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Average minimum January temperature fC]

In the deciduous forest regions in the mid-latitudes of the northern hemisphere, a green wave of leafing out can be seen sweeping north on satellite images as spring temperatures warm up. The relative timing of this green wave follows the climate so closely that it can be predicted using a simple mathematical formula based on winter temperature (see Figure 2.18). At the northern end of the temperate deciduous biome, leafing out occurs months later than in the south, even in the same species of trees. A similar green wave occurs on the outer edges of the tropics as the monsoon rains move gradually out from the equator.

Leaves must unfurl rapidly to take full advantage of the temperate zone spring, but they must be able to do it without tearing. Most leaves in cold climates have teeth or lobes at their edges (Figure 2.19*), and it has been suggested that this feature may help them to avoid getting torn as they open. Or it may be that the thin leaves of deciduous species need to flex in the wind without tearing when they are fully grown, and the teeth may help them to do this. Another possible explanation for the toothed leaves is that the presence of teeth helps to promote gas exchange for the rapidly photosynthesizing leaves in the spring when CO2 supply is limiting and evaporation rate is low, due to the relatively cool temperatures. Some types of trees that occur across a wide range of temperatures even show the same temperature-related trend in leaf shape within a species. For example, leaves of red maple populations that come from cold climates in eastern Canada are "toothier" than those from Florida

Figure 2.19. Toothed or lobed leaves are far more prevalent in cooler climate forests. One example is beech (Fagus grandifolia) in North America, which has small teeth along the edges of its leaves. Source: Author.

(see Figure 2.20), with a gradation in leaf shape with latitude between these two extremes.

Even though the underlying reasons are not well understood, the relationship between average temperature and the prevalence of toothed leaves is very predictable. The cooler the climate, the higher the proportion of trees in the local flora that have teeth on their leaves (Figure 2.21). This relationship is so predictable that geologists use fossil leaf floras as a thermometer for the climates of particular regions in the geological past.

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