Assessing clinal variation and elevational range limits in Ipomopsis aggregata

Background/Question/Methods

In montane environments, extreme environmental variation across small geographic scales can result in very different selective pressures on populations of a species that range across an elevational gradient. Climate change may in fact alter the selective pressures acting on populations and influence the ecological and evolutionary dynamics of plant populations, with such effects being especially strong at range boundaries. Determining the spatial patterns of selection and trait variation for species that range across elevational gradients and what factors limit a species’ elevational range will help predict species-level responses to climate change and the potential for range expansion.  We examined clinal variation in ecologically important plant traits in three natural populations of Ipomopsis aggregata, including the highest elevation population known, across a 1000 m elevational gradient in Colorado. In addition, we conducted a fully-reciprocal transplant experiment with seeds collected from these three populations, including transplantation to a site just beyond the current elevational range limit. We assessed germination and survival over the first year and measured the same suite of plant traits in the reciprocal transplant as in the natural populations to assess the extent of local adaptation and phenotypic plasticity in those traits.

Results/Conclusions

We found that populations of I. aggregata exhibit striking differences in many traits. Leaf length, leaf water content, specific leaf area, and leaf nitrogen content increased monotonically with elevation, while leaf carbon content and leaf C:N ratio decreased with elevation. Percent germination of seeds decreased with elevation, while survival of seedlings during the first growing season rose from 74% at low elevation to 100% at high elevation. With respect to genetic variation among populations, survival differed across source populations only for plants in the lowest elevation garden. Interestingly, percent germination and survival did not differ between the high elevation site within the species range and the site outside of the species range. In the first year, plant growth increased with elevation within the species range, however, plants established beyond the species range were significantly smaller than plants at any other site. We hypothesize that there is selection at high elevation for large seeds and fast growth rates, corresponding to our observation that seeds from high elevation plants are ~70% larger than seeds from plants at lower elevation. Taken together, these data provide mixed evidence of local adaption across this species’ range and indicate phenotypic plasticity with regards to some traits.