Why does she say that northwestern europeans were most fully participating in the southernization process only after they acquired tropical colonies?

The northern countries were unable to produce the types of goods that they were searching for (such as rice and sugar), and were only able to do so once they had claimed a number of the more southern nations. These countries had the climate for growing these types of plants, so the ability for northern countries to be more "southern" only came about once the newer colonies were brought into the fold.