Academic Reading — Test 67

Reading passage

Every flowering plant, tree and grass releases vast quantities of pollen into the air and water each year. Most of these grains drift only a short distance before settling, yet a remarkable number come to rest in places where they are preserved for thousands or even millions of years. The study of this preserved material is called palynology, and it has become one of the most powerful tools available for reconstructing the vegetation that once clothed an ancient landscape. The reason pollen is so useful lies in its outer wall, which is built from a substance called sporopollenin. This material is extraordinarily resistant to decay, surviving conditions that would destroy almost any other plant tissue. Where leaves and wood rot away within a season, the tough shell of a pollen grain may endure indefinitely, locking inside it a record of the plant that produced it. The places that preserve pollen best are those where oxygen is scarce, because the bacteria and fungi that normally break down organic matter cannot thrive without it. Peat bogs, lake beds and the floors of slow-moving estuaries are therefore ideal. As sediment accumulates layer upon layer, year after year, the pollen that falls onto these surfaces becomes trapped and sealed. The deepest layers are the oldest, and the sequence builds up like the pages of a book, with the most recent chapters lying nearest the top. By extracting a vertical column of sediment, known as a core, a scientist can sample this archive from the surface downwards and read the changing vegetation backwards through time. Cores are usually taken using a hollow tube that is pushed or drilled into the ground, allowing the layers to be removed without mixing them. Identifying the pollen is painstaking work. A small portion of sediment is treated with chemicals that dissolve the surrounding minerals and leave the durable grains behind. These are then mounted on a slide and examined under a microscope at high magnification. Although the grains are tiny, often no wider than the width of a human hair, their surfaces carry distinctive patterns of ridges, pores and spines. Each plant family, and frequently each genus, produces grains with a characteristic shape and sculpture, so a trained analyst can name the parent plant with considerable confidence. By counting hundreds of grains from a single layer, the analyst works out the proportion of each plant type present and plots these figures against depth to produce a diagram. Such diagrams reveal, at a glance, how forests gave way to grassland, or how one species of tree replaced another, across the centuries. Pollen evidence has transformed our understanding of how landscapes responded to past changes in climate. After the last ice age, for example, the warming of the northern hemisphere allowed trees to spread back into territory that glaciers had stripped bare. Pollen cores show this recolonisation unfolding in a clear order: hardy birches and pines arrived first, followed later by oaks, elms and limes as the climate grew milder. Because the sediments can be dated, often by measuring the decay of radioactive carbon in the organic material, the timing of each arrival can be fixed with reasonable precision. In this way palynologists have built detailed timelines showing not only which plants lived in a region but when they did so, and how quickly the vegetation shifted as temperatures rose or fell. The technique has also shed light on the long relationship between people and their environment. When early farmers cleared woodland to grow crops and graze animals, they left an unmistakable signature in the pollen record. The grains of forest trees decline sharply, while those of cereals, weeds of cultivation and open-ground herbs increase. Some plants, such as the ribwort plantain, thrive on trampled and disturbed soil and are regarded as reliable indicators of human activity. By tracing these signals, researchers can estimate when and where agriculture began, and how far it reshaped the countryside long before written records existed. Pollen, in this respect, is a witness to history as well as to nature. Like any method, palynology has limitations that must be kept in mind. Not all plants produce pollen in equal amounts: those that rely on the wind for fertilisation, such as grasses and many trees, scatter enormous clouds of it, whereas plants pollinated by insects produce far less and may be under-represented or missing altogether. Some grains travel great distances on the wind, so the assemblage in a sediment does not always reflect only the vegetation growing immediately nearby. A careful analyst allows for these biases rather than reading the diagram too literally. Even so, the grains preserved in mud and peat remain among the most eloquent records we possess of vanished forests and fields, and of the slow, ceaseless transformation of the living landscape.