Thanks for the scene. Testing it, we could find the problem.
Currently, Forest doesn't clear its cache for animated geometry between frames. We choosed thia approach for efficiency reasons, because extract meshes from animated objects may be slow.
For some animation modes, Forest needs to fetch the same samples on different frames multiple times. In this way, these samples are cached along the animation, and it's not necessary to compute them each frame.
But as side effect, the render cache requires more memory to store them.
I just added a mechanism to check the memory used. In this case specific scene, Forest cache takes 53Gb. just for 100 frames. That's is excessive.
We will try to optimize that for next updates.
Until then we suggest to divide long renders in several jobs. For example, instead to render from frame 0 to 100, create 10 separated render jobs of 100 frames. Cache is cleared when each render job is completed.
This can be done in Backburner, Batch render, or any other tool you use to render.
Please note this issue happens only with animated models, and Forest animation mode different from 'Disabled'. Static models have a single sample for the full animation, so render cache doesn't grow.