Answer: There is a perfectly natural explanation.
Though pines and most other conifers are called evergreens, their needles do not stay alive and green forever. Generally, new needles are produced every spring and summer and last for two to four or more years. So, as the tree grows larger year-by-year, newer needles are always at branch ends and older needles are farther back in the crown.
As needles age, they become less efficient at producing food for the tree. They also become more shaded by newer needles. For these reasons, old needles finally turn brown and drop off. This doesn't hurt the tree because several year's worth of newer needles are always there to replace the old ones.
Do be concerned, however, if your tree is losing needles at the branch tips. These needles are young and have not out-lived their usefulness. The culprit is probably some type of disease or insect.
So, if one-third to one-fourth of the needles on the inner parts of your evergreen tree are falling off, it is probably just a normal sign of aging. Just rake up the dead needles, or better yet, leave them under the tree for a good mulch.
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