Spruce and pine differ physically in needle length, stiffness, and flex.
Evidence
Spruce, like the pines, is a needle-bearing conifer but has short (1/2 to 1 in.) stiff needles held rigidly on the twigs; the needles of the pines, on the other hand, are 2 to 6 in. long, thinner and comparatively flexible. — T. F. W. Embleton, Sound Propagation in Homogeneous…, 29
Categorization
- XREF: connects to: Tree branches, not leaves or twigs, resonate as forced acoustic oscillators near 275-450 cps. — T. F. W. Embleton, Sound Propagation in Homogeneous…, 49