Arboreal sex

At this time of year, the reserve is full of pollinators carrying pollen from tree to tree in a kind of reproductive frenzy.

Self fertilisation, for trees, is not a good thing. Cross fertilisation is nearly always the preferred option with self fertilisation as a back up strategy: emergency reproduction in a time of crisis.

Trees have three ways of avoiding self fertilisation. Firstly, they may have their male stamens and female pistil on the same flower but ripening at different times. These are hermaphrodites. The park’s blackthorns, coming into flower at the moment, are hermaphrodites; they ripen their pistil first and pollinating insects foraging for nectar bring pollen from other blackthorn trees to fertilise them.

Blackthorn is a hermaphrodite

When fertilisation has occurred and the pollen tube is burrowing down through the pistil towards the ovule, the stamens ripen, the orange anthers open and the pollen is released. Now, visiting pollinators carry pollen away to other trees.

Another way trees avoid self fertilisation is by having separate male and female flowers on the same plant but, again, ripening at different times. These trees are call monoecious. Hazel and larch, both wind pollinated, are monoecious. Hazel catkins, the showy, yellow male flowers, produce their pollen first and the tiny, red female flowers open afterwards. Larch does it the other way round: the pretty green female flowers open first and then the tiny brown male flowers ripen.

Larch is monoecious

Dioecious species have separate male and female trees, a sure fire way to rule out self-fertilisation. Trees that find themselves without a tree of the opposite sex nearby have had to evolve another emergency strategy: dioecious trees can clone themselves. They reproduce vegetatively, their roots spreading out sometimes many metres and producing new trees, or their branches dipping down until they touch the earth and take root.

Somewhere in America there is a single male quaking aspen tree that has spread vegetatively over more than 100 acres and is believed to be one of the largest and oldest living organisms on the planet.

Willows are dioecious

Willows are dioecious. We are all familiar with the male tree’s catkins but not so much with the female tree’s less dramatic flowers. It is easy to assume the two are different species.

Willows can reproduce vegetatively if left to age naturally. Their branches dip towards the ground, the parent tree splits, the branches root where they touch and a ring of clones form around the remains of the original tree.

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