The protagonist makes a mistake by building a fire under a spruce tree because it is completely covered with snow. The man has already had his hands out of his mittens and his fingers have gone numb. He can barely pull the twigs off of the spruce tree to strengthen his fire, but when he does, the spruce shakes and drops piles of snow onto his fire, burying it.

This could be called the definitive turning point of the story. It is at this point that the protagonist begins to panic upon realizing his life is in very real and present danger. His inability to grip the matches with his freezing fingers proves that by attempting to build his fire under the spruce tree, he did indeed botch his last chance at survival.

Strangely, there are times where it would be a really good idea to build a fire under a tree.  If you are trying to escape detection and want something to try and dissipate the smoke and prevent people from far away finding your fire, it can be a good idea.  It can also act as a shelter from rain if you are lighting a fire in the rain.

The problem is that the main character in the story is lighting a fire under a spruce tree that is absolutely drenched and hanging with snow.  When some of it melts, you are creating rain or even bringing down a lot of snow onto your fire, in this case leading to the man's death.