As the Cascade Mountains formed and became larger, the climate was altered due to the mountain-barriers. Summers were very dry while winters were wet and snowy. These conditions do not favor the hardwoods, and so the conifers were able to capitalize on the changing of the Earth. In the Cascade summers, there is less water when it is most needed. Hardwoods are poorly adapted for such an environment because they use most of the water they get immediately, and much water leaves through transpiration.
Conifers necessarily use water as well.
The difference is a conifer s ability to store extra water in their trunks. If water from the soil and rain becomes scarce, they have gallons of water available in their trunks that can travel up through the porous sapwood. Ray parenchyma store carbohydrates for the tree, and so a conifer has reserves of both food and water. In addition, conifers can photosynthesize all year. Hardwoods shed their leaves in the fall and are therefore unable to make food all year, which is unfortunate for they have a higher nutrient requirement.