The USDA Forest Service is now accepting applications for jobs that start spring/summer of 2019. You will need to apply by October 12, 2018 to be considered for seasonal work. Not sure what you will be doing next summer but may want a botany, recreation, fire, wildlife... job with the Forest Service?
Go to USA jobs https://www.usajobs.gov/ now and begin the process. This will be the only opportunity to apply and the only way to get seasonal work with the Forest Service!