The pecan tree is 1 of 17 species of hickory trees. Hickories are native to North America and Asia and formerly occurred in Europe, but Ice Ages, beginning about 2.5 million years ago, wiped them out there. European mountains have an east to west orientation, while American mountains are oriented north to south. Hickories prefer temperate climates, and the east-west mountains blocked their retreat in Europe during glacial expansions. This explains why hickories and so many other tree species survived Ice Ages in North America but not in Europe.
Evidence of fossil pollen grains suggests hickory trees grew alongside dinosaurs during the late Cretaceous, though the oldest fossil hickory nut dates to about 34 million years ago. Most early hickory species had thin shells, but they evolved thicker shells about 38 million years ago in response to the evolution of tree squirrels. Squirrels love the nutrient rich nuts, so hickories evolved nuts with thicker shells, and the squirrels in turn evolved greater gnawing power. Evolution is a constant struggle. However, pecans retained the thinner shells of their early ancestors. This puzzled me because it seems as if squirrels would have eliminated all hickory species with thinner shells because they were easier to exploit. I wondered if pecans were a recent species, cultivated and spread by Native Americans. I’ve concluded however, based on certain lines of evidence, that pecans are an ancient species of hickory, not a recently evolved species manipulated by man.
Wild pecan tree.
Native range of wild pecan trees. Man has greatly expanded this range by planting pecan orchards. Georgia is now the leading producer of pecans, though they are not native to this state. Pecans need longer growing seasons than other species of hickory because their nuts mature later.
Genetic studies determined pecans have a large genetic diversity within populations. If pecans descended from human cultivation, they would have low genetic diversity because they would descend from a small population initially cultivated by man. I was also mistaken in considering squirrels the only major predator of hickory nuts. The pecan weevil ( Curculio caryae ) infests all species of hickories, and pecan trees growing in mixed stands with other hickory species have an advantage over their cousins. Pecans mature later in the season than other species of hickory. The pecan weevil hatches and emerges in August and will infest whichever hickories have developed kernels. Because thick shelled hickories mature before thin-shelled pecans, the pecan weevil will infest them first and go through their life cycle without ever infesting the thin-shelled pecan. Weevils will wait for pecans to mature, if no other hickory trees are available. So though thin shelled pecans may suffer heavier squirrel predation, they are less likely to have their nuts destroyed by weevils, if they grow near other species of hickory.
Pecan weevil larva. Pecans mature later than hickories. Though squirrels favor pecans over hickories, the later maturing pecans are less likely to be attacked by pecan weevils in mixed forests, giving pecans an advantage over other hickory species.
Pecans are native to river bottomland terraces where they grow in forests dominated by sycamore, sweetgum, and elm. Other subdominants in these terrace forests include water oak, box elder, silver maple, cottonwood, green ash, hackberry, and other hickory trees. Pawpaw, bamboo cane, pokeweed, grape vine, poison ivy, and green brier make up the thick undergrowth of bottomland forests.
Pecans hybridize with 5 other species of hickory. The nuts produced by wild pecans and hybrids vary in quality. Most are smaller and have somewhat thicker shells than cultivated varieties of pecans, and some even have high amounts of bitter tannins–all part of their ongoing evolutionary war with squirrels and weevils. Human cultivation of pecans on a large scale began circa 1900. Though Georgia isn’t part of the pecan’s native range, the state is the leading producer, and there is a large demand in China and India, resulting in high prices at the grocery store for the nuts.
Most cultivated varieties of pecans don’t mature before the first killing frost occurs in Midwestern states. A few small early maturing varieties can produce in the Midwest as can the hican–an hybrid cross between pecan and shellbark hickory.
Hican nuts–an hybrid cross between a pecan and a hickory. They produce earlier maturing nuts for northern locations that have frosts before pecans can fully develop.
The southern Mississippi River Valley has long served as a refuge for pecans and other hickory trees during glacial expansion cycles. Here, they grew in mixed Ice Age forests with spruce, beech, walnut, and oak. Pecans expanded their range north up the Mississippi River Valley following the end of the last Ice Age.
The oldest written recipe for pecan pie dates to 1886. It was a custard pie made with sugar, eggs, and butter. (Sugar/custard pies originated during the Middle Ages.) Pecan pies became more popular in the 1930s when some unnamed employee of Karo syrup invented a recipe for a pecan pie using syrup as well as sugar. I prefer my pecan pie made with maple-flavored corn syrup.
Pecan pie with whiskey maple cream sauce. I like my pecan pies made with maple flavored corn syrup.