Is this true in general or only because we plant understory species in full sun parts of our yards and usually get away with it?

Shade, grass competition for light rain and exposure to winds all come to mind.

Then again it is not like I go on woodland hikes on 100 degree days.

Contrast to a yard environment (which is really a grassland Savannah environment in most cases). Trees are exposed to hotter ground temps resulting in increased drying, some level of compaction, competition from grasses, exposure to drying winds, exposure to direct sun (trunk branches and all), at most a small mulched area, etc.

So in short, your on the right track. A yard is a much tougher environment in many cases.

Arktrees

Mycorrhiza association is another biggie, and may especially be important (more so, in other words) with some species. Multiple factors, from soil profile disruption to topsoil removal and from the flora profile of the site to how the area is maintained, can all play an important role with this factor.

And the third key thing that came to mind after reading the OP, was the environment (moisture, winds, light exposure, etc) fostered by the forest canopy compared to that of a yard.

I'd say there's both scientific and observational evidence out there that trees and other plants, when dense enough, benefit the water system significantly

a good example is how the state of MD requires any new construction to build in stormwater systems / habitat offsets that appear to include ponds, appropriate native tree planting and other native shrubs and grasses.

I lived up there quite a while, and the difference between there and the free-for-all, socialized cost private development in Alabama is simply amazing. Even a quick t-storm here fills the streams with red clay silt from construction sites and overwhelms water systems. (It does not help that they let farms pull drainage ditches from their field into the county storm water evacuation system that we paid for, sad to see it happen, especially when the extra water caves in the side of the road)

How does that relate to the topic of this thread? Am I missing your point?

It shows deliberate projects use dense planting to preserve or restore areas in comparison to singular or sparse yard- like planting or in comparison to areas with no formal program for such....

Back to forests, the woods around here still look okay. I've got a few shrubs I planted in the woods that don't look too well, but they ARE still alive (there's always next year...)! I think the shade and the wind reduction is the most important thing for reducing moisture loss. Our ground could easily absorb 2 inches of rain right now without it running off - Some fields have cracks so big that you can reach your arm into them! This is the worst dry spell we've had since 1908 or something - We're dryer than the dust bowl years!

We're due to get a little rain tonight; emphasis on "little". The weatherman already warned ahead of time to not get excited, because the rain would not be nearly enough to help the vegetation around here. In other news, the grape vines are thriving...

hogmany, too broad of a brush to paint our whole state, IMO, at least from the view down here. Just be glad there IS construction near you. Nothing much going on around here... for decades. I used to live up north, too, and am so glad to be away from contrived neighborhoods of pointlessly curved streets and cookie-cutter housing with identically landscaped lawns (that some hired hands installed) that nobody ever ventures into, all consisting of the same plants in the same "foundation planting" with the lawn-tanker-truck making its' rounds, and those little signs in every yard reminding everyone to stay off of the toxic "barefoot lawn."