SOME plants are profligate with their seeds. Take the gorse shrub. Half a hectare of the stuff sheds about 250 million seeds; as its pods dry, they twist open and hurl their contents through the air. Other plants have serotiny 鈥� the habit of hanging on to their seeds for dear life, keeping them locked up in a protective case such as a pine cone, often for years. Serotiny is the norm for some plants, including the lodgepole pine, the rock rose and South African proteas.
Why do they do it? The main reason seems to be so that they can regenerate after fires. Forest fires tend to destroy everything in their path, so species that can protect their seeds from the heat have an obvious advantage. Serotinous species (the word is derived from the Latin for late) actually thrive on fire 鈥� indeed they depend on it to open their protective seed cases and germinate the seeds.
鈥淪erotinous species thrive on fire. Indeed they depend on it to germinate their seed鈥�
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And they can wait years, even decades, for the opportunity. Forest fires tend to occur in cycles: between 10 and 25 years in the Mediterranean, 10 and 13 years in Australia and a staggering 40 years in South Africa鈥檚 fynbos region. In the northern hemisphere, the fire season is about to begin.
So how does fire bring serotinous seeds to life? It works both physically and chemically. The heat scarifies the hard seed cases of plants such as rock roses and South African proteas, allowing water to seep in and making germination possible. It also opens pine cones so they can release their seeds. Then smoke from burning plants and trees kicks off germination. Although people have known for centuries that smoke plays a role in germination, it was only last year that Australian researchers discovered the component of smoke that is responsible (Science, vol 305, p 977). The key is butenolide, a previously unknown compound in smoke that acts as a trigger, although the precise mechanism is still unclear. Butenolide initiates germination in many kinds of seeds, not just the serotinous ones. Appropriately enough, it is effective in the wild mountain tobacco, for example.
To get an idea of how well serotiny works, consider Scotland鈥檚 Caledonian Forest. The swift response to fire that serotiny provided to the lodgepole pine allowed this alien species to dominate over native species: after a fire, its cones released rapidly germinating seeds before local trees had a chance to compete.
The seeds of the lodgepole pine can also germinate without fire, something that the serotinous Mediterranean rock rose cannot do. Pity this species if no fire breaks out during its 15-year life 鈥� it simply dies out. Until the next fire, that is.