Common oak trees great for a forest scene.

For each type of tree model there Is a copy that will allow you to edit the leaves (size, rotation and number of the leaves). The leaves are joined on the other tree models. This will then mean the trees are unable to be edited. But if you want to apply these trees to the particle properties the leaves will apply with the trees.
If you can't get the leaves to appear on the trees when using the particle properties Click on the treeGo to Modifiers > ConvertSelect the trunk and all the branches > Ctrl+J to joinGo to the landscape meshRemove the tree collection from the particle system and re add itThe origin for the tree may change so make sure it's correct:

The total value of polygon and vertices for the all 11 tree models.
Polygons: 53,902.952Vectors: 23,952

The total value of polygons and vertices for the different types of tree models Thick tree oak tree Vertices: 3,542 Polygon: 8,932

Thin tree details Vertices: 4,616Polygons: 10,703

Bush Vertices: 8,376Polygons: 19,284

Thick dead tree Vertices: 566Polygons: 2,216

Tree 4Vertices: 2,760Polygons: 6,653

Dead tree
Vertices: 188Polygons: 740