Heater air intake blows out the flame
Providing general wind break in the area will help. Regarding flow, you don't necessarily have to block where flow enters; you can also block the escape. 

I don't think it would hurt to plant some evergreen shrubs along the side of the wall and perhaps some trees around other areas of the (north) yard. A small fence around this area immediately in front of the vent/intake would certainly work better than nothing. 



Could you use wind resistant wall vent? 



Since the air is blowing down from above, the wall vent pictured above should work to shelter the immediate venting area. In the moderate range a fence should work pretty well. And general wind breaks in the yard will be effective on the larger scale.