When the sun comes out, the snow melts and settles a little. At night it all freezes and if there are any branches protruding from the snow, the frozen snow attaches itself to that branch. As more snow falls and the snows settle those branches may be torn downward and broken or ripped from a small tree. Even plants selected from other very cold climates still may not have characteristics which allow them to survive our snow. Sierra snow pack may drown, suffocate or cover for too long some plants.
Some plants simply break in a heavy storm.
Select the most suitable plants for our climate to be planted in our landscapes, however, as in the wild, sometimes damage may occur on the toughest plants. I believe that winter staking and tight wrapping decreases the chances of damage in most situations and it should therefore be done. Every summer in our Mediterranean climate we have a little drought and every winter it rains and snows. Some plants have very deep roots which follow the deepest soil moisture as the surface moisture is sucked up and away by the dry air. Many other plants only survive in or near perennial streams and wet meadows.