[geenew]

It’s a bit trickier than that; some of the photons from the pulse pass through the canopy to hit the ground. In other words, a single pulse can have multiple detected returns, some from the top of the canopy, some the middle, some the ground.

It’s important to realize that the laser pulse doesn’t remain at the same diameter the whole time; it’s path looks like a cone, albeit a very narrow cone. By the time it hits the ground it can be a couple of metres in diameter.

That two metre wide bunch of photons is what is travelling through the canopy. Some fraction hit the top of the canopy and are reflected; some fraction of the reflected photons travel back in a straight line to the detector, the rest are lost to the environment. The same process repeats until the pulse hits the ground, and whatever photons are left reflect back.

The energy returned to the detector is a tiny fraction of the energy sent out.