Are you asking which tree(s) cast the most dense shade?

I would say Norway Maple, its shade is so dense/cool hardly anything will grow under it. Then again its one of the worst invasive species out there and it will completely take over your yard and neighborhood.
I think white oak is an even better option although it does take some time to get to a large enough size to even cast shade.

What deciduous species have the coolest shade temperature in your option?

The ones with the highest Leaf Area Index. Buckeye is a good one, but your clue comes from driving around - the large trees with no grass underneath are a good place to start.

j0nd03, not necessarily the most dense shade, but I guess that should create the coolest temperatures, unless leaf size or leaf thickness can make the temperature even cooler like WxDano mentioned.

Thanks everyone, I think trees with little to no grass underneath could also be that way if they have very fibrous and shallow root systems that suck up lots of water (maples have that). So no grass underneath I bet is not necessarily always a Tell-tale sign of the coolest shade perhaps. But dense shade I bet is a large factor as well to no grass growing under the Norway maple.

So maybe the question might should be: What is a deciduous tree species with the most dense shade that also has large leaves or leaflets, and has relatively thick and glossy leaves as well?

This post was edited by treeguy123 on Sun, Jan 27, 13 at 0:16

Sugar & Norway maples. Beech too.

Morus alba here.

-Sugar maple

-American basswood

-American beech

-Red oak

-Tulip tree

Sycamore or London Planetree. Leaves aren't glossy, but thick and fast growing trees strong wood they shed twigs and bark but not horribly IMO. L. Planetree is less likely to get that fungus in spring that ruins the leaves, I can't think of it but it makes the trees have to put out new leaves, it isn't too horrible for healthy trees. The trees live a long time usually. The disease is Anthractnose, now I remember.

Me again. Beech may be the most dark shade but they grow slow. I would plant one and maybe some faster growing temporary trees, but the temporaries may hog up the light and water. But maybe that would make the Beech put on some height more than thickness, they can grow in shade and seem to be thin and upright in the woods, so maybe some competition would get them to stretch upward. Then get rid of the competition and it'll fatten up. Mine in full sun is short and stocky. The European Beech is said to grow faster, I never tried one though.

Poaky, sycamore lets a fair bit of sun thru even when it is directly overhead -- mine does. Grass grows right up to the trunk.

I posted this pic before of a small pine, but look at the background Sugar maple & its shade -- solid from a relatively high sun angle (the ball of light upper left isn't the sun -- it's a reflection). Grass grew scantily near the trunk, so I just dedicated that area to spreading out yard waste & creating a mulch zone. Irises grow fine right up against the trunk, tho. Go figure...

acer pseudoplatanus makes a very thick shade. Otherwise magnolia grandiflora, beeches, and hemlocks. Typically it seems that every plant that tolerates shade will also make the darkest shade. Look for any so-called climax plant: these are the ones that shade out the competition in forests and are the only ones that can develop in their own shade.

I read somewhere once that Beech uses twice as much water for transpiration than oaks. I don't know if this means it'll be cooler underneath one though.

I just got done with a Board meeting where we were discussing costs for reducing water usage, and whether we need to change species palette for the increasing drought from man-made climate change.

A decent plan for shade 20 years from now is an anticipation of the projected climate at that time - likely a bit warmer and more of what you have in precip. You can likely stretch a half-zone if you are looking to expand your choices. Surely in that state there are maps with projected changes.

.02

Thanks everyone, it looks like Beech is in many of your options.

And lcadem, You made some good points about shade tolerant species. Never really thought a whole lot about that, and the shade they produce. Beech does grow on south facing shady slopes. Hemlock seedlings can take deep shade, and the adults make very deep shade so that makes sense.

This post was edited by treeguy123 on Mon, Jan 28, 13 at 21:31

I would say an American Beech. I never see anything growing under those things.

Once you have enough leaves to filter out the infrared rays coming from the sun. I wouldn't think that additional leaves would necessarily make much difference in temperature.

The other consideration here would be the rate of evapo-transpiration--natures swamp cooler--in reducing the temperature under the tree. So, a tree transpiring a lot of water would generate cooler air than one that transpires little. Having plenty of growth--grass, ferns, moist soil, etc. under the tree would also increase evaporation and lower the temperature. That said, probably the thicker the canopy the higher the transpiration rate and thus better cooling.

I wouldn't think that additional leaves would necessarily make much difference in temperature.

The more leaves, the more evapotranspiration. Thus more cooling.

I believe the most important factor has to be a dense canopy. After the sun becomes completely blocked, then evapotranspiration may make a small difference but how many *F can that really alter the ambient temperature? I am sure the cumulative effect is greater from this process in a forest than an open grown tree (which is what the OP was referring to - 1 single tree). Surely the overlying airmass and soil saturation levels are so overbearing, evapotranspiration is the ant on the mountain WRT the ambient temperature.

As beng and others have said, beech, sugar maples and norway maples are notorious for their dense shade AND thirsty roots.

John

This post was edited by j0nd03 on Tue, Jan 29, 13 at 10:27

Aren't beech trees noted for smooth bark that lends itself to vandals carving on it with knives?

Unless you're in an area devoid of ill-raised, unsupervised youth, might be worth a thought. That, and the slow growth.

How tall & how wide a plant do you want? Density of the shade will impact temp., but so will the surface area shaded. A larger patch of shade will tend to be cooler.

Richard.

I believe the most important factor has to be a dense canopy. After the sun becomes completely blocked, then evapotranspiration may make a small difference but how many *F can that really alter the ambient temperature?

One of the main drivers of cooling is ET. Nevertheless, one single tree of species X with dense shade is about the same temp reduction as species Y with dense shade, as species Z with dense shade. Might as well choose for form, fall color, bark texture, whatever.

I read something a couple years ago about leaf size playing a role in a tree's ability to cool the air. Planetrees were at the top of the list for cooling and carbon sequestration.

I would think crown height would also improve a tree's ability to cool the air.

I've driven past big trees in warm weather -not under them -and felt a noticeable difference in the coolness of the air.

Interesting points brought up. I'd be inclined to think it would be difficult for a human to notice any temperature differential in a single specimen tree related to it's transpiration rate. But, considering the enormous surface area of a heavy canopy of dense leaves, I don't think I'd like to place a bet on it either. Evaporative cooling would be grossly impacted by the relative humidity and turbulance of the air around the tree and light/dark response of the stomata too.

You could even go up one level, and ask which forest type is the coolest. Around here, the oak-hickory forests (dryer upland sites) tend to be a bit warmer than the moist, beech-maple forests. That could be more related to site conditions, though - Beech-Maple forests grow in cooler valleys and ravines, where the exposure isn't as great anyways.

As to the beeches being slow-growing, I know some around here that would beg to differ. They can grow a foot or two a year, in good soil with normal rainfall. They do prefer slightly cooler climates, though. To me, they are one of the nicest trees (besides sugar maple!).