2 MILLION BLOSSOMS
Above left: Crab apples, an excellent example of how pollination increases carbon sequestration and adds to the soil composition from above. © Jeff Ollerton Above right: An early purple orchid, one of the many understory plants that attracts pollinators in the spring. © Jeff Ollerton
most plant species use animals as pollinators. Animal-pollinated plants vary in their reliance on these
pollinators. Some are completely dependent upon pollinators for all of their sexual reproduction, particularly those that are dioecious and have separate male and female individuals. Others are completely self-pollinating and have dispensed with the pollinators altogether, though that’s rare. Most species sit somewhere in the middle and, while they can produce some seeds via self-pollination, they need pollinators to ensure that there’s genetic mixing within and between populations. Having sex with yourself for a long period of time is rarely a good evolutionary option. For these forests and grasslands to maintain their
diversity and abundance of plants in the future, they require abundant and diverse communities of pollinators. These animal-pollinated plants help ecosystems directly lock up carbon from the atmosphere. But there’s a more subtle role for pollinators to play when it comes to accumulating carbon dioxide. Both woodland and grassland soils lock the plants. Pollinators contribute substantially to the build-up of that soil.
Soil From Above stigma it sets off a process that leads, ultimately, to a plant producing seeds which then germinate and ensure the survival of that plant population. But along the way a whole series of structures are produced that support reproduction of the plant, many of which are then cast-off after playing their part. Woody casings protect developing seeds, and structures promote seed dispersal, for example the wings of maple seeds. Many of these protective elements or seed dispersal accoutrements are dry, woody, and contain a high proportion
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of carbon. They protect or help disperse the seeds, then having served their purpose, drop to the soil, and begin to decompose, locking in carbon along with decomposing leaves and fallen wood. If you’ve ever walked around beneath a large horse
chestnut tree, and felt the crunch of the spiky, dried-out seed casings beneath your feet, you can appreciate how much carbon-containing material insect pollination contributes to soils. Ecologists who study forest carbon dynamics assess the litter, often the product of pollination, comprises 10 percent to 20 percent of that total mass dropped per year. Frequently reproductive litter is very woody and, compared with leaves, this litter is less likely to be munched on by invertebrates or broken down by fungi. Surviving untouched increases their value for carbon storage. In addition to the by-products of reproduction, many of the seeds produced do not germinate immediately. They end up in the soil as part of the “seed bank,” waiting for the right conditions to germinate, which sometimes takes decades. Many seeds never complete the cycle, dying within the soil, again adding to the soil’s carbon content. The soil seed bank can be huge: in the top 10 cm layer of a temperate woodland per square metre. Seeds can persist in such a seed bank for hundreds of years before germinating. Without pollinators to fertilise ovules and turn them into seeds, this seed bank would be a fraction of its size. Crucially, soils are more important than vegetation when it comes to locking up carbon. Three-quarters of the carbon on land is locked in soil; only 25 percent is contained in the living plants. Soil is thus the second-most important carbon store after the oceans.
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