Nutrients And Evergreenness

Evergreenness is not only determined by climate—soils can have a lot to do with it too. Part of the reason why boreal forests tend to have evergreen conifers may be that the soils underneath them are nutrient-deficient. Each time a plant changes its leaves, some nutrients fail to be re-absorbed before the leaves are dropped, and are lost. If nutrients are in short supply, other plants that keep their leaves will grow faster and overtop this plant, and their roots will also grow fast and grab even more nutrients first. Hence there is selection against dropping leaves unnecessarily where nutrients are scarce.

POLAR

TROPICAL

POLAR

Figure 2.22. Latitudinal bands of alternating evergreen and deciduous forest. Idealized arrangement of evergreen vs deciduous forest types on the earth's surface. In reality, this is complicated by non-forest zones, oceans, and climate and soil differences affecting the evergreen strategy.

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POLAR

TROPICAL

POLAR

Figure 2.22. Latitudinal bands of alternating evergreen and deciduous forest. Idealized arrangement of evergreen vs deciduous forest types on the earth's surface. In reality, this is complicated by non-forest zones, oceans, and climate and soil differences affecting the evergreen strategy.

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Treeless reen boreal Deciduous temperate

Evergreen warm temperate Deciduous tropical Evergreen tropical Deciduous tropical Evergreen warm temperate Deciduous temperate rgreen boreal

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EVERGREEN (WARM TEMPERATE)

EVERGREEN (BOREAL EVERGREEN)

DECIDUOUS (COOL TEMPERATE)

DECIDUOUS

(TROPICAL DECIDUOUS FOREST)

EVERGREEN (TROPICAL RAINFOREST)

EVERGREEN (WARM TEMPERATE)

Figure 2.23.

Map of eastern Asia with zones of evergreen vs. deciduous forest.

DECIDUOUS (BOREAL DECIDUOUS)

EVERGREEN (BOREAL EVERGREEN)

DECIDUOUS (COOL TEMPERATE)

DECIDUOUS

(TROPICAL DECIDUOUS FOREST)

EVERGREEN (TROPICAL RAINFOREST)

But why are the soils in the boreal zone nutrient-deficient? Partly because there are conifers! The conifers produce a nutrient-poor litter which gives rise to organic acids that cause leaching. To some extent it seems a chicken-and-egg situation, although the fact that there are short summers selecting for evergreenness is probably the underlying cause for conifers being present in the first place.

In other climate zones that have mainly deciduous forest, there can be patches or whole broad swathes of evergreen forest where nutrients are deficient. One example is the local areas of white sand forests in the tropics again, where the trees are holding on and keeping nutrients. In the southeastern USA, conifers (mostly pines, the genus Pinus) predominate on the nutrient-poor exhausted soils of the coastal plains. Where the soils are good (e.g. along the Mississippi river floodplain) deciduous trees outcompete the pines. Eucalyptus—the Australian "gum tree" genus of some 500 species that predominates across a full range of climates in Australia—is usually evergreen even in areas with a strong dry season. It has adopted the same strategy: "holding on" to its leaves come what may. This probably has something to do with nutrient-poor soils predominating across Australia.

Even in forest biomes, shrubs and herbaceous plants exist as "subordinates". Where trees dominate the vegetation, there is always a contingent of herbs and shrubs to get in quick and reproduce where there is disturbance (before the trees can outcompete them). Other species live as "understory" shrubs or herbaceous ground cover, tolerating low light levels. In effect, they get the scraps of light and nutrients the trees leave behind. But, in defining the biome we pay attention to the trees which are the most noticeable part.

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