North American trees
Grouped by leaf arrangement, because that is the order to check them in and how you should think about it in the field.
Opposite leaves
A short list — maple, ash, dogwood, the viburnum family and horse chestnut. Recognising opposite arrangement puts you here immediately.
Cornus florida
Tear a leaf slowly: fine white latex threads stretch across the break. Nothing else in the eastern woods does this.
Acer platanoides
Snap the leaf stalk from the twig: milky white sap oozes out. No native North American maple does this.
Acer rubrum
Opposite leaves with sharp V-shaped notches between the lobes.
Acer saccharum
Opposite leaves — only a handful of tree groups have this, which narrows things enormously.
Fraxinus americana
Opposite AND compound — very few trees are both, which is why this combination identifies ash quickly.
Alternate leaves and conifers
Everything else, which is the majority of trees. Narrow these with leaf type, edge, fruit and bark.
Fagus grandifolia
Smooth pale grey bark at all ages, never furrowed.
Platanus occidentalis
Mottled bark flaking in jigsaw patches to reveal white underneath.
Juglans nigra
Green husked nuts the size of a tennis ball, staining everything they touch.
Tsuga canadensis
Two white stripes on the underside of each flat needle.
Cercis canadensis
Perfectly heart-shaped leaves with completely smooth edges.
Pinus strobus
Five needles per bundle — count them. This alone identifies it among eastern pines.
Quercus rubra
Bristle tips at the end of each lobe — the defining red oak character.
Betula papyrifera
Chalky white bark peeling in horizontal papery strips.
Populus tremuloides
Flattened leaf stalk — roll it between your fingers and it is a ribbon, not a rod. This is why the leaves quake.
Rhus typhina
Twigs densely velvety-hairy, like antlers in velvet.
Ailanthus altissima
Crush a leaflet: a strong unpleasant smell, often described as rancid peanut butter or burnt rubber.
Quercus alba
Rounded leaf lobes with no bristle tips. This defines the whole white oak group.