Ecology of past forests

9.1.1 The Palaeozoic era and tectonic plate theory

Woodlands and forests of the past were often very different from those we know today. In this matter, as in so many others, the Greek philosopher Heraclitus was correct in stating that only change is constant. Moreover, the changes known to have occurred in the world's flora and fauna have sometimes been very abrupt (Toghill, 2000), as in the biggest mass extinction of all time which occurred at the end of the Permian period (245 Ma i.e. millions of years ago - see Fig. 1.1). The causes of these mass extinctions may well have been either large asteroids striking the earth or the eruption of supervolcanoes such as that now simmering beneath Yellowstone National Park, Wyoming, USA. Each of these possesses a huge magma chamber, the contents of which contain vast amounts of mainly sulphurous gases. Once an eruption begins these previously dissolved gases assume the vapour phase, hurling much of the magma high into the stratosphere where some of it remains as fine ash for such a long period that it entirely disrupts the climate, lowering the temperature to such an extent that few plants and animals survive. Many past changes, however, have been slow and gradual as climate has changed and new types of trees appeared.

If we are to understand why the distributions, as well as the nature, of the forests of the past were often so very different from those of today, we need to understand the basic structure of the planet we live on. At the centre of the Earth is a mainly nickel-iron core (Briggs etal., 1997, p. 31). The crust is the outermost of the five successively less dense layers that surround this core. It consists of a number of abutting tectonic plates that form both the continents and the ocean floors (see Section 2.5.2 regarding continental drift). The positions of these plates are not fixed but move large distances over the millennia; our increasing knowledge of the time taken for these movements to occur is gradually enabling us to relate the tectonic history of the Earth to the evolution of forest populations in various parts of the world as Section 9.1.3 illustrates. Plates have in the course of history moved from areas with a warm climate to those which are much colder, as well as vice versa, which explains why forests formerly existed in what is now Antarctica. At destructive plate margins, where one plate is being forced down below another, crustal shortening occurs. The still-ascending Andes of South America exemplify the mountain building that often occurs on the leading edge of an overriding plate, thus leading to the geographical isolation of what were once continuous populations.

Sometimes pressure that has built up over many years between adjoining plates is suddenly released with dramatic results, as when the Burma plate continued to force itself over the Indian plate at the end of 2004. The resulting earthquake, which measured 9.0 on the Richter scale, had an epicentre a little to the west of North Sumatra, and itself caused considerable devastation to the forests of the immediate area. It also triggered off a huge wave that travelled as far as Sri Lanka, India and Kenya reaching 10 metres high and killing many hundreds of thousands as it crashed on the shore. It devastated all in its path, including many coastal forests, whose soils were also rendered saline, especially those of outlying islands. Such a seismic surface wave, which is triggered at sea by seismo-volcanic activity is called a tsunami. Relatively low in the open ocean, it rises in elevation on reaching shallow coastal waters.

Severe though this damage was, such waves have caused even greater havoc in the past. Some tsunami are caused by volcanoes. The repeated eruptions of Krakatoa in 1883 initiated a series of tsunami and when the entire island blew up in late August - the loudest event ever recorded - the resulting wave was well over twice as high as that of 2004, though its influence was more local and the number of people killed far fewer. This volcanic region is particularly interesting to a forest ecologist. In 1927 there was a violent eruption just below the surface of where the old volcano had been. Since then a large volcanic island has built up and become forested in its cooler areas. We appear to have a cyclic process in which new forested land develops from molten magma welling up from deep within the Earth's crust, only to be destroyed in another vast explosion a few hundred years later.

