Seedlings are more sensitive to severe environmental conditions than both adult plants and seeds of the same species, making germination a risky one-way transition in the plant life cycle. Consequently, strong selection pressures act on germination responses, resulting in a range of strategies to time germination appropriately to places or times that are suitable for seedling survival and onward growth. A range of environmental conditions tell seeds whether or not they are in a place suitable for germination, such as light conditions, smoke after burning and moisture. Timing strategies for germination are often cued by temperature and cold stratification. It is often argued that life under severe and unfavourable climatic conditions will select for increased environmental tolerance in local populations.
Calluna is the keystone species of Europe’s heathland systems and occurs throughout a broad geographical and climatic range, being found along Europe’s western coast from the Strait of Gibraltar to northern Norway, from sea level into the alpine zone (Pyrenees, Alps, Scottish Highlands and Scandinavian Mountains) and even in continental Western Russia. A new study in Annals of Botany investigates germination behaviour along climatic gradients in heather, Calluna vulgaris. The finding of a conditional cold-avoidance strategy for Calluna germination together with previous records from Scotland, France and Spain support a theory of gradual replacement of cold as the main hazard for seedlings as we move south in Europe by first competition and then, further south, possibly drought, that explains varying germination patterns in relation to temperature. Our main results suggest that Calluna in Northern Europe generally avoids hazards imposed by cold climates by cueing germination towards the relatively warm frost-free late spring to early summer season. In populations from less adverse climates, the species’ cold-avoidance strategy seems to be weakened in favour of earlier germination, which would allow the species to address other limitations of, for example, light and space as a consequence of higher competition under warmer climates.