Knowing that biomes are in a general way related to climate, ecologists have wondered if it is possible to predict which biome will occur in a particular place, 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 ecologists to look both forwards and backwards in time. They can be used (1) 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 "biome indicators'', or conversely to reconstruct past biome distributions from certain climate indicators.
Perhaps the earliest serious attempt to express how climates relate to vegetation was by the German climatologist Vladimir Koeppen (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 Koeppen 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 warmer 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, Koeppen's scheme does broadly predict types of vegetation that will occur in different parts of the world, ecologists 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. He 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 heat. 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 place 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 Holdridge'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 are 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 climate that is —30°C for six months of the year 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 Holdridge'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, Holdridge'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 2.24) 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 are really more important, and also because different types of soil, exposure, relief, and many other geological and geographical factors strongly influence vegetation. Partly this must be because Holdridge'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 Koeppen, Holdridge and others to come up with schemes that are 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 biome categories using the vegetation-climate scheme. The result 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 BI0ME3 scheme. In their major aspects, such schemes tend to resemble Koeppen or Holdridge'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 BI0ME3 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 Koeppen 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.
Critical Temperature Line
Figure 2.24. Holdridge's predictive scheme for relating biomes to climate.
Critical Temperature Line o
Figure 2.24. Holdridge's predictive scheme for relating biomes to climate.
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.
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