Vascular plants, which transmit fluids by means of internal plumbing of either tracheids or open-ended vessels, did not arise until the Silurian (about 440-410 Ma) with tree-like forms developing in the Devonian (410-360 Ma), so as Fig. 1.1 shows, the period in which parts of the Earth have been clothed with woodlands and forests is relatively short. The same is true of the Vertebrata, the largest and most important of the four main groups in the phylum Chordata. Fossil traces of invertebrate animals with hard parts, such as belemnites, ammonites and trilobites are found from the beginning of the Cambrian. Traces of typical fish-like vertebrates are found in rocks of Silurian age. These evolved from early chordates not represented in the fossil record, but probably similar to the modern lancelet Amphioxus. This animal has an incompressible and elastic rod, the notochord, running down the centre of the body. The earliest vertebrates appear to have been jawless. These early Agnatha included the heavily armoured cephalaspids and pteraspids which were not parasites and scavengers like the modern lampreys and hagfish, some of which still parasitize the fish of woodland streams and rivers. The future, however, lay with the jawed vertebrates (Gnathostomata), which developed in parallel with them, eventually giving rise to the birds and mammals.

The xylem of vascular plants provides a skeleton that supports the body of the plant above the ground, as well as transporting water and mineral salts. Together these plants form the Tracheophyta whose four classes, the Psilopsida, Lycopsida (=Lycopodiopsida), Sphenopsida (=Equisetopsida) and Pteropsida, are shown in Fig. 1.1. Members of the Psilopsida were abundant in the Silurian (in which they arose) and Devonian periods during which the other three classes evolved from them. Like the modern ferns, plants belonging to this ancient group of vascular plants showed a distinct alternation of generations between a small haploid sexual gametophyte and a much larger diploid asexual generation (the sporophyte) that produces spores that give rise in turn to further short-lived haploid gametophytes (Bell and Hemsley, 2000). The present-day order Psilotales contains two small genera, Psilotum and Tmesipteris, whose anatomical and reproductive features recall members of the fossil class Psilopsida. There is, however, no continuity in the fossil record, and for various reasons (Bell and Hemsley, 2000) they are now placed in the ferns. The Lycopsida and Sphenopsida are today in relative decline, while the Pteropsida dominate the terrestrial vegetation of the world. This class contains the ferns, conifers, ginkgoes, cycads and the angiosperms, whose most important trees are dicotyledonous. The commonly employed term Pteridophyta refers to all tracheophytes apart from the true seed plants i.e. the gymnosperms and angiosperms. Amongst the gymnosperms, the area of the world covered by conifers remains very great.

Two of the most important events which have occurred in the evolution of our woodlands and forests are the rise of the gymnosperms, whose ancestors originated around 370 Ma in the Devonian, and of the angiosperms, whose first fossils date from the Early Cretaceous (130 Ma) in the upper Mesozoic and which include the broadleaved trees so important today. Though the number of species in the gymnosperms is much less now than in the Cretaceous period, they provide a vast amount of softwood timber. The present diversity of the angiosperms is immense, and the dicotyledonous trees covering much of the Earth's surface produce a wide variety of hardwood timbers. The mono-cotyledonous palms do not yield useful timber, but the dates and coconuts they produce are a valuable food source. Although the plants and animals of many earlier forests were very different from those now existing, general ecosystem function including soil relationships was broadly similar.

A brief review of past forests helps us to appreciate just how radical past changes have been. The types of ancient forest which will be briefly described here are the clubmoss (Lepidodendron) forests of the Carboniferous period (363-290 Ma), the forests which existed when the dinosaurs roamed, and the more recent interglacial forests which are rather like our own but which were more extensive and possessed far more large animals.

