Landscape
Since I read the books as a child, I had no real concept of what different kinds of trees looked like. Growing up in Southern United States, the most common trees around me were birch, maple, oak, and ash. Tolkien only uses two of these species in his descriptions (oak and ash), but I undoubtedly pictured The Shire with the same trees that I encountered every day.
Tolkien Included:

Oak Trees

Ash Trees
I added:

Birch Trees

Maple Trees
At first, this may seem like a trivial matter, but it can have a very large impact on the visual landscape in the reader’s mind. For example, in Chapter three, the travelers (Frodo, Sam, and Pippin) stop in a “fir-wood” and make camp for the night:
“Just over the top of the hill they cam on the patch of fir-wood. Leaving the road they went into the deep resin-scented darkness of the trees, and gathered dead sticks and cones to make a fire” (FR, I, iii, 72).
Tolkien describes the hobbits setting up camp under a group of trees similar to this:

However, I had never seen fir trees. Using the context clues, I assumed that it had to be a kind of tree that had “cones.” Well, I was certainly familiar with one of those, we had dozens of pine trees in my back yard! So I pictured the three hobbits sitting in a group of trees that looked something like this:

Now I know that many readers are saying, “They are all evergreen conifer trees, this really isn’t a big deal!” But I would ask you to look closer. Here is a close comparison of branches from each (images from http://www.finegardening.com/fir-vs-spruce-vs-pine-how-tell-them-apart):

Fir Needles

Pine Needles
This view shows you the very different appearance of each tree. Also, if you grew up with pine trees (or used one for your Christmas/Yule celebration) then you know that they have two very unique characteristics: they shed their needles and their sap is very sticky and odoriferous. This means that in order to start a fire under the pine trees, the hobbits would likely have had to clear a space among the fallen, dead needles of a pine so as not to start a larger fire than they intended! So while this distinction of trees is very small, it makes a very large difference in the impression it leaves on the reader.
In the end, The Shire that I pictured as a child had a few more birch and maple trees than Tolkien probably envisioned, and all of the fir trees were replaced with pine. This leads to a very different mental image and also changes the associations that the reader has with the trees. These differences of experience lead to different individual interpretations and responses to the text.
Old Man Willow
This actually explains why many of the artists who have portrayed Old Man Willow have made the dangling limbs shorter than I always imagined them. I had always thought that it was largely artistic license, since a curtain of dangling limbs is less appealing than a clear view of the action,. Perhaps this latter consideration still plays a role, but the fact that the types of willow in England and that many of them have characteristically shorter limbs than a Black Willow certainly reaffirms their decision.

Where Do We Go From Here?
I intend to have the first reflection on the character of a hobbit soon. I also want to look at the events with Farmer Maggot and the Barrow Downs before we head on to Bree.
What Do You Think?
What trees have you always pictured in the Shire?
Do you think that the kind of trees you imagine change the way you think about the setting?
How have you always pictured Old Man Willow?