It depends...

It depends on where you are modeling. In the Pacific NW we have trees that tower over everything, 150 to 200+ feet tall, likely the region where the photo in your post was taken. 

But in northern Manitoba where I model, 30-50 feet is more common due to poor soil, short growing season and forest fires on a regular basis.  Big trees in this area are a very rare sight. The photo below shows what you would normally see, trees slightly taller than the power poles.  

Tall Trees

If you surf the web for You Tube Layouts I have always been impressed by trees that tower over the trains, most times full grown trees are at least 3 times the height of the train. In my area the Northern Black Hills Of South Dakota most full grown Ponderosa Trees come in at between 90 and 110 feet. Now those would make a loco look small  

Selective compression

Most trees are huge and when converted to scale size can overwhelm a scene. A 96 foot tree in O scale would be 2 feet tall to scale. Just like our structures, trees should be compressed to fit the restricted space most of us have for our layouts. I believe a good general rule is approx. 75% of the actual scale size. That O scale tree would still be 18 inches tall.

Rick

Don't be scared of scale trees...

Dear MRHers,

Let's not be scared of scale trees...

Don't let the NG and logging predominance get in the way, near-scale trees are very doable if one is so-inclined...

Happy Modelling,
Aim to Improve,
Prof Klyzlr 

I have to agree the scale

I have to agree the scale trees look more realistic. Now in HO scale in the Appalachia region trees a foot tall would be the norm for most trees and trees in the 8 to 9 inch range would look good as well depending on the area. 60 to 90 foot tall would represent most mature trees as they did not reach the heights of some of the Western areas in the time periods after the 1930s as most areas had already had the old growth logged and things like Dutch Elm disease and things like the gypsy moth devastated forests. There are still the occasional monster trees that get past 120 feet but they are rare.

Prototype modeling

guess thats the hardest part for trees. Build several for my layout and still wonder about size. Trees are really big buggers. So I do believe that a scale tree looks out of proportion on what ever scale you're modeling like the use of real color. So selective compression is in place on the matter of trees. They just have to look good..

Like on these modules I tried to give the feel of small trains big trees. Overall most ones are between 10 and 15 inch, to small for scale trees but I do think they look pretty good..

...

We're just unused to seeing scale sized trees alongside the trains, particularly when we're trying to over exaggerate how big the trains are.  I mean, they're big, but...that image of the SD in those pines really puts the size in perspective.

It looks similar to that around the flagstaff area , where you find old growth 100' ponderosa.  Even the new growth isn't short...

Those are some wonderful forest scenes - and I think you've got the right balance, Marty, that's a believable forest there!

I make my own trees from sage

I make my own trees from sage brush and agree with the 60 to 90 scale foot height of mature trees. This photo shows an area currently under construction where trees have just been added.  

Depends!

We model Chama to Antonito on what is now the Cumbres and Toltec Scenic Railroad formerly DRGW narrow gauge.  Chama is 7863 foot elevation (small trees) up to Cumbres Pass 10015 foot elevation (smaller tress) so size of the trees depends on where and what you are modeling.  Where I live in southern rural Oregon our little sticks here go around 150 footers!    Russ

Trees about 2 to 2.5 times as tall as the engine/cars/caboose.