Topographic heterogeneity lengthens the duration of pollinator resources

The availability of sufficient and diverse resources across time is important for maintenance of biodiversity and ecosystem functioning. In this study we examine the potential for variation in environmental conditions across topographic gradients to extend floral resource timing. Flowering time on a landscape may vary across topography due to differences in abiotic factors, species turnover, or genotypic differences. However, the extent to which this variation in phenology affects overall flowering duration on a landscape, and the components of diversity that influence flowering duration, are unexplored. We investigate whether differences in flowering time due to topography yield an overall extension in duration of flowering resources in a northern California grassland. We recorded flowering time of pollinator resource species across four successive spring growing seasons (2015-2018) on paired north and south aspects. Flowering time differences were evaluated both at the community level and within species present on both paired aspects. The role of plasticity was examined in an experimental case study. We found that aspect is a strong determinant of phenology, with earlier flowering on warmer south-facing slopes. Aspect differences resulted in complementarity in timing of flowering resources across sites, as aspects that started flowering earlier also ended earlier. Complementarity between north and south aspects served to extend the flowering time of pollinator resources by an average of 4-8 days (8-15%), depending on the year. This extension can be attributed to both within species responses to aspect differences as well as species turnover. Flowering of L. gracilis genotypes was distinct across aspects, demonstrating that plasticity can drive the extension of flowering duration. Our findings indicate that heterogeneous topography can extend overall flowering time of pollinator resources, which may support pollinator biodiversity. Extension was most pronounced at the community level, which incorporates species turnover as well as plastic and genotypic differences within species.

Methods
Description of methods used for collection/generation of data:

GrasslandPlotTemps2015_2018.csv

Ibutton temperature data collected at Pepperwood Preserve in 2015-2018. Paired grassland sites on North and South facing slopes. Temperatures were collected once per hour at 10cm below soil surface.

Temperature and moisture were recorded at the plots to quantify microsite differences between aspects. Temperature was recorded at each aspect with an iButton (Thermochron, N=8) placed 10 cm below the soil surface and set to record every hour. Temperature measurements were taken from March 1st through June 30th in all four years to compare aspect temperature differences during the flowering season. Additionally, temperature measurements were collected throughout the year in 2016 to capture full growing degree day accumulation curves.

Phenology data collection methods:

(for SpInfo.csv, PollResDates5.csv, SPECIESPollResDates.csv, LASPollResDates.csv)

In each plot, we recorded flowering throughout the spring growing season (March-June) in 2015, 2016, 2017 and 2018. Flowering phenology was observed for all pollinator resource species in the plots, including native and non-native species, annuals and perennials. Species status as a pollinator resource was identified by direct observations of animal visitation during the study, together with outside sources, including information provided by the Xerces society (Mader et al. 2011). Richness of pollinator resource species in each plot varied from 1 to 22 species over the entire season. Inflorescences in flower for each species in each plot were counted weekly to determine start, middle, and end flowering, as well as the length of the flowering season. For species with inflorescences that had more than one phenology stage present, an inflorescence was counted as flowering when at least 50% of it was in flower. Not all species were present in all sites, or on both slopes.

Mader E, Shepherd M, Vaughan M, Hoffman Black S, LeBuhn. G (2011) The Xerces Society Guide to Attracting Native Pollinators: Protecting North America’s Bees and Butterflies. Storey Publishing, North Adams MA.

Methods for data processing:

PollResDates5.csv

Raw flowering time data was collected as the number of inflorescences in flower for each species on each census date. Data was filtered to species that were determined to be pollinator resources.

Phenology date columns calculation: The number of inflorescences on each census date in each plot was divided by the total number of inflorescences counted in the plot for the full season to determine proportional flowering on each date. This proportion was then summed to create a cumulative sum.

Community flowering dates for pollinator resource species were calculated based on cumulative plot flowering over the season, as follows: start date as the date when 5% of the cumulative number of flowers in a plot (summed over the season) had been reached, mid-flowering date as the date when 50% flowering was reached, and end date as the date when 95% of flowering had been reached. Flowering duration was defined as the total number of days between start and end dates (when 5% and 95% flowering had been reached, respectively) for each plot.

SPECIESPollResDates.csv

Raw flowering time data was collected as the number of inflorescences in flower for each species on each census date. Data was filtered to species that were determined to be pollinator resources.

Phenology date columns calculation: The number of inflorescences for each species on each census date in each plot was divided by the total number of inflorescences for that species counted in the plot for the full season to determine proportional flowering on each date. This proportion was then summed to create a cumulative sum for each species.

Species flowering dates were defined as: start date as the date when 5% of the cumulative number of flowers in a plot (summed over the season) had been reached, mid-flowering date as the date when 50% flowering was reached, and end date as the date when 95% of flowering had been reached for each species. When flowering was only observed for a species on one survey date, the duration of flowering was calculated as 1 day (although it is likely that flowering occurred for two days or longer depending on the species). There were instances of gaps between flowering time on north and south aspects for individual species, and these were removed when calculating site flowering duration.

LASPollResDates.csv

Experimental plots of goldfields (Lasthenia gracilis (DC.) Greene) were set up just outside of main phenology plots in 2017 and 2018, with 3 subplots per aspect at each site (n=24). These 30x30 cm subplots were planted with 30 seeds each, collected from two grassland locations on Pepperwood Preserve from 10 maternal lines (3 seeds per line per plot). Seeds were planted in the fall and marked with toothpicks to differentiate them from any other Lasthenia individuals occurring at the site. Flowering time was recorded for these subplots in the same way as the study phenology plots – with all open inflorescences from all individuals counted each week from March through June. Counts were conducted only on planted and marked Lasthenia individuals within each plot.

Flowering dates were defined as: start date as the date when 5% of the cumulative number of flowers in a plot (summed over the season) had been reached, mid-flowering date as the date when 50% flowering was reached, and end date as the date when 95% of flowering had been reached for each experimental plot.