Check your bylaws to see if this committee is even required, or if it's optional. Typically, committees serve at the pleasure of the board, members can be removed with or without cause, and the entire committee can be disbanded altogether unless it's required by the bylaws.
If the committee members are buddies with the board, you may have an uphill battle, though. In which case...
Aside from handing the board the bill for the damaged trees, you may also want to discuss liability - this should get the attention of competent board members.
In addition to damage to personal property, the board needs to consider what can happen if uninsured volunteers are doing physical labor or are dealing with chemicals. (Seriously, is nobody paying attention to the
Roundup lawsuits?) Even if these committee members are covered by workers comp and other insurance, that may not protect the HOA if a committee member ends up with cancer and decides to sue the HOA because of alleged chemical exposure. Competent boards generally employ lawn care professionals who are trained and insured by their employers - clearly not what's going on in this case.
Use of volunteers for physical labor would get my attention, even if they hadn't damaged my property. I'd be sending the board a letter specifically addressing insurance needs and the possibility of lawsuits.