Which is typically one of the early symptoms of dehydration?

Symptoms of dehydration:
Thirst
Dry mouth
The pee being dark yellow
Fatigue
Headache
Muscle cramps
dry skin
dizziness
Rapid heartbeat
Rapid breathing
Sunken eyes
Sleepiness
confusion
Fainting

The earliest are probably the feeling of dry mouth and thirst.

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.