Carbon dioxide limitation of the photosynthesis of Prunus avium L. seedlings inside an unventilated treeshelter

The equilibrium level in CO2 concentration inside tight treeshelters sealed in the soil and protecting young seedlings of Prunus avium L. has been mea sured. High night concentrations have been observed. Ar dawn, a fast decrea se occurred, and values as low as 100 mu l l(-1) were recorded in the morni ng. An increase during the afternoon was observed, with values remaining us ually below the ambient outside value in spring and in early summer, but ri sing above this value on hot days in summer or autumn. The significance of such unusual values is discussed. Net assimilation rates are diminished as the assimilation becomes chiefly CO2 Limited. CO and radiation dependant as similation curves for control and sheltered trees were recorded. The net as similation Aux density of the leaves of a sheltered tree during a standard sunny day was only 62% of the rate observed on non sheltered trees. Most of this reduction was a result of the low CO2 concentration in the shelter. C O2 concentrations above the ambient value were interpreted as the result of a negative assimilation rate inside tree shelters, resulting from temperat ures above 35 degrees C. The ventilation rate resulting from free convectio n through the top of the shelter increased with air temperature, bur remain ed too low to supply the tree with enough CO2. The impact of the soil CO2 r elease on the results is discussed. The results are in accordance with the reduction of the tree biomass accretion in treeshelters, and the need for v entilated shelters to overcome this problem is evidenced. Optimal ventilati on rates for maintaining the CO2 level at a minimum 300 mu l l(-1) value in the shelter are proposed. Such results contradict most previous publicatio ns and the possible explanations for the discrepancies are discussed, (C) 1 999 Elsevier Science B.V. All rights reserved.