Nhận biết (M1)

Some climate models predict that the habitat ranges of North American trees will expand northward as average temperatures warm. However, some tree species, like sand post oaks and limber pines, haven't expanded. This is called "migration lag"—when species fail to expand their range, even when conditions seem right. Scientists have proposed several possible explanations: fewer species of soil fungi available at the northern edges of the trees' habitat, physical barriers blocking seed dispersal, or deer overgrazing preventing new trees from establishing.

According to the text, what unexpected phenomenon are scientists observing with North American tree species?

• A.

  The ranges of sand post oaks and limber pines are primarily expanding southward instead of northward.

• B.

  Some North American tree species aren't increasing their habitat ranges despite suitable climate conditions.

• C.

  The ranges of sand post oaks and limber pines are expanding more rapidly than climate models predicted.

• D.

  Some North American tree species are developing resistance to fungal partnerships at the northern boundaries of their ranges.