Predicting Where Vegetation Types Will Occur

Knowing that bionics arc in a general way related to climate, ccologists have wondered if it is possible to predict which biomc will occur in a particular placc, using some simple set of rules based on climatic conditions. As well as providing a satisfying explanation of the present-day world, these predictive schemes are useful in enabling ccologists to look both forwards and backwards in time. They can be used (I) to predict how biomes will shift in the future in response to human disruption of climates (e.g., under global warming due to the "greenhouse effect"; see Chapter 3), and (2) to reconstruct past climates from fossil "biomc indicators", or conversely to reconstruct past biomc distributions from certain climate indicators.

Perhaps the earliest serious attempt to express how climates relate to vegetation was by the German climatologist Vladimir Koeppcn (1846-1940). who presented his global scheme in 1918. Koeppen noted that particular types of vegetation (biomes, essentially, though he did not use this term) are associated with particular climates, such that a map of vegetation can more or less be predicted from a map of climates.

The sort of feature that Koeppcn used to demarcate a climate zone was the mean rainfall, and the extremes of monthly temperatures. The tropical zone, for example, included areas with every month of the year on average wanner than 18 C. Polar climates, by contrast, had a mean temperature for the warmest month of less than 10 C. Using formal rules like these, Koeppen marked out several very broad ecological zones which had different combinations on the scale of warmth and dryness. So for example he distinguished zones of "wet tropics" and "dry tropics". He also recognized that the distribution of rainfall during the year was very important. For example, one of his major categories is for areas with a Mediterranean climate—a marked dip in rainfall during the summer and plenty of rain during the cool winter. Mediterranean climates occur in several parts of the world and tend to have similar-looking vegetation and even closely related genera of plants between the different places.

Although, in many respects, Kocppen's scheme docs broadly predict types of vegetation that will occur in different parts of the world, ccologists were aware of its imperfections. In many areas, what Koeppen's scheme would predict does not quite match what is seen on the ground. These mismatches prompted others to try to come up with schemes for linking climate and vegetation, which used slightly different features of climate chosen from a consideration of what would really matter in the ecology of plants.

In 1967 an American ecologist, L.R. Holdridge, put forward a rather different scheme that incorporated the balance between precipitation and evaporation, lie wanted to emphasize that in a warm climate a certain amount of rainfall goes much less far in terms of keeping plants alive, because evaporation is so much stronger in the heal. The net "water balance" is surely what really determines whether a plant experiences drought, and the sorts of plants that will be able to survive in a place. As an example, lowland England which has a notoriously damp climate has an annual rainfall of around 700 mm. This is enough to sustain closed forest vegetation and to keep lawns green year round. Yet an area in equatorial Africa which has this amount of rainfall will be a dusty, dry placc most of the year, with only an open scrub vegetation. In the much warmer climate in Africa, water evaporates faster and so more rainfall is needed to keep things moist. The important difference is not the amount of rainfall, but how rainfall compares with the temperature, and Iloldridge's scheme recognized this.

Holdridge also emphasized that, in terms of judging the favorability of the climate to plant growth, only temperatures over a certain threshold should really matter. Holdridge drew the line at 0 C; he suggested that we should not bias temperature averages during the year by counting anything lower. Below that threshold level, plants arc essentially dormant, so we can ignore those parts of the year—no matter how cold—because it makes no difference. So. for example, one might have a climatc that is —30 C for six months of the vcar and 30 C for the other six months of the year. This would have a mean temperature of 0 C, implying that just about no plant could grow there, yet as a matter of common sense we know there would be forest vegetation able to thrive in the warm temperatures during half of the year. Taking a simple yearly average would obviously be a misleading way of classifying the world in terms of vegetation and it would have much less predictive value. In Iloldridge's scheme, months below 0 C on average default to 0 C, and the average temperature derives only from the "important" temperatures, which are those above freezing. With these sorts of refinements, Iloldridge's scheme did rather better than Koeppen's scheme at "predicting" vegetation based on climatic rules.

Holdridge also came up with his now-famous "triangle" (Figure 2.19) with three axes of classification in terms of climate. The world's vegetation types were arranged like cells in a honeycomb within this triangle, each with its particular range of temperature and water balance. While visually appealing and easy to read off. it would be surprising if the world's vegetation types neatly fitted in this way on the diagram in a perfect geometric pattern! Although it does better than Koerner's scheme, the Holdridge model has not been found to be very practical at predicting in many parts of the world. It seems that vegetation does not always follow the rules, perhaps because other climatic factors arc really more important, and also becausc different types of soil, exposure, relief, and many other geological and geographical factors strongly influence vegetation. Partly this must be because Iloldridge's scheme does not recognize patterns in the seasonality of rainfall or temperature, which can be all-important (e.g., does the rainfall all come in glut in part of the year, leaving the rest of the year dry?). In this sense it is more limited than Koeppen's old scheme. Nevertheless, the Holdridge scheme is still quite widely used because of its familiarity, and one often sees maps of "Holdridge vegetation-climate zones" presented for particular parts of the world.

