If we look at the climate map, we see that the climate patterns are similar to the patterns that the biomes form on the landscape. A map of soils would show similar patterns as well. The chart below shows another way of looking at the relationship of biomes to climate. Look at a few examples of different biomes, and see where they are located along these gradients. Biomes, as I've said, are distributed according to climate.
Mountains are a special case, because the climate on mountains is different from that in flat lands.
The main differences in climate of mountains are temperature and moisture. The temperature decreases 3.5 degrees F. for every 1000 feet. This is called the lapse rate. So, at any given latitude, if there is a mountain system, the climate on the mountains will be cooler than the climate in the surrounding flat lands. Your text expresses this by saying that the biomes are depressed along mountain ridges.