Trees of the clubmoss forests (Fig. 9.1), which were up to 40 m tall, are of great economic importance because their fossil remains are the major constituents of coal. The Lepidodendrales (ancestors of our extant, herbaceous clubmosses) had several genera, notably Lepidodendron and Sigillaria. They possessed a limited degree of secondary thickening and the equivalent of a cork cambium (further description of the biology and evolution of early trees can be found in Ingrouille, 1992 and Thomas, 2000). Lepidodendron had spirally arranged leaves and a trunk which reached a metre in diameter. Unlike modern relatives in the Pteridophyta, it produced seeds borne in large cones on the dichotomous branches of the canopy, but was not in the line which subsequently gave rise to the true seed plants of today. Sigillaria had a tall columnar stem which was either simple or slightly branched and bore a tuft of simple leaves up to a metre long at the stem apex or apices. The climate of the Carboniferous period was moist and the soils waterlogged; the rhizome-like rhizomorphs of both these trees, which bore slender roots that subsequently broke off, were shallow and repeatedly dichotomously branched. Trees of Calamites, a relative of our modern horsetails, were also in the canopy, growing from huge underground rhizomes. Although a variety of invertebrates scavenged and assisted with soil processes, only three of the major vertebrate groups were present in these forests. These were fish, which first appeared in the Silurian period; the amphibia, that arose in the Devonian period (and of which the labyrinthodont shown in Fig. 9.1 was an early representative); and the reptiles, whose earliest known fossils date from the Carboniferous itself.

9.1.2 Forests of the Mesozoic era

The trees, ferns, herbs and lower plants of the world's woodlands have altered almost beyond recognition since Carboniferous times. During the Mesozoic

Pityostrobus

Figure 9.1 Margin and trees of a clubmoss forest of the Carboniferous Period. The cone-bearing trees in the distance are of Lepidodendron, as is the central fallen tree. There is a vertical dead trunk (hulk) of an early cordaitalian member of the gymnosperms on the left; with an unbranched species of Sigillaria placed more centrally. The trunk on the right is of a tree fern, some of whose foliage is seen in the top centre of the picture. Grasses and other flowering plants had not yet evolved, but bryophytes, ferns and small horsetails abounded in the undergrowth. The labyrinthodont amphibian shown in the foreground was typical of this early stock that spent much of its life in the water, but had the ability to walk on land. Resting on the fallen trunk is a member of the extinct 'griffenflies' (Protodonata) which included the largest insects that have ever lived and gave rise to the Odonata, the dragonflies and damselflies of today (Grimaldi and Engel, 2005). The biggest of all was Meganeuropsis permiana from the early Permian, which had a wingspan of approximately 71 cm. (Drawn by Peter R. Hobson.)

Figure 9.1 Margin and trees of a clubmoss forest of the Carboniferous Period. The cone-bearing trees in the distance are of Lepidodendron, as is the central fallen tree. There is a vertical dead trunk (hulk) of an early cordaitalian member of the gymnosperms on the left; with an unbranched species of Sigillaria placed more centrally. The trunk on the right is of a tree fern, some of whose foliage is seen in the top centre of the picture. Grasses and other flowering plants had not yet evolved, but bryophytes, ferns and small horsetails abounded in the undergrowth. The labyrinthodont amphibian shown in the foreground was typical of this early stock that spent much of its life in the water, but had the ability to walk on land. Resting on the fallen trunk is a member of the extinct 'griffenflies' (Protodonata) which included the largest insects that have ever lived and gave rise to the Odonata, the dragonflies and damselflies of today (Grimaldi and Engel, 2005). The biggest of all was Meganeuropsis permiana from the early Permian, which had a wingspan of approximately 71 cm. (Drawn by Peter R. Hobson.)

