Decoupling the effects of parental and offspring warming on seed and seedling traits

Global warming may pose a serious threat to seed germination and establishment in alpine ecosystems, given that temperature is a primary factor in stimulating seed germination and regulating changes in seed dormancy. However, to date, little is known about the relative importance of temperatures experienced by parents versus by the seeds (after dispersal). In this study, we tested the effects of warming at different stages on germination success and timing in the Australian alpine herb Wahlenbergia ceracea. To decouple the effect of parental warming from that of offspring warming, we raised parental plants (in both outcrossed and selfed lines) in both current (benign, cool) and future (warm) temperature conditions, and then sowed the seeds they produced back in either current or future conditions. We quantified (1) the effects of parental and/or offspring warming on (i) the percentage of germination and (ii) the season of germination (i.e. effects on dormancy); (2) whether the season of germination affected seedling growth; and (3) the effects of inbreeding and its interaction with temperature. We found that the percentage of germination decreased slightly with parental warming, but increased greatly with offspring warming. Parental warming also increased the fraction of dormant seeds, indicating a shift from predominately autumn to spring emergence. Spring-emerged seedlings grew slower than autumn-emerged seedlings, but the growth rate of spring-emerged seedlings were not detrimentally affected by warm offspring temperatures. In this facultatively autogamous species, inbreeding magnified the negative effects of both parental and offspring warming. Our results illustrate the value of tests of the effects of warming across generations and on multiple life stages for improving our understanding of the ecological processes behind plant germination and establishment, and of plant responses to climate warming.