Returns a vector with the classification of tree.sp into one of these
four groups: spruce, pine, birch, and other. spruce group is defined by
species.spruce, pine group is defined by species.pine, and birch is
defined by species.harw. If a species does not fall within any of these
three groups is classified as 'other'.
Arguments
tree.sp
Tree species code.
species.spruce
A vector with the codes used to define 'spruce' group .
species.pine
A vector with the codes used to define 'pine' group .
species.harw
A vector with the codes used to define the 'birch' group .