Tree species richness is far smaller than that in the temperate forests to the south, where more than 100 species are typically observed in 2.5° x 2.5° quadrats in eastern United States. Species richness clearly declines from south to north in the taiga. Whereas 40 or more tree species can be found in the southern taiga in Canada, this declines to 10 or so species near the tundra boundary. Animal species also show strong gradients. Reptile and amphibian species are almost nonexistent above 55°. Mammal species richness declines from close to 40 species to about 20 going northward in the boreal forest biome in North America, while bird species decline from about 130 to less than 100.
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