Turnover and reliability of flower communities in extreme environments: Insights from long-term phenology data sets
Introduction
Flowering phenology plays an important role in the structuring of plant communities as well as the consumers that depend on them. These community-level patterns are affected by their component plant species which have different levels of flowering reliability. Plant phenology studies have long sought to discern which environmental cues yield observed phenological patterns of various individual plant species (Abd El-Ghani, 1997; Adondakis and Venable, 2004; Berlin et al., 2000, Bowers, 1987; Bowers and Dimmitt, 1994, Friedel et al., 1993, Friedel et al., 1994, Lambert et al., 2010, Miller-Rushing and Primack, 2008, Pavon and Biones, 2001), related species (Miller-Rushing and Inouye, 2009), or all species in a community (Beatley, 1974, Crimmins et al., 2010, Kemp, 1983).
Plants are sometimes constrained by their phylogeny or evolutionary history (Franks et al., 2007, Kochmer and Handel, 1986, Willis et al., 2008), but many species also exhibit flexibility in their life history to survive climate fluctuations (Willis et al., 2010). At the community level, no two species of plants (Beatley, 1974), not even congeners (Miller-Rushing and Inouye, 2009), respond identically to the array of environmental cues. At the population level, there is a wide range of individual responses, most likely due to a combination of microclimate variability, genetic variation and phenotypic plasticity. All of these factors come together to make the flowering community apparently stochastic from year to year, especially where environmental conditions fluctuate markedly such as in deserts and subalpine areas (Holway and Ward, 1963, Huelber et al., 2006, Kudo and Hirao, 2006). The projected increase in climate variability in the southwest (Overpeck and Udall, 2010, Sheppard et al., 2002) may result in increased variability in phenology in the future.
Few phenology studies look at all species in a community and only a handful of studies have examined the community of flowers from a resource availability perspective (Alarcón et al., 2008, Aldridge et al., 2011, Anderson and Schelfhout, 1980, Dupont et al., 2009, Moldenke, 1975, Petanidou et al., 2008, Rabinowitz et al., 1981). Most that do are often only a few years in duration. Our study utilizes three long-term flower phenology datasets from the Sevilleta National Wildlife Refuge in the Chihuahuan desert of New Mexico (9 years), an elevation gradient in the Sonoran Desert outside of Tucson, Arizona (20 years), and a subalpine area at the Rocky Mountain Biological Laboratory in Gothic, Colorado (34 years). These data offer novel insights into long-term reliability of flower communities. In addition, the majority of published phenological studies focus on temperate areas in which temperature dictates phenology. In contrast, arid and semi-arid systems are typically driven by precipitation, not temperature. Phenological studies in these precipitation driven systems are few (Primack and Miller-Rushing, 2011).
To investigate these disparate communities from a resource perspective, we determined the proportion of years that flowering species were available for pollinators in any particular month, averaged across all species in a community and separately for annuals and perennials. In other words, how reliable are the flowers, within a particular community, at a monthly resolution? We also analyzed the data seasonally. For a particular season, what are the chances that a species will flower the following year? We used turnover rates to address reliability of flowering resources from one season to the next. We hypothesized that the sites with the highest variation in precipitation would have the highest turnover of flowers and the lowest frequency of bloom. We also hypothesized that annuals would be less reliable than perennials because they cannot store resources from one season to the next.
Section snippets
Study location 1, New Mexico
The Sevilleta National Wildlife Refuge is located at the northern tip of the Chihuahuan Desert about 80 km south of Albuquerque in central New Mexico, USA. The refuge lies across an ecotone between Chihuahuan Desert and semi-arid shortgrass steppe. Three sites, representing different ecosystem types were sampled by the Sevilleta Long-Term Ecological Research Program field crew over nine years; Chihuahuan desert black grama grassland (NM-G) dominated by Bouteloua eriopoda (elev. 1616 m),
Frequency of bloom
Completely reliable plant species that bloomed in the same month every year of each study were rare. Only 77 of 393 (19.6%) species in AZ bloomed in the same month every year, 4 of 71 (5.6%) in CO, and 9 of 109 (8.3%) in NM. Averaged over all years of the study, for all species at each site, the percentage of years in which the average plant bloomed in the same month was 53.3% in NM, 55.4% in the low AZ sections (1 + 2), 50.3% in the highest AZ section (5), and 61.5% in CO (Table 2). Annuals
Frequency
Different conclusions can be made from perennials as compared to annuals when looking at frequency of bloom in particular months. Overall, perennials support the hypothesis that areas with low and inconsistent rainfall have the lowest frequency of bloom. The only site that did not fit this pattern was the highest AZ section (5), which had substantially lower frequency than all other sites. No independent weather data are available for the high-elevation site and the difference in elevation of
Acknowledgments
The New Mexico study was supported by National Science Foundation award DEB-0620482 to the University of New Mexico for Long-term Ecological Research. We would also like to thank Dave Lightfoot for starting this research project and the Sevilleta field crew for collecting the data. The raw data from New Mexico is available online at http://dx.doi.org/10.6073/pasta/432f8ca8bdd4155e5cf506b8c630c4f7. National Science Foundation awards DEB-94-08382, IBN-98-14509, and DEB-0238331, and DEB 0922080
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