Extraordinary drought of 2003 overrules ozone impact on adult beech trees (Fagus sylvatica)

Abstract

The extraordinary drought during the summer of 2003 in Central Europe allowed to examine responses of adult beech trees (Fagus sylvatica) to co-occurring stress by soil moisture deficit and elevated O 3 levels under forest conditions in southern Germany. The study comprised tree exposure to the ambient O 3 regime at the site and to a twice-ambient O 3 regime as released into the canopy through a free-air O 3 fumigation system. Annual courses of photosynthesis (A max ), stomatal conductance (g s ), electron transport rate (ETR) and chlorophyll levels were compared between 2003 and 2004, the latter year representing the humid long-term climate at the site. ETR, A max and g s were Communicated by H. Jones lowered during 2003 by drought rather than ozone, whereas chlorophyll levels did not differ between the years. Radial stem increment was reduced in 2003 by drought but fully recovered during the subsequent, humid year. Comparison of AOT40, an O 3 exposure-based risk index of O 3 stress, and cumulative ozone uptake (COU) yielded a linear relationship throughout humid growth conditions, but a changing slope during 2003. Our findings support the hypothesis that drought protects plants from O 3 injury by stomatal closure, which restricts O 3 influx into leaves and decouples COU from high external ozone levels. High AOT40 erroneously suggested high O 3 risk under drought. Enhanced ozone levels did not aggravate drought effects in leaves and stem.

The dependency of the size-growth relationship of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica [L.]) in forest stands on long-term site conditions, drought events, and ozone stress

“…In terms of the combined effect of ozone and drought stress, our results match the findings of Löw et al (2006), who shows that drought counteracts the impact of O 3 in adult beech by stomatal closure.…”

Responses of native broadleaved woody species to elevated ozone in subtropical China

“…E-O 3 caused a large reduction in chlorophyll content and TAC, accompanied by a significant increase in MDA content (Tables 4 and 5), confirming that there was oxidative stress in most E-O 3 plants. Many studies have reported an increased concentration of antioxidants or elevated defense enzyme activities in response to O 3 exposure, which interacts with O 3 to neutralize its effect on plant tissues (Löw et al, 2006). Furthermore, the generation and accumulation of superoxide and/or peroxide may differ among species (Wohlgemuth et al, 2002).…”

The incorporation of extreme drought events improves models for beech persistence at its distribution limit

“…However, long-term climatic means do not express the effect of extreme weather events, which trigger vitality decline, appearance of pests or diseases leading to mortality (Löw et al 2006, Geßler et al 2007Lakatos and Molnár 2009). The probability of drought events is expected to increase in the second half of the twenty-first century (Schär et al 2004).…”