With each level, you have +1% chance to automatically use spring after the last tree has been chopped. So if you have it on level 100 there will always automatically spawn new trees after all trees have been chopped
"The browser could not allocate enough memory for the WebGL content.If you are the developer of this content,try allocating less memory to your WebGL build in the WebGL player settings." this shows up on chorme.
We are sorry to hear that the game is running your machine hard, we are trying to lower the poly count to help with performance more. The next update should include this, please let us know if this helps you!
Unfortunately with the game engine I used it's not possible.