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 measured. High night concentrations have been observed. Ar dawn, a fast decrease occurred, and values as low as 100 mu l l(-1) were recorded in the morning. An increase during the afternoon was observed, with values remaining usually below the ambient outside value in spring and in early summer, but rising 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 assimilation curves for control and sheltered trees were recorded. The net assimilation 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. CO2 concentrations above the ambient value were interpreted as the result of a negative assimilation rate inside tree shelters, resulting from temperatures above 35 degrees C. The ventilation rate resulting from free convection through the top of the shelter increased with air temperature, bur remained too low to supply the tree with enough CO2. The impact of the soil CO2 release on the results is discussed. The results are in accordance with the reduction of the tree biomass accretion in treeshelters, and the need for ventilated shelters to overcome this problem is evidenced. Optimal ventilation 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 publications and the possible explanations for the discrepancies are discussed, (C) 1999 Elsevier Science B.V. All rights reserved.