Diversity of pollinators and pollenizers is key to successful chestnut pollination
Chestnuts are an ecologically and economically important fruit and forest tree species that have been cultivated for several centuries. Annual chestnut production now reaches over two million tons, yet little is known about the ecology of chestnut pollination. How is chestnut pollen dispersed? Is the quantity and quality of pollen limiting? What are the determinants of chestnut pollination success? Without an in-depth study of these mechanisms, it is difficult to design highly productive orchards. During his PhD studies and post-doctoral contract with INRAE in Bordeaux, France, Clément Larue has attempted to tackle these questions. Are chestnuts pollinated by wind, insects, or both? A few years ago, there was no available study to give a definitive answer to this question. Over several years, Clément performed pollinator exclusion experiments and monitored insects visiting chestnut flowers. In pollinator exclusion experiments, fruit set (i.e. the percentage of flowers that bear fruit) of branches accessible to insects were compared with fruit set of branches that could only be pollinated by wind as they were enclosed by insect-proof nets. Within these nets, scarcely any fruit was formed. Chestnut is therefore strictly entomophilous. During insect monitoring experiments, insects visiting chestnut flowers were photographed and identified. Only those insects that regularly visited both male and female flowers were considered as pollinators. Clément and his team found that calyptrate flies and to a lesser extent beetles were the main pollinators of chestnut trees. Bees – whether domestic or wild – only visited male flowers of chestnuts and therefore did not participate in pollination. The installation of beehives, therefore, will not improve orchard pollination. Instead, the preservation of non-bee pollinators is crucial to guarantee fruit production. In another study, Clément identified another important issue for chestnut pollination: pollenizer limitation. If the number and diversity of pollen donor trees planted in orchards are insufficient, fertilization will fall, resulting in very low fruit set in monovarietal or bivarietal orchards. Pollinators and pollenizer diversity are therefore key to the design of highly productive chestnut orchards.
Clément Larue won the ISHS Young Minds Award for the best poster presentation at the VII International Chestnut Symposium in Spain in June 2023.
The article is available in Chronica Horticulturae