At first there is two terms that needs to be clarified: Sadakat (donations) and Zakat.
Sadakat (or donations) are any amount of money you pay for any charity purpose. To poor people, to public welfare, even to rich people some times: you donate for public schools that educate people regardless of their financial state, or hospitals that treat the public masses without paying any fees, and so on.
Zakat are specified ratio of money paid at specified time to specified recipients. There are several types of Zakat, I'll mention two here due to the purpose of question:
Zakat Almal is 2.5% of your liquid savings that remains in your possession for a complete lunar (Hijri) year. And is paid for poor people and other recipients all mentioned in Surat Altauba, Ayah 60.
Zakat Alfetr is about 3 kilos of the most common food in your country: wheat, rice, corn, and so on depending on your country. It is paid in the month of Ramadan to poor people only.
So the answer is: if by "donations of Ramadan" you mean Zakat Alfetr, then no it doesn't go for trees, you go as described in case 2. If you mean Zakat Almal which you want to pay during Ramadan to get more Hasanat (good deeds), then also no it does not go for environmental causes, you should follow case 1.
But if you mean by "donations of Ramadan" any Sadakat, then the answer is yes, you can pay it for planting trees or any other public welfare purpose. But remember, you will still be liable to pay Zakat Alfetr before the end of Ramadan.
God bless you.