Trees are busy having sex with other trees for a fair part of the year. They cross-polinate all the time when they bloom. Were us better humans who might bang 10-20 of the opposite sex a year, Trees can do it with as many brethren as the wind and bees and birds can reach.
To think they are combined by roots? LOL, NO, trees are are far more advanced and have sex with 100's of their kind in a day.