Revising the story, I think Derek's suggestion of making the tree a yew tree would work well. I looked up more information on them today finding out more about the history of them. For example some of them are thought to have been around for thousands of years, many planted by Celts in places they thought were sacred, which is why they are often found in church yards and churches were often built in the middle of a group of trees as they were thought of as the biggest protection against evil. They have also been said to 'teach us to see death as a transformation that is never final', 'bring a greater stillness to life' and they were used for summoning spirits, understood as a link between death and rebirth. They sound like a very spiritual and meaningful species of tree with an interesting history. Using this as my tree would also solve the issue of the changing seasons and not having to worry too much about how it looks.
So, now being a little happier with changes I made to the script and doing some editing, I started to get through the research I will need to do to be able to complete this piece well. Thinking things through, using the story I have I will need to research into at least 10 different time periods for the clothing/landscape along with Celtic England, more on yew trees, churches, various wildlife, army uniforms from various times, railways, bomber planes, hippies and building and architecture, and after that, hopefully get it into a style I really like and that works.
Below are some of the images I found when researching that I will use as reference for some of the items and characters in the piece.
Images I found from my research on yews
Gathering images of British landscapes from farmlands to cities so I can create a gradually developing landscape.
Thinking about the church at the beginning i did some research into old churches and industrial/factory buildings for later when the landscape develops
Images I can work from when designing some of the characters. From Celts to fashion of the early 20th century
In terms of style, I would really like to create the tree as an actual standing object out of mixed materials. I think this would be quite effective as it would stand out as the big feature of the animation and could be really textured. I would need to make sure that I filmed enough footage of it for each scene before moving onto the next one where the tree might change, for example if one of the characters marks it or breaks a branch etc. and make sure I use a green screen so that I can then fit it in with the 2D drawn animation. This means that storyboading will be key and one of big things next on my list. I will need to do some screen tests before getting too into this idea to make sure that the mediums will work together and not look a bit odd having the two quite different styles together.