The effect of a latitudinal temperature gradient on germination patterns

Premise of research. Germination timing determines a plant’s exposure to environmental factors and has strong impacts on fitness. Despite this key role in the life cycle, relatively little is known about variation in germination within species. This study examines whether geographic variation in local environmental conditions affects germination in populations of Campanula americana throughout eastern North America. Methodology. We determined the temperature of spring and fall germination through a survey of 16 natural populations along a latitudinal transect over 4 yr. We then examined germination propensity and rate in a subset of these populations under controlled temperature conditions. Pivotal results. Northern populations germinated at cooler temperatures than southern populations regardless of season, and fall germination occurred at warmer temperatures than spring germination across all populations. Northern populations germinated later in the spring than southern populations. Under controlled conditions, northern populations germinated more rapidly than southern populations, while warm temperatures accelerated germination, with all populations germinating at similar rates. However, under controlled conditions fewer seeds germinated from northern populations in comparison to southern populations, and the decrease in propensity to germinate at higher latitudes was pronounced under cool conditions. Conclusions. Temperature influences germination timing in C. americana. In the field, germination occurs at a consistent time across a wide range of climates, but populations spanning the latitudinal gradient germinate under different temperatures. Under controlled conditions, warmer temperatures accelerate germination. Populations differ in plasticity of germination propensity to temperature. This reduced germination of northern populations, especially under cooler temperatures, may serve as an adaptive plastic response to the later arrival of suitable temperatures for growth. The influence of environmental variation on germination in natural populations and under ecologically relevant experimental conditions indicates that novel environmental conditions imposed by climate change may alter life-history patterns.