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Flowering time advances since the 1970s in a sagebrush steppe community: implications for management and restoration

Citation

Bloom, Trevor; O'Leary, Donal; Riginos, Corinna (2021), Flowering time advances since the 1970s in a sagebrush steppe community: implications for management and restoration, Dryad, Dataset, https://doi.org/10.5061/dryad.tmpg4f50r

Abstract

Climate change is widely known to affect plant phenology, but little is known about how these impacts manifest in the widespread sagebrush ecosystem of the Western US which supports a number of wildlife species of concern. Shifts in plant phenology can trigger consequences for the plants themselves as well as the communities of consumers that depend upon them. We assembled historical observations of first flowering dates for 51 species collected in the 1970 and 80s in a montane sagebrush community in the Greater Yellowstone Ecosystem and compared these to contemporary phenological observations targeting the same species and locations (2016-2019). We also assembled regional climate data (average spring temperature, day of spring snowmelt, and growing degree days) and tested the relationship between first flowering time and these variables for each species. We observed the largest change in phenology in early spring flowers, which as a group bloomed on average 17 days earlier, and as much as 36 days earlier, in the contemporary data set. Mid-summer flowers bloomed on average 10 days earlier, nonnative species 15 days earlier, and berry-producing shrubs 5 days earlier, while late summer flowering plants did not shift. The greatest correlates of early spring and mid-summer flowering were average spring temperature and day of snowmelt, which was 21 days earlier, on average, in 2016-2019 relative to the 1973-1978. The shifts in flowering phenology that we observed could indicate developing asynchronies or novel synchronies of these plant resources and wildlife species of conservation concern including: Greater sage-grouse, whose nesting success is tied to availability of spring forbs; grizzly bears -- which rely heavily on berries for their fall diet; and pollinators. This underscores the importance of maintaining a diverse portfolio of native plants in terms of species composition, genetics, phenological responsiveness to climatic cues, and ecological importance to key wildlife and pollinator species. Redundancy within ecological niches may also be important considering that species roles in the community may shift as climate change affects them differently. These considerations are particularly relevant to restoration and habitat-enhancement projects in sagebrush communities across western North America.

Methods

Study Site

Craighead’s historical data and our contemporary observations center around Blacktail Butte, an isolated outcrop of vegetated limestone with elevations ranging from 1,990-2,343m in GTNP within the Middle Rockies, EPA Ecoregion Level III. The plant community at the base of Blacktail Butte is best described as montane sagebrush steppe (Innes and Zouhar 2019), where mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana) is the dominant species along with other shrubs including antelope bitterbrush (Purshia tridentata), rubber rabbitbrush (Ericameria nauseosa), mountain snowberry (Symphiocarpus oreophilus), and diverse perennial and annual forbs. Blacktail Butte also harbors patches of mixed conifer or aspen overstory with a native forb and tall shrub understory. There is a moderately-used day hiking trail and climbing area at the base of Blacktail Butte, but other human disturbances are minimal, aside from a small homestead site on the NW flank that was removed in the 1930s. Blacktail Butte is important habitat for a diversity of wildlife including grizzly bears, black bears, moose, elk, mule deer, wolves, songbirds, raptors, and sage-grouse.

Historical Observations

We retrieved hand-written notes made by Frank Craighead from his family archives containing nearly 800 phenological observations of native and non-native angiosperms observed in 1974-1979 and 1988 (Fig. 1) and entered these into digital form. During each of these years’ spring and summer seasons, Craighead typically made two to four observations per week of various phenological events including the timing of plant emergence and flowering, migratory bird arrivals, mammal behaviors such as bears emerging from hibernation, fish spawning, and weather events. His notes include observations of 258 species of flowering plants, although not every species was observed every year. We categorized plant observations representing first presence of leaves, first presence of buds, first flower, peak flower, and occurrence of fruits or seeds. All data were sorted by species, year, and phenological event. The vast majority of the observations were of first flowering date, and we have therefore focused our analyses on this phenological phase (phenophase).

We identified 51 species that had at least three first-flowering observations in our historical dataset. We reviewed the written locations of each record using the Spatial Analysis Georeferencing Accuracy (SAGA) protocol (Bloom et al. 2018a), and identified a 2.7 km trail that Craighead’s surviving family members told us he used routinely to make his phenology observations (C. Craighead and S. Craighead, pers. comm.) and confirmed by his notes. The trail ranges from Craighead’s cabin just north of Blacktail Butte and the area along the northwest edge of Blacktail Butte through a gradient of ecosystem types (Appendix S1: Fig. S1).

