Peter Young - Oak

T o a layman an oak is a tree; to a botanist it is one of a family of trees. While the layman admires a single tree or a group of them in a forest, the botanist enjoys studying their differences. The genus Quercus embraces some 600800 species of hardwood trees and shrubs. It is impossible to be more precise because there are many hybrids, some from the natural fertility of adjacent native oaks, others the work of breeders experimenting with species from distant geographical sources. Among them is one between a Turkey oak (Quercus cerris) and a cork oak (Quercus suber) from the western Mediterranean. A fast-growing, straight tree reaching as high as 100 ft (30 m), it varies in exhibiting the main characteristics of one of its ancestor species. Some resemble the deciduous Turkey oak, others have a corky bark and evergreen leaves. It adds elegance to parklands but has none of the sturdy qualities commonly associated with oak. Its wood tends to shrink and warp, reducing its use to firewood. Some trees, although so called, are not true oaks. For example, the Australian silky-oak looks like an oak but is about two-thirds the weight of a genuine oak and, though used as a veneer or in steam-bent form, for example in furniture, is not strong or durable enough for outdoor applications.

There are many variations among species. These depend upon the suitability of soils, location and climate, and the diseases to which they are prone. Profiles differ in height and spread, as does the girth of the trunk, appearance and texture of the bark (which changes colour over time), size and setting of the acorns, the character of the twigs and stalks, and the shape and colour of the leaves. Their lobes may have rounded or pointed ends, their top and under side subtly different greens. The trees are usually deciduous; the holm oak (Quercus ilex), a native of the Mediterranean but introduced to other climates, is an evergreen with pointed leaves. When oak timber is cut, the interior reveals more differences. The weight of the wood reflects its density and porosity. Colours of the timber and regularity of the grain distinguish species. Expert eyes can identify timbers that will dry well, are hard or easy to work and what applications they are suitable for.

Evergreen oak at Westbury Court Gardens in Gloucestershire.

A mature cork oak (Q. suber) in Portugal.

Differences between species are the result of their origin and evolution. Forests grew after the melting of the most recent ice sheets: the glaciers withdrew around 10,000 BCE and the landscape of tundra and steppe was gradually transformed. A patchwork of woodland, in places dense, came to cover the plains of Europe, North America and Asia. The types of succeeding forest depended upon latitude, altitude, underlying geology, climate, distance from the sea and other local factors. Wood and tree pollen preserved in peat bogs shows the sequence of types of trees that grew. Oak typically developed after aspen, birch, willow, pine and hazel. Not that there was a rigid division of types of wood: trees grew in mixed forests, for example of oak and ash. Evidence they have left also outlines the rise and fall of different civilizations, as in the Bronze Age culture of southeastern Spain, a dry area of Europe. Pollen sequences show how vegetation changed over thousands of years. An urban society, the Argaric culture, emerged during the third millennium BCE, lasting only some 700 years before disappearing from the archaeological records around 1600 BCE. Originally, it flourished in an area of diverse forest dominated by deciduous oaks and other broad-leaved trees. The presence of significant levels of charcoal in the retrieved pollen sequence points to this natural resource being over-exploited. Slash-and-burn cleared the forest for mining and grazing, leaving the ground to grow fire-prone secondary vegetation. Following the transformation of the environment, partly aided by a progressive change to a drier climate, the Argaric civilization disappeared.

Holm oak (Q. ilex).

Climate change is recorded in tree rings. Under good growth conditions, with plenty of water and nutrients, tree rings are broad and well separated; during a drought rings grow closer together. Data collected from several sites in central Europe has made it possible for personnel at the Swiss Federal Institute for Forest, Snow and Landscape to construct a climate chronology spanning a period of 2,500 years. This has been related to the fortunes of societies, as in the rise, decline and fall of the Roman Empire. Wet and warm summers coincide with periods of prosperity. Conversely, dry summers and marked variations in climate are factors in economic and other downturns. Climate change was by no means the only cause of the barbarian invasions and the fall of the Roman Empire, but it may well have been a contributory factor. It is also a warning of the kind of long-lasting effects that, at times of global warming, could result from lack of adequate adaptation to a greener way of life.