What does a Tree mean in a dream?

A Tree in a dream can represent:
Spiritual/universal connections
Stability
Sustenance
Growth
Do you need to 'branch' out on your horizons in life?
Do you feeling like you are just 'hanging out on a limb’, not feeling like you have stability, taking risks and feeling vulnerable?
A tree that has withered away or is dead may signify some type of loss of growth or stability.  A tree can also represent your 'family tree', and a dead tree may represent infertility or literally the 'end of the line' for the family tree.
Climbing a tree in a dream can often be looked at as symbolism for `climbing the ladder to success`, and better you are at climbing, or the easier it is, the more success you will have or the higher the position etc.
Lush, green and vibrant trees in a dream can symbolize growth and new hope.
The `Tree of life` can represent knowledge and the essence of life.
The `Tree of the Knowledge' that Adam and Eve took the apple from signifies temptation, evil and knowledge.
Trees in a dream can symbolize the qualities of strength, stability, protection and shelter.
You are concentrating on your own self-development and individuation. 
Trees that are barren and dry and have not leaves can symbolize infertility, loss of vitality and energy.  It can also be a symbol for òld age`, as trees eventually do wither and die.
Trees are life sustaining as they provide oxygen, food and shelter.
If you are dreaming of trees often, you may be a desire for protection or stability from someone or a certain situation.
A tall tree can symbolize wanting to 'reach for the sky/heaven' or spiritual growth and maturing.
A tree with many roots can symbolize that you have your feet firmly planted in the ground, and that you have a very solid footing underneath.
Trees also symbolize the unity or connection to Heaven and Earth, as their roots are firmly planted in the earth, and the branches are reaching high into the Heavens.

What does a Friend mean in a dream?

A Friend in a dream can represent:
Close ties
Loyalty
Fun times together
Intimacy
Shared history
Fondness

What does a Pearl mean in a dream?

A Pearl in a dream can represent:
The 'Pearls of wisdom' - gaining wisdom over time/age
Irritations
'Making the best of a situation' (the oyster makes a pearl out of a grain of sand that is irritating to it)
Is there something in your life or an area of your life that is becoming an irritation?
Has something that once was an irritation to you, now turned into something good/benificial?
Pearls are also considered a symbol of purity, status, dignity and grace.
Pearls can also represent conformity, as the more perfect and round a pearl is the better, and a string of pearls on a necklace is more valuable, the more similar the pearls are.

See also 'Oysters'

What does a Boil mean in a dream?

A Boil in a dream can represent:

See 'Abscess' and 'Acne'

What does a Unicorn in a dream mean?

A Unicorn in a dream can represent:
Innocence
Mythical
Pure
Unique
Elusive
Can be seen as a phallic symbol.
Chooses its friends carefully.
Veiled / hidden power  
Graceful / charming

What does an Old Friend mean in a dream?

An Old Friend in a dream can represent:
Dependence
Faith / loyalty
Shared history
Fondness

See also 'Friend'

What does Fog mean in a dream?

Fog in a dream can represent:
Obstacles and blockages
You need to know what`s out there, but you can`t see - you may have to take big risks.
Wisdom / lack thereof.
Lack of understanding - confusion about where you stand in a stituation.

See "Mist"

What does a Dragon in a dream mean?

Dragons in dreams can represent:
Powerful and Potent
Breath fire - the life force
Agressive and Dangerous
‘Slaying the dragon' - conquering your fears
Power in isolation
Extreme wealth
Cryptic wisdom
Exotic knowledge
Far-reaching threat
In Chinese culture Dragons are thought to bring good luck and to posess imperial power and wisdom.

What does the Wind mean in a dream?

The Wind in a dream can represent:
‘The winds of change’
The element of air representing thought and intellect.

What does a Willow mean in a dream?

A Willow in a dream can represent:
The 'weeping willow tree' – Sadness
Being able to bend or move within a situation.