A. Although it is fairly typical that leachates from coniferous trees, and the soil below them, are acidic, it is less easy to translate this to stream water chemistry because many factors interact to produce the pH you find in a stream. Coniferous leachates are high in organic acids such as fulvic and hummic acid; these would tend to contribute an excess load of hydrogen ions into the ground water and thence into the stream - thus lowering the pH. However, geological processes play an important role in stream ph also. If your stream is high in a watershed, groundwater has had less time to interact with geological materials and is usually more acidic. Certain rocks leach unique chemicals that interact to produce the water chemistry you find. Specific tree species also have unique chemical interactions. White pine, for instance, leaches high amounts of nitrate to the soil below the trees, and this could influence acidity.

On a practical note. I tried growing some vegetables in a small garden in my back yard; part of the plot was under a pine tree. The vegetables grew just fine - but those under the pine tree were practically inedible, and I'm sure it was from the leachate under the pine tree because those vegetables outside of the "drip zone" were fine.