"Picea" mean Spruce or pitch pine?

Question

While going through the name of the tree I find that Oxford Dictionary translated "picea" as a Spruce and while lewis translated it as a pitch pine. Both these are different trees. Why different translations?

Answer 1

Picea is the modern scientific name for the spruce genus, but its application to just spruces, like the application of Pinus to just pines and Abies to just firs (and indeed the narrowing of those categories in English), is a modern convention that basically became a thing faute de mieux.

The Latin word picea is derived from pix 'pitch', and in principle could be applied to any tree whose primary use to humans was to produce resin, which is why L&S have it as 'pitch-pine' (they also somewhat untenably specify that it's Pinus silvestris, the Scots pine, which you'll note is not actually the tree conventionally called the pitch pine today, Pinus rigida—a New World tree the Romans didn't know about).
Classifying trees by use is historically obviously more common than classifying them by genetic affinity, which meant names could apply to multiple species of trees at the same time or change over time. Compare e.g. Latin fagus 'beech', mainly used for its beech nuts as animal feed, to the Greek cognate φηγός 'oak', mainly used for its acorns as animal feed; this is a clear example of a phenomenon that happens in every language, and its often impossible to determine which specific species of tree any given instance of a tree name refers to in a text. When a pre-Linnaean Latin author uses picea, just assume they mean a resin-producing conifer or a tree that looks like one, and translate it accordingly.

(Compare also the modern Christmas tree, which, depending on the region, can refer specifically to the Norway spruce; specifically to the Scots pine; to almost any coniferous evergreen; or even to any tree that will hold Christmas lights.)

Answer 2

I would like to add that even the modern nomeclature and the genus boundaries are far from granted. There is a consensus now, but it was not clear when the modern Linnean names were creted as many synonyms document. The common European spruce Picea abies was named Pinus abies by Linneus, but was later put to other genuses, namely as Abies abies and Picea abies, by other authors. The European fir Abies alba was named Pinus picea by Linneus, later put to genus Abies as Abies alba and independently as Abies picea.

What is Pinus, what is Picea and what is Abies, was far from granted in the times when modern conifer taxonomy was being created.