Answer: By brown spots, I'm guessing that you mean some branches have bunches of brown needles. If you're sure the branches weren't damaged by heavy snow load or wind, then the cause may be a combination of the soil, fertilizer, and weather conditions. I'm glad to hear that you amended the soil before planting your pines, but the trouble with amending a planting hole in clay soil is that it acts like a moisture sink. Moisture will run off the surface of clay and sink in where there's less resistance - right into the planting hole. It's best to amend a whole plot of land instead. Fertilizer spikes have the ability to actually draw water away from plant roots if water is scarce, but the pines did fine last summer, so that may not be the cause. However, moisture was scarce going into winter, and winds were heavy, the combo may have caused desiccation of some of the branches and needles. Those are my guesses - you'll get a more accurate and thorough answer from a local forester, ag extension agent (ph# 879-0825) or nursery person who can actually look at the trees. Hope this sheds some light!
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