Nutrients And Evergreenness
Hvcrgrccnncss 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 arc nutricnt-deficicnt. Each time a plant changcs 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.
But why are the soils in the boreal zone nutrient-deficient? Partly bccausc there are conifers! The conifers produce a nutrient-poor litter which gives rise to organic acids that causc leaching. To some extent it seems a chickcn-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 arc deficient. One example is the local areas of white sand forests in the tropics again, where the trees arc holding on and keeping nutrients. In the southeastern IJSA. conifers (mostly pines, the genus Finns) 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 out-compete 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 out-compete 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.
Continue reading here: Other Trends In Forest With Climate
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