Just looking to get some input on how trees react to rainfall? Standing under a tree you're obviously drier, with occasional large droplets breaking through the canopy. The trunk doesn't appear to have an visible water running down its surface. Is the water just being shifted out to the perimeter of the tree? Or is something else happening? Thoughts / info welcome.

I once took a slow hike through the California Redwoods.
Never felt a drop of water, but when I got back to my car, the road was flooded.
Those redwoods are so massive that the rain never seems to reach the bottom.

If you look at most deciduous trees, you will notice that most leaves face away from the trunk, and slope outward.
So, a rain drop would wash away from the trunk, then fall on a leaf that is further away from the trunk.
Progressing downwards and away from the trunk...to the drip line.

I would imagine that on a mature tree, in full leaf stage, a rain gauge at the drip line might show an inch of rain, whereas a rain gauge near the trunk might not show any rain after the same rain. This 'trains' the tree to keep extending its feeder roots outwards, thereby increasing its chances of survival.

Many trees including the redwoods that John referred to, will open their stomata and absorb quite a bit of rain. In certain locations, redwoods and sequoias can get half of their total moisture in the form of fog or mist that is absorbed without ever reaching the ground.

Dale's answer is right on the money. The surface area of a leaf is much larger than that which we perceive as the surface. The surface of the leaf is contains many vase-shaped stoma, each stoma in turn has even smaller stoma radiating off of it - all of these absorbing rain. When all the leaves become saturated (you've seen trees where the leaves and branches look "weighted down" with water), you then get "through fall". However, as John pointed out, most leaves are stacked so that water gets shunted off to the "drip line" as opposed to directly under the tree. That's why you can stand under a tree for a long time without getting wet.

Great answers! Thanks everyone.

Then there is the other effect that trees actually cause rainfall. It has been well documented in scientific studies that deforested areas get less rainfall than forested areas, and areas downwind of forested areas get greater precipitation. So before the rain comes down, the transpiration of water up into clouds from trees can provide the water that comes down a few miles downwind.

Not to mention that trees can make rain, even when it's not raining. If you have ever been in a eucalyptus forest when the fog rolls in along the Pacific coast, you will swear that it is raining, but it is actually fog condensing on leaves and then dripping to the ground. This is why eucalyptus can thrive in areas with meager levels of actual rainfall.

I'll throw in an observation in support of what has been said. The pinon and juniper trees and the sage and chamisa shrubs that dominate the high desert around here quite noticeably green up immediately upon the first rains of a normal monsoon season. Last year it took 6 weeks for the rains to soak through the parched upper 6 inches of soil. The dry season dusty green of these woody perennials had quickly turned to a more saturated "forresty" green with the absorption of moisture through their needles/leaves.

I have personally never done this, but it would make a fun experiment with "reverse transpiration.....I have read that an ordinary tomato plant growing in a pot can be carefully depotted, all the soil washed from the roots, and the plant quickly repotted into a container of dry sand. The top of the plant is then misted with water and bagged, so as to create a 100% humidity atmosphere around the top of the plant only, with the misting being repeated from time to time. Supposedly within 24 hours the sand in the pot around the roots will be abundantly moist. I know that there are close relatives of the tomato that are from fog deserts in South America where it hardly ever rains, and this brings up further implications about what trees and other plants can do with airborne moisture.....

In reply to Alder - http://priyamsez.blogspot.com.au/2010/12/fog-harvesting.html

Casuarina trees are possibly an interesting example of this.

Trees made rain in my backyard when we had a few days of heavy fog last week. Went out for a smoke and it was pouring fat drops under the Doug Fir canopy, but just a nice moisturizing pea soup in the clearing.

I am currently reading Toby Hemenway's gaia's garden and just got through his discussion of this exact question. As others have said, much simply gets absorbed by leaves, some gets shunted out to the drip line, but some will wind up running back down twigs and branches until it gets to the trunk and then down the trunk. Probably takes quite a bit of rain before enough starts running down the trunk for us to perceive.

I always wonder how active trees are in taking in water when they are dormant. I live in a place with a relatively frost-free winter than can get 30 inches of rainfall just while the trees are dormant, so I'm always wondering whether the trees are sucking it up from the roots while they are leafless.