Concepts for Understanding Fruit Trees, by T.M. DeJong
Professional fruit growers looking for a good understanding of the principles of how fruit trees function on a fundamental level have much to gain from Dr. Ted DeJong’s Concepts for Understanding Fruit Trees. Packed into just over 100 pages is a systematic description of how shoots, roots, leaves and fruits play a part in the tree’s carbon economy.
DeJong’s thesis is that “There are a few unifying concepts that make much of the general behavior and responses of fruit trees to the environment or management fairly easy to understand and predict.” He sets forth in the introduction how fruit production is ultimately a function of two factors: how efficiently the tree captures external resources (sunlight, carbon dioxide, nutrients, water) and how efficiently it distributes those resources. After a short description of the resource capture (carbon production) process, he focuses for the rest of the book on how the various organs of the tree — shoots, roots, fruits, etc. — compete for the distribution of that carbon. DeJong argues that “understanding the scheme of assimilate distribution within fruit trees allows for a more complete understanding of how fruit trees function and how horticultural manipulation of fruit trees can influence the growth of various organs, as well as how specific organ growth can be optimized to meet specific objectives.”
The book is short but dense, with plenty of helpful illustrations. There is lots of terminology, though, much of which — depending on the reader — requires keeping a good reference guide or the internet close at hand.
Unpacking these terms is the foundation of the book, though. Understanding the differences between, for example, proleptic, sylleptic and epicormic shoots, and how they are produced and grow, helps explain why different types of pruning cuts lead to different tree architecture. Heavy pruning produces epicormic shoots — commonly known as water spouts or suckers — which can take energy away from the tree and can shade out leaves that are feeding fruit. But hard heading cuts do stimulate strong vegetative growth, which is desirable at certain times and places.
Thinning is also a big takeaway of the book. DeJong highlights how earlier thinning is better because it preserves energy from going into fruit that will later be thinned off; this saved energy is like a compounding bank account.
In the final chapter, DeJong describes the integration of tree energy sources and sinks and the challenges in developing models for perennial plants. Models for annual plants are much simpler, but DeJong and his team at UC Davis succeeded in developing a computer model for peaches, L-Peach, which can graphically depict what happens when various pruning practices or other external factors are considered. Surely this is an area that AI will eventually be able to contribute to — simple-to-use software that shows the effects of any given pruning cut and the regrowth of various shoots, roots, fruits and leaves over time, for various types of tree crops, would be an invaluable educational and research tool for professionals and hobbyists alike.
Concepts for Understanding Fruit Trees is not a comprehensive treatment of pomology, and some of the topics ecological growers are likely to consider aren’t touched upon — notably the role of soil, leaf and endophytic microbes in receiving plant-produced carbohydrates and returning vital nutrients. And DeJong’s focus is on the tree’s individual organs instead of the tree as a wholistic system: “The control of what trees do and how they function lies in the individual organs that make up the tree, not in the tree as a whole.” Trees are “[c]ollections of semi-autonomous organs that are dependent on one another for resources but control their own destiny.”
But the book is an indispensable guide on the “why’s” of important tree management practices such as pruning and thinning and is a worthwhile part of any orchardist’s bookshelf.
