Got the 3x3 array built, and just finished removing a fair number of individual trees and replaced them with this smaller array that can fit in where the 6x6 is too large. I checked the fps readings before and after, and obtained a 50% increase.
I also reoriented the surface normals on the trunks of my two new single palm trees, so as to be more in line with the approach used for the palm blocks and arrays. In short, for surfaces facing out to the side, the normal is not directed horizontally, but instead is oriented almost vertically, (20 degrees from straight up). The reason is this. When a side-facing surface has its surface normal oriented perpendicular to the face (as is the normal state of affairs), it is kost brightly lit when the Sun is essentially on the horizon (if the surface faces it.) This means that when the Sun is overhead, such faces are rather more dimly illuminated. For such objects as plants, which generally exhibit much irregularity and so do not behave like flat walls, it is much better to orient surface normals so the the plant is brightest when the Sun is high in the sky.
The result is that the 3D trunks of the single trees now change their illumination in keeping with their own fronds (made of simple vertical polygons) as well as the palm block and array objects. Similar enough that a casual glance might not permit to distinguish between them.
I'm getting close to releasing this latest version of the map! And the new objects will be supplied, naturally.