The Quaternary The Last 24 Million Years

Even before we humans began our grand experiment with greenhouse gases, we had always lived in a time of dramatically unstable climate. Such variability was unusual even against the standards of the changeable history of the earth. The impression of ever-lasting stability one might get from seeing the world over a few decades is an illusion: on the timescale of a several thousand years, the climate in any place in the mid-latitudes can plunge to near-Arctic temperatures, and then after a few hundred or a few thousand more years shoot back up again to be warmer than today. On the same timescale, smaller fluctuations in temperature can also chill the tropics, but more importantly for the ecology of these regions there are large fluctuations in rainfall. There have been several times in the past few tens of thousands of years when the tropics were far drier than today, and there were also times when they were much moister. All these changes must have had dramatic effects on the distribution of biomes, and the individual species within them.

450000 400000 350000 300000 250000 200000 150000 100000 50000 0

Figure 3.1. The global temperate history of the last 450,000 years showing a sawtooth pattern which appeared by 700,000 years ago. Temperature in °C compared with 1960-1990 baseline. Source: CDIAC.

450000 400000 350000 300000 250000 200000 150000 100000 50000 0

Figure 3.1. The global temperate history of the last 450,000 years showing a sawtooth pattern which appeared by 700,000 years ago. Temperature in °C compared with 1960-1990 baseline. Source: CDIAC.

Instability in the earth's climate started in earnest about two and a half million years ago, and since then it has intensified into broader, longer term temperature fluctuations. Over the last 700,000 years, these big swings in climate have tended to occur on roughly a 100,000 year rhythm (Figure 3.1).

Each pendulum swing in climate begins with a warm phase much as we are in now, lasting maybe 15,000 years. These warm phases are known as "interglacials", and the latest one that has so far lasted for about the last 10,000 years or so is called the Holocene. In each of the last several interglacials for which we have a good climate record, global temperatures reached an early peak and then slowly declined afterwards. Eventually, the rate of decline became faster and often ended in a sudden, dramatic plunge in temperatures around the world. After a few thousand more years there might be a partial recovery of temperatures, but this would be a very temporary respite. Within a few hundred or a few thousand more years it would plunge again, often further than before. The decline would continue as a halting, reversing trend— two steps forward, one step back—until eventually after tens of thousands of years the earth's temperature arrived at a low point, known as a glacial maximum. As well as being much colder than the present, these glacial maxima tended to be much drier on a global scale. All the time as the earth's temperature was declining, great ice sheets would build over North America and Europe until at the glacial maxima they covered the northern half of both these continents, reaching several kilometers in thickness.

After maybe 10,000 years in this glacial maximum state, the earth would then be seized by a sudden warming. It is thought that these warming phases often occurred over just a few decades, raising annual temperatures by 5, 10 even 20°C—depending on the geographical region—to begin the next interglacial. The ice would begin a dramatic meltback, but taking several thousand years to disappear completely, because of its sheer bulk.

This overall pattern in climate change is known as the "sawtooth" cycle, so-called because it begins with a sudden rise in temperature to a peak, followed by a much slower decline (Figure 3.1). It is thought that the sawtooth cycle, plus the many sudden jumps in temperature that occur within it, results from a complex series of amplifying factors within the earth's climate which magnify small triggering changes into something much bigger. Such amplifying factors are known as "positive feedbacks" (see Chapter 5). The underlying control on the timing of the 100,000 year cycle seems to be caused by a series of wobbles in the earth's position relative to the sun. These affect the relative proportion of sunlight that hits the earth's northern hemisphere during summer rather than winter, and this is ultimately thought to control temperature change through a complex assortment of amplifiers, some involving melting of snow and ice, others involving capture of heat by vegetation (see Chapters 5 and 6).

As recently as 16,000 years ago, during the most recent glacial maximum, our planet was a very different place. Seen from space, the outlines of continents would have been recognizable and yet oddly unfamiliar, because sea level was lower and land extended out for many kilometers. What are now separate land masses were joined together by plains that are now drowned below the sea. For instance, Alaska was joined to Siberia by low-lying land across the Bering Straits, and most of the islands of southeast Asia formed a single land area. Another striking difference about that time would have been the huge white ice sheets—ice as thick as a mountain range—covering Canada and northern Europe. And the land surfaces themselves, in areas that are now dark with dense vegetation when seen from above, would then have tended to be much lighter with the yellows, reds and browns of bare soil. The global climate at that time was colder and more arid, and for the most part regions that would naturally now be forest were covered by drier vegetation such as open woodland, scrub, grassland or desert (Figures 3.2, 3.3). The great forest belts of Canada, Europe, Siberia and eastern Asia were almost absent at that time, because the climate was too dry or too cold for any dense tree cover. In the tropics also, the large block of rainforest that covers central Africa seems to have been largely absent, and replaced by savanna. The great Amazon rainforest was fragmented and shrunken down to a smaller core area surrounded by savanna or scrub.

