Crash is caused by the Collisions feature. Scattering area is too dense, and Forest cannot allocate the data structures to compute it.
Although the total number of items is not very high, there is a lot of empty space between the grass areas, which must be evaluated as well (red bounding box in the image).
If surface is planar, i suggest to assign individual area splines to Forest (one for each small area), and not use any surface at all.
Not only Splines are more efficient than Surfaces, but also in this case there is a noticeable improvement, if Forest doesn't need to evaluate the empty space between them.
When surfaces are not planar, i suggest to combine both: small spline areas to define the scattering limits, and surfaces just for elevation ('Surface Area' in Areas rollout must be off).
In this way, items are scattered using the splines, and in a second step are aligned over the surface. Even in this case, several smaller surfaces are more efficient than an unique large surface (when there are empty spaces between them).
Also if possible we suggest to disable Collisions checking, and use only when strictly necessary. It's a feature with a huge performance hit, both in CPU and memory.