Contemporary Observations

In spring 2016, we initiated contemporary observations of the focal 51 species (Appendix S1: Table S1). Our study site was identified to match Craighead’s path as best as possible, representing a gradient of ecosystem types, from the dry, exposed mountain sagebrush steppe near Craighead’s home, through a mixed conifer forest, across another mountain sagebrush steppe, and into an aspen grove with tall shrub and forb understory. The study site’s extent along Craighead’s regular walking path was chosen to represent the major ecosystem types that were present, nearby, and collectively contained all the species observed by Craighead. Similar to other studies that have replicated historic phenology observations (Bradley et al. 1999, Miller-Rushing and Primack 2008, Jones and Daehler 2018), we made the assumption that Craighead was aware of where along the path he was likely to encounter earliest flowering plants of each species and made his observations at these locations.

During spring, summer, and early fall 2016, 2017, 2018, and 2019 we visited the study site a minimum of twice weekly (every 2 to 4 days), which is considered relatively high frequency (Miller‐Rushing et al. 2008), to record the phenophase of our focal species. Although we collected contemporary data on every phenophase, for this paper we are reporting on first-flower, the only phenophase with sufficient historical data to allow direct comparison.  For each plant species, we recorded the Julian day (0-365) for the first flower observed each year (day of year, DOY). We also grouped species into one of six ecological groups (Appendix S1: Table S2) based on average first flower date for the entire study period (1974-2019) and species’ ecology. We categorized most species into groups that split the flowering season approximately into thirds (April- early September) based on the average first flowering date of the entire dataset (contemporary and historic data), a common practice in phenology studies (Moore and Lauenroth 2017, Arfin Khan et al. 2018, Pearson 2019). We defined Early Spring flowers as having an average first flower before June 15, Mid-Summer between June 16-July 31, and Late as any species with an average first flower date after July 31. To explore other ecological patterns, we also defined shrubs that produce berries edible to bears and birds as “Berries” and species that are not native to the region as “Nonnative,” the smallest group since there were few nonnative species in the historical record.

Climate Data

We assembled several sets of climate data in order to (a) quantify changes in climate variables relevant to flowering over the study period, and (b) test the relationships between these variables and observed changes in first flowering date for individual species and across different ecological groups. Data were derived from both the Moose Weather Station (National Oceanic and Atmospheric Administration 2020) and the Parameter elevation Regression on Independent Slopes Model (PRISM) (PRISM Climate Group 2018).

The Moose Weather Station is located at the GTNP headquarters less than 1.5km from the base of Blacktail Butte and is managed by the National Park Service. This weather station has been continuously recording daily temperatures and daily snow depth from 1958 to the present within a 100m area. Unfortunately, data were missing for April 1979 and could not be related to first flowering time for this particular year. Spring temperature was defined for each year as the mean of the mean daily temperatures for all days in the months of March, April, and May. To complement the fine temporal and spatial resolution from the Moose Weather Station data, we also derived temperature data from PRISM (Daly et al. 1997). PRISM data were used to help validate the Moose Weather Station data. The use of a single station is often questioned, as there can be variation in instruments over time, and often these stations move locations while maintaining the same name. Although the Moose station has not been moved significantly and is close to our study site, its data still carry some of the limitations of being derived from just one station. PRISM data are derived from multiple climate observations from a wide range of networks overlayed on Digital Elevation Models to develop sophisticated spatial climate datasets which are considered very accurate and widely used (Daly et al. 1997, Bloom et al. 2018b, 2018a, Buban et al. 2020). Using PRISM data focused on a 4km grid cell centered on Blacktail Butte, we averaged monthly spring (March, April, May) minimum, mean, and maximum temperatures from 1970 to present.

Usage Notes

First flowering dates of 51 species for years 1974-1979, and 1988 collected by Frank Craighead and 2016-2019 collected by authors are included in the attached CSV. Dates are displayed in Julian Day of Year format (1-365). Spring temperature data for March, April and May (MAM) is provided from both PRISM and Moose Weather Station in seperate CSVs. Missing data is marked by NA.

Funding

Meg and Bert Raynes Wildlife Fund

Community Foundation of Jackson Hole

University of Wyoming/AMK Ranch

National Science Foundation GRFP, Award: DGE-1322106