Aim: Climate affects the flowering time of many species.  Little is known, however, about how climate influences the properties of regional floras, including the rate at which taxa sequentially flower throughout the flowering season. This study is the first to detect geographic variation in this rate across North America.  In addition, we tested for the independent effects of intraspecific variation in flowering time and taxonomic composition on the rate of sequential flowering among regional floras distributed across a temperature gradient.

Location: North America

Time period: This study examined > 59,000 herbarium specimens that were collected in flower from 1901 to 2013.

Major taxa studied: 2,803 angiosperm taxa. 

 Methods: We identified 51 climatically homogeneous regions across the continental, mostly western U.S. in each of which at least 100 species were represented by herbarium specimens. We then examined the effects of mean annual temperature (MAT) on the rate of sequential flowering among species in each region.  We also evaluated whether geographic variation in the rate of sequential flowering was due to intraspecific variation in the flowering time and/or the taxonomic composition of regional floras.

 Results: As MAT increases over space, the rate of sequential flowering (standardized by the absolute length of each region’s flowering season) increased among relatively early-flowering taxa, but decreased among the latest-flowering taxa. Both intraspecific variation and shifts in taxonomic composition among floras contributed to this pattern.

 Main conclusions:  Among floras throughout North America, the rate of sequential flowering among co-occurring taxa changes with MAT.  Intraspecific phenological variation primarily affects the rate of sequential flowering during the first half of the growing season, consistent with the inference that future warming will most strongly affect flowering synchrony among early-flowering taxa.