Evergreens often die slowly and parts of the tree might show signs of life even when there's no hope of recovery. Yellowing needles could be part of the tree's normal life cycle, since evergreens shed needles in stages. Evergreen foliage lasts from two to 10 years depending on the species. Some evergreens shed needles gradually, while others, such as white pine, drop old needles over a short period in autumn. New needles replace those that fall, but most new growth takes place at the outer border of the canopy. Interior branches gradually shade out and die back.
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Inspect the tree's canopy. Healthy evergreens show mostly green foliage even when shedding old needles. Branches with only brown needles, interspersed with healthier branches with green foliage, mark a tree that's under stress but not dead. Drought, disease and soggy ground all could cause this damage.
Test green needles and evergreen leaves by bending them between your fingers. Healthy foliage flexes without breaking. Dead needles need some drying time before turning brown, but dead foliage dries out quickly and snaps when flexed even if still slightly green.
Test the bark on the tree's trunk at the base if you've found extensive damage to the tree's canopy. Cut a slit in the bark with the point of your knife and chip away or pry aside the dead dry outer bark. Look for a green cambium layer above moist sapwood. Dry brown bark all the way to the sapwood means at least a major injury to the tree. If dead bark girdles the tree, it can't be saved.