Trees

Peter Thomas

• Publisher: William Collins
• Year: 2022
• ISBN:
  9780008304522
• Topic: Biological Sciences
• Subtopic: Natural History
• Index: Biological Sciences

CHAPTER 1

Setting the Tree Scene

Trees are amazing things that provide us with many products while giving us shade and adding beauty to our surroundings. Towering above us, trees, not surprisingly, are the largest and longest-living things in the world. Their very size and age lead to problems that we animals do not readily appreciate, such as how they can possibly arrange thousands of leaves to receive the best light, and how they can take up huge amounts of water from the soil. They also have to detect and respond to attacks when they can’t run away and yet survive for sometimes thousands of years. Fortunately, we are increasingly learning their inner secrets to how they live and survive. Yet they can still puzzle us, even to the extent of defining just what a tree is!

DEFINITION OF A TREE

What is a tree? Surely this is a stupid question, since we all recognise a tree when we see one! The problem here is that most plant groups occur within a single plant family, sharing a common ancestor and so sharing characteristics found in the whole group. Thus orchids are all in the Orchidaceae family, grasses in the Poaceae, and you can tell by the structure of the flowers. However, the woody tree habit has evolved independently in a wide range of families. Trees are found in at least 20 families in temperate areas and so probably hundreds worldwide. Even the 220 native woody plants of the Canary Islands have at least 38 independent origins (Lens et al. 2103). Many unrelated plants have evolved the woody habit, allowing them to grow taller than neighbours in the competition for light, which makes it harder to pin down a good biological definition of a tree. Moreover, the woody habit is just as easily lost through evolution by some plants as it is gained by others so it is even harder to pin down any genetic differences between woody and non-woody plants – there are no unique tree genes (Groover 2005). The best we can say is that a tree is a perennial woody plant with a self-supporting main trunk and branches, which grows fatter over time and forms a distinct crown (Hirons & Thomas 2018). The ‘wood’ itself is defined as a mixture of cellulose fibres forming tubes that are strengthened with lignin.

Others would argue that trees also tend to be tall, and so the IUCN’s Global Tree Specialist Group defines a tree as a woody plant with usually a single stem growing to a height of at least 2 m, or, if multi-stemmed, then with at least one vertical stem that is 5 cm in diameter at chest height (Beech et al. 2017). These definitions are helpful but, except for the perennial bit, there are many exceptions. A number of trees like Hazel Corylus avellana normally have multiple thin trunks rather than one main stem; strangler figs that germinate in the crown of another tree and grow ‘strangling’ roots down to the ground are not self-supporting for the first part of their life; and many trees such as those growing near the polar treelines would never attain more than a few centimetres in height. When it comes down to it, a tree is just recognisable as being a tree. Indeed, Lord Denning, who had to define a tree as part of a judgement under the UK Town and Country Planning Act 1990, stated that ‘anything that one would ordinarily call a tree is a “tree” within this group of sections in the Act’.

As far as the remit of this book goes, while trees can be towering giants over 100 m tall, they can also be small arctic willows just a few centimetres high – they both share a woody skeleton that they reuse each year and so work in roughly the same way. Equally important, using the above definitions, some plants are not considered to be trees. Lianas and vines are not self-supporting and so are not really trees (although some will be discussed as we go along), and plants with woody stems which die down to the ground each year, such as Asparagus Asparagus officinalis, do not have a ‘perennial woody stem’. Similarly, bananas are not trees because they have no wood – the trunk is made from non-woody leaf stalks squeezed together. Nor are bamboos, since they are just very hard grasses that do not grow fatter over time, even though they can be up to 25 m tall and 25 cm thick.

What then is the difference between a tree and a shrub? This is really a horticultural distinction that separates growth forms such that a ‘tree’ has a single stem more than 6 m tall that branches at some distance above ground. By contrast, a shrub has multiple stems from or below ground level and is less than 6 m tall. From a biological viewpoint there is no real difference in how they work, so both are included in this book.

Structure of a tree

Trees are primarily made up of three main parts: leaves, shoots and roots. These will be considered in more detail later in this book, but for now a simple overview may help. The leaves produce the food of the tree – sugars – by photosynthesis. The ‘shoots’ are made up of the trunk (biologically, the stem) and branches which hold the leaves up to the light, forming the crown of the tree, and also hold the flowers and fruits. The crowns of several trees combine to make up the woodland canopy. The trunk and branches also act as a conduit for water to be brought from the roots to the leaves via the wood (xylem), and for sugars to be transported around the tree in the inner bark (phloem). The water flowing through the wood contains nutrients such as nitrogen dissolved from the soil and also carries plant hormones which transmit messages from the roots to the crown.

The roots have the dual role of holding the tree up and absorbing water from the soil. The above-ground part of the tree is perhaps intuitively much bigger and heavier than the roots below ground, but, as shown in Table 1, which uses Scots Pine Pinus sylvestris as an example, as a general rule of thumb the weight (biomass) of the fine roots that take up water and nutrients is roughly equal to the weight of the leaves (or, in this example, needles). As the tree matures, the trunk makes up a progressively larger proportion of the biomass while the coarse or big woody roots that hold the tree up stay around the same percentage of the biomass through the life of the tree. The weight of the fine roots, which absorb most of the water, and the weight of the needles make up a decreasingly small proportion of the tree’s biomass. Reproduction, in the guise of the weight of cones, is a very small part of the tree’s total biomass.

table 1. Amount of biomass (as a percentage of the whole tree) in Scots Pine Pinus sylvestris at three different ages.

Data from Helmisaari et al. (2002).

