In Carrara trees are constructed as a single mesh for the trunk, branches, and twigs and one or more instance collections for the leaves. Taken as an individual object this is a fairly efficient model. However, when used with a replication surface this can lead to out of memory conditions. Octane is limited in the way it handles nested instances. Instead of true nesting it flattens out all nested instances into one expanded collection. So a tree with 10,000 leaves replicated 10,000 times will result in a scatter array of 100,000,000 instances. Since each instance takes 48 bytes of memory that would be 4.8 GB of memory needed to replicate the one tree.
To help manage the scattering of trees the plugin allows you to collapse the leaves and the tree into a single mesh. This usually allows you to fit the tree and the instance array into memory. However, be warned, flattening a tree can also result in out of memory conditions depending on the number and complexity of the leaves. In general, when you want to replicate a tree, keep it simple. Create single non-flattened trees for close up shots and simplified flattened trees for replication in the background.
