Variation in ripening years of seed cones of Juniperus communis L.

Many Juniperus communis populations are in decline, often with poor reproductive performance. Biological and phenological studies have shown that seed cone ripening time takes between two and three summers after pollination, and occasionally four. Individual junipers differ: in samples from northern England some were almost entirely three year, some were two year and others had mixed proportions. The limited evidence suggests that two year ripening is commoner in warmer areas such as southern England, and three years more frequent in northern and mountainous parts of Europe. Suggestions about the causes of these differences include the effects of climate on pollen tube growth rates, phasing of the seed cone crop production and genetic background