The CONIFER LIFE CYCLE starts from the point where a mature tree produces cones and describes the reproductive steps that then take place in order to produce a seed which will then go on to grow into another mature tree. The Conifer life cycle is similar to that of the Broadleaf tree but usually takes longer. Elms, for example, can be pollinated in spring and shed seeds 8 to 10 weeks later but some Conifers can take up to 2 years to produce seeds after pollination. In Tree Guides the term ‘flower’ is often used when describing the reproductive structures of Conifers. In fact Conifers, unlike Broadleaf trees, do not have flowers or fruit. They have male and female cones. The botanical name for a cone is a strobilus. Pollen cones, like male flowers, release pollen and Seed cones, like female flowers, receive pollen and develop seeds. Seed cones are usually woody to protect the developing seeds.
A fully grown conifer tree, the Atlas Cedar
These are the pollen cones (male ‘flowers’) of the Atlas Cedar, shedding yellow pollen onto the wind in November. Most Conifers have male and female ‘flowers’ on the same tree and so are monoecious. Male ‘flowers’ are low down on the tree and female ‘flowers’ higher up. Yews and Junipers, however, have male and female ‘flowers’ on different trees and are dioecious. Each pollen grain contains half the DNA required to make the final plant. It has a single set of chromosomes.
This is a female ‘flower’ or seed cone of a Cedar of Lebanon in October – ready to receive pollen in November. The seed cones of Pines, Spruces, Firs, Cedars and Larches have a series of overlapping woody scales arranged spirally along the central stem. These form the outer protective coating of the cone and are known as cone scales or seed scales. Pollen grains, arriving on the wind, are trapped deep within the scale, close to the ovule which contains the female egg. The pollen grain then germinates and starts to grow a pollen tube towards the ovule.
This photo shows that the cone is still green in August of the following year (after pollination took place in November). Unlike Broadleaf trees, fertilisation does not take place immediately. In some conifers it takes place 15 months after pollination. During this period the pollen tube slowly works its way to the ovule. When it arrives, the sperm cells reach the egg and fertilisation takes place. The 2 sets of chromosomes are combined and the seed starts to develop
The cones turn grey during the following winter and are fully mature after 2 years when they are ready to shed seeds.
This is a cross-section of a mature Cedar seed cone. The seed scales fit very tightly together and protect the ovule and seed during their long development. It is difficult to see the seeds but they are close to the central axis of the cone, between the seed scales.
The cone of this species then breaks up on the tree and the seeds are dispersed by the wind. Some Pines have cones that open on the tree to release seeds and others have cones that fall intact and open on the floor.
This is a photo of a seed scale with two winged seeds lying on it. This Cedar of Lebanon cone has been broken open to show the seeds. Normally, the cone breaks up layer by layer on the tree from the top downwards and the seeds are released onto the wind for dispersal
This is a cross-section of a mature Corsican Pine seed cone. The woody cone scales are packed closely together and the seed is protected between them. When conditions are suitable the cone ‘opens’ whilst still on the tree and the seeds fall out.
This is the tiny winged seed of the Corsican Pine. When the cone opens it falls out and and can be carried away by the wind. Some conifers e.g. the Stone Pine have large seeds that can be dispersed by birds or animals. Some cones e.g Monterey Pine only open to release seeds after the intense heat of a forest fire.