Many ecologists have tried to build from the legacy of Kocppcn, Holdridge and others to come up with schcmcs that arc better at predicting vegetation from climate. These schemes are particularly useful when it comes to predicting how the ecology of the world may look in the future under the global warming of an increasing greenhouse effect. Basically, one generates a climate for the high C02 world on a computer, and then slots in the biomc categories using the vegetation climate scheme. The result

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Hgurc 2.19. Holdridgc's predictive scheme for relating hiomes to climate.

Critical Temperature Line

Hgurc 2.19. Holdridgc's predictive scheme for relating hiomes to climate.

to is a vegetation map for the future-changed climate, that one can compare with the present-day vegetation.

Examples of vegetation schemes that are used by modelers include the aptly-named BIOME3 scheme. In their major aspects, such schemes tend to resemble Kocppen or lloldridgc's schemes, but they have many minor refinements in terms of where the boundaries are drawn. In many cases, what has been altered in these latest schemes are the lowest temperatures that occur in local climate records, which appear to predict the limits of certain growth forms of plants. For instance, in the BIOME3 scheme, the "tropical rainforest" biome is said to be limited to areas where the mean temperature of the coldest month is above 15.5 C (rather than 18 C as Kocppen suggested). The explanation put forward for this is that 15.5 C is closely correlated with the real factor that limits where tropical rainforest can occur: the occurrence of occasional frosts on the time scale of decades. These frosts cannot be tolerated by many typical rainforest tree species, so their distribution limit (and the drip tips and buttress roots that go along with them) ends there.

Though they are useful, generalized bioclimatic schemes such as BIOME3 can never get it completely right. The vegetation environment relationship is just too complicated to be completely predictable. Also, it is important to remember that such schemes are ultimately based on people just looking at biome maps drawn by ecologists, and choosing something in the climate that seems to correlate well with these limits. Although that choice may have a reasonable plant physiological basis, it is ultimately only chosen because it corresponds to what is on the map. In the case of tropical rainforest, the limit is drawn at 15.5 C, because that is the temperature at the point where, in vegetation maps of southwestern China, tropical rainforest reaches its most northerly point in the world, at 26 N. Although the vegetation boundaries on a map might look as if they are beyond dispute, the real world tends to be much more complicated. In many areas of the tropics (including southern China) the equatorial forest grades almost imperceptibly over hundreds of kilometers into the vegetation of cooler and drier climates—with drip tips and buttress roots becoming progressively less common—so that there is no single point where one can truly objectively say that equatorial forest ends and another vegetation type begins. For example, many ecologists would disagree with the idea that the southwest Chinese evergreen forest is really tropical rainforest at all. In order to make sense of the world, it is necessary to chop it up into neat categories such as biomes. But we should also bear in mind that even the maps that bioclimatic schemes are based on are somewhat subjective.

2.18 SPECIES DISTRIBUTIONS AND CLIMATE

Each biomc is made up of many species of plants, and each one of these species has its own particular "distribution range", the area in which it grows naturally. Sometimes, towards the edge of a biome a lot of plant species seem to reach their limit at just about the same point. For example, in eastern North America, quite a few species of deciduous trees die out at the southern edge of the boreal forest in Canada. This is not surprising, because there must be a point along a temperature gradient at which the strategy of being deciduous is no longer quite so viable. Thus a lot of trees that use this strategy will tend to reach their limit at about the same place.

However, most of the species of plants present in each biomc have their own idiosyncratic distribution ranges that do not show much relationship to the boundaries of the biomc. In many cases the spccics range boundaries do seem related to aspects of climate, though not necessarily the same factors that define the edges of the biome.

Often, if one plots the distribution limits of a particular species it turns out to correspond quite closely to a climatic parameter such as the mean temperature of the warmest summer month, the annual rainfall or the yearly minimum winter temperature. It is perhaps too easy to keep trying different climate parameters until one "fits", but often the correlation between a parameter and the spccics range limit is so striking that it is hard to believe that it could be just coincidence. It seems that beyond a certain extreme of temperature or rainfall conditions, each species of plant is physiologically unable to survive (e.g., it cannot survive the frosts, or the summer drought, etc.). But the tolerance limits vary greatly between different species of plants, according to their own anatomical and physiological peculiarities.

In parts of the temperate latitudes, many plant spccics have what is called an "oceanic" distribution pattern that roughly follows coastlines, even though they may-extend inland several hundred kilometers away from sea shores. The oceanic distribution tends to occur because these are plants that do best under cool summers and/or mild winters, in the climates which result from the moderating influence of the ocean. One example of an oceanic species is the ivy, Hedcra helix. which is concentrated along the western fringe of Europe. Another is the strawberry tree (Arbutus unedo) which occurs along the extreme western fringe of Ireland, Spain and Portugal, and also around close to the Mediterranean Sea.

