Hello ,
Unfortunately, it isn't possible to apply a white tint to trees/vegetation since the white value is used as the neutral tint. I will pass on your feedback to the team to see if this is something that can be adjusted for the future so that we have a separate neutral/transparent tint value that will let you apply a white tint to vegetation.
For now, you will likely need to import an aspen tree model from a third party library for your needs.
Best,
Raghib