Aesthetics
I think more wars have been Fought over Aesthetics than any other issue.
Whilst technically what you describe is alteration to common property, another interpretation could be garden maintenance. Trees and shrubs get larger over time and need trimming. They can also get too large, or just go out of fashion.
Just make sure this is a really important battle you want to take on. And that you have support of a majority of owners lest you get labelled that crazy person.
Get 25% of owners to call a general meeting and put your motion on the agenda. Better than going to mediation since the majority vote is the rule.