What is a tree?
Find out what makes a tree different from other types of plant
WHAT IS A TREE?
A TREE is a large, perennial plant with a single woody stem which is hard and strong. This allows a tree to grow tall or very tall, to stay upright without flopping over, and to withstand wind and other pressures. Plants are built basically of cellulose, but lignin in trees is what makes wood hard. Tannins and resins in wood give each species a distinctive colour and odour.
On the other hand, a SHRUB is a medium-sized, perennial, woody plant which branches prolifically into many stems from the base; shrubs do not usually grow as tall as single-stemmed tree in Britain and Ireland.
A HERBACEOUS PLANT or HERB is a plant in which there is little strong, supportive wood in the stem or stems. It is soft, sometimes floppy, cannot grow tall or long unless it lives in water – which then supports it.
Trees are classified within two advanced groups of plants, the Gymnosperms (conifers, cycads and ginkgoes) and Angiosperms (flowering plants).
Gymnosperms have an ancient lineage: they started to evolve in the late Carboniferous geological period, 359.2 to 299 Ma (million years ago) when large horsestails, clubmosses and ferns were being fossilised into coal. They flourished, gradually evolving into thousands of species. Gymnosperms then dominated the Earth’s vegetation for over 100 million years (a long time!). Then, from 60 Ma, many succumbed to competition from the evolving flowering plants and became extinct. Later, the huge ice sheets of the Ice Ages (within the past 1 Ma) killed many more species over vast geographical areas. There are now less than one thousand species of gymnosperms left on Earth. With many gymnosperms becoming extinct, Conifers are currently the most important group remaining.
Angiosperms or ‘flowering-plants’ have a much shorter lineage: they are the most recent and most evolutionarily-advanced plants on Earth. Angiosperms originated about 140 Ma, in the early years of the Cretaceous (chalk) period. During the remainder of the Cretaceous period (up to 65.5 Ma), angiosperms evolved rapidly and diversified into many new forms, contemporary with rapid evolution of insects and dinosaurs. The Ice Ages stimulated their evolution and adaptation to new landscapes.
Not all angiosperms are trees: many are shrubs, but most are herbs. Including grasses. Most gymnosperms are trees or shrubs, and none are herbaceous.
Conifer trees have a simple, pyramidal shape because of their regular branching in the form of whorls. Nowadays, conifers are usually evergreen (but larches, dawn redwood and swamp cypress are deciduous) and they have leaves which are hard needles or overlapping green scales. Their reproductive apparatus usually develops in the form of separate female and male cones.
The ovules are borne naked on the female cones, so that pollen from male cones can be blown and land directly onto them. After fertilisation, the resulting seeds drop or are blown from between the scales of the female cones.
Being a conifer tree signifies close genetic and evolutionary relationship with other conifers. Being an angiosperm tree does not signify close relationship, as they belong to many, different, unrelated groups. It does signify that the tree has flowers and that the leaves are wide and flat (broadleaved).
Angiosperm trees grow thin, flat leaves with a large surface-area. The leaves are deciduous in the north-temperate zone of Earth, dropping all together each autumn, with new leaves appearing and opening out the following spring. We all know one of the exceptions – holly.
Last autumn, I was startled by the sound of a horse-chestnut tree whose orange, autumn leaves were dribbling noisily through the canopy to the ground. I stood for ten minutes below it, listening to the rustling noise in the morning air.
Angiosperm trees do not bear cones: their ovules are enclosed within a container called the carpel or ovary. The whole reproductive structure is called a ‘flower’.
But unlike a conifer, the pollen cannot land directly onto the ovule, which is enclosed within the carpel. Pollen lands on a stigma (with the help of a bee or wind),
and the pollen cell grows a tube through to the ovule, allowing the male and female gametes to join. After fertilisation, a seed or seeds develop. The seeds are enclosed within the ripened carpel or ovary, which becomes a fruit. We eat many broadleaved tree fruits and seeds, such as hazel nuts, apples, cherries and rowan berries.
Gymnosperm seeds are rather small and not enclosed within a fruit. We eat only a few of their seeds, such as nuts of a few pines and monkey-puzzle seeds.
There are only 3 species of gymnosperm tree native to Britain and Ireland:
Yew (Taxus baccata)
Juniper (Juniperus communis) and Scots pine (Pinus sylvestris subs. scotica). Two are exceptional in producing berries not cones!
Scots pine is native only in Scotland and not to other parts of Britain or Ireland, where it became extinct during later post-glacial times.
All the other conifers you see growing in gardens, the British and Irish countryside, have been introduced from foreign parts, for instance Norway spruce (Picea abies), Nordmann fir (Abies nordmanniana), Cedar of Lebanon (Cedrus libani) and Korean pine (Pinus koraiensis).
There are about 35 species of trees native to Britain, but only about 25 native trees in Ireland. This may be the result of Ireland being cut off from Britain and Europe by the new Irish Sea soon after the ice melted, while the English Channel and North Sea separated Britain from Europe much later. So there was much longer for tree species to migrate across land to Britain than to Ireland (the trees didn’t migrate of course, their seeds did!).
Beech (Fagus sylvatica) and Hornbeam (Carpinus betulus) are examples of trees not native in Ireland or Scotland. However, the strawberry tree (Arbutus unedo) is a species of tree native to Ireland but not Britain.
Examples of interesting and beautiful angiosperm trees native to Britain and Ireland are: Rowan (Sorbus aucuparia), Chequer tree (Sorbus torminalis), Wild cherry (Prunus avium), Aspen (Populus tremula) and Field maple (Acer campestre).
Many angiosperm trees have been introduced to the UK, for instance: Walnut, (Juglans regia), Horse chestnut (Aesculus hippocastanum with white flowers, and A. x carnea with red flowers), Sweet chestnut (Castanea sativa) and Grey alder (Alnus incana).
Introduced trees outnumber our native trees by at least ten times. We are obviously not satisfied with what nature provided us in our extreme north-west European islands! Perhaps humans have caused the landscape to deteriorate so much that we need some of those foreign species to grow where native species cannot?
Ruth Tittensor 190313