Near the dam entrance in mystery lake you'll find plenty of birch saplings, especially around that chain-link fence area, while maple tends to show up more around the ravine grove and plateau. The basin clearing in mountain town is another solid spot, work your way through milton park's outskirts too since they often spawn there. Pleasant valley's got some nice clusters near the farmhouse road, and those signal hill slopes are worth checking, just watch for wolves while gathering. For quick runs try coastal highway around the fishing village and jackrabbit island, lots of thickets there with both types.

Good to know maple is always gonna be rarer than birch no matter where you look, and these spots aren't guaranteed every time since it's all random spawns. Maybe interesting that checking higher elevation spots often pays off, especially around cliffsides and clearings where you get better visibility. When you're gathering try to plan your route between shelters, weather changes fast and you don't want to get caught out there with a full inventory of saplings.