Although the world was generally a lot drier during the last glacial, a few areas were instead wetter. For example, the southwestern USA was much moister than nowadays, with dense scrub vegetation and deep lakes in areas that now have only sparse semi-desert and dry salt pans. The salt flats at Salt Lake City, Utah were part of a huge lake—Lake Bonneville—which stretched hundreds of kilometers through

i j Extreme desert

Figure 3.2. Distribution of forest vs desert, (a) present day and (b) last glacial maximum (18,000 14C years) compared. Source: Author.

i j Extreme desert

Figure 3.2. Distribution of forest vs desert, (a) present day and (b) last glacial maximum (18,000 14C years) compared. Source: Author.

the mountain valleys. The reason for the wetter climate in the American southwest at that time was that it was receiving the belt of rain-bearing winds (linked to the northerly part of the ocean gyre) off the Pacific that nowadays hits Seattle and Vancouver; this wind belt had been diverted more than 1,000 km farther south by the presence of the vast ice sheet that covered Canada and northern Washington State. Although weakened and giving much less rain than it does now, it was able to make a real difference to the ecology of the region.

In the mid-latitudes of Europe, above about 45°N, plants that nowadays grow above the Arctic Circle were common where there is now temperate forest. For example, the pollen of the Arctic rose, Dryas, turns up commonly in the muds of ancient European lakes from that time. Temperatures in the southeastern USA—in Tennessee, for example—were comparable with the climates we presently see at the border with Canada (Figure 3.4*). The cooling during glacial phases affected not just the high and mid-latitudes, but the tropics as well. There are numerous indications, from preserved pollen and ancient glacier limits, that the tropical lowlands were perhaps 5 to 6°C cooler on average than nowadays. On mountains in the tropics, vegetation zones were moved downslope by about 1,000 m because of colder temperatures (although possibly complicated by effects of decreased CO2 on plants; see Chapter 8).

During the glacials, the difference was not simply that biomes changed places. Some biomes that are widespread today possibly did not exist at all during the last ice age. One example may be the lowland tropical rainforests: even just 15,000 years ago they apparently did not occur anywhere in a truly modern form. Pollen records extracted from lakes show that tropical mountain species of trees lived mixed in together with the lowland rainforest species of today. For example, in South America, South-East Asia, and Africa, the coniferous tree Podocarpus that is now usually only found high on mountain slopes was present down close to sea level. Does that make the vegetation different enough to put these forests in a different biome from tropical rainforest? Some ecologists would say it is, others would say it is not, because biome categorizations are always to some extent subjective.

Conversely, some biomes which were widespread during the last ice age do not exist today, or at most they only barely exist. An example of such a "vanished" biome is the steppe-tundra. This open and rather arid vegetation type covered most of northern Eurasia and parts of North America during the last ice age. It combined tundra and steppe plants that do not normally grow together nowadays, plus others that are nowadays more typical of sea shores. Ecologists have wondered what caused this strange combination to prevail over such vast areas. Was it due to peculiar climates at that time—types of climate which no longer exist? Or perhaps it had something to do with the effects of low CO2, bending the ecological requirements of plants so that species that now live in quite separate environments could grow side by side (see Chapter 7)? A type of vegetation rather resembling the steppe-tundra does actually still occur as isolated patches on south-facing slopes in the mountains of northeastern Siberia; at least, it combines a more limited subset of steppe and tundra plants, with some extra species that apparently did not occur in the glacial steppetundra. What makes for this vegetation is a combination of short but warm and dry summers, with extremely harsh winters. This might have been the sort of climate that was much more widespread during the last glacial. But, once again reflecting the slippery nature of biome categories, many ecologists who study the ice ages do not accept that this eastern Siberian steppe-tundra is the same biome as the steppetundra that once covered northern Eurasia.

* See also color section.

