Well, as i said it depends on improvement... But since you are not telling me which one...
If the Nif of the improvement has a TreeCut object, you can increase the radius of the tree cut object (Use NifScope).
If the Nif of the improvement not has a TreeCut object, you can add one (best is copy&paste it from a existing improvemenat) and then increase set the radius to what you want (Again, use NifScope).
The ForestPreserve.nif has a TreeCut object, so you can chek it out for reference.
To find out what Nif is used, check out the Civ4ArtDefine_Improvement.xml.
Some [many] improvements are handled via Civ4PLOTLSystem.xml. To find out if, look for the <bUseLSystem> tag in Civ4ImprovementInfos.xml
With these you have additional options in the LSystem.
Relevant Attributes there are bCutTrees:1 to enable tree cuting and fCenterCut:xxx to define its size (can be used on Artrefs and Goals). Serch for "goal:IMPROVEMENT_COTTAGE" in the file for refrence.