- You may be looking for the engineer.
By 2050, oak trees were extinct (at least in England), all having been cut down or died of pollution. The last oak tree, Major Oak, was kept on display in a museum in London. This tree was stolen by a Centuripede and used to build a pod for her eggs. After Starkey and K9 helped save her eggs from Drake's bomb, she rewarded them by saving their lives and giving them the acorns she had saved from Robin Hood's time. (TV: The Last Oak Tree)
While trying to work out how a forest had grown on Earth overnight, the Twelfth Doctor compared a tree to a time machine, and determined that this had to have happened due to messing with the fabric of time, as "you [couldn't] create a whole overnight forest with extra special fertiliser". He referred to a hypothetical scenario, where "[y]ou plant[ed] a little acorn in 1795, and in the year 2016, there [was] an oak tree, there in the same spot, with a tiny, little bit of 1795 still alive inside of it." (TV: In the Forest of the Night)