San Francisco isn't even as dense as Queens, and isn't anywhere close to Paris or Barcelona. Its problems aren't a result of density, but rather economic stress.

Moreover leaving a few trees up in a lower-density development isn't a way to respect nature; whatever trees are left are there to decorate human properties.

We need new cities designed to be high-density from the start, and we need higher density in most existing cities. When humans live in urban areas, nature can be left alone, and you don't have to go 100 miles outside the city to reach undeveloped land.