Contribution of ambient ozone to Scots pine defoliation and reduced growth in the Central European forests: A Lithuanian case study
Introduction
Ambient ozone (O3) is considered to be one of the most important and pervasive phytotoxic agents whose effects are likely to increase in the future (Krupa and Manning, 1988, Hutunnen et al., 2002, Percy et al., 2003, Vingarzan, 2004). Its effects on plants are the result of a three-step chain of events: exposure, uptake, and biological effect (Tausz et al., 2006). Ozone can cause a wide range of symptoms: increase in respiration (Reich, 1983), mineral nutrient deficiencies (Schmieden and Wild, 1995, Utrainen and Holopainen, 2000), decrease in foliar chlorophyll content and development of necrotic spots (Reich, 1983, Smith, 1981, Chappelka et al., 1999), acceleration of leaf senescence and reduction in leaf life-span (Pell et al., 1999, Skelly et al., 1999, Stow et al., 1992), reduction in photosynthesis (Reich, 1983, Reich and Amundson, 1985, Utrainen and Holopainen, 2000), and ultimately reduced growth and productivity (Karnosky et al., 1996, Karnosky et al., 2006, Laurence, 1998, Matyssek and Innes, 1999, Manning, 2005).
Leaf and needle drop (reduction in foliage biomass, defoliation), and reduction in stem increment (radial or basal area) were chosen as the main objects in order to explore the possible effect of O3 on trees. Forest monitoring usually concentrates on these non-specific response indicators, which are the object of many stressors other than O3. However, due to the expected increase in future ambient O3 concentrations (Fowler et al., 1999, Percy et al., 2003) it is necessary to determine if exposure to ambient O3 levels actually affects tree growth and crown condition in natural forests (Manning, 2005).
Acid rain and ambient O3 are among the key factors resulting in spatial and temporal changes of tree crown defoliation (Reich, 1987, Guderian, 1985, Takemoto et al., 2001, Sandermann, 1996). Their combined effects differ significantly from the sum of individual effects due to their complex synergistic or antagonistic interactions (Bytnerowicz et al., 2007). Therefore, the process base of O3 damage to plants is still not fully clarified (Zierl, 2002, Matyssek et al., 2005) and the relationship between forest tree crown condition and O3 effects is still not well established (Manning, 2005, Ollinger et al., 1997, Percy and Ferretti, 2004, Paoletti, 2006). Most of the authors investigating O3 effects on forests suggest a need for studying O3 impacts in the context of other environmental factors (Kolb and Matyssek, 2001).
Relationships between ambient O3 and tree growth, focusing on physiological or biochemical effects on tree seedlings, have been extensively documented in artificial conditions (Chappelka and Samuelson, 1998, Skärby et al., 1998, Krupa and Kicker, 1989, Manning et al., 2004). However, too little is known about the effect of O3 on tree growth on a regional scale, where its effects may be subtle and difficult to detect (Paoletti, 2006, Percy and Ferretti, 2004) and studies often fail despite using sophisticated statistically-based approaches (Muzika et al., 2004). Data obtained in artificial conditions do not apply well to actual forest conditions and the growth of large trees (Sandermann, 1996, Manning, 2005, Kolb and Matyssek, 2003). Due to differences in the physiological behavior of seedlings vs. adult trees and chamber effects, extrapolation of these results to trees in the forest is impossible (Kolb et al., 1997, Manning, 2005).
Effects of O3 exposure have to be evaluated in the context of changing climate, i.e. increasing temperature, changes in water availability, increasing CO2 concentrations, increased available nitrogen [N] due to elevated levels of N deposition, and many other factors (Bytnerowicz et al., 2007). In this study we attempted to investigate the possible effect of natural and anthropogenic environmental factors on pine defoliation and stem growth, and quantify O3 contributions to the integrated impact of these factors. A working hypothesis was that in the central European forests peak O3 concentration could be one of the key factors resulting in Scots pine defoliation and stem increment reduction, reinforcing the effects of the other natural and anthropogenic stresses.
Section snippets
Materials and methods
Data on Scots pine (Pinus sylvestris L.) from permanent observation stands (POS) of the local forest monitoring network in National Parks over the period since integrated monitoring (IM) stations have been in operation (Aukstaitija IMS, LT-01; Dzukija IMS, LT-02; Zemaitija IMS, LT-03) were used.
Changes in ambient ozone and acidifying air compounds and their deposition
Data from IMS indicated a significant decrease in air concentrations of sulfur compounds and ammonia and their deposition until 2000 (Fig. 1). The air concentration of SO2 at Aukstaitija IMS (LT-01) decreased by 82% (from 2.73 to 0.49 μg S m−3), at Zemaitija IMS (LT-03) by 79% (from 2.22 to 0.47 μg S m−3), and at Dzukija IMS (LT-02) by 57% (from 3.0 to 1.3 μg S m−3). Wet sulfur deposition at LT-01 decreased by 67% (from 685 to 225 mg S m−2), at LT-03 by 58% (from 750 to 312 mg S m−2), and at LT-02
Discussion
The detection of the negative ambient O3 effect on mature trees under field conditions is more complicated than under artificial conditions, where experiments have not yet precisely determined if ambient O3 levels actually impair it (Manning, 2005). Years may be required before ecological change within ecosystems resulting from continuous exposure to toxic airborne concentrations become evident (Szaro et al., 2002). Numerous studies underscore the negative effect of ambient O3 on tree
Conclusions
In the Lithuanian forests, peak ambient O3 concentration is the key factor resulting in Scots pine growth reduction, while the key factors resulting in its defoliation seem to be air acidifying compounds and their deposition (O3 only predisposes these effects). However, when the effect of air acidifying compounds, their deposition and climatic factors was accounted for, the contribution of peak concentrations of ambient O3 to pine defoliation changes remained significant (p < 0.05). The effect of
Acknowledgements
Thanks are due to Dr Almantas Kliucius from the Lithuanian University of Agriculture for the help in the forest. We thank Dr Rasele Girgzdiene and Dr Dalia Sopauskiene from the Institute of Physics for providing data on environmental pollution. The authors also thank Ingrida Augustaitiene who helped prepare the manuscript.
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