A common suggestion in ecologically minded circles is for tree and perennial crops to replace annuals as the cornerstone of our agricultural systems. Doing so would avoid the constant soil disturbance and erosion associated with annual agriculture and feature plants with deeper drought resistant root systems. Trees also have a positive influence on local hydrology and climate. Yet despite all these supposed advantages tree dependent agricultural systems are a rarity. Chesterton’s fence is the notion that reforms should not be made until the reasoning behind the existing state of affairs is understood. A reformer often tears down old structures and erects new ones before they understand the situation. Could it be that the world didn’t develop tree based agricultural systems for sound reasons?

Chestnuts were collected from wild since long before agriculture and are still used in mountain regions where grains cannot grow. Yet they were only domesticated around 2000 years ago (much later than grains, and even later than fruit trees). This slower domestication is a major disadvantage to most tree crops, especially when coupled to their slower turn-over of generations. The first annual grains that mature every year have had around 10 000 generations since they were adopted by humans, while chestnuts with their minimum 5 year maturity and later start have at most 400 generations of domestication. Perhaps in another 50 000 years tree crops will have been as thoroughly domesticated as today’s annuals.

Rapid turnover of generations is also essential for managing pest and disease threats. These blights emerge on a fairly regular basis but resistance can emerge quickly and spread rapidly in annual crops. In the meantime fields emptied of the blighted crop can usually be sown with a minor crop species that isn’t affected. By contrast serious pests and diseases in tree crops are much more difficult to manage. For example when chestnut blight was introduced into North America it rapidly destroyed a continent’s worth of highly productive wild trees. Even though resistant Chinese and European chestnuts were available they were not replanted in any significant numbers. One possible curse of “zero maintenance” crops is that when they do eventually need maintenance nobody knows how to do so.

For more modern examples almost every major tree crop is facing serious and often intractable pest and disease issues. Industrial cash crops such as coffee and cacao are now grown in mass scale monocultures and both are experiencing serious disease threats. Apples and citrus are likewise under threat. This is the same unavoidable ecological consequence of increasing scale while decreasing diversity that is seen in monocultures of annuals, but the solutions are more difficult to find for trees. It isn’t only an issue of modern industrial agriculture. When peaches were introduced to the new world the American natives rapidly adopted them, growing large orchards from seed. Within a few generations pests and diseases emerged and the crop was abandoned in some places.

Tree crops suffer some serious disadvantages in terms of their agronomic properties. Most trees produce fruit that are highly perishable and contain mostly water. They need careful handling to eat fresh (limiting commerce without refrigeration) or time and energy intensive preservation. Tropical climates can feature more such crops since their production isn’t as highly seasonal, lowering the need for preservation. Nut crops are better candidates as staple crops but also have some complications. Most nuts are oil rich meaning that they are prone to rancidity after a few months of room temperature storage. Starchy nuts like chestnut and bunya can last a bit longer but need energy intensive drying in order to extend their shelf life. Grains and legumes by contrast dry readily with little additional energy provided the harvest season is not too wet.

The stature of most semi-domesticated trees also means harvesting can be difficult, or trees must be managed by intensive pruning, or breeding and grafting for dwarfing. In this current industrial age tree crops also often lack specialised machinery, with a chicken and egg problem of it not being worth developing specialised machines for a minor crop, and being unable to scale up a crop without suitable harvesting machinery. Trees also have undesirable habits such as masting, where a very large crop is produced at random intervals, or biennial bearing where a large crop is produced every second year. The final major issue is their slow establishment and maturity. Even if the eventual crop is larger and more valuable than an annual the reality is that farmers need to survive, either in terms of calories or dollars, in the short term. Would you take a job offering $10 000 per year for ten years, paid weekly, or another that will pay you a million dollar lump sum in a decade? While the latter makes more sense the former would be the only practical option for someone who needed to eat every day.

A survey of history reveals that there were a few tree based agricultural systems around the world. One mysterious example is the pre-Columbian culture throughout the Amazon basin. Analysis of forest composition and arrangement and extensive earthworks has revealed that the entire basin was at one time intensively managed, with tree crops like Brazil nuts distributed and managed by humans. This society collapsed so thoroughly due to the spread of epidemic diseases that we have only recently realised they existed. By contrast the annual crop dependent cultures in other regions were able to recover from the epidemics and continue to this day. If tree based agriculture is more resilient and needs less maintenance then why did the survivors in Amazonia not bounce back during the many decades between the epidemics and the arrival of European colonists?

The Pacific islands were populated by Polynesian peoples that carried several tree crops with them, including the highly productive breadfruit and breadnut trees. Colonists found the natives were too lazy to work in the sugar plantations because they had a ready form of food available from the breadfruit trees, so responded by cutting them down. Navajo peach orchards were similarly destroyed by colonial Europeans to cripple their agriculture. The vulnerability of slow maturing trees to war and conflict is another major weak point, along with more mundane pressures for timber and firewood. Similar tree destroying strategies are found in ancient history during war and continue to this day, with a recent report of a Palestinian olive grove being levelled.

So where does this leave the place of tree crops in agriculture, especially as we move toward the twilight years of industrialization? As always tree crops have the potential to serve important accessory roles, providing diversity in the diet and continuing to produce something during years when annuals fail and serving a wide range of important ecosystem functions. With the passing of many thousands of years they have the potential to improve, though will always be limited due to the inevitable impacts of serious disease epidemics. More intensive breeding to improve traits will likely make this problem worse by narrowing the genetic base over time. Instead tree crops are better off being left as semi-domesticated species that populate the fringes of our agricultural systems while annuals continue to take center stage. Many of the problems attributed to annuals are really issues of industrialization, economic exploitation and over-population, issues that no amount of plant breeding, whether of annuals, perennials or trees, can ever address.

Captain Bligh transplanting breadfruit trees from Tahiti to the Caribbean. The African slaves there decided they didn’t like eating them. Thomas Gosse 1796.