What is alternate bearing in fruit trees?

L’alternance (ou « portance alternée », ou encore « biennalité ») est un phénomène fréquent chez de nombreux arbres fruitiers où l’arbre produit une récolte abondante une année, puis très peu ou pas du tout l’année suivante. Autrement dit, épisodes de forte fructification et années maigres se succèdent de façon cyclique. Causes principales - Épuisement des réserves énergétiques : une année de forte production mobilise beaucoup de glucides et d’acides aminés nécessaires à la formation des bourgeons floraux pour l’année suivante. - Inhibition physiologique : la présence d’un fort chargement en fruits émet des signaux hormonaux (auxines, gibbérellines) qui empêchent la différenciation des bourgeons floraux. - Stress abiotiques ou biotiques : sécheresse, gel au moment de la floraison, carences nutritives, attaques de parasites ou maladies aggravent le phénomène. - Prédisposition variétale : certaines variétés sont naturellement plus sujettes à l’alternance. - Mauvaise gestion culturale : taille inappropriée, fertilisation déséquilibrée, pollinisation insuffisante ou faible formation de bourgeons en raison d’un excès de vigueur végétative. Signes observables - Année « pleine » : beaucoup de fruits souvent de petite taille, branches fortement chargées. - Année « creuse » : très peu de floraison, peu de fruits ou fruits très clairsemés. - Variations de calibre et qualité des fruits liées au manque d’éclaircissage. Mesures de prévention et de gestion - Éclaircissage des fruits (manuel ou chimique) pendant l’année de forte production pour réduire la charge et favoriser la formation de bourgeons floraux. - Taille équilibrée visant à maintenir un bon rapport bois/fruits et à limiter l’excès de production. - Fertilisation équilibrée : apporter azote, potassium et éléments secondaires en quantités adaptées pour reconstituer les réserves sans stimuler excessivement la végétation. - Irrigation régulière en période critique pour éviter le stress hydrique. - Choix de variétés et de porte-greffes moins prédisposés à l’alternance. - Contrôle des maladies et ravageurs pour éviter d’ajouter du stress à l’arbre. - Dans certaines cultures, recours à des produits régulateurs de croissance ou agents de formation florale sous conseil technique (usage encadré selon réglementation). En pratique, la lutte contre l’alternance combine interventions immédiates (éclaircissage, irrigation) et mesures à long terme (taille, fertilisation, choix variétal). Avec une gestion adaptée on peut réduire fortement l’intensité du phénomène et obtenir des récoltes plus régulières. Si le problème est marqué, n’hésitez pas à demander conseil à un technicien arboricole local pour diagnostiquer causes spécifiques et proposer un plan cultural adapté.

This phenomenon is defined as the irregularity of a tree’s production from one year to the next. In other words, fruit trees will produce an abundance of fruit one year and an almost total absence of fruit the following year.

This phenomenon is a major problem for professional fruit producers who see the price of their harvests vary greatly from one year to the next. For incredulous amateur gardeners, it often raises many questions.

This major orchard problem mainly affects high-stem orchards (fruit trees whose graft union is located at 180-200 cm in height) and especially species known as pip-bearing orstone : Apple trees, Pear trees, Cherry trees and Apricot trees.

In warmer Mediterranean-type climates, Olive trees and certain citrus trees are also sensitive to alternate bearing.

Apple trees and pear trees are sensitive to alternate bearing, as are apricot trees

What are the causes?

Both external factors (climate) and intrinsic factors (tree genetics, species, variety, rootstock and age of specimen) explain alternate bearing in fruit trees.

Climate

As any good gardener knows, climate in one year is never the same as in another. Productivity of a fruit tree depends on climatic conditions during flowering. If, unfortunately, during opening of a tree’s flowers a succession of rain and frosts were to occur, almost an entire harvest would be lost that year. This can start a cycle of alternate bearing, as your tree will instinctively (to reproduce) produce a profusion of flowers the following season to make up for its losses. This will cause an overabundant fruit set that will inevitably lead to reduced fruiting the following year and so on… Alternate bearing can therefore occur because of an adverse climatic event.

Droughts that have affected our temperate climates for years can also induce a cycle of alternate bearing. Indeed, fruit trees are then deprived of water and certain minerals, causing fruit abortion. Thus begins the vicious circle of alternate bearing.

Factors intrinsic to trees (endogenous)

When a tree bears fruit, seeds forming inside the fruits produce hormones (called gibberellins) which will inhibit formation of fruit buds (spurs) for the following year. This competition between fruit formation and vegetative differentiation of buds is main cause of alternate bearing in orchards.

When fruits form, they also draw huge amounts of sap and mineral elements to develop, which leads to fewer fruit buds forming for the following spring.

Frost as well as drought particularly affect fruit production (here a pear tree)

How can I limit the phenomenon?

There are several preventive measures against this phenomenon, but it remains very difficult to counter completely even for professionals.

Here are the best ways to limit alternate bearing:

• Buy varieties least sensitive to this phenomenon by asking advice from your nursery.
• Thin out excess fruit in years of “overproduction”: After pollinators have fertilised the flowers, assess fruit load of your tree. If tree appears to be carrying too many fruits, shake some branches and remove fruits that are too close together. This operation will give you fruit of good calibre and also encourage tree to form fruit buds (called courçons) for following year. This operation is usually carried out in June.
• Ensure fertility and water supply for the tree: Ensure soil is sufficiently manured (amended) with organic matter beneath tree and that it suffers as little as possible from summer droughts.
• Don’t forget to prune your trees (during appropriate period) so that crown of your tree is airy and well lit. This will stimulate production of future fruit buds. Overall, we train tree to a conical rather than rounded shape to improve sunlight exposure.
• Treat fungal diseases affecting foliage that could compromise production.
• Limit growth of overly vigorous shoots and bend branches to encourage formation of flower buds.
• Combat late frosts… this is almost impossible for amateur gardener but professionals have a whole range of tricks to raise temperature during late‑spring frosts (braziers, hot‑air blowers, helicopters flying over the orchard…).
• If your tree does not produce many flowers, do everything possible in your garden to attract pollinators: sow melliferous plants, install insect hotels, leave a pile of dead wood near tree, adopt or sponsor a hive…

Fruit thinning (here on an apple tree) is carried out in June when fruits are about 2 cm in diameter