Preservation

Plant tissues decay by being consumed by microorganisms. They will be preserved if they cannot be consumed by these organisms or in the absence of such organisms. Preservation normally involves charring, desiccation (drying), or waterlogging. Charring removes hydrogen and oxygen and leaves a carbon skeleton of items such as wood, a seed, or fruit and occurs when plant remains are exposed to excessive heat near, or in, hearths, or when a structure burns such as in a house fire. In some cases, plant material such as chaff is mixed with clay to make pottery and the subsequent firing will preserve some of the chaff in the pottery wall. Only in the relative absence of oxygen will charring occur. The higher the temperature, the worse the preservation will be. Delicate materials such as leaves and fibers of herbaceous plants rarely survive unless the temperature is quite low. Tougher plant tissues such as seed coats, particularly of nuts and other dry seeds, are retained at higher temperatures. Plant remains stored in containers, under soil, or otherwise protected from the air will usually be the best preserved by charring. Carbon is relatively stable so does not decay when exposed to oxygen.

At dry sites such as protected caves and rock shelters (see Caves and Rockshelters) as well as at sites in deserts, plant tissue preserves because of the lack of moisture that microorganisms require. A wider range of plant tissue such as leaves and other soft tissue survives in such contexts. The plant remains may appear almost fresh. At the Guila Naquitz site in the dry highlands of Mexico, bean pods and quids of chewed maguey have been recovered. At Mammoth Cave, Kentucky, thousands of objects made from wood, gourds and other plant material left behind by indigenous miners are preserved. Mud to make adobe walls is usually mixed with plant stems and chaff and when the bricks are sun-dried the stems and chaff are preserved inside the bricks. In extremely wet conditions, preservation occurs when oxygen cannot be replenished in the water. This often occurs in deep marine or lake settings, alluvial flood deposits, bogs, and landslides. In these settings, aerobic (oxygen consuming) microorganisms that are mainly responsible for the decay of organic material cannot survive. Sites such as Torihama in Japan, Hemudu in China, and Scowlitz in British Columbia, Canada are examples of wet sites with excellent preservations of plant remains.

In other instances the plant remains themselves are not preserved, but their impressions are. Impressions of seeds and fruits in contact with clay during the manufacture of pottery occasionally occur on the interior and exterior surfaces of pottery. These impressions retain the surface form and sometimes the cellular pattern of the seed, fruit or stem that made the impression. At Pompei in Italy, plant remains are preserved as casts of the original item in the volcanic ash or as charred remains.