You get an error message displaying "The density of items has reached the Max. Density limit"
This message is triggered when the density limit has been exceeded. It is defined in the Distribution Map rollout:
Defines the limit of possible tree positions in the area (by default 10 million). Take care: this includes both white and black pixels in the distribution map, so you may reach the density limit using an empty map and low X/Y values, but not get a single tree in the scene.
To define the position of each item in the scene Forest creates internally a virtual "grid", defined by the Distribution Map and the X/Y Density values. If for example, you use a low density map (as "Very Scattered"), you may be tempted to increase the density lowering X/Y Size. This is not efficient, because most of the positions in the map will be gaps (black pixels). We use this parameter to avoid getting higher densities of the grid (but with few items at all).
To create dense distributions, we suggest first to set a high density map (as "Full"), or lower the "Threshold" value. Then, adjust the X/Y parameters to get the desired result. To prevent the "grid" look of the "Full" map, turn on Transform->Random Translation to 10 or 20%.
We added this limit because it is very common than novice users try to create huge distributions, in their early scenes with the plugin. We prefer they get a warning message, instead a crash of Max or the render engine.