Types of trees

Trees come in two main types (Fig. 1): gymnosperms and angiosperms. Both of these are seed plants but only the second produces flowers. The oldest group are the gymnosperms (from the Greek gymnos, meaning ‘naked’, and sperma, meaning ‘seed’; gymnos is also the root of ‘gymnasium’, since Greeks exercised naked). These are the conifers and their relatives, all of which have seeds borne in cones (or fruits in junipers and yews) that are unenclosed such that the seeds can be seen without breaking anything open. Although they are seed plants they do not have flowers as such.

The other main group are the angiosperms (from the Greek angeion, meaning ‘case’, and sperma, meaning ‘seed’). These are the flowering plants: the name comes from the fact that they also have fruits (or cases) enclosing the seeds. This includes many of the common broadleaf trees, such as oaks, beeches and ashes that mostly have flat leaves, in contrast to the conifers that mostly have needle leaves. There are many names that one could use for the angiosperms, the most common being hardwoods (the wood is generally harder than the conifers but there are exceptions) and broadleaf trees (the leaves are mostly but not always flat and broad). Neither term is completely accurate, but we will use broadleaf trees as shorthand for angiosperm trees. These broadleaf trees can be divided into the dicotyledonous (dicot) trees and the monocotyledonous (monocot) trees. The monocots are defined as having a single seed leaf (cotyledon) and include a number of trees in a few families, notably:

• Palms (the family Arecaceae, previously Palmae or Palmaceae)
• Dragon trees Dracaena species, cordyline palms Cordyline spp., European butcher’s broom Ruscus spp., Yuccas such as the Joshua Tree Yucca brevifolia (Aspagagaceae)
• Screw pines Pandanus spp. (Pandanaceae)
• Grass trees Xanthorrhoea spp. of Australia and aloes Aloe spp. from South Africa (Xanthorrhoeaceae, but this is likely to be absorbed into the family of Asphodelaceae in the future)
• Traveller’s Palm Ravenala madagascariensis (Strelitziaceae).

The dicots with two seed leaves form the rest of the familiar broadleaf trees.

FIG 1. The two main types of trees. Conifers and their relatives make up the gymnosperms, including (a) the pines (Pinaceae) but also including the flat-leaved Maidenhair Tree Ginkgo biloba (Ginkgoaceae), (b) monkey puzzles (Araucariaceae), junipers, redwoods, cypresses and many more in the Cupressaceae, and the yews (Taxaceae). The angiosperms, or broadleaf trees for convenience, mostly have flat, wide leaves like (c) the oak but include some with needle-like leaves such as (d) gorse Ulex spp. and broom (Cytisus and Spartium species). Photographs of (a) Hartweg’s Pine Pinus hartwegii in Honduras, (b) Monkey Puzzle Araucaria araucana in Staffordshire but native to Chile and Argentina, (c) Georgian Oak Quercus iberica in the Caucasus of Azerbaijan and (d) Gorse Ulex europaeus on the coast of Devon.

GEOLOGICAL HISTORY: WHERE DO OUR TREES COME FROM?

Plants colonised land around 465 Ma (mega-annum, or millions of years ago) in the Middle Ordovician, and the first vascular plants (those with internal plumbing) appeared by 400 Ma in the basal Devonian (formerly the Silurian). These vascular plants had simple stems just a few millimetres wide and a few centimetres tall with no leaves, typified by Cooksonia. It was another 40–50 Ma before leaves were common. These early land plants were small because the vascular tissue was just for conducting water up the plant without giving any great structural strength. Taller plants – the first trees – became possible with the evolution of stiffer vascular tissue and a more developed root system to give support. These first trees, reproducing by spores like their primitive ancestors, evolved in the mid-Devonian around 393–383 Ma and were capable of living for several decades and reaching up to 30 m tall and 1 m wide. They spread to produce widespread forests dominated by the first tree-like plant, Archaeopteris, not to be confused with Archaeopteryx, the bird-like dinosaur. This was a turning point that helped change the world’s climate.

A wider diversity of trees, referred to as the progymnosperms, evolved in the mid-Devonian. Within 100 million years, the coal-producing swamps of the Carboniferous (359–299 Ma) were dominated by lush forests. We would have recognised the tree ferns from today’s forests but the others – fast-growing giant horsetails and clubmosses – have long since disappeared, leaving us with just a few small relatives. The horsetails such as Calamites were up to 15 m tall and 30 cm in diameter but would have been dwarfed by the clubmosses (notably Lepidodendron) which reached 45 m in height and 2 m in diameter (Fig. 2). These can be considered the dinosaurs of the plant world: an extinct group that achieved a huge size (Thomas & Cleal 2018). The forests were accompanied by giant insects (such as dragonflies with wingspans up to 63 cm), all made possible by oxygen levels in the atmosphere rising up to 35 per cent (compared to the 21 per cent today). This rise in oxygen may have been due to the evolution of lignin which provides structural support to wood, but which microbes had not yet evolved the ability to digest, leading to carbon being locked up and oxygen released in photosynthesis left in the atmosphere (see David Beerling’s book The Emerald Planet [2007] for a very readable account of these changes). In these forests the first seed plants appeared, called pteridosperms or seed ferns. They resembled tree ferns but, importantly, produced seeds rather than spores. From these, the first primitive conifers appeared around 300 Ma at the end of the Carboniferous. The earliest-known conifer was named Swillingtonia denticulata after Swillington Quarry near Leeds in Yorkshire where it was found, dating back to 301 Ma (from Scott & Chaloner 1983).

FIG 2. An artist’s model in the Singapore Botanic...