Selection on Flowering Time Differs on Solidago altissima in Sites with Contrasting Water Availability

Abstract

Earlier flowering has been observed in many plant species in recent decades as climate has changed. However, many plant species encompass more than one ploidy level (e.g. diploid and tetraploid) and it is not known whether ploidy levels within a population are responding to climate change in the same way. For example, earlier flowering may be more strongly favored in one ploidy level over another and this may vary in contrasting environments. To test this hypothesis, a phenotypic selection study was performed on experimental populations of diploid and tetraploid Solidago altissima that were planted into in two sites that differed in water availability. Multiple clones from 15 genotypes from early- and late-flowering artificial selection lines of each ploidy level were planted (N=480). Measurements including survival, size, and flowering phenology were taken every other week for the duration of the growing season (14 wks). Clone survival was high overall, with the highest mortality in more mesic eastern site (3%). At the time of flowering, eastern plants had more leaves, were taller and had more reproductive branches. Additionally, eastern plants flowered earlier and produced more basal and stem branches. The plants of differing ploidy level also differed in that the tetraploids had more leaves, were taller and had more reproductive branches. Both diploid and tetraploid plants that flowered earlier had greater reproduction at the end of the growing season at both sites. At the end of the growing season, the diploids and eastern location plants were more reproductively advanced. Overall, this research suggests selection differs on flowering time primarily among sites that differ in water availability and not between ploidy levels. Whether plants will be able to adapt to these changes at the speed that they occur is not yet clear, and plants will need to be able to adapt at a rate that is equivalent or faster than the changes or they will face extinction. Research of this kind is important because it will allow for more accurate predictions for the fate of future plants in changing climates.