential early onsets, and ranges from 11 February to 15 May 2019. We delineate the anomalous flowering times (magenta dots) as those occurring from 1 November 2018 to 10 February 2019, well outside typical known time frames. These are superimposed on non‐flowering records (gray dots) from the same time periods. Points outside of the map boundary (e.g., for Y. schidigera) are located in Mexico, which is not shown here.

Figure 3

Percentage of cases of full matching, conflicting, and uncertain records per species. Uncertain cases are those in which at least one classifier reported they could not ascertain presence or absence. Larger, tree‐form Yucca species often have increased rates of uncertainty in documenting flowers, whereas some smaller shrub species (e.g., Y. filamentosa) proved challenging for documenting whole versus portion of a plant.

Figure 4

The spatial distribution of occurrences of the six focal Yucca species shown with different colored dots indicating the occurrence source. Species’ range maps from eFlora.org were digitized and included in the background.