Allthough I'm not 100% certain but I strongly suspect that the Eucalyptus (Gum) tree's that are everpresent in Galicia (North Western Spain ) and in Northern Portugal where I did long distance walking the last few years are from seeds that were brought over on ships by religious monks in the early 1800's and planted all over the northern part of the Spanish peninsula .. I believe that these seeds were collected by returning catholic monks from a Spanish monastery in Western Australia . I was very curious when I saw so many gum trees the first time in October 2015 (as I approached Santiago on pilgrimage ) and I did a little library research later on when I returned .
It's not conclusive yet .. because I didn't make notes or keep records of the books & information I sighted one afternoon at the city library .. it was just something to do one afternoon .. but I do know that Eucalyptus is native to Australia , and therefore these trees are an invasive species in Iberia . They are beautiful to see though as one goes through the forest on the approach to Santiago de Compostella.
As I understand these Australian natives were brought over as a means of providing a relatively fast growing timber that could be sold and thus produce an income.
The one unfortunate by-product is that when it gets very. very hot and dry (as it did in Portugal last northern summer) and a wild fire breaks-out these trees, due to the Eucalyptus Oil in the leaves & sap burn at temperatures more than twice those of European Native trees (Pine, Oak, Elm etc). They quickly get out of control and cause devastation. Cheers