It's easy for Tyki to sink the tree into it's reflection. Once a good chunk of the tree is in the lake's reflection, it remains relatively stable and harder to wiggle. And once there's not enough space for the tentacles to grab onto, a simple push will trap it fully.

The tree sinks into the reflection itself. It doesn't look like it's going into the lake and there's no signs of the tree hitting the bottom, disturbing the water, or anything like that. It simply passes into the reflection, much like Strange himself passes into mirrors when he's traveling between them. Once the tree is entirely in the reflection, it can be viewed head-on. Looking down on the tree looks exactly like what would happen if Tyki were looking down on the tree from above.

In the lake, a few fish swim underneath the reflection. The tree trapped in the water's reflection is still technically trapped in water. Any ripples briefly change the shape of the tree, but it returns it's normal appearance once the ripples disperse.

Strange, still on the shoreline, is super curious to see what it looks like. The main problem here is the fact that he sinks. So, he turns himself into smoke and lets a breeze drift him across the lake, until he's floating next to Tyki, near the reflection.