Europe Biomes Ice Age Biome During Last Glacial Maximum

Figure 3.3. Biome distributions of Europe, North America at the present day (a, b) and last glacial maximum (22,00014,000 14C years ago) (c, d). Part (d) is a map of the result of a collaboration between the author J. Adams, A. Beaudoin, O. Davis, P. and H. Delcourt, and P. Richard Source: Author.

H r j'j'j i1 i [.l,j In: .. j- ■_>1 (i.l. h there were no u ijI

H r j'j'j i1 i [.l,j In: .. j- ■_>1 (i.l. h there were no u ijI

Figure 3.3. Biome distributions of Europe, North America at the present day (a, b) and last glacial maximum (22,00014,000 14C years ago) (c, d). Part (d) is a map of the result of a collaboration between the author J. Adams, A. Beaudoin, O. Davis, P. and H. Delcourt, and P. Richard Source: Author.

Map Europe 20000 Years Ago

Figure 3.4. (a, b) Temperature zones in the USA for the last glacial maximum 20,000 years ago and the present day compared. Climates now associated with the border region with Canada (lighter grays) came down south as far as Tennessee and North Carolina at that time. Source: Author, with William Hargrove.

3.3 BIOMES IN THE DISTANT PAST

In the distant past, millions of years ago, both biome distributions and the types of biomes which existed were far more different from now. For example, 55 million years ago the world seems to have been much warmer and much moister than it is now. There was forest almost everywhere on land—even at the north and south poles—and no desert or dry grassland existed anywhere, apparently (at least no-one has found evidence of them). The warmth of this time was so dramatic that subtropical palms and alligators occurred as far north as Spitsbergen Island, well within the Arctic Circle. If tundra occurred anywhere then, it must have been confined to the tops of very high mountains.

Still further back in time, before the rise of the flowering plants about 120 million years ago, there could not have been quite the same "tropical rainforest" that we see today. In its place, in the wet climates close to the equator, various sorts of conifers as well as other gymnosperms such as ginkgo trees seem to have made up the tropical forest of the time. There were drier climates then, but no grasslands, because grasses had not yet evolved. Their place was apparently taken by ferns that may have grown in extensive savanna-like meadows.

3.3.1 Sudden changes in climate, and how vegetation responds

It used to be thought that all climate change in the geological past was very slow, taking thousands or even millions of years. With more detailed understanding of climate indicators in the geological record, we now know that many past climate changes occurred extremely rapidly. During the glacial-interglacial cycles of the past couple of million years, it seems that climates in the mid-latitudes often took just a few decades to switch from near-Arctic to temperate conditions, or back again. For instance, in the mid and high northern latitudes, the sudden global warming that occurred at the end of a cold phase known as the Younger Dryas 11,500 years ago seems to have been largely completed in under a century, with most of the change occurring in less than 50 years. Some geologists who work on this timeframe suggest that most of that change actually occurred in under 5 years. Similarly abrupt changes in temperature and rainfall may also have occurred in the past in the low latitudes: for example, in the Saharan and Arabian deserts according to some interpretations of data from sediments.

If and when climates changed so suddenly in the past, how long did the biomes take to move? We can get clues to the speed of change in vegetation from the pollen record preserved in lake and sea floor sediments. Each particular species or genus of plants tends to have its own distinctive-looking pollen grains. Thus, when a particular type of tree or herbaceous plant moves into an area it is possible to pinpoint its arrival from looking at the preserved pollen in sediments. The best evidence is heavily biased towards the mid-latitudes of the northern hemisphere, where there are a high concentration of botanists, a relative abundance of research funding and conditions favorable to preservation. Data from hundreds of sediment cores in North America, Europe and eastern Asia gives a general picture of how the temperate and boreal

I 15-20% I 120-40% >40% Laurentide Ice Sheet

Figure 3.5. Maps of migration rate of eastern North American trees (a) spruce {Abies) and (b) oak (Quercus) in the pollen record, starting from the last glacial maximum (ka = thousands of years ago). From Davis et al. (1988).

forest biomes migrated and changed after the last glacial ended. It is possible that some of the general lessons learned from these regions also apply to vegetation change in the tropics after the last glacial (Figure 3.5).

Looking at the pollen evidence from the mid-latitudes, one thing that is clear is that the new biome distributions suited to a changed climate do not snap into place overnight. In many areas the sudden warming phases seem to have left vegetation way behind, so that it took hundreds or even thousands of years to catch up. For example, just before a sudden warming event 14,500 years ago (known as the Late Glacial Interstadial), the climate of England had been much colder than it is now. The summers were too cool and dry for trees to grow, so there was a tree-less tundra vegetation (except perhaps a few shrubby clumps of birch—Betula—in moist valley bottoms). Suddenly, the warming event hit (Figure 3.6), and we can see the change by an influx of warm climate beetle species and plankton, as well as more direct chemical indicators of temperature which turn up in the lake sediments. It seems that in almost no time at all, insects that can live in open grassy vegetation spread northwards from where they had survived in southern Europe.

