Data used in: Phenological sensitivities to climate are similar in two Clarkia congeners: Indirect evidence for facilitation, convergence, niche conservatism, or genetic constraints

To date, most herbarium-based studies of phenological sensitivity to
climate and of climate-driven phenological shifts fall into two
categories: detailed species-specific studies vs. multi-species
investigations designed to explain inter-specific variation in sensitivity
to climate and/or the magnitude and direction of their long-term
phenological shifts. Few herbarium-based studies, however, have compared
the phenological responses of closely related taxa to detect: (1)
phenological divergence, which may result from selection for the avoidance
of heterospecific pollen transfer or competition for pollinators, or (2)
phenological similarity, which may result from phylogenetic niche
conservatism, parallel or convergent adaptive evolution, or genetic
constraints that prevent divergence. Here, we compare two widespread
Clarkia species in California with respect to: the climates that they
occupy; mean flowering date, controlling for local climate; the degree and
direction of climate change to which they have been exposed over the last
~115 years; the sensitivity of flowering date to inter-annual and to
long-term mean maximum spring temperature and annual precipitation across
their ranges; and their phenological change over time. Specimens of C.
cylindrica were sampled from sites that were chronically cooler and drier
than those of C. unguiculata, although their climate envelopes broadly
overlapped. Clarkia cylindrica flowers ~ 3.5 days earlier than C.
unguiculata when controlling for the effects of local climatic conditions
and for quantitative variation in the phenological status of specimens.
However, the congeners did not differ in their sensitivities to the
climatic variables examined here; cumulative annual precipitation delayed
flowering and higher spring temperatures advanced flowering. In spite of
significant spring warming over the sampling period, neither species
exhibited a long-term phenological shift. Precipitation and spring
temperature interacted to influence flowering date: the advancing effect
on flowering date of high spring temperatures was greater in dry than in
mesic regions, and the delaying effect of high precipitation was greater
in warm than in cool regions. The similarities between these species in
their phenological sensitivity and behavior are consistent with the
interpretation that facilitation by pollinators and/or shared
environmental conditions generate similar patterns of selection, or that
limited genetic variation in flowering time prevents evolutionary
divergence between these species.