era (215-65 Ma) most important forest trees were gymnosperms, but the angiosperms became increasingly important from their appearance in the early Cretaceous 140 Ma (Fig. 1.1). It is often difficult to identify fossil wood, of which there have been many re-assignations in recent years. The pines (Pinus spp.) are now thought to have originated in the early-middle Mesozoic. Palaeozoic fossils formerly ascribed to this genus have been reclassified into extinct pinaceous genera including Pityostrobus and Pseudoaraucaria. The world's forests changed considerably during the Mesozoic era, when the dinosaurs roamed. The period when almost all the land vertebrates more than a metre long were dinosaurs was initiated by the Permian-Triassic extinction some 250 million years ago when 90% of all animal species, including the trilobites, were eliminated when a meteorite struck what is now Wilkes Land, Antarctica, making a crater 30 miles wide. Within 20 million years the archosaurs, whose descendants the crocodiles and alligators still survive, gave rise to the first primitive dinosaurs. Following another mass extinction 200 Ma, the dinosaurs virtually ruled the land for 135 million years, only to be exterminated when another meteorite struck Mexico at the end of the Cretaceous period, forming a crater six miles across at Chicxulub. Between these two extinctions caused by meteorite impacts, very large land and marine dinosaurs flourished and pterosaurs flew overhead. Tyrannosaurus rex and Baronyx, another flesh-eater 10 m long discovered as recently as 1986, both lived in the Cretaceous and were amongst the many carnivores preying upon herbivorous dinosaurs and other animals which lived in and around the Mesozoic forests. It is now possible to date the ages of both ancient and modern reptiles by examining the annual growth rings in their weight-bearing solid bones. Using this technique it has recently been shown that T. rex put on 4 tonnes in a teenage spurt between the ages of 14 and 18. Interestingly, modern elephants do much the same but live to 70, instead of dying at around 30 as did the tyrant lizard king, which also finally attained around 6 tonnes.

Subsequent changes amongst animal populations have been even greater. Today, one of the most characteristic features of woodlands is birdsong, yet the first bird fossil (Archaeopteryx), with its toothed beak, huge eyes, expert co-ordination and feathers, arose from an early dinosaur line as late as 147 Ma near the junction between the Jurassic and Cretaceous periods. This animal is now known to have been capable of true flight; fossils of its precursors have yet to be discovered. Ground-dwelling dinosaurs appear originally to have grown feathers for insulation and warmth and only later did the various lines gradually develop the ability to glide or fly. Microraptor gui (128-124 Ma), which was about 60 cm long, used its four feathered limbs and tail to glide from tree to tree in much the same way as the modern flying squirrels (all of which are nocturnal) employ the membranes between their front and hind legs. Being warm-blooded (homoeothermous) gives both birds and mammals the advantage that they can remain active under cold conditions, their body temperatures being both high and constant. The other groups of vertebrates are coldblooded (poikilothermous), with temperatures only slightly above that of the surrounding water or air, one of the reasons why snakes in cooler countries hibernate in winter and sun themselves in summer.

The most primitive of the three major groups of mammals are the mono-tremes, which derive their name from the fact that a single tube - the common cloaca - is used for reproduction, urine excretion and the egestion of faeces. The species that persist today are very specialized and unusual, but J. Z. Young's (1950) prediction that fossil evidence regarding the origin of this ancient group would eventually be discovered has been justified by the finding in the Australian Cretaceous of jawbones of species with a dentition similar to that of the juvenile modern monotremes. Modern placental mammals appeared in the Cretaceous, a period which ended 65 Ma with the extinction of 75% of all living species including the mighty dinosaurs. Dominance was now assumed by the mammals which increased rapidly in both diversity and size during the ensuing Tertiary (65-2 Ma). Recent changes have been just as dramatic. The most recent remains of mammoths discovered in western Europe were found in Shropshire in 1986. The animals had been caught in thick mud above permafrost which collapsed on melting into a kettlehole a mere 12 700 years ago.

Knowledge of mammals that could glide from tree to tree in the Late Jurassic or Early Cretaceous forests is very recent (Meng et al, 2006). The discovery in Mongolia of the fossilized remains of Volaticotherium antiquus (meaning ancient gliding beast) is one of the most important discoveries of mammals in the Mesozoic era for over a century. This animal, which weighed a mere 70 g, could glide using the membrane stretched between its arms and legs, and is so unusual that a new order of mammals (Volaticotheria) has had to be created in order to accommodate it. It steered itself with a long stiff tail and existed 130 million years ago, whereas the earliest previously known flying mammals were the bats (from 51 Ma) and the flying rodents (from 30 Ma). Its remains are of truly remarkable quality; impressions of the fur and part of the gliding membrane survive in the rock where it was found.

