Perhaps the most striking thing about tree leaves is their tremendous diversity in size. The Arctic/alpine willow (Salix nivalis), which grows around the northern hemisphere, can have leaves just 4 mm long on a sprawling ‘tree’ less than a centimetre high (Figure 2.1a). Smaller still, the scale needles of some cypresses are nearer a millimetre long. Amongst the largest of leaves are those of the foxglove tree (Paulownia tomentosa), which on coppiced trees can be over half a metre in length and width on a stalk another half metre long. Such large sail-like leaves are at risk of being torn by the wind (as in the traveller’s palm – Ravenala madagascariensis; Figure 2.1b) so it is perhaps no surprise that big leaves are usually progressively lobed and divided up into leaflets to form a compound leaf. This can lead to even larger leaves: the Japanese angelica tree (Aralia elata) can have leaves well over a metre in length (Figure 2.1c). Many palms have feathery leaves over 3 m long and in the raffia palm (Raphia farinifera) up to 20 m (65 ft) long on a stalk another 4 m long.

The leaves are the main powerhouse of the tree. Combining carbon dioxide from the air with water taken from the soil they photosynthesise using the sun’s energy to produce sugars and oxygen. These sugars (usually exported from the leaf as sucrose, the sugar we buy in packets) are the real food of a tree. They are used as the energy source to run the tree, and they form the raw material of starch and cellulose and, combined with minerals taken from the soil, allow the creation of all other necessary materials from proteins to fats and oils.

What is a tree?

Everyone knows what a tree is: a large woody thing that provides shade. Oaks, pines and similarly large majestic trees probably come immediately to mind. Such big trees are characterised by the enormous changes in size from seed to mature tree: a mature giant sequoia (Sequoiadendron giganteum) is a billion, billion times heavier than the seed it came from (that’s 1 with 12 zeros after it). A stricter, but more inclusive, botanical definition is that a tree is any plant with a self-supporting, perennial woody stem (i.e. living for more than 1 year). The first question that normally comes back at this point is to ask what then is a shrub? To horticulturalists, a ‘tree’ is defined as having a single stem more than 6 m (20 ft) tall which branches at some distance above ground, whereas a shrub has multiple stems from the ground and is less than 6 m. This is a convenient definition for those writing tree identification books who wish to limit the number of species they must include. In this book, however, shrubs are thought of as being just small trees since they work in exactly the same way as their bigger neighbours. Thus, ‘trees’ cover the towering giants over 100 m through to little sprawling alpine willows no more than a few centimetres tall.

Some plants can be clearly excluded from the tree definition. Lianas and other climbers are not self-supporting (although some examples are included in this book), and those plants with woody stems which die down to the ground each year, such as asparagus, do not have a perennial woody stem. Bananas are not trees because they have no wood (the trunk is made from leaf stalks squeezed together). Nor are bamboos since they are just hardened grasses even though they can be up to 25 m tall and 25 cm thick (see Box 1.1).

Trees form the visual backdrop in most people’s lives whether in an urban or rural setting. In England alone we have more than 90 million trees outside of woodlands in either small groups, hedgerows or as individual trees. But of what value are they to us apart from enhancing our surroundings?

The value of trees

Over their long history, trees have played an important part in our lives that goes beyond just the supply of wood. Trees have been (and still are) sacred to many peoples; oaks were sacred to the European Druids, baobabs (Adansonia digitata) to African tribes, the ginkgo (Ginkgo biloba) to the Chinese and Japanese, sequoias (Sequoiadendron giganteum) to N American first people, and monkey puzzles (Araucaria araucana) to the Pehuenche people of Chile. Indeed, many of our words and expressions are derived from a close association with trees. Writing tablets were once made from slivers of beech wood (Fagus sylvatica), and ‘beech’ is the Anglo-Saxon word for book. Beech is still called ‘bok’ in Swedish and ‘beuk’ in Danish. Romans crowned athletes with wreaths of the bay laurel (Laurus nobilis); this was extended to poets and scholars in Middle Ages, hence Poet Laureate. Similarly, Roman students were called bachelors from the laurel berry (baccalaureus) leaving us with bachelor degrees (baccalaureate) and, since Roman students were forbidden to marry, unmarried bachelor males.

The whole point of a woody skeleton is ultimately to get the leaves above competitors to ensure a lion’s share of the light. And from this simple goal comes an enormous range of tree shapes, from the unbranched stems of palms and tree ferns to the tall spires of conifers, the broad spreading crown of oaks and the multiple stems of an old yew. What governs the shape of trees? How are trees organised to display what often looks like an impossibly large number of leaves?

Trees of distinctive shape

It is usually possible (but not always!) to identify a conifer from a distance by its conical outline. Within the cone there are usually horizontal plates of foliage showing where the branches are produced in whorls around the main stem, usually one whorl per year (Figure 7.1). This contrasts with the wide dome of a hardwood where the leading shoot of the young tree gives way to a number of strong branches giving the whole canopy a rounded shape.

Within these two main shapes it is possible (with a little practice) to distinguish different species simply by their shape. This book is not the place to list the distinctive features of common species but one example will illustrate the point. In common lime (Tilia x europaea) the main branches develop in great arching curves which in time lose the terminal buds. New growth comes from near the top of the branch end resulting in another arch, creating the effect of multiple rainbows joined together at their ends (Figure 7.2). Also, epicormic buds (Chapter 3) characteristically produce a mass of sprouts around the base of the trunk and frequently a congested growth of small twigs in the centre of the canopy. Some of these young growths escape to produce vertical branches through the crown, parallel to the main stem.