In Juneau, Alaska we recently designed a heated pedestrian plaza in front of a transit center with landscaping. My research indicated that you need coordination with the mechanical/electrical to understand the heat requirements/output based on degree cooling days. In areas that the temperature only needs to be raised a few degrees to get melting, these systems have little negative affect on the pits, especially if insultated. Areas that require significant heating due to extreme cold temeratures will transfer heat to the pits even with insulation. Insulated pits will take longer to freeze naturally and adding a heat source slows the freezing process even more. Keep in mind that the most sensitive roots are near the surface of your soil. In the end I kept the trees out of the heating zone to ensure the melt system did not affect the trees.
I contacted Jim Urban (LA and arborist) during design and he indicated that there was little research on this but he agreed that keeping trees out of the heating area was the best course of action for the trees.
One item related to snow melt systems that is often overlooked is where does the melt water go? Once the water moves off the heated surface it will quickly refreeze once it makes contact with a cold surface and cause glaciation.
