Characteristics of woodlands and forests

1.1.1 Wooded environments

Forests often appear monumental and unchanging. This is, however, mostly an illusion caused by our short human perspective. The earliest green plants possessing both roots and tissues specially adapted for the transmission of water belonged to the Psilopsida, which gave rise to the ferns and fern allies. It is from the ancestors of this group, which arose in the Silurian (c. 440 million years ago), that all trees - both ancient and modern - are ultimately derived (see Fig. 1.1). Amongst the many evolutionary trends found within this group were tendencies towards the production of (a) tall trunks and (b) seeds from which young plants, including trees, could develop relatively rapidly. Tree ferns, cycads, maidenhair trees, conifers, palms and the very large number of broadleaved genera remain in our woodlands and forests to this day (further detail on past forests can be found in Chapter 9). The amount and composition of the world's wooded areas have changed continuously over geological time, sometimes more rapidly than at others, and continue to do so, helped especially now by human activities. This book is mainly concerned with understanding today's forests in that light.

Wooded land currently covers between 30-35% of the world's land surface (depending on what is counted as forest) or around 39-45 million km2. The Food and Agriculture Organisation of the United Nations (FAO) figures used in Table 1.1 give 30.3% or 39.5 M km2 of the world's land area as forested, with 2.8% of that being under plantation (i.e. purposefully planted). Forests are obviously not equally spread around the globe, their distribution being very dependent upon climate (this is expanded upon in Section 1.6 below). This can be seen in Table 1.1 which breaks down forestry cover into world regions. Some of the least forested countries have primarily desert environments (Gulf

(years 109)

Global tectonics

Global tectonics

Typical Features Tropical Forests

Age Global Sea Level

Precambrian (Proterozoic)

Vascular plants

Figure 1.1 The stratigraphic column. Geological eons, eras, periods and time scale, indications of the origin and duration of the major vertebrate and vascular plant groups, together with estimated variations in sea level. Earth became solid c. 3.9 billion years ago at the end of the Hadean, and the beginnings of life were present within another 50 million years. Stromatolites and blue-green algae (known as cyanophytes or cyanobacteria) were present early in the Archaean era. The latter were the first photosynthesizing organisms on Earth. At first the oxygen they produced combined with iron-forming ferric oxides which sank to the bottom of the primitive seas. It then transformed the initially very adverse atmosphere and provided the oxygen required by animal

Vertebrates

Precambrian (Proterozoic)

Vascular plants

Age Global Sea Level

200 220 240 260 280 300

- 320 340 360 380 400 420 440 460 480

- 520 540 560 580

Falling Rising

Figure 1.1 The stratigraphic column. Geological eons, eras, periods and time scale, indications of the origin and duration of the major vertebrate and vascular plant groups, together with estimated variations in sea level. Earth became solid c. 3.9 billion years ago at the end of the Hadean, and the beginnings of life were present within another 50 million years. Stromatolites and blue-green algae (known as cyanophytes or cyanobacteria) were present early in the Archaean era. The latter were the first photosynthesizing organisms on Earth. At first the oxygen they produced combined with iron-forming ferric oxides which sank to the bottom of the primitive seas. It then transformed the initially very adverse atmosphere and provided the oxygen required by animal

Vertebrates

Table 1.1. Forest cover in world regions, defined as including all natural forests and plantations. Taken from FAO (2005)

Per cent of

Per cent of forest

Forest area

land area

area that is

World region

(M km2)

forested

under plantation

Africa

6.4

21.4

2.5

Asia

5.7

18.5

7.8

Europe

10.0

44.3

2.2

N and Central Americaa

7.1

32.9

2.5

Oceaniab

2.1

24.3

1.9

South America

8.3

47.7

1.4

World

39.5

30.3

a Including Greenland.

b Including Australasia and surrounding islands.

countries such as Kuwait and Egypt are all below 0.3% cover, and according to FAO figures, Oman and Qatar have no forest cover), or cold inhospitable climates (Iceland also has 0.5% forest cover). At the other end of the scale, the highest forest cover is found in northern boreal climates (Finland 74%) and on moist islands in the equitable Pacific such as the Cook Islands (67%) and the much larger Solomon Islands (78%). The five most forest-rich countries (the Russian Federation, Brazil, Canada, the United States and China) account for more than half of the world's total forest area (21 Mkm2 or 53% - FAO, 2005). Deforestation and afforestation play an overriding part in how much forest is left in many areas of the world. After the last ice age the UK would

Caption for Figure 1.1 (cont.)

life. The Cambrian explosion, which began the Lower Palaeozoic, involved the sudden and abrupt production of myriads of life forms, including the complex and very varied trilobites, from simple precursors. Note that the Quaternary Period consists of the Pleistocene Ice Age, in which there have been a number of interglacials including the ongoing Holocene, which began 10 000 years ago and is part of the present Flandrian temperate stage with its global warming and ice melt. Our views on tracheophyte (vascular plant) relationships are constantly being modified as more and more fossil evidence accumulates: Bell and Hemsley (2000, p. 141) should be consulted for a more complex view. The Lycopsida (shown by the asterisk) included the clubmoss trees Lepidodendron and Sigillaria, and the Sphenopsida included Calamites (#). (Mielke, 1989; Benton, 1991; Briggs et al., 1997; and After Bryson, 2004.)

have been fairly extensively forested but by World War I was down to around 6% cover, a figure which has now increased to 11.8%. From Table 1.1 it can be seen that the degree of planting of artificial plantations (as opposed to regenerating 'natural' forest) varies tremendously around the world. Forest area under plantation is greatest in Asia (7.8%); elsewhere it is less than 2.5%, giving a figure of 2.8% for the world forest as a whole. This adds further complexity to the way in which humans have influenced forest cover.

Nevertheless, it is estimated that about half of the forest that has grown under modern climatic conditions since the end of the Pleistocene, around 8000 years BC, has been lost, largely due to human activities. The spread of agriculture and domesticated animals, increasing population and cutting of forests for timber and fuel have all taken their toll. Some 13 M ha (0.13 M km2) of forest are being lost globally each year (FAO, 2005). When new forests are taken into account the net loss of forest between 2000 and 2005 was still 7.3 M ha per year, an area the size of Sierra Leone or Panama. The only silver lining is that we are not losing forests as quickly as we did between 1990 and 2000 when the net loss was 8.9 M ha per year. Global net loss of forest has been estimated as 0.18% per year between 2000-2005 (FAO, 2005). It is perhaps not surprising that the UNEP World Conservation Monitoring Centre has identified over 8000 tree species that are threatened with extinction at a global level and are concerned for the estimated 90% of all terrestrial species that inhabit the world's forests. See Section 11.1.2 for rain forest losses.

1.1.2 Differences between woodlands and forests

The terms forest and woodland are commonly used almost interchangeably, and if there is any differentiation, then most people see a forest as a remote, large, dark forbidding place while a woodland is smaller, more open and part of an agricultural landscape. These views are very close to the normally accepted definitions of the two terms. A woodland is a small area of trees with an open canopy (often defined as having 40% canopy closure or less, i.e. 60% or more of the sky is visible) such that plenty of light reaches the ground, encouraging other vegetation beneath the trees. Since the trees are well spaced they tend to be short-trunked with spreading canopies. The term forest, by contrast, is usually reserved for a relatively large area of trees forming for the most part a closed, dense canopy (although canopy closure as low as 20% is accepted in some definitions). A forest does not have to be uniform over large areas, and indeed is often made up of a series of stands, groups of trees varying in such features as age, species or structure, interspersed with open places such as meadows and lakes and areas where grazing animals are limiting

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