Manipulation of flowering time: phenological integration and maternal effects

Timing of flowering is central to reproductive success and is currently advancing in many natural populations due to a warmer climate. However, we have little understanding of how earlier initiation of flowering influences subsequent reproductive phenology or the expression of traits in the offspring. To evaluate the consequences of an altered flowering phenology we manipulated cohorts of Campanulastrum americanum, an herb with annual and biennial growth forms, to flower and disperse seeds up to a month earlier, at the same time, and up to a month later than a natural population in two separate years. Relative to the date of first flower, the temporal patterns of flower production and the timing of fruit maturation and seed dispersal were similar among individuals that initiated flowering over the expanded reproductive season, indicating strong phenological integration of reproductive traits. However, plants that initiated flowering substantially outside the natural window showed a change in the rate of reproduction, with a compressed reproductive schedule for early-flowering individuals and an expanded one for late-flowering plants. Changes in flowering time had more dramatic effects on the offspring generation. Initiation of flowering two weeks earlier would result in a fourfold increase in the frequency of annual offspring, and four weeks earlier would result in a tenfold increase. The frequency of annuals was less sensitive to modest delays in flowering time but decreased with greater delays in flowering time. Collectively, these results reveal a tightly integrated reproductive phenology that shifts with timing of flowering within generations but may lead to more dramatic responses to climate change between generations.