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If the yellowing is occuring in the fall it is most likely fall needle shed, this is normal. Pine, spruce, cedar and juniper shed their older needles in the fall. Spruce hold their needles for a couple years before shedding them. Pine on the other hand only keep them a short time. The amount of shedding is increased when the tree is stressed (Drought, insects).
Other common causes of browning are:
-fungus starts on the newest growth and moves inward. Small blackish balls seen in the browning area release spores that spread the infection. Makes sure to collect all the fallen needles to control the spread of the fungus. Chemical control can also be used, for this the identification of the fungus has to be known. The chemical has to be sprayed just before the spores are released.
-Browning on the bottom is usually an indicator of dog urine. Usually this happens to cedars but can also happen to other small shrubs. On cedars the foliage turns black, this can just be pruned off.
-Winter injury also causes browning, this is more noticeable in the spring. It is usually worse on plants exposed to the weather. This can be controlled by planting Hardy plants, make sure the tree is watered-in before it freezes. Plant in desireable locations.
-Insects pests and mites causes browning throughout the growing season
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