Trees are a significant part of the natural world which no magician of sense can afford to ignore. John Uskglass certainly did not, for as is well-known he not only surrounded his capital city of Newcastle with four* obliging and valiant forests that destroyed his enemies, but established good relations on England's behalf with all the major species of trees. Jonathan Strange once summarized the properties of trees in the following speech:
"Oak trees can be befriended and will aid you against your enemies if they think your cause is just. Birch woods are well known for providing doors into Faerie. Ash trees will never cease to mourn until the Raven King comes home again."
An English folk rhyme expresses similar beliefs:
The ivy promised to bind England's enemies
Briars and thorns promised to whip them
The hawthorn said he would answer any question
The birch said he would make doors to other countries
The yew brought us weapons
The raven punished our enemies
The oak watched the distant hills
The rain washed away sorrow
One is inescapably reminded of the actions of the ivy, the brambles, the hawthorn and the birch which absorb Christopher Drawlight into themselves upon his death. (And the rhyme is also perfectly accurate in claiming that "the yew brought us weapons", since the famous English longbow was made of yew.)
The gentleman with the thistle-down hair has some remarkable observations to make on the nature of trees. He appears to respect them and to treat them with some courtesy - witness his care to make the passage of the moss-oak into the world as easy as may be - yet later animadverts on the innate stubborness and pride of wood. (As is his way, he deals with this recalcitrance by the summary method of reducing the wood to ashes, which he considers a much softer and easier substance to deal with.)
According to Jonathan Strange, a rowan-tree has an inhibiting effect upon his power to do magic.
See also Moss-oak