Synopsis of the CASIROZ Case Study: Carbon Sink Strength of Fagus sylvatica L. in a Changing Environment - Experimental Risk Assessment of Mitigation by Chronic Ozone Impact

Stuttgart

Thieme

2007

S. 163-180

4 Abb., 2 Tab., zahlr. Lit. Ang.

Unselbständiges Werk

200144117

Plant Biology 2007, 9(2) : Ozone sensitivity of adult forest trees, S. 163-180

Databases are needed for the ozone (O3) risk assessment on adult forest trees under stand conditions, as mostly juvenile trees have been studied in chamber experiments. A synopsis is presented here from an integrated case study which was conducted on adult Fagus sylvatica trees at a Central-European forest site. Employed was a novel free-air canopy O3 fumigation methodology which ensured a whole-plant assessment of O3 sensitivity of the about 30m tall and 60 years old trees, comparing responses to an experimental 2 x ambient O3 regime (2 x O3, max. 150 nl O3 l-1) with those to the unchanged 1 x ambient O3 regime (1 x O3 = control) prevailing at the site. Additional experimentation on individual branches and juvenile beech trees exposed within the forest canopy allowed for evaluating the representativeness of young-tree and branch-bag approaches relative to the O3 sensitivity of the adult trees. The 2Ø3 regime did not substantially weaken the carbon sink strength of the adult beech trees, given the absence of a statistically significant decline in annual stem growth; a 3% reduction across five years was demonstrated, however, through modelling upon parameterization with the elaborated database. 2Ø3 did induce a number of statistically significant tree responses at the cell and leaf level, although the O3 responsiveness varied between years. Shade leaves displayed an O3 sensitivity similar to that of sun leaves, while indirect belowground O3 effects, apparently mediated through hormonal relationships, were reflected by stimulated fine-root and ectomycorrhizal development. Juvenile trees were not reliable surrogates of adult ones in view of O3 risk assessment. Branch sections enclosed in (climatized) cuvettes, however, turned out to represent the O3 sensitivity of entire tree crowns. Drought-induced stomatal closure decoupled O3 intake from O3 exposure, as in addition, also the Ńphysiologically effective O3 doseń was subject to change. No evidence emerged for a need to lower the "Critical Level for Ozone" in risk assessment of forest trees, although sensitive tree parameters did not necessarily reflect a linear relationship to O3 stress. Exposure-based concepts tended to overestimate O3 risk under drought, which is in support of current efforts to establish flux-related concepts of O3 intake in risk assessment