These shields are slightly different from what's happening on the weather-system scale. Those winds are low to the ground, and when you obstruct them, the wind is deflected upward and diffused by the trees absorbing some (a very tiny amount) of the wind's energy. So it's a highly local effect. Forests can also modulate properties of the atmosphere around them, which can do various things like alter cloud formation or even stimulate rain. Rain stimulation is still be studied as I recall, but it has huge implications if it's true.

When you've lived in the country most of your life, it's weird when you hear people discover basic things like windbreaks or the fact that tree shade is cooler than other shade. My great-grandparents would have said, "Well, duh."