Juglandaceae

The Juglandaceae, commonly called the walnut family, are a group of flowering plants in the order Fagales. They are mostly trees, though occasionally large shrubs, and are found naturally across the Americas, Eurasia, and Southeast Asia. The family contains about nine or ten genera and roughly 63 species, including well-known nut trees like the walnut (Juglans), pecan (Carya illinoinensis), and hickory (Carya).

The Persian walnut (Juglans regia) is a major global nut crop. Beyond nuts, walnut and hickory provide valuable timber, and pecan wood is prized as cooking fuel. Recent studies have examined the genetic relationships among the family's members in greater detail.

Description

Members of this family are typically trees, rarely large shrubs, with aromatic leaves that are usually arranged alternately, though in Alfaroa and Oreomunnea they are opposite. The leaves are pinnately compound or ternate, generally 20 to 100 cm long.

Pollination is by wind, and both male and female flowers usually grow in catkins. Most species are monoecious, but some are dioecious. The fruits are often mistaken for drupes but are actually accessory fruits: the outer covering is an involucre, not part of the carpel, making them drupe-like nuts.

Climate change

Climate change is projected to shift the range of Juglandaceae. Models for Europe suggest that Juglans regia will move northward to stay in suitable climates, with more severe shifts under greater temperature changes.

This expansion could extend beyond its native range, potentially making it invasive. Some members of the family form associations with both ectomycorrhizal and arbuscular mycorrhizal fungi. The significance of these dual partnerships is still under study, but they may offer ecological benefits, and this remains an active research area.

Biogeography

The family's distribution spans temperate regions of North America, Europe, and Asia, as well as tropical parts of Asia, Central America, and South America. The subfamily Juglandoideae likely originated and diversified rapidly in North America during the early Cenozoic, later spreading to Asia and Europe via the Bering and North Atlantic land bridges, as shown by phylogenetic studies and fossils.

Seed dispersal

Seeds are dispersed by rodents, especially squirrels, and by members of the crow family. Squirrels bury nuts for winter and often forget them, aiding dispersal. Humans have also spread seeds through farming.

Many members, particularly the black walnut (Juglans nigra), produce juglone (5-hydroxy-1,4-naphthoquinone), a toxic compound that hinders the growth of nearby plants. This chemical is present in most parts of the plant, though the nut kernel is edible.

The living genera are grouped as follows: Subfamily Rhoipteleoideae includes Rhoiptelea. Subfamily Engelhardioideae includes Alfaroa (gaulin), Engelhardia (cheo), and Oreomunnea. Subfamily Juglandoideae includes Tribe Platycaryeae with Platycarya, and Tribe Juglandeae with Subtribe Caryinae (Carya, including hickory and pecan, and sometimes Annamocarya) and Subtribe Juglandinae (Cyclocarya, Juglans, and Pterocarya).

Modern molecular phylogenetics, using chloroplast and nuclear DNA sequences, have clarified relationships within the family. Evidence suggests a whole genome duplication occurred in an ancestral species. Recent studies have also identified two new species within Engelhardia: Engelhardia anminiana and Engelhardia borneensis, both described in 2018.

For food, walnuts and pecans are widely farmed and eaten. The United States leads in pecan production, followed by Mexico, while China produces most of the world's walnuts, followed by the United States. Native Americans collected sap from hickory trees to use as a sweetener.

As timber, walnut wood is valued for its rich dark brown color and is used for cabinets and furniture. Butternut (Juglans cinerea) wood is lighter tan. Hickory wood is flexible and shock-resistant, making it popular for sporting equipment like bows, ax handles, skis, and lacrosse sticks, and was favored by American pioneers for building cabins.