Q.Colorado Blue Spruce tree
Our Blue Spruce tree is mature and about 50 feet tall, but the problem is that now the lower branches (4-5 level of limbs) have lost their needles and appear “dead”. My question is: Can these limbs be removed and not harm the tree? And when is the best time for removal? Also, would the addition of organic mulch around the base be helpful?

There isn't a simple answer as there are several potential causes. Two are simple - if the lower branches aren't getting any sun, the "leaves" will die. These trees are shallow rooted and must be watered in hot or dry weather. But blue spruces are prone to a fungal canker disease. You may be able to spot the symptoms. You must remove all branches showing evidence of disease; better yet, get an arborist's opinion. There are a few more conditions you should read about. Try this website, my state's extension service: http://msue.anr.msu.edu/news/what_is_spruce_decline_and_what_should_you_do_about_it