Early emergence enhances plant fitness:: A phylogenetically controlled meta-analysis

The time at which a seedling emerges can determine its future success as a plant. Despite the large number of studies that have examined the effect of emergence time on. different components of plant fitness (survival, growth, and/or fecundity), the potential evolutionary response to selection on seedling emergence date is still poorly known. In this study, we review 55 of those studies by a random-effects meta-analysis, considering the phytogenetic relatedness among taxa. We test the following hypotheses: (1) early emergence increases seedling survival, growth, and fecundity, (2) early emergence is more advantageous to large-seeded species than to small-seeded ones, as the former can compensate for the lower number of seeds by increasing seedling survival, (3) perennial plants benefit more than annuals from early emergence, as the iteroparity of the former allows them to risk seedling emergence to the best conditions each year, whereas the semelparity of the latter forces them to spread the risk of emergence over time, and (4) the effect of emergence time may depend upon the experimental conditions (field vs. controlled experiments in a greenhouse or laboratory). Our results show that early emergence differentially affects components of plant fitness, with no effect on seedling survival but large benefits to seedling growth and fecundity. Such effects vary depending upon intrinsic factors like seed size or life-form, and also upon methodology (census time and experimental conditions). Large-seeded species gain from emerging early by growing more during their first growing seasons, although they survive and reproduce similarly to small-seeded species. The survival benefit of early emergence is greater in perennial than in annual species, thus supporting hypothesis 3. The relationship between emergence time and seedling growth appears to be stronger under controlled conditions than in the field, probably as a result of the unlimited nutrient and water resources of the former. In field conditions, in contrast, limited resources probably decelerate the growth of early seedlings, precluding the detection of differences between these and late seedlings.