Moving farther inland, the "oceanic" species drop out and are replaced by-certain other species which seem to thrive under the hotter summers, colder winters and lower rainfall conditions. These are known as "continental" species, because they are associated with the more extreme continental climates. In England, continental species of wildllovvers—whose ranges tend to extend to the steppe environments of Ukraine—are found in meadows in southeastern England, especially on warmer drier south-facing slopes and on sandy soils which tend to imitate the warm droughty conditions of the steppe grasslands. In the wetter, cooler west of England, these species are absent. An example of a continental species in Europe is the stemless thistle, Cirsium acaule, which towards the more oceanic northwestern limits of its range is confined to the warmer, drier more "continental" south-facing sides of hills.

The individual shapes of ranges cannot always be put down to climate. They also seem to be affected by soils, and in some cases chancc aspects of history such as where that particular species managed to survive during glacial times and how far it managed to disperse out of these refuges before reaching topographic barriers (sec Chapter 3). An example of this historical effect from Europe is the purple-flowered rhododendron. Rhododendron pontiewn. It thrives when introduced to Britain and Ireland, and has escaped to fill many woodlands there, yet its natural range was confined to the mountains of southern Spain, the Balkans and Turkey. The same plant turns up as fossils from an earlier warm period in Britain, so we know that it once grew there too. It seems that Rhododendron was pushed back by ice age cold and aridity and then never managed to regain its former range, largely due to bad luck by being hemmed in by areas of unsuitable climate. It was only when humans helped it out by importing it as a garden shrub that Rhododendron politician managed to make the leap to favorable climates in northwest Europe.

2.18.1 Patterns in species richness

When the ranges of individual species are superimposed on one another and countcd up, striking patterns in the total numbers of species become clear. Spccics richness, as it is called, tends to be greater at the warmer end of each biomc in the mid and high latitudes, and in the wetter parts in the tropics. In general, there is a strong trend towards more spccics of trees in forests at lower latitudes. This trend is most obvious in eastern Asia where the climate is uniformly moist from north to south and the only major trend in climate is in terms of temperature (Figure 2.20). Some areas of the world show trends related to both temperature and rainfall: for example, the species richness of the deciduous forest in eastern North America which increases towards the south but also dccrcases into the dry interior of the US.

No-onc is quite sure why spccics richness tends to be higher in warmer and moistcr environments. A range of hypotheses have been put forward during the last 150 years, but each of them starts to look paradoxical when examined in detail.

One popular idea amongst ccologists notes that the latitudinal difference in tree species richness correlates strongly with net primary productivity, the growth rate of vegetation. According to this idea, if there is a bigger "cake" of resources enabling and resulting from faster growth, there is more chance for species each to take their own "slice" (or niche). However, when we look in detail there is not really much evidence that species are on average more specialized in species-rich environments than in species-poor environments.

Another idea suggests that, because the world was nearly all warm and moist around 50 to 60 million years ago when the flowering plants were busy diversifying, most lineages became fundamentally adapted to living in the tropics. Over more recent time, the cold and dry environments that have become much more widespread have presented a new challenge that few lineages of plants have been able to adapt to. If this is the case, surely we would expect to see the levels of botanical richness of the high latitudes increasing in the fossil record, as more groups of plants overcame these barriers. Also, the earliest groups of plants that made it out into colder and drier environments should have been busy diversifying into more and more forms over time. Yet. in the fossil record we see almost no signs of such a build-up in diversity. Essentially the same groups of plants have been important for the past 30-40 million years in the eolder temperate forests, with nothing much added. Contrary to the expectations of this hypothesis, diversity in the temperate forests has if anything declined somewhat over the past few million years (see below). Essentially, then, the causes of these grand geographical gradients in richness remain a mystery to ccologists.

Fossil Grid Sampling Map

Figure 2.20. Tree species richness map of parts of eastern Asia (eastern Russia. Japan, Taiwan). These are the numbers of wild tree species occurring per cell in a geographical sampling grid, based on published tree species range data. There is a very strong latitudinal gradient. Source: redrawn from Author.

Figure 2.20. Tree species richness map of parts of eastern Asia (eastern Russia. Japan, Taiwan). These are the numbers of wild tree species occurring per cell in a geographical sampling grid, based on published tree species range data. There is a very strong latitudinal gradient. Source: redrawn from Author.

Even if we cannot really explain latitudinal gradients, certain other broad-scale patterns in species richness can be explained more convincingly in terms of past events which destroyed most species of plants in some places but left many more to survive in others. For example, the tree flora of temperate eastern Asia is a lot richer in species than climatically similar parts of Europe and North America, even though all three areas show a strong underlying trend in species richness which parallels the average temperature. The reason for this difference between the regions may be the fact that during glacial phases over the past 2 million years, the climate in parts of east Asia stayed a lot moister than anywhere in Europe or the eastern USA.

Many drought and cold-sensitive types of trees that existed in all three regions before about 3 million years ago would have been able to survive in Asia, whereas they died out in Europe and North America.

In this chapter we have considered how vegetation is shaped by climate in a relatively static sense. Even when including the extremes of its changes, we have merely touched upon the ways in which plants might have moved from one place to another when climate shifted. Chapter 3 is devoted to these transformations in vegetation in response to climate.

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