Warmer-climate snails, which we might think of as being slow-moving, turned up only a few decades later than the insects. But the trees that could have thrived in this climate remained absent for several hundred years. Forest had still failed to arrive when eventually another sudden cold snap occurred around 12,500 years ago

Figure 3.6.

Temperature history of the late glacial. There were two main sudden warming phases, separated by a cold phase known as the Younger Dryas.

Figure 3.6.

Temperature history of the late glacial. There were two main sudden warming phases, separated by a cold phase known as the Younger Dryas.

years before present

(Figure 3.6), making the climate too cold for forest once again. What had existed in the meantime before this cooling was a meadow-like grassy vegetation, consisting of just a few tundra and weedy grassland plants that could also grow well under the warmer summers. This was apparently a climate perfectly suited for trees, yet it was a landscape almost without any trees!

The reason for this delay in the arrival of forests may have been that before the warming, the only populations were more than a thousand kilometers away in southern Europe, mostly as scattered woods clinging to rainy mountainsides and moist gulleys. The rarity with which they show up in the pollen record of the cold phases shows just how restricted the populations of many common European tree species must have been at the time.

It simply took a long time for these tree populations to begin to disperse outwards from their glacial-age refuges, and establish extensive populations that could then send seeds further on their way. Something that slows trees down particularly is that it takes quite a few years for them to mature to the stage where they produce seed, so each little "hop" northwards tends to take decades.

One might expect that tree species with light, wind-dispersed seeds would have spread north fastest in Europe and North America. In fact, there is no sign of this difference in seed characteristics showing up in the migration speeds recorded in the pollen record. Some light-seeded trees spread relatively fast, others much more slowly. Exactly what caused the differences in rates of spread is not certain. Some of the heaviest-seeded trees that rely on animals to help spread them were also amongst the fastest to spread (e.g., the hazel tree, see below). It is uncertain whether trees sometimes made huge leaps of tens of kilometers in a single generation with a single "lucky" dispersal event, such as a bird carrying an acorn a very long way before accidentally dropping it.

It has been suggested that prehistoric humans played an unintentional role in dispersing many of the European trees that have edible seeds, such as beech, hazel and some oaks. People could have gathered nuts and then migrated many kilometers in search of game, accidentally dropping the seeds along the way and allowing such sudden jumps in ranges of trees. Some studies by Oxford University anthropologist Laura Ravel and colleagues of Amazonian forest Indians in the present-day world suggest that they often plant seeds of useful trees from the rainforest along trails or out into forest patches in the savanna, to ensure that the products of the trees are always on hand. If this sort of deliberate planting occurred in the past, it could have aided the spread of trees out from their refuges after the last ice age, in both temperate and tropical environments. An example of a tree that could have been spread this way is the hazel tree in Europe, which perhaps explains why it turned up so early in eastern England.

The influence of humans on the world's vegetation might thus be even more pervasive than we would expect, extending back even to the broad-scale migrations of biomes after an ice age. Whole continents could actually turn out to be gardens, at least in a very loose sense.

Some geologists who have studied the post-glacial movement of trees suggest that the delays in migration in northern Europe were actually dictated by the climates of the time. Although warmer, the climate just after the end of a glacial may have been quite hot and dry in summer—too arid for trees to establish. However, this does not seem to tally with the evidence for fossil beetles and other invertebrates which indicate a moist, warm and mild climate even while trees were absent.

An additional factor that may have slowed down the migration of trees northwards in Europe was a lack of the symbiotic mycorrhizal fungi that trees need to help them take in nutrients, especially in rocky, newly colonized soils. Often when they are planted on mine tailings, trees do much less well if they lack these fungi. In the landscape that followed on from these warming events, there were actually a few shrubby birch (Betula) trees that had been confined to moist valley bottoms during cold phases. Even during several hundred years in the new warmer, moister climate of the interglacial, they steadfastly refused to spread out across the landscape, and lack of mycorrhizal fungi seems one possible explanation for this.

