DOI: 10.5061/dryad.tmpg4f50r
title: Flowering time advances since the 1970s in a sagebrush steppe community: implications for management and restoration

description: 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.
description: <i>Study Site </i> 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 (<i>Artemisia tridentata Nutt. ssp.</i>
 <em>vaseyana) </em><em>is the dominant species along with
 other shrubs including antelope
 bitterbrush</em><em> (Purshia tridentata),
 </em><em>rubber rabbitbrush
 </em><em>(Ericameria nauseosa),
 </em><em>mountain
 snowberry</em><em> (Symphiocarpus oreophilus),
 </em><em>and
 diverse perennial and annual forbs. Blacktail Butte also harbors patches
 of</em><em> </em>mixed conifer or aspen
 overstory with a native forb and tall shrub understory.<i>
 </i>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.
 <i>Historical Observations</i>
 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). 
 <i>Contemporary Observations</i>
 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. <i>Climate
 Data</i> 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.
description: 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.