The reproductive strategies of trees

In the bright early morning Lisbon light stands a single olive tree. Behind it is the white facade of the Igreja de Santa Isabel de Lisboa whose weathered doors have faded to a scratchy grey green. On the white limestone and black granite cobblestones that surround the tree lie the first ripe olives of the season.

The tree is isolated and yet it is heavy with fruit and despite the long, hot summer, its evergreen leaves are thick and healthy.

In spring and early summer, the pollen of Lisbon's plane trees blow freely across the city. There are almost 3,500 plane trees in Lisbon, many mature. Each spherical male inflorescence releases millions of pollen grains. It's a successful reproductive strategy but one which triggers hay fever in many people.

Olive trees are fewer in number and inconspicuous. They are 8-15 metres in height (plane trees vary between 30 and 50 metres and have a far larger crown) and typically appear in ones and twos or forgotten in old parks such as the Tapada das Necessidades.

Plane and olive trees are chosen for their resistance to pollution and drought tolerance and for the many benefits they bring to city streets and parks. Plane trees are allergenic. Olive trees are highly allergenic and although non-fruiting cultivars have been introduced they remain a major cause of seasonal allergies.

It is not possible to plant female trees in order to reduce the number of pollen because plane trees are monoecious - female and male flowers are always present on the same tree. Monoecy is only one of the sexual systems by which trees, and plants in general, reproduce.

15–20% of tree species worldwide are monoecious, including about 70% of the gymnosperms (conifers). Amongst the angiosperms (flowering plants, including trees) the number is 10-20%. By number of individuals, roughly half of trees worldwide are monoecious, mainly due to the dominance of conifers and broad-leaved forest trees. That figure is similar in Lisbon but for different reasons.

Three of the top ten most common trees in Lisbon are monoecious: the European nettle tree, limes (especially the small-leaved lime or linden) and plane trees (especially Platanus × hispanica). They are selected for their adaptations to long, hot summers, and resilience to urban stresses such as pollution, and compact soils.

Dioecious plants have female and male flowers on different trees. Aside from several poplar species there are only two common dioecious trees in Lisbon, the Canary Island date palm and the ginkgo.

The majority of trees, however, are neither monoecious nor dioecious, but hermaphroditic. Instead of having distinctly male and female flowers as is the case for dioecious species, individual flowers contain both male and female reproductive organs.

Hermaphroditic trees have “perfect flowers” (bisexual flowers), which means each flower can, in principle, both donate and receive pollen.

Two of the top ten trees, jacarandas and Tipuana tipu, are hermaphroditic. Their insect-pollinated flowers support biodiversity and have low allergenicity.

Olive trees don't fit into any of these categories. They are polygamo-monoecious: they bear perfect flowers with both stamens and carpels, as well as male-only flowers.

This variation on a theme is not uncommon in trees and plants more generally. Every imaginable combination exists for one reason or another and they are all adaptations that ensure reproduction takes place.

The allergenicity of species is a selection criterion but it is generally secondary to adaptability and resilience. Variety also minimises tree losses. The composition of Lisbon’s public street trees increased from 21,822 individuals and 33 genera in 1929 to 70,792 individuals and 140 genera (and 365 taxa) in 2021.

The decision to plant a wider variety of species is intended to prevent the loss of many specimens of a single species due to disease as happened when Dutch elm disease devastated scores of elm avenues and plantations across the world in the later half of the twentieth century.

Species variation also mitigates pollen counts because species release pollen and flower at different times. Cypress, holm oak, cork oak, and olive, which are all highly allergenic, release their pollen at staggered intervals from midwinter to early summer.

The olive tree outside the church is isolated. Its closest neighbour is a couple of hundred metres away down the road behind a cemetery wall. It's unlikely but possible pollen from this tree reached some of the stigmas on its perfect flowers.

Cross fertilisation is favourable for reproductive (and evolutionary) success and self-fertilisation is usually a last resort, avoided by male and female parts maturing at different times.

Although we are accustomed to seeing olives in groves, their reproductive strategy has evolved in trees which may be isolated, where conditions are variable and drought is possible. In a good year, olive trees produce more perfect flowers capable of developing ovules into fruit. In a poor year, the percentage of male-only flowers (which are less resource-intensive) increases, ensuring that the ovaries, though fewer in number, have a greater chance of being fertilised.

The fruit on this olive tree is abundant. By one means or another, pollen did land on the stigmas of its perfect flowers and fertilisation did take place.