DO HABITAT SHIFTS ALTER FLOWERING PHENOLOGY OVERLAP IN CLOSE RELATIVES? IMPLICATIONS FOR LOCAL COEXISTENCE

Abstract

Flowering time in plants is a highly variable trait that influences species' resource use and exchange of pollen with con- and heterospecifics. Levin (2009) suggested that habitat shifts within species might cause plastic shifts in flowering phenology, reducing pollen exchange across habitats. Coupled with divergent selection across habitats, diverged flowering time might thus pave the way towards ecological speciation. Some of these ideas may apply across species as well. If close heterospecific relatives share phylogenetically conserved flowering times and negatively affect each other's fitness, habitat shifts to microallopatry might provide a means for local coexistence by close relatives by reducing resource competition, shared enemies, or negative interactions via pollination. Habitat shifts might also select for diverged flowering time, or cause flowering time divergence, if phylogenetically conserved cues arrive at different times across habitats. Here, we ask if flowering phenology is phylogenetically conserved for 208 species at our coastal field site in northern California, whether flowering phenology differs systematically across habitat types, and whether habitat shifts are associated with phenological separation, especially in congeners. Because annuality and perenniality have been shown to be associated with habitat traits and flowering time, we included life history in our analyses as well. We also explore the frequency of habitat shifts between congener and noncongener pairs. We use both field observations and data from Jepson eFlora/Jepson Manual 2 (Baldwin et al. 2012) to explore patterns in flowering phenology. The two data sources were well-correlated across 59 species. Phylogeny, habitat, and life history all influenced flowering time, and habitat and life history were also phylogenetically conserved across 208 spp. Congeners differed in habitat more often than noncongener pairs, and also overlapped more in flowering time. Habitat shifts were not associated with shifts in flowering time in congeners, despite mean peak flowering time differences across habitats, and phylogenetic conservatism in habitat use. Congeners that differed in both habitat use and life history, however, did have the greatest difference in peak flowering dates. Habitat shifts likely play a role in local coexistence of close relatives, but our data do not support habitat-mediated changes in phenology as a possible mechanism. Experimental approaches may elucidate the role of phenology, resource competition, pollinators, and other associates in mediating coexistence of congeners at our coastal California field site.