Isolated faunas are frequently of very different origins, but their members often show parallel evolution of features found in other groups in distant forest regions. Thus, the three remaining species of monotremes (the duck-billed platypus of Australia and the two spiny echidnas of Australia and New Guinea) like other mammals are warm-blooded and feed their young on milk, but lay eggs (though unlike those of reptiles, these develop in the mother's pouch). The tiny young of marsupials grow rapidly in the pouch of the mother, while young placentals (now dominant in most of the world's forests) are born at a very advanced stage. The island continent of Australia, so long isolated from the rest of the world, developed a most remarkable Pleistocene marsupial population including elephants, lions, large cow-like animals and a huge blunt-faced kangaroo which may have been 3 m tall. All these megafauna are depicted in cave pictures made by the aboriginal population 20 000 years ago in the last Australian ice age (Vandenbeld, 1988). The giant browser Diprotodon optatum, nearly 3 m long and 2 m high at the shoulder, was the largest marsupial that ever lived. The demise of this megafauna, as elsewhere, appears to have coincided with the rise of humans.

Though the present separation of New Guinea from northern Australia is relatively recent, their faunas have already diverged. The long-beaked echidna Zaglossus bruijni, which feeds on earthworms, is much larger than its Australian equivalent, the burrowing spiny anteater Tachyglossus auleatus. Moreover, far more of New Guinea's kangaroos live in the trees, where they move rather clumsily, feeding on the foliage and occupying a niche similar to that of many Old World monkeys.

9.1.3 The Cenozoic: Tertiary and Pleistocene forests

In the early Tertiary, which began some 65 Ma, there was a northern forest containing firs, birches, pines and poplars in what is now the Arctic Circle. The more varied temperate angiosperm forest to the south contained maples, buckeyes, horsechestnuts, chestnuts, beech, ash, walnuts, oaks, lime and elms together with evergreen conifers including Douglas fir, firs, hemlocks, neo-cypresses (Chamaecyparis) and spruces. At this time the break-up of the ancient supercontinent Pangaea, now understood in terms of tectonic plate theory, was at a relatively early stage and a land bridge connected North America and Asia. The straits between Greenland, Iceland and northern Europe were relatively narrow, and many tree genera were dispersed right across the northern hemisphere. The southward movement of the temperate Tertiary forest (shown in Fig. 9.2), which gave rise to four major forest regions that subsequently developed differently, was initiated by a drop in temperature towards the end of the Tertiary which culminated in the Pleistocene ice age. The upthrust of the Rocky Mountains in North America created the treeless rain shadow of the Great Plains and caused the formation of distinct western and eastern forests. The western forests of North America became dominated by conifers, while angiosperms remained abundant in the east. In Eurasia

Rainforests The Northern Hemisphere
Figure 9.2 Southward movement of the southern Tertiary forest to give the four distinct forest regions now found in the northern hemisphere. Stippling indicates arid areas. (From Gibbs and Wainhouse, 1986. Forestry, 59, 141-153, by permission of Oxford University Press.)

the low rainfall of the central area, which was caused by its distance from the sea, separated the eastern and western forest regions.

The present geological period is the Quaternary, which has had a duration of two million years and is divided into two epochs, the Pleistocene and the Flandrian, of which the latter has existed for only 10 000 years. The Quaternary began with the onset of the Pleistocene ice age and has been subject to repeated climatic changes. Toghill (2000, chapter 13) describes the sequence of 17 cycles of cold and temperate climates known to have occurred so far. The end of the successive glaciations of this was marked by a brief, relatively warm period known as the Allerod interstadial, during which the east-west alignment of the Alps and Pyrenees prevented the easy migration of trees and other plants to and from the refuges to the south of them. In eastern Asia, as in North America where the major mountain ranges run north-south, the major elements of the rich Arcto-Tertiary forests retreated south along continental migration routes, rather than being destroyed by ice as were the complex forests of western Eurasia. Even at the end of the Pleistocene glaciations, however, at least one modern species of many tree genera existed in all four major forest regions. These have continued to evolve together with their associated fauna, flora and microbial populations including pests and pathogens. The effects of moving these last two groups from one major region to another can be very serious and are considered in Sections 5.3 and 5.4. Today the magnificence of the Arcto-Tertiary forests is approached only by the mixed mesophytic stands of the southern Appalachians or eastern Asia, which are far richer in trees and understorey species than any other temperate forests. In the early Tertiary the tulip tree Liriodendron was widespread and included stands in Iceland and Europe; now it is restricted to eastern China (L. chinense) and south-eastern USA (L. tulipifera).

