Effect of ventilation on growth of Prunus avium seedlings grown in treeshelters

Treeshelters are widely used to protect tree seedlings against animal browsing or rubbing. They reduce the tree transpiration as a result of the low ventilation rate of the shelter. Photosynthesis is impeded by low levels of available CO2 due to poor gas exchange with the outside. We assess the impact of ventilating a treeshelter on the microclimate, on the transpiration and on the photosynthesis of protected Prunus avium seedlings grown either in containers with different water regimes or in the field. Ventilation was achieved either by numerous holes scattered along the shelter, or by a unique inlet of air at the bottom of the shelter, inducing a draught effect. Ventilation significantly reduced air temperature and increased CO2 availability. After one growing season, an increase in tree biomass resulted from the ventilation, ranging from +50% (container-grown trees without irrigation) to +55% (field-grown trees) and +85% (container-grown irrigated trees). These results confirm that most of the previous studies on treeshelters erroneously interpreted high CO2 concentrations in treeshelters. The draught effect with a single air inlet at the bottom of the shelter was as effective as numerous holes. Both tree transpiration and photosynthesis were stimulated by the ventilation. Irrigated trees took more advantage of the ventilation than non-irrigated trees. All treeshelters favoured above-ground biomass irrespective of ventilation. The size of the air inlet inducing an effective draught effect is discussed. Practical recommendations regarding the design and use of improved treeshelters are finally examined.