there are two files in the attached zip:
1. the model of the trees (BrokenTrees15_01_backup.nif)
2. the texture (helltrees_1024.dds)
What you have to do:
get evergreen01_01 up to evergreen14_01 found in the unpacked assets fpk of civilization IV. The path is: art/terrain/features/treeevergreen
now you have to repeat the following steps for every *.nif (14 times in total):
1.) select the trees and scale vertices: z = 1.5 (only do that if you want the trees taller than normal, else you have to scale the trees of BrokenTrees15_01_backup by z = 0.67)
2.) change the texture, so the trees as well as the shadows use helltrees_1024.dds (this needs only one texture change because both meshes refer to the same texture property)
3.) copy the the branch of the shadow mesh and paste it back on the scene root node
4.) change the UV map of the new shadow mesh and simply drag the the square over the shadow to the bright spot in the bottom right of the texture
5.) adjust the position (x,y coordinates) of the new shadow mesh so the bright spots are where the tree bases are.
6.) set the alpha property of the new shadow mesh to 4097
this sounds perhaps a bit much, but it should not take you longer than 1 to 3 minutes per *.nif. if you have questions feel free to ask. also you may adjust the uv maps a bit, because the firaxis guys have chosen them poorly when they did their trees (for example some branches overlap with the bole, thus giving you some weird artifacts at the edges of the branches). You can simply compare the UV maps of the nif in the zip to see where is fixed stuff, for comparison. Finally you should probably rename the BrokenTrees15_01_backup.nif to NameOfYourTrees15_01.nif