Many of the trees, herbs and animals of the Pleistocene interglacial forests were similar or even identical to those which exist now in the current interglacial. Angiosperm trees including birches, aspen, willow, hazel, beeches, elms, oaks and maples were prominent but the firs, pines and spruces continued to dominate many large areas, particularly those that were colder and more exposed. One of the biggest differences was in the animals; the adaptive radiation of the mammals, which at the beginning of the Mesozoic had been only a small group of mammal-like reptiles, was astonishingly rapid during the Tertiary, persisting into the Pleistocene and up to the present day. Pollen diagrams for the last three complete British interglacials, the Cromerian, Hoxnian and Ipswichian (we have yet to reach the next ice age so the current interglacial is as yet incomplete) give us an understanding of the plants, and consequently the climates, which prevailed. At the start of each interglacial there was a gradual rise in temperature which caused the ice sheets to wane and the sea level to rise, reached a peak in the climatic optimum, and then declined. Precipitation increased at the same time, often remaining high for most of the interglacial. Towards the end of the interglacial, heavy leaching took place under cold conditions in which chemical weathering proceeded very slowly. Mineral nutrient levels in the soil dwindled, particularly in upland areas, and did not rise substantially until warming took place in the next interglacial. Plants which escaped the glaciers in elevated areas or survived in refuges to the south and in the case of Britain, to the east, accomplished recolonization of areas devastated by ice.

Interglacial forests contained an astonishing variety of birds and mammals, but some of the latter, particularly the elephants and closely related mammoths, were much larger than those which exist today. The woolly mammoths which died out 10 000 years ago were related to mastodons which also died out at the end of the last ice age. Male mastodons weighed six tonnes each; recent evidence shows that they fought duels to the death using their curved tusks to crush areas of their opponents' skulls. There was also a gradual impoverishment of the tree flora as glaciation succeeded glaciation. Both Norway maple (Acerplatanoides) and Norway spruce were native to Britain in the Ipswichian, the last complete interglacial; silver fir (Abies alba) and Pterocarya (wingnuts) were present in the preceding Hoxnian, and even Carya (hickories) occurred in the early Pleistocene. A major climatic warming 10 000 years ago marked both the end of the Pleistocene, and the beginning of the Holocene (or Recent) epoch which still continues. In the last millennium the Little Ice Age which began in the mid-1440s, and ran until the mid-1800s, was succeeded by the present period of global warming (see Section 11.3).

9.1.4 Holocene changes and the origins of British woodlands

British woodlands and forests have, like those of almost the entire world, been subject to the increasingly intensive influence of humans for the whole of the Holocene and ancient semi-natural woodlands (land continuously wooded since AD 1600) are the closest approach to truly natural forests that remain; they now cover only about 2.3% of the British landscape. They form a cultural landscape that has often been cut over, frequently being used for coppicing (see Section 10.3). Recent woods are defined as having been planted or developed naturally on open vegetation at some time in the past 400 years. Semi-natural woods are defined as being predominantly composed of trees and shrubs native to the site which have developed from stump regrowth, as in former coppices, or have regenerated naturally from seed rather than plantings; they may be either ancient or recent. The terms old-growth and virgin forest are frequently used in North America and Europe, respectively, to denote forests which are natural or have been left virtually undisturbed, especially by logging, for very long periods. Subsequent changes to forest vegetation can often be very complex due to human intervention; an example is given in Box 9.1.

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