Trees not casting shadows properly

While using Unity 5, I removed all of my tree GameObjects, so I could replace them with Terrain based trees. I discovered that the trees do cast shadows, but only for about 20 units. Anywhere farther the shadows do not show up. I already tried checking my Quality settings, and the shadow distance is up to 1000, so that isn't the problem. It's also important to note that all other shadows (shadows from GameObjects) are showing up just fine, it's only the trees on the terrain that don't work.

Does anyone know what is wrong?

By the way, I do not want to "Bake" lighting, as the game is a driving game, and I need the shadows to adjust to the cars, and also change with the sun as it moves through the sky.

As you can see in the picture I've uploaded, the trees (marked by green) are on not casting shadows, but the building, farther back (marked by blue) is casting shadows just fine.

In the second picture, here, the problem is much more obvious, as you can see that almost all of the trees in the frame are not casting shadows.