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