When the sun is shining, sometimes with 'burning hot' intensity, the rocks absorb the heat too and alpine plants therefore need to be very resilient towards high temperatures but on the other hand also be able to cope with frosts and snow in wintertime. In addition to that these plants often grow with hardly any soil to mention, which would offer nutrition and the means for storing water and still they survive well.
Due to these amazing properties, alpines are ideal plants for rock gardens or for planting over flat roofs.