I love the shading you have on the tree.  It's got a great sense of lighting and color that I think gives it a great environment.  Also there's some subtle perspective changes in each of the levels of the tree that just makes it look great.  One problem that I see is that it's mirrored, which means the light is coming from the top-down.  You should give more thought to the environment where the tree is placed (a Christmas tree outside?), and where the light is coming from.  The only location for a Christmas tree I can imagine the lighting coming from directly above is a streetlight.  So think about where you really want the tree to live, and then give it a suitable light source.  (I hope you keep those colors, though.  They're great). 

Also, Aelyrin's right in that a real Christmas tree doesn't work like that.  Usually it's a lot thinner (meaning it's easier to see the trunk), and less like fur.  The branches are perfectly straight, and they actually stick up (unless you have some pretty heavy ornaments).

This calls for a reference photo!

yosh64 is basically right: you need to block out basic areas of light and shadow.  What will really make your piece look realistic is realistic lighting.  This is going to be a sort of long-winded explanation of a basic concept, but it's valuable!  We all live on the planet Earth, so we all can see how the sun interacts with things like grass, trees, buildings, and roads.  The trick is to translate what we all see around us to your piece of art.  So start with the thing that allows us to see at all: the sun! 

The sun casts white light for most of the day, and at sunrise and sunset it turns to yellow or orange (depending on how close to nighttime you are).  So you should decide what time of day you want your piece to be, and then color your piece accordingly.  The sun also scatters its light through the atmosphere, creating blue ambient light that shines from every point in the sky.  So begin to place areas of light and areas of shadow: the areas of light should have a lighter white or yellow hue shift, and the areas of shadow can shift towards blue.  Remember that you don't just put the color blue in areas of shadow: you should keep the color of the object (for example, a yellow house), but just have it be TINGED blue. 

For an illustration of this concept, look at this reference picture of a yellow house.

Notice where the light is coming from.  The white lines represent sunlight.  They travel parallel to each other.  The blue lines represent the ambient light from the sky.  They travel from every point in the sky.  Notice how parts of the roof, the porch, the trees all cast shadows on the objects around them. The parts of the house that are facing the sun are almost entirely white with a slight yellow tinge (a combination of a strong white light, a weaker blue light and a yellow house).  The parts of the house that are not facing the sun are an olive green (a combination of a weaker blue light and a yellow house).  Notice that there are very few parts of the house that actually look yellow; most of these areas are areas where sunlight is shining on the house with only some of its intensity, because it is partially blocked.  Our brain tells us it is a yellow house, so it can be tempting to just use yellow when you are drawing a yellow house.  But in order to successfully represent it, you need to get past that mindset and just let your eyes and your true judgment of color do its thing.

You can apply this to any of the objects in your piece.  Take the grass.  "Is grass really all one shade of green?  No, grass can be green where it is well-watered, or light brown where it dies or gets too much sun.  How does sunlight interact with the green grass?  When green grass is in sunlight, it becomes whiter, but the color also intensifies because the sunlight shines through the blades.  In areas of shadow, it becomes darker, grayer, and slightly blue..."  ETC., ETC., ETC.

This may sound like a long process, but in your brain you can think it through in a matter of seconds.  It's just a matter of choosing the color that you would SEE if you were there, not the color you THINK YOU KNOW is there.

Good luck!

There's people innovating in that artistic field like anywhere else they just do it - as usual - against the grain. It's the 'churn them out we have a schedule to keep' anime that makes the whole field look bad.

I'm finding that to be true about any area of art.  (It's the same way in music:  for example, I generally disliked country music until I discovered that the majority of the country music I've been hearing is processed and formulaic.)  When a lot of love and commitment goes into creation, that's when you get works of art that really shine, and are beautiful, no matter what the type or style.  But any type of art and music that is commercially-driven with no artistic ingenuity or effort turns out to be soulless.

I'd say the best way to improve your colors and shape is by looking at a real tree.  Be sure to look at it objectively as a collection of forms and colors, and not as what you know to be a "tree".  Except for pruned bushes, no real tree has such a spherical clump of leaves.  Also your trunk is a little bit too big for the size of the leaf clump; there's just nowhere for all of that trunk to go. 

Faktableed: Great song, I really liked listening to it!

Cool, thanks!

Adarias: The statue of the woman on the right is really great.  These are really nice sketches.