pollination distance for plums, and the NAP system (Sobkowiak)

I'm planning an orchard area on a new property, and when it came to picking out plum trees, over a few sources I came across various and sundry things I hadn't read about other fruit trees, like:
-I should be planting in a "grove" where the branches of different varieties could physically overlap
-I should be planting American Plum seedlings as pollinators (again very close) to a group of "regular" plum trees
-I should be planting a pollinator mate for every plum and they should be no more than 10' from each other

How does this square (or does it?) with the Permaculture Orchard Trio system, which recommends that no trees of like species be planted near one another, but always separated by other species (in which case we are talking distances of 30-40' from one plum to the next)?

Since this advice appeared pretty much only for Plums, it made me wonder whether I should plant 2-3 plums (rather than one) in the place of the "P" in the N-A-P array...

Or should I just disregard the above advice in an orchard of about an acre.. planning on 80-90 trees, of which 10-12 plums?

Thanks in advance for any advice or experiences you can relate.

Pollination is highly localized, and quadratic in nature... What that means in practical terms is that doubling the distance between two trees cuts the pollen that they share with each other to 1/4th. Tripling the distance cuts the shared pollen to 1/9th. Etc... Many (most?) plums are self-incompatible, so they require a pollinator of a different variety nearby... As a plant breeder that has watched how pollen interacts in the vegetable garden, I would plant my orchard in clumps of similar type trees. When I planted a hazel orchard, I planted clumps of genetically unrelated trees into the same hole so that pollen sharing would be facilitated.

An alternate way to insure that plums get cross pollinated well would be to graft 2 or 3 different varieties onto the same rootstock.

Cynthia Quilici wrote:Permaculture Orchard Trio system, which recommends that no trees of like species be planted near one another

I had never heard of this; it certainly doesn't seem to emulate nature.

Joseph, thank you for your thoughtful reply. When you say "quadratic", though, that makes me think of wind-pollination mostly. Would insect-pollination follow that same drop-off?
Your information about plums seems to reinforce advice to plant them in close contact, but isn't this true of apples as well (not usually self-fertile)? The grafting is an excellent idea.

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Tyler, I don't think I adequately described the "Permaculture Orchard" concept, and used a word ("near") without defining it in that context.

My understanding is that a primary goal in not allowing the same species to be near neighbors (near=the next tree over) is to reduce pest problems. If a pest finds a tree, it won't be so likely that it can easily expand itself to the next tree over. This makes sense especially when one thinks of how intensely-planted orchards are with a monocrop, usually. N-A-P is Nitrogen-fixer of some kind, then Apple, then Pear or Plum. I don't know why Sobkowiack didn't include cherries.. I'm going to include them as a P.

A sample design can be found here:

although I see that the Ps here are all Pears.

Here's a Permies comment of Stefan's where he references this:
https://permies.com/forums/posts/list/36614#285510

Joseph Lofthouse wrote:Pollination is highly localized, and quadratic in nature... What that means in practical terms is that doubling the distance between two trees cuts the pollen that they share with each other to 1/4th. Tripling the distance cuts the shared pollen to 1/9th. Etc... Many (most?) plums are self-incompatible, so they require a pollinator of a different variety nearby... .

While a simple distance^2 relationship for pollination makes a lot of sense among fly or beetle pollinated varieties, among bee pollinated varieties it seems like things would be a lot more complicated. Bees have "floral consistency" and tend to forage from a single type of flower per trip. How the trees are oriented relative to the location of a hive may have a significant impact on pollination, perhaps even more so than their actual spacing.

I acknowledge that honeybee pollination may be more complicated than wind pollination. Wind-pollinated plants are also limited to a single type of flower... My best guess is that honeybee pollination is a close approximation to distance^2. When I watch honeybees pollinating flowers, they tend to go from the current flower to the next closest flower, to the next closest, etc... They don't typically fly from one end of the row to the other end of the row, or from one tree to a different tree unless the trees are close together. With insect pollinators, the pollen from the most recently visited flower tends to be on top of the collected pile, and thus be much more likely to be dislodged onto the current flower than previously collected pollen. Even with honeybees, I believe that pollination is most typically a highly localized event.

There's no debating against the success Stefan is having with his method, so it seems like your assumption of the difficulty of dislodging 'lower pollen' might be a bit off Joseph.

That's not to say that one might get even better pollination with trees in extremely close proximity, but there's a limit to the degree of pollination one wants [unless you like the idea of thinning fruit, perhaps because you're a smallholder who uses the thinnings as an additional yield for family pork production.] At broad scale thinning fruit is a ridiculous amount of work.

I'm not capable nor willing to critique the success rate of other people's methods.

I'm a lazy farmer... If it means that I can avoid the labor of thinning, and thus end up with lots of small plums, I'll do that every time instead of thinning and getting fewer larger plums. If it's mostly raining while the bees could have been pollinating the plums, or if a cold snap takes out most flower buds, or if hail damages most of the fruits, I prefer to have more fruits on the tree, since in my world view, more fruits on a tree means more opportunities to avoid losing all of the fruit. I believe that, the closer self-incompatible trees are planted to each other, the better chance there is to have more pollinated flowers and thus more fruits. My plums are planted close enough together so that their branches intertwine.