I could play this all day! Such a beautifully simple concept and lovingly executed.
The only negative here is that making the tree fall is actually more fun than making it tall...
So as a tweak here I'd suggest adding some fun elements to growing the tree - like birds that come an nest in it, or fruit that grows on it once it's big enough. Something to "reward" the player for making it grow.
Another idea would be to give some warning shakes, sways or noises once the tree is nearly in danger of falling, just to give the player a chance to react.
Still a great little game and very relaxing to play