Leaves evaporate billions of tons of water vapor every day all around the world during the late spring and summer. The evaporation of water (transpiration) allows the leaves to get rid of the heat burden from sunlight and send the heat up higher into the air. Some of this water vapor ends up in the clouds. What the trees are “trying to do” is to keep its leaves cool. But this also means that when we or our houses are down in the shade, it is cool shade, cooler than the shade of a carport or building. I explain this in my book Green Planet: How Plants Keep the Earth Alive.
One would think that trees have to do a lot of work to pump all this water up from the ground, through the roots, through the trunk, and up to the leaves. But, actually, the trees allow the laws of physics to do the work for them. The water molecules cohere to one another. As water molecules transpire from the leaves, they pull the water molecules behind them. The little columns of water in the pipe cells of the trunk are stretched like tiny rubber bands. In some cases, botanists have been able to measure the tree trunk getting narrower when transpiration begins. During the daytime in the growing season, the leaves simply open their pores and let the laws of physics pull the water up. The water is under tension, which is the opposite of pressure.
But before the leaves emerge from the buds, there is no transpiration. The water cannot be pulled up to the top of the tree. Yet, we all know that the sap rises. In this case, the tree generates water pressure (by accumulating sugar and minerals) in the roots, and this pushes the water up to eventually make the buds burst open.
If the tree generates water pressure but the buds are not yet open, the water can occasionally leak out of damaged wood. The damage can be intentional, for instance when the rising sap of maple or birch trees is made to drip into little buckets. Or it can be accidental. If you are standing underneath a birch tree when the sap is rising but before the buds have opened, some of this water might drip down on you. You may think a squirrel has peed on you, but it was actually the tree. This little drop of water, coming out of what seems like a clear blue sky, hitting your head can have some of the same effect that the apple had on Newton: opening your mind to discovering something new about the world of nature—in this case, trees.
I have posted a video about this: Darwin gets peed on by a tree.
Water cannot be pushed (pressure) and pulled (tension) at the same time. The tree pushes the water into the expanding leaves and other structures. Once transpiration starts, the pressure stops and tension takes over.
All this invisible activity is going on right before your eyes!