After looking at this tree, capable of producing 40 different varieties of fruit, the why (cobbler) was pretty clear. But just what makes it possible for all those different fruits to grow on a single tree?
The combination of multiple fruits onto one true is accomplished by grafting shoots of new plants onto a tree. But, long before anyone picks up a pair of pruning shears, whether a graft will work or not is determined by the characteristics of the plants used and, in this case, the fact that all the fruits in the tree share a genus:
They also call it "bud grafting"... My uncle taught me to do it. It works when you swap genus to genus, so you can graft citrus to citrus, malus to male, or, as with the tree above, prunus to prunus. It's tedious, but less traumatic to the host tree. It works like gangbusters.
Almonds (Prunus dulcis), peaches (Prunus persica) and cherries (Prunus avium or Prunus cerasus) can be grafted together because they are of the same genus. Notice how cherry and peach pits are kind of shaped like almonds? They are botanical relatives.
Though that similarity of genus helps the graft to take and the plant to eventually thrive, though, it's not the only factor at play. Growth times, hardiness, and vigor are also key. In fact, in rare instances, plants that are unusually good grafting hosts can even cross genera, as the University of Missouri's Horticulture department explains:
Most varieties of a particular fruit or flowering species are interchangeable and can be grafted. Because of differences in vigor, some are better able to support others as understocks. For example, although a union is possible, sour cherry is not a good understock for sweet cherry. Sweet cherry is more commonly grafted onto Mazzard (Prunus avium) or Mahaleb (P. mahaleb) seedlings. Plants of the same botanical genus and species can usually be grafted even though they are a different variety. Plants with the same genus but of a different species often can be grafted. But the result may be weak or short-lived, or they may not unite at all.
Plants of different genera are less successfully grafted, although there are some cases where this is possible. For example, quince, genus Cydonia, may be used as a dwarfing rootstock for pear, genus Pyrus.
The process that makes these fruit cocktail-style trees possible isn't just limited to trees, though. It's also the same process responsible for producing other dual-producing plants, including the pomato (just like it sounds, folks, potato + tomato).