The Queen, who is the society's patron, and the Duke of Edinburgh, ranger of Windsor Great Park, were shown a Cypress oak commemorating the anniversary.
Society president John Besent said: "We have enjoyed the patronage of the monarch since 1905 and trust we have been worthy of it."
Some of the society's 4,000 members including arborists, landowners and students, enjoyed a forest tour led by Bill Cathcart, Windsor Great Park superintendent.
- The society is inviting applications for travel bursaries covering all or part of travel and subsistence in the UK and abroad for independent learning-based projects or study tours relating to trees and forests.
Bursaries are usually worth between £300 and £600, and applicants must submit an article on their experiences. There are no age restrictions. Deadline is 11 March.
Visit the RFS website or phone 01442 822028.