Here's a short recap on IDing the 6 spruces on the national list:
The Engelmann Spruce (Picea engelmannii) has blue-green, needle-like leaves 1.5 to 3 cm long. Leaves have a square cross section. They are arranged spirally on the twig, and leaves below the twig curve slightly towards the tip. From the picture they look rather pointy.
The White Spruce (Picea glauca) has blue-green leaves that are shorter than the Engelmann's – 1-2 cm. They have a rhombic cross-section and are more densely arranged on the twig. They are stiff and curved but have blunt tips.
The Black Spruce (Picea mariana) is what I posted above. It's needles are blue-green and shorter than P. glauca's, and it has dark reddish-brown bark. The cones are also distinctive – they are the smallest of all spruces and are purple or reddish.
The Blue Spruce (Picea pungens) has curved grayish to blue needles (lots of variation in color), about the same length as the Engelmann Spruce's leaves. It gives the entire tree a gray-blue hue. Needles are pointy and are arranged in dense clusters. The tree here has very pale gray/whitish needles, but there are plenty of others that are bluish or even light green. I think most people can ID these pretty easily since they're common ornamental trees, even if their natural range is small.
The Red Spruce (Picea rubens) has short, yellow-green leaves, extending from all sides of the twig. The needles are sharp and very slightly curved. The twigs are orangish brown.
The Sitka Spruce (Picea sitchensis) has blue-green leaves that are stiff, sharp and needle-like, and fairly long. Nothing much to distinguish it from the Blue or Engalmann spruces; ideas would be welcome.
