| | 1. Leaf base cordate; fruits 1–2 per catkin, not winged, each enclosed in a papery involucre of 2 fused bracts |
| CORYLUS (no key) |
| | 1' Leaf base ± truncate, rounded, subcordate, or tapered; fruits many per catkin, winged, each subtended but not enclosed by 1 bract |
| |
| | 2. Pistillate catkin cone-like, with woody bracts remaining attached after fruit release |
| ALNUS |
| | 2' Pistillate catkin not cone-like, with papery, lobed bracts released with but not attached to fruit |
| BETULA
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