You asked for the "Forestry Service" and there are two possibilities, neither one of them with that name. First, there is a state agency called the Texas Forest Service which has responsibilites on state and private lands. Second, there is the United States Forest Service which manages the 191 million acres of National Forest and National Grassland lands throughout the U.S. I'm not familiar with the state agency so I can't tell you if any offices or other facilities are located in that area.
For the U.S. Forest Service the Sam Houston National Forest is located to the north of Conroe. There is a Ranger District office in New Waverly, a bit further north of Conroe. All National Forest lands in Texas are administered by the Forest Supervisor's office in Lufkin, headed by a Forest Supervisor for the "National Forests in Texas". These lands are included in the Sam Houston, Sabine, Davy Crockett, and Angelina National Forests, as well as the Caddo and Lydon B. Johnson National Grasslands. The dispatcher for this forest is likely in Lufkin, unless it is located somewhere else in an interagency communications center.
My information shows a frequency of 171.475 MHz for the Angelina NF and 170.575 MHz for the Davy Crockett. I don't have any listings for the Sabine and Sam Houston National Forests. I would bet the frequency for the Davy Crockett is used on the Sam Houston and the Angelina's frequency would be used on the Sabine due to their locations. Divided up in an east and west arrangement would be my guess.
Also listed for the National Forests in Texas is a frequency of 168.750 MHz for "air detection" which would be aerial fire patrol. The first two frequencies I gave you show as being used with repeaters and the air patrol frequency is used simplex. Other frequencies used would be 168.625 MHz (National Air Guard), 168.650 MHz (National Flight Following), and 169.900 (Air to Air Tactics or possibly air to ground assigned to the NF's in Texas). I posted further information in Wiki under something like "National Interagency Incident Radio Support Cache" which shows additional frequencies used for large incidents and some of the listed tacticals may be in use by the U.S. Forest Service on a rountine basis. 168.350 MHz is very commonly used by federal natrual resource management agencies and is a good one to try.
As I'm always looking for feedback on the lists I have I would like to hear back from you if any of these frequencies are in use. My lists are official lists obtrained when I worked for the USFS, but like me, are getting older all the time. I don't visit the Texas forum very often so a private message to me would really be appreciated.