There are two main forces:
(1) Each molecule of water is made of 1 atom of oxygen and 2 atoms of hydrogen. Molecules of water are not found singly or in small isolated groups but one molecule attaches to the second and the second to the third and so on. Due to this property of bonding a quantity of water in the form of a drop, lake, river or sea is formed. This natural bonding takes place because a molecule of water does not have one single electric charge. Where two atoms of hydrogen form the hydrogen bond the electric charge is positive whereas at the other end it is negative. Wherever asymmetrical electric charges of two molecules meet bonding is bound to take place due to mutual attraction.
It will be relevant in this context to know that a large canopied tree loses 1000 liters water daily through evaporation. As each molecule of water comes out of a pore of the leaf another molecule is automatically pulled behind it because their attraction known as ‘hydrogen bond’ is not broken easily. The link is strong. The second molecule similarly pulls the third one by the force of that attraction. In this manner armies of billions of molecules gradually inch up the capillaries having only one millimeter diameter.
(2) Chemical attraction is another force which assists this process. Rots of the tree absorb salts in the soil which make the water inside the roots saline. As salt content in the rain water is low it flows towards saline water in the roots in accordance with the principle of chemical attraction and accumulating in the roots pushes the water in capillaries upward.
The summary of this answer is that in case of very tall trees like coconut palm the water in the higher reaches of the tree experiences the force pushing it upward.