In traveling from a forest ecosystem to a grassland, one notes that the trees gradually give way to prairie grasses. The critical factor governing this shift is usually the (A) length of the growing season (B) annual mean temperature (C) availability of carbon dioxide (D) availability of water (E) availability of oxygen

Answer: Availability of water

Explanation:

When a person is travelling from a forest ecosystem towards grassland ecosystem the number of trees gets reduced in number.

The trees are replaced by prairie grasses. This change is because of less water availability in the area having grass.

The amount of precipitation decides the type of vegetation found in that area. So the area having trees would be having more precipitation as compared to the area having prairie grass.