Protected Areas Database of the United States (PAD-US)
PAD-US provides spatial data on parks, forests, wilderness areas, and other types of protected lands.

Recreation.gov
While primarily focused on recreational opportunities, it includes statistics on visitor use and reservations, which can provide insight into public land recreation trends.

Bureau of Land Management (BLM)
The BLM provides data on land management, use statistics, and reports on recreational activities on BLM-managed lands.

U.S. Forest Service (USFS)
The USFS publishes reports and data on forest lands, including statistics on visitor use, trail systems, and resource management.

National Park Service (NPS)
The NPS provides extensive data on visitor statistics, park usage, and management practices for national parks.

Outdoor Industry Association (OIA)
The OIA publishes annual reports and surveys on outdoor recreation participation, economic impact, and trends in public land usage.

The National Recreational Trails Database
Offers data on recreational trails across the U.S., including usage statistics and trail types.

U.S. Census Bureau
While not specifically focused on public lands, the Census Bureau provides demographic data that can be useful for understanding how different populations access and utilize public recreation spaces.

National Oceanic and Atmospheric Administration (NOAA)
Provides statistics related to coastal and marine public lands and recreational activities, including fishing and tourism data.