Used attributively or in the possessive to designate concepts originated by Wallace or related to his… a hypothetical boundary proposed by…

a divide separating two river systems of a continent; spec. the divide in North America separating rivers flowing into the Atlantic from those…

In the northern hemisphere, the line north of which no trees grow.

the line beyond which trees do not grow, with reference to either altitude or latitude; cf. tree-line n. (a).