Why Tree Ranges are Different Sizes

Scientists have uncovered the overriding principles that govern why some tree species are confined to a narrow range of latitudes while others thrive across a continent. They examined the distribution of 234 North American and European trees that grow anywhere from just north of the Arctic Circle to Honduras or North Africa.

Consistent with an ecological law called "Rapoport's rule", the species growing closest to the north pole also have the largest latitudinal distributions. The further north a tree can survive, the broader its range. The reason for this has to do with how a species copes with climate, energy and competition.

Northern regions present stressful conditions for plants due the low amount of energy available from solar radiation and the wide swings in temperature over a year. The trees best able to compete under these conditions are pioneer species that establish quickly. The researchers found for boreal and temperate forests, that pioneer species of trees become more frequent as latitude increases.

Trees with large ranges also produce light-weight seeds, since northern trees cannot afford to invest much energy in making seeds. Furthermore, species nearer the pole are shorter and younger when they start reproducing. These traits enable a species to endure when energy is relatively scarce.

Late-successional species, on the other hand, compete best when growing in a stable climate where they receive plenty of energy. They are limited to where these conditions exist at low elevations and latitudes towards the equator. There are exceptions to this pattern however, such as European beech, a late-successional species that grows as far north as 60 degrees.

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