It is a well known fact of Latin grammar, that trees follow natural gender and are always feminine, even when the word form would suggest masculine gender, as in populus "poplar".
What does motivate this kind of gender assignment? Is there an accepted explanation of that fact?
EDIT: fdb brought up this forum thread and the main information is that the gender assignment of trees was instable in the development of modern Romance languages, with Portuguese and Sardinian being the most conservative ones in this respect. There is a hint, that the predominance of the feminine gender might be inherited from Proto-Indogermanic.