Maples are not the only species with these characteristics - most large growing, wide canopied, shade trees will produce similarly difficult to habitat areas. Even conifers!

Ability to tolerate dry shade is the primary concern. And that and you hardiness zone/climate will limit what will work well for your purposes.

BLE shrubs that fit the criteria are few and far between for your area. Mahonias, possibly aucuba, possibly fatsia, possibly sweetbox or silverberry. But anything that will get to 10' tall and is alive and growing is going to be extremely difficult to achieve.

As with anything that one would attempt to grow under these conditions, you want to start small and coddle for the first few growing seasons.

Try Ilex opaca, look for plants in the 4-6ft range because they are slow growing. Eventually, in tree time, they will get larger than 10ft, but you will have time to prune them before then. Is this a woodland edge or within the understory?

Deer will eat ilex opaca I’m pretty sure. Let me pose the question differently. How much would I have to thin the maples to be able to plant between them?

I’d call these areas woodland fragments. The remains of woodland left by clearing for houses and driveways, say 30-50 feet wide?

Deer will eat anything if they are hungry enough, but Ilex opaca is rated in the highest category of deer resistance by Rutgers. Deer eat a lot of my plants but my neighbors American Hollies look good. Asimina triloba and Aesculus parviflora are also in that deer resistance category, and shade tolerant.

Its hard to say how much you have to thin them, I would want 4-6 hours of direct sunlight or at least dappled sunlight. That could involve thinning out trees or pruning out individual branches to let more light in. Keep the ground covered in leaf litter to help with moisture retention. The lower hudson valley is extremely rich and lush in vegetation, I suggest you look around at surrounding landscapes and natural places for inspiration. If you are on a slope, a low spot, or a ridge that could also have a huge impact, so look for places with similar exposure.

Think about a layered planting too, you can have grasses and perennials in the lower 3-5ft and "leggy" trees covering above 5ft.

I do look around the woods in my area and search in vain for any understory left besides barberry, winged euonymus and invasiveslike mutiflora rose and bittersweet. Deer have given us a very transparent landscape. I will look into the ilex more. Thank you for directing my focus to the canopy rather than the spacing of trunks it may be better to eliminate some of the larger maples. Opening up the canopy by pruning would be the best albeit expensive route. And also require frequent upkeep I assume.

“How much would I have to thin the maples to be able to plant between them?”

IME you will be better served to plant in front of or behind the maples rather than between or under them. They are large, spreading trees both above and below the ground and IME typically outcompete most desirable shrubs that one might plant in their vicinity. Trying to prune the canopy or remove trees to allow other plants to establish seems a bit of an exercise in futility to me since the already existing trees will fill in faster than anything new you add. Limbing up or thinning canopy will be moving you in the wrong direction.

Alternatively or in addition to planting out from under the maples, add fence panels as needed to obscure particular views.

I have the same issues with deer eating everything and leaving behind only a tangle of multiflora rose, winged Euonymus and many other undesirable spreading plants. I tried to grow a privacy screen starting with small shrubs but deer and shade were issues. I even had a couple white mulberry trees removed to increase light levels, which left a 50ft souther-facing gap between my house on the west and a pair of towering Pin oaks to the east. I finally decided to put up a fence. Its the least natural looking option and cost a small fortune, but it provided instant privacy, a haven for my own garden, and a real barrier for my dog and child from a busy road. Now I get to enjoy planting along the entire perimeter of the fence. The most deer-proof remains of my initial privacy screen effort are the shrubs Viburnum prunifolium, Viburnum nudum "Branywine", Aesculus parviflora, and the perennials Rudbeckia laciniata, Veronicastrum virginicum, Panicum Virgatum, Senna hebecarpa, and Agastache scrophulariifolia. I also picked up Staphylea trifolia as a deer resistant, shade tolerant shrub. Aralia spinosa, the Devils Walking Stick, is another deer proof option I am looking at for an out of the way area. If you have any direct light I would look into a combo of perennials and small trees or shrubs. Every option is going to take maintenance and fencing of some type might be something to consider.

Well this is probably the reality check I needed to hear. In my case, it will either be get rid of enough of the maples entirely and plant conifers and shrubs in their place, or live with the view through their legs. So rather than thin, I should remove them an area at a time...

I have no idea if this would work in your setting but I have similar issues with redwoods - you really can't plant anything underneath them due to their roots aggressively seeking water wherever they can find it. So I put together a grouping of large pots, raised up on bricks (otherwise the redwood roots find their way into the pots) and planted in those. I have a combination of Japanese maples, Mahonia, Azalea, dwarf Metasequoia and ferns in the smaller pots on the outside of the grouping. The tallest is about 7-8' at this point, but I wasn't going for height per se.

No im afraid containers won’t work in my situation. I haven’t the means to water them.

Crazy idea... try cutting down a small sugar maple close to the ground, and let it resprout from the stump. In a year or three it should regrow looking like a multistem shrub, then afterward prune off the largest branches each winter to maintain size.

Hmmm! Anyone ever do this??

No but seems like a reasonable possibility. Red maples resprout well from cut stumps. I think a three year cycle might work well based on experience with red maples.

I did it with a Norway maple unintentionally, the thing just wont die. I also did it (coppiced) 2 white mulberries and a crab apple that where growing around my power lines, and 2 black cherries that were storm damaged. Im only going into year 2 but they have all resprouted to around 3ft already. That reminds me I have to go prune them soon. I looked up coppiced sugar maples specifically yesterday, and it seems there is a maple syrup producer in Vermont using some new vacuum assisted syrup production method using coppiced saplings, so it must work on some level.

I imagine you'd need to protect the coppiced stools from the deer. The species we have here certainly eat new tree growth when they can reach it.

Wouldn’t a coppiced maple require as much sun as a shrub?