Trees are almost entirely client side that uses a single, special prim to mark their location and allow you to resize them and such.
They're much like particles, then.
What we need is an "llTreeSystem()" call analogous to "llParticleSystem()" that you can pass parameters like
TREE_BRANCH_TEXTURE,key
TREE_LEAF_TEXTURE,key
TREE_BRANCH_FORK_MIN,float
TREE_BRANCH_FORK_MAX,float
TREE_BRANCH_FORK_ANGLE,float
TREE_LEAF_FORK_MIN,float
TREE_LEAF_FORK_MAX,float
TREE_LEAF_ANGLE,float
TREE_BRANCH_SIZE,vector
TREE_BRANCH_DEPTH_MIN,float
TREE_BRANCH_DEPTH_MAX,float
I won't explain these parameters, they should be obvious when you think about how fractal trees work...
The tree itself would be rezzed inside the prim's bounding box, so the prim's drag corners and attitude would define the tree's size.
The people currently making prim trees would go wild. A lot of their existing textures and techniques would be immediately applicable.