The direction that a slope faces has a lot to do with the microclimate on that slope. In the Northern Hemisphere, a slope that faces south will get a lot more sun than a slope that faces north; this means that on the south facing slope springlike conditions will come earlier and fall conditions will arrive later. This means that a south facing slope will have both a warmer microclimate and a longer growing season than a north facing slope. In areas where there is snow in the winter, the direction of the slope determines how late the late snow melts away, again affecting growing season.
Additionally, if a slope faces toward the prevailing winds, it may receive more precipitation than a slope that faces away in the lee direction, but the plants on the windward slope will have to contend with more frequent winds. In some cases this situation can create plant communities that are quite different on two sides of the same mountain.