Flowering first assists pollination

John Hale, of Dunedin, asked:

In spring, some trees and plants produce their flowers before any leaves appear. Most produce the leaves first. What decides the sequence? Are there any species which don’t have it fixed?

Janice Lord, a botanist at the University of Otago, responded:

The sight of a deciduous magnolia in full flower can be breathtaking, as there are no leaves to obscure our view. However, despite our love of spring flowers, they are not produced for our benefit.

Flowers are a necessary first step for the process of sexual reproduction and fruiting. The timing of flowering is particularly critical because flowers not only need to be pollinated, but there also needs to be enough time for fruits and seeds to mature before the weather again turns cold.

Most plants benefit from, or even require, crosspollination with another plant of the same species. So, flowering out of sync with neighbours could cost the plant dearly in terms of failing to produce seeds. However, flowering at the same time as neighbours is only part of the process — pollen needs to get from one flower to another. In windpollinated plants, such as birch and elms, flowering before leaves are fully expanded means pollen can more easily be blown from one plant to its neighbour.

For insectpollinated plants such as fruit trees, the massed flower display makes the plant highly conspicuous from a distance, prompting insects to move more readily between plants. So, flowering before leafing can improve crosspollination.

However, it is also risky, as earlyflowering plants often preform their flower buds in autumn, which means they can be damaged by weather and pathogens before spring arrives. An earlyflowering plant also has to use stored reserves to produce the flowers before there are any leaves able to photosynthesise. So, more frequently, deciduous plants adopt the conservative strategy of flowering at the same time as they unfold their new leaves.

Flowering in colder parts of the world is often triggered by increasing day length, as well as increasing temperature following a period of chilling, which mostly prevents plants from flowering during brief warmer periods in autumn and winter. In comparison, leaf unfolding is more simply related to warmer temperatures following a chilling period.

It is easy to see how in some species the environmental cues for flowering versus leaf unfolding are so distinct that flowers always appear before (or after) leaves. However, in other species, if differences in cues are small, variable spring conditions between one year and the next might easily change the order of leaf and flower appearance.

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