Abstract
Climate change is known to affect regional weather patterns and phenology; however, we lack understanding of how climate drives phenological change across local spatial gradients. This spatial variation is critical for determining whether subpopulations andmetacommunities are changing in unison or diverging in phenology. Divergent responses could reduce synchrony both within species (disrupting gene flow among subpopulations) and among species (disrupting interspecific interactions in communities). We also lack understanding of phenological change in environments where life history events are frequently aseasonal, such as the tropical, arid, and semi-arid ecosystems that cover vast areas. Using a 33-year-long dataset spanning a 1,267-m semi-arid elevational gradient in the southwestern United States, we test whether flowering phenology diverged among subpopulations within species andamong five communities comprising 590 species. Applying circular statistics to test for changes in year-round flowering, we show flowering has become earlier for all communities except at the highest elevations. However, flowering times shifted at different rates across elevations likely because ofelevation-specific changes in temperature and precipitation, indicating diverging phenologies of neighboring communities. Subpopulations of individual species also diverged at mid-elevation but converged in phenology at high elevation. These changes in flowering phenology among communities and subpopulations are undetectable when data are pooled across the gradient. Furthermore, we show that nonlinear changes in flowering times over the 33-year record are obscured by traditional calculations of long-term trends. These findings reveal greater spatiotemporal complexity in phenological responses than previously recognized and indicate climate is driving phenological reshuffling across local spatial gradients.
Highlights
Flowering phenology is a key biological indicator of current climate change [1,2,3,4,5]
We address the following questions: (1) at the metacommunity level, has flowering phenology diverged or converged across elevations; (2) can variation in temperature and precipitation across elevations explain shifts in flowering phenology; and, (3) within species, has flowering become more, or less, synchronous for subpopulations across space? Our approach reveals spatially divergent, temporally nonlinear changes in flowering phenology among communities and subpopulations that are obscured by the pooling of phenological data across space and the simple linear calculation of long-term trends
Change in Flowering Phenology from 1984–2016 The relationship between flowering time and year varied by elevation band, indicating elevation-specific phenological responses of communities (Data S1A)
Summary
Flowering phenology is a key biological indicator of current climate change [1,2,3,4,5]. The magnitude and direction of changes in flowering times appear to vary significantly both among species and among communities, in part because of variation in how much abiotic factors such as temperature and precipitation have changed and in how sensitive species are to those changes [6]. In some communities, flowering phenology responses are related to plant traits, such as flowering season [5], whether species have an annual or perennial life cycle [2,5], and whether species are wind or animal pollinated [5]. Species that flower early in the season or are annuals show the greatest advances in phenology, whereas evidence regarding pollination mode is mixed [2,5]. Closely related species exhibit similar responses; phylogenetic relationships can be important predictors of shifts in flowering time [7,8]
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