Alder vs poplar

Red alder and yellow-poplar are the light, easy-working hardwoods of the West and East. They are close in weight; alder is a little harder and moves a little less.

Red alder is about 9% harder than yellow-poplar.
Yellow-poplar is about 2% heavier than red alder.
Yellow-poplar has about 13% more movement across a flatsawn board than red alder.

Each column is the species its page leads with: yellow-poplar for poplar.

Side by side

Property	Red alder	Poplar
Janka hardness, 12% MC	590 lbf	540 lbf
Weight, 12% MC	28.7 lb/ft³ (459 kg/m³)	29.4 lb/ft³ (470 kg/m³)
Specific gravity, 12% MC	0.41	0.42
Tangential shrinkage	7.3%	8.2%
Radial shrinkage	4.4%	4.6%
12 in flatsawn board, 6% → 12% MC	0.184 in	0.208 in
12 in quartersawn board, 6% → 12% MC	0.109 in	0.114 in
Bending strength (MOR), 12% MC	9,800 psi	10,100 psi
Stiffness (MOE), 12% MC	1.38 million psi	1.58 million psi
Crushing strength, 12% MC	5,820 psi	5,540 psi

One source: USDA Forest Products Laboratory, the authority for species properties it measured

Wood Handbook Tables 5-3b, 4-3 and 13-5; weight and movement worked with its Eqs. 4-14 and 13-3. A dash means the Handbook prints no value.

What sets them apart

Color: Red alder is almost white to pale pinkish brown, with no visible line between heartwood and sapwood. Yellow-poplar heartwood is yellowish brown, sometimes streaked purple, green, black, blue or red. (Wood Handbook, p. 2-3 and 2-11)
Shrinkage: Red alder shrinks relatively little. Yellow-poplar shrinks a fair amount from green but dries easily and is stable afterwards. (Wood Handbook, p. 2-3 and 2-11)
Uses: Red alder goes mostly into furniture and sash and door panel stock; it is the main commercial hardwood of Oregon and Washington. Yellow-poplar goes into furniture, molding, siding, cabinets and musical instruments. (Wood Handbook, p. 2-3 and 2-11)

For any other pair, open each species page from the wood hub; every page uses the same layout and the same sources.