After a physical (ie not digital) way to recreate the sounds of light wind through tree tops. Need the the ability to simulate with intermittence like in real life. Question for a foley artist....
You could get some leaves and branches and moves them around, perhaps swinging a leafy branch or two gently near your mic. You could even blow eaves in a fan and take out all the lows of the fan, automating the volume envelope of the result and perhaps the amount of lows to taste to go along with the places where your strong gusts are.
I've not tried either of these methods personally, but they'd be where I'd start and the experiment won't cost you anything if it turns out not to work as you'd like.
Some time ago I did it like Justin said, with a some branches and moving in circles.... Here is the sound I got (this is the raw sound without any process or edit). I recorded in a room without acoustic treatment and the mics are a bit noisy. I think this way is hard to get long "modulation" (i dont know how to say...), its more like a whooshes, maybe overlaping many layers it will work, for what i needed it doesn´t.
And in this video the Master Ben Burtt Shows another way (7:10min):