Sediments as natural archives
Every time a plant produces pollen and spores, most will end up landing on the ground somewhere eventually, some quite close to the parent plants (like the bird-pollinated rātā or pōhutukawa trees (Metrosideros spp.) and others great distances away (like the wind-pollinated beech trees, Nothofagus spp.). While most pollen and spores will decay and rot soon after landing on the ground, in some environments such as peat bogs and lake sediments, they will be preserved and accumulate in chronological order along with the dead remains of plants, microscopic animals and possibly mineral materials for hundreds, thousands, and in some cases millions of years. The depth of sediment (and pollen and spores) therefore relates to age. Given that pollen and spores are the key to revealing what plants lived on the landscape in the past, peat bogs and lake sediments can be looked upon as important historical archives of past environments. Pollen grains and spores are usually abundant and well preserved in peat bogs, lake and pond sediments, and it is these kinds of sediments that are most often used for reconstructing past vegetation patterns.
Pollen is transported in different ways from the parent plants growing on the landscape to the sites of preservation in peat bogs and lakes. It is important to be able to consider these sources when it comes to interpreting the fossil pollen records. Local plants growing directly on a bog surface or wetland plants growing in shallow lake waters will deliver a large proportion of their pollen to their immediate surroundings, as it falls directly from the plants. In contrast, pollen from taller vegetation surrounding the peat bogs, or plants growing in the lake catchment area, can be transported great distances away from source. Pollen escaping from tall forest canopies can be lifted by thermals to high altitudes in the troposphere, which may carry the pollen many hundreds of kilometres away. For example, mataī (Prumnopitys taxifolia) pollen is found in peat deposits on New Zealand’s subantarctic Auckland and Campbell islands – a long way from the southernmost limits of their natural distribution on the mainland. Pollen-bearing soils, sediments or riverbed sediments can get eroded by water and reworked, transporting additional older pollen and spores into a mixture of contemporary sediments. This can often confuse the interpretation of a pollen record, but fortunately the surface walls of reworked pollen and spores can often be recognised by their poorer degree of preservation.
Larger lake bodies receive pollen from a much wider source area than small peat bogs, but both can be used together to piece together the regional and local vegetation history of an area. Pollen and spores are also found in lower concentrations and generally less well preserved in soils, caves (both sediments and speleothems), glacier ice, polar ice caps and ocean floor sediments. The extent to which pollen and spores are preserved in sediment deposits relies on a number of factors, the most important of which are fine grain size and a low oxygen environment from the time of deposition to minimise microbial and aerobic attack. The degree to which different types of pollen and spores are preserved is also related to the thickness of their walls; some very fine structured pollen and spores never get preserved in the fossil record.