Indeed, it's a strategy that a fair number of tree types employ. Off the top of my head, various pines and eucalyptus do the same. Usually part of the secreti

Indeed, it's a strategy that a fair number of tree types employ. Off the top of my head, various pines and eucalyptus do the same. Usually part of the secretion happens via the roots, but a large part is done via leaves. Presumably the same oils that deter insects from feeding on the leaves / needles tend to leach out into the soil over time. And all things being equal, the leaves are most likely to fall near the trunk, making it an excellent delivery mechanism, too.