I might be going out on a limb, but just bear with me for a moment. The Whomping Willow acts as a literal guardian of Hogwarts, roots descending into a tunnel system (i.e. the Underworld), the trunk, where the deactivation knot sits is the physical world (what we can physically interact with), and the whomping branches are the many-layered arms and interconnected aliveness of the Overworld, spirit realm, or cosmic consciousness, depending on the interpretation.
The World Tree, in its many forms, is often depicted with the Overworld, the branches, representing the future, the spirit realm, and sometimes thought and possibility. These are all unknown quantities most of the time and often thought to be subject to change based on current events. The Middle World, Midgard, or the physical plane, is relatively stable most of the time, as it is the place where our physical reality exists. This is also the place that change begins, that shapes the future occurring in the branches. The roots both hold past memory, keeping it safe, and ensuring that the rest of the tree does not topple. A tree, especially a mighty one carrying the literal world, cannot stand if it has a faulty root system, just as a house cannot stand without a firm, stable foundation.
I had initially thought that as a result of the Whomping Willow's tendency to literally lash out at anything moving around it, this aggression and movement might eliminate it from consideration from the World Tree analogy. However, when the rest of the Potterverse is analyzed, much of the story reflects the same constant change that the tree depicts.
With the existence and use of time-turners, the past loses some of it's static memory. Additionally, the passage of characters through the Whomping Willow's tunnel (from the physical world, into the underworld), fundamentally changes their perceptions of each other and other characters in the story-line. The most obvious of these being in Harry Potter and the Prisoner of Azkaban when Remus' lycanthropy and Peter Pettigrew's state of aliveness being confirmed or revealed. These changes stayed as the characters emerged back into physical reality. The damage to the tree's trunk at the beginning of Harry Potter and the Chamber of Secrets illustrates how the physical, corporeal world can negatively impact the immediate present (the trunk), and the future (the tree's branches), the tree shown later bandaged with its limbs in slings as a human would. Crashing into something head-first without first looking is problematic. Perhaps the result is not always as visible as the back emd of a flying car sticking out of a hedge after asaulting the school's arboreal guardian, maybe it is. The point is, put at least some thought into your actions before commiting to them.
Once someone knows how to interact with the tree, with the connected whole of existence, it is possible to more easily pass between realms, climbing or descending as necessary for a given goal. The roots, fully grounded and constant, allow for a transformational passage to another place (the Shrieking Shack), where transformation and further character development can occur.
Also, with all the upheaval surrounding Voldemort, the future was fully unknown. The only thing truly certain was that there would be a confrontation and though each character hoped their side would be victorious, predictions could be changed or fullfilled with a wand-flick, much as the Willow's limbs flail when defending itself: disruption, change, and upheavel the only constants in much of the later books.
I had forgotten when I began writing this several days ago, but the Whomping Willow was styled after an ancient Yew tree at Ashridge Estate. Unfortunately, this tree collapsed in 2014, due to age, but it has been left to decompose en situ, beginning its next great adventure. They can live for centuries and are often seen as having healing abilities, removing pain or giving nightmares, depending on how they are approached (hint: don't fall asleep beneath one or the night mares will come riding). The yew also have poisonous berries and seem to bleed when cut, red sap oozing from wounded areas. It is also called "the tree of the dead," because it was often planted in churchyard cemeteries, protecting those burried there.
The short-stubby limbs with long whip-like branches that the Whomping Willow is known for would have occurred in the process of pollarding the tree (1). "Pollarding is a method of pruning that keeps trees and shrubs smaller than they would naturally grow. It is normally started once a tree or shrub reaches a certain height, and annual pollarding will restrict the plant to that height" (5). This would likely have occurred several centuries ago, the locals harvesting the cut branches for fire wood.
While the Whomping Willow lacks the stability and apparent strength generally associated with the World Tree, there is no other tree so revered in the Harry Potter series. The Willow is a protector and dealer of perceived justice, defending those sheltering under its branches, even after they have escaped through a tunnel. Unless you have correctly identified its secret (the hidden knot), the tree is indiscriminate in its offense. This holds true throughout the rest of the Potterverse, as unless a character has played whatever game is afoot and won, they are often smacked down forcefully. This is also often the case in our reality. The future is unknown and dramatically mercurial at times. However, if we can work with it, whatever species is growing in each of our own lives, than some of the struggle and hardship can be alleviated. That is not to say that once you go hug a tree all your problems will disappear. What I am saying is that if you look within and come to truly know yourself, your motivations, struggles, and dreams--if you know why you do what you do, it becomes easier to prepare for future events and calm whatever upheaval is presently happening. If you know fully how your reality functions, and realize that other individuals are essentially existing in their own, when they clash or overlap, it is not so shocking or strange an idea.
Let me know in the comments what you thought of this. I know it's a bit long and perhaps tangential. The idea popped up when I was trying to work on another post and I've not been able to write anything else for the past three days.
Until next time,
1. For more information on the ancient yew's demise and decomposition: https://www.berkhamstedtoday.co.uk/news/end-of-an-era-whomping-willow-of-harry-potter-fame-splits-in-half-1-6115452.
2. Langley Yew image from pixabay.com
3. Actually Mummy, "Day Out: Visit the Whomping Willow of Harry Potter Fame" https://www.actuallymummy.co.uk/2013/08/22/the-real-whomping-willow-from-harry-potter/.
4. The Paris Review, "History of the Yew Tree, 'The Tree of the Dead,'" https://www.theparisreview.org/blog/2015/10/31/beneath-the-yew-trees-shade/.
5. The Royal Horticultural Society (RHS), "Pollarding" https://www.rhs.org.uk/advice/profile?PID=156.