B.Sc Forestry

No, it's not mandatory. You can apply for M.Sc in forestry. As M.Sc. Is a good option. As Masters degree can give you good career opportunities. You can go through this.
Good luck.

Dear Pratikshya,
Reference to your query, we appreciate your willingness to pursue higher studies with us. We are happy to inform you that we have few seats available under direct admission category.
BFIT runs B.Sc forestry, 4 year, regular degree program affiliated to H. N. B. Garhwal Central University. You are eligible for the same. You can go ahead for the admission process by filling free of cost online admission form at:
https://www.bfitdoon.com/online_registration.php

No. To become an IAS officer, you have yo be graduated and that graduation can be of any course.

There are over 17 B.Sc in Forestry colleges in India. Please refer to the link provided below for details regarding these colleges. You should be able to navigate to a college of your choice using the filters on the web page.
Regards.
https://www.shiksha.com/science/colleges/b-sc-forestry-colleges-india

Uttar Banga Krishi Vishwavidyalaya offers this course.

Yes, you are eligible to pursue B.Sc Forestry from Guru Ghasidas Vishwavidhyalaya. The minimum criteria to apply is a score of 50% in class 12th with mandatory subjects: Biology, Agriculture and Maths.
Age-Maximum 22 years
Admission Process
Admission is through counselling based on the rank obtained in Vishwavidyalaya Entrance Test (VET).

Hi,
The subjects your desired course includes are botany, urban forestry, forest insects & disease, land surveying, silviculture, forest vegetation and renewable wood products.
Forestry science courses teach about general knowledge, management principles and environmental concerns related to forest communities. You can also take individual classes if you personally like to.
Graduates of this program can work as professional foresters in any of the public and private industries. You also may apply for a research positions for government.