Wheat can produce 3-4 million calories per acre and potatoes can produce 6-8 million calories per acre. But what about apples? I've harvested one Gravenstein tree and will do the next one today. I got 288 pounds of fruit off the first tree and my orchard is on a grid of 200 trees per acre. That means this tree produced the equivalent of 57,600 pounds per acre. At 236 calories per pound for raw apples (Source: www.caloriecount.about.com), this equals 13,593,600 calories per acre for an apple tree producing less than 300 pounds per tree. This is 3.4 times the calorie production for wheat and 1.7 times the value of potatoes (using 4 million calories per acre for wheat and 8 million for potatoes - the upper end of the spread).
Now let's consider commercial apple production. Back when I was a migrant worker in the 70's, most of the apple orchards I worked at were on a 200 tree per acre grid and a common yield was half a bin per tree. A bin is 25 boxes and a box is 40 pounds, so a bin = 1000 pounds of apples. Many times I picked a whole bin per tree, so averaging a half bin per tree is a robust average. At half a bin, or 500 pounds per tree and with 200 trees per acre, the calorie value of commercial apple production jumps to 100,000 pounds per acre, or 23.6 million calories per acre. This is nearly 3 times the calorie yield of the most optimistic calorie value for potatoes and almost 6 times the most optimistic calorie value for wheat! Obviously, apples are a good deal for farmers (like me) who want to get the maximum calories per acre, while still maintaining diversity of crops. And this does not even address the health aspects of eating apples.
When we purchased our little farm in 2004, one of the first things we did was put down more apple and pear trees. I was a migrant worker for 8 years, from 1974 to 1982, and I went back and picked apples again in 1993. I have always liked apples and I have quite firm ideas on pruning, thinning and harvesting. We have mostly semi-dwarf trees and a mix of 16 varieties of apples, 10 varieties of pears (including Asian pears), 2 varieties of plums and 4 kinds of cherries. Our orchard space is only 1/4 of an acre and most of the trees are still quite young, as there were only 7 fruit trees on the place when we started. I have Mason bees for pollination, but I also plant specific crops, like borage, that are good feedstock for bees. Thus, we attract plenty of native bees and we have no problem with pollination.
As we prepare for peak oil and the next economic shock, it is imperative that homeowners (and even renters!) grow some of their own food. Since apples can produce so many calories per acre, it is to everyone's advantage to plant more apple trees. Planting trees is an investment in the future and it takes 4-5 years for production to come on, but once the apples start producing, the amount of high-quality food produced is astonishing. Apples do take some management, but the labor costs are quite low compared to vegetables. If there is a perfect food, from both a farming and a consumer perspective, it is an apple. Johnny Appleseed was no dummy - even though he focused on cider production, his great idea led to many beneficial results for eating apples.