If you want to go for Forestry department after B.tech you have to clear the exam IFS exam conducted by UPSC.

Below is the eligibility criteria

Pass graduation / Bachelors degree in any subject or in any engineering stream or equivalents from a recognised institute / university.

2. Age - 21 Years to 30 Years for General candidates, 21 Years to 33 Years for OBC candidates and 21 Years to 35 Years for SC / ST candidates.

3. Nationality - Indian