Something that is obvious from the pollen record after these past warming events is that each tree species migrates in its own individual way, not depending on other types of trees being around it. In both Europe and North America, different species of trees spread out from their glacial-age refuges at very different rates, the slowest arriving thousands of years after the fastest. They also took quite different routes that probably reflected chance dispersal events, and also the locations of the source areas where each had survived the last glacial. Some ecologists have also suggested that the different routes trees took were individual responses of species to changing combinations of climatic parameters, each species adjusting its range to the climate that suited it at the time.

After the most recent big warming event 11,500 years ago marking the end of a cold phase known as the Younger Dryas (Figure 3.6), trees gradually spread northwards from where they had been surviving in southern Europe. They eventually arrived in northern Europe where they blanketed the landscape in forest. But, it was a matter of chance which tree species got there first, and the earliest arrivals often

Figure 3.7. The hazel tree (Corylus avelana), shown here with its distinctive flowers known as "catkins", blanketed much of eastern England after the end of the last ice age. This seems to have been due to sheer luck in arriving before other trees.

initially multiplied to form great forests of only one type of tree. For example, after this warming event the hazel tree Corylus avelana (a small nut-bearing tree; Figure 3.7*) initially formed great uninterrupted forests in eastern England. The most reasonable explanation for this is that hazel was simply the first tree to arrive from the south following the change in climate. After dominating for hundreds of years, it began to lose out as other tree species arrived and began to compete with it. Nowadays, hazel is still a common tree in northwest European forests, but always mixed in with and overtopped by other species of trees.

Although there was an element of unpredictability in terms of where and when each tree species turned up as the ice age ended, there were some kinds of trees which seem to have been destined to spread in early and play an important part in the forests. For example, the pollen records show that around 10,000 years ago Scots pine (Pinus sylvestris) and silver birch (Betula pendula) spread quickly across northern Europe, and in most areas they became the major trees of the first forests of our present interglacial. It is thought that in part these two species did so well at first because they were good at getting to the right place at the right time. Both were able to disperse their seeds on the wind. However, many other wind-dispersed species were actually much slower in migrating, so it is uncertain how much this helped them. Perhaps more important is that pine and birch had already been surviving quite far north in Europe as isolated groves of trees during the glacial time, because they are both very cold-tolerant—so they did not have too far to move when the climate warmed. Another reason for their early success, though, may be that pines and birches were good at surviving on the relatively shallow, poorly developed soils of lands that had previously been ice-covered or barren steppe-tundra during the ice age. In most areas, within a few thousand years other tree species arrived or became more abundant, steadily pushing out the earlier colonists. In England, Scots pine even seems to have gone completely extinct before it was reintroduced as trees planted by humans. In other areas of colder climate (northern Russia and Scandinavea, and high mountains around Europe) both pine and birch are still an important part of the forests, because the other tree species that might outcompete them cannot live in such cold. The fossil pollen records of previous interglacials show that there was always a similar pattern of early abundance for birch and pine in northern and central Europe, followed by a decline after a few thousand more years. This seems to make the point that this broad-scale pattern of succession was not just chance: it was the inevitable result of certain underlying ecological factors.

It is a moot question as to whether some trees are still migrating out from their glacial refuges to fill their potential ranges. Certainly, once forest has established across a broad area, this is likely to slow down the migration rate of any new species arriving, because large competitor trees dominate the space where seedlings of the new arrival would otherwise be able to establish. After the last major warming event 11,500 years ago, the trees that migrated certainly seem to have spread north a lot faster through western Europe—which had previously been a barren, tree-less land-scape—than through eastern North America which retained a covering of trees throughout the last glacial. However, since most trees in both regions stopped moving north several thousand years ago (or have even retreated slightly south since then), it is thought that they have probably reached the boundaries of their own climatic limits.

Even when a species of tree arrived in a particular area, it only became an important part of the forest after its population had had a chance to multiply up. In eastern England after the last big warming the pollen record from lakes suggests that tree populations doubled every 31 to 158 years, depending on the species (Figure 3.8). It took several doubling times (varying from several centuries to a couple of thousand years) before each reached a roughly stable "plateau" level of abundance. Interestingly, a rare piece of information from tropical tree com

Figure 3.8. Increase in pollen abundance of Scots pine (Pinus sylestris) in eastern England after the end of the ice age, indicating an expanding population of the tree after it colonized the area.

munities—the Queensland rainforests of northern Australia—indicates very similar doubling times for tree populations following the global moistening of climate that occurred in the early part of the present Holocene interglacial.

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