The outer bark (rhytidoma) of tree trunks (Fig. 1) is exposed to long-acting effects of air pollution and stemflow transporting atmospheric deposits from tree crowns. We have been engaged in determining selected physical and chemical parameters of bark extracts (pH, electric conductivity, concentrations of SO₄^--, NH₄⁺, NO₃^-, Cl^-, F^-, selected metals) as suitable indicators of air quality since the 1980s. Bark extract monitoring has been used in the following projects:

• Surveys of the bark extract parameters of spruce trees along mountain altitudinal profiles (1989–2010)

We have repeatedly determined the parameters of spruce bark extracts (e.g., pH, E. C., selected anions or metals) along linear altitudinal profiles in the Krkonoše Mts. (Malý Špičák – Vrchlabí), in the Kozákov-Tábor range (Lomnice nad Popelkou – Kyje), and in the Ore Mts. (Výsluní – Kadaň). A reference forest stand is situated near the Košetice meteorological observatory in the Pelhřimov district (e.g., [1], Tab. 1 and Fig. 2).

[1] Suchara I. (1992): Experience in air quality indication through leaf enzyme activities and bark extract characteristics.- In: Boháč J. (ed.) Bioindicatores Deteriorisationis Regionis, Proceed. Sixth Int. Confer., České Budějovice 1991, Inst. Landscape Ecol., České Budějovice, p. 109–116.

• Determining tree bark extract parameters in Prague’s parks (1989–1990 and 2009–2010)

We determined the basic extract parameters of oak, pine and spruce bark collected in parks in inner Prague and in a suburban zone (e.g., Karlovo náměstí, Riegrovy sady, Havlíčkovy sady, Stromovka, Petřínské sady, Hvězda, Hloubětín, Michelský les) at the end of the 1980s [2]. More recently, we collected tree bark specimens in the parks on the same sites to determine the bark extract parameters repeatedly.

[2] Suchara I. (1993): The use of some urban park woody species in an estimation of air pollution level. Tree bark extract characteristics.- Zahradnictví, Praha, 20: 241–260.

• Determining the extract parameters of spruce bark from Průhonice Park (1990–1991 and 2009)

We collected spruce bark specimens at about 20 sites in Průhonice Park. In the bark extract we determined the pH, E. C., concentrations of SO₄^2-, Cl^-, F^-, NO₃^-, NH₄⁺ ([3], Fig. 3). At the same sites in the park we collected spruce bark samples for repeated analyses of the bark extract parameters.

[3] Suchara I., Sucharová J. (1992): Potential use of tree bark as a universal (bio)indicator of environmental level. A glimpse at its history and some perspectives.- In: Hrubík P. (ed.) Int. Symp. Occas. 100th Anniver. Arboretum Mlyňany Foundation 1892–1992, Veda, Bratislava, p. 144–153.

• Monitoring the extract acidity of spruce bark collected across the Czech Republic (1995, 2005, 2010)

We repeatedly collected spruce bark samples at 180 - 260 sites evenly spread throughout the Czech Republic. In addition to other parameters, we repeatedly determined the bark extract acidity (pH). The investigations document a clear tendency toward a reduction in bark extract acidity throughout the county (e.g., [4], [5], [6], Fig. 4, Fig. 5).

[4] Suchara I. (2010): Kyselá reakce smrkové kůry a kvalita ovzduší v ČR během posledních dvaceti let.- Ochrana Ovzduší 22, 3: 28–34.

[5] Suchara I., Sucharová J. (2007): Možnosti využívání stromů jako bioindikátorů kvality ovzduší.- Ochrana Ovzduší 20 (39), 1: 22–26.

[6] Suchara I. et al.(2014): The influence of contrasting ambient SO2 concentrations in the Czech Republic in 1995 and in 2010 on the characteristics of spruce bark, used as an air quality indicator.- Ecological Indicators 39: 144–152.

• Determining total mercury concentrations in oak bark around the Spolana chlor-alkali plant, Neratovice (2004)

We determined the distribution of total mercury in oak bark in the surroundings of the Spolana chlor-alkali plant near Neratovice after a catastrophic flood episode in 2002 ([6], Fig. 5).

[6] Suchara I., Sucharová J. (2008): Mercury distribution around the Spolana chlor-alkali plant (central Bohemia, Czech Republic) after a catastrophic flood, as revealed by bioindicators.- Environ. Pollut. 151: 352–361.

• Determining of selected radionuclide activity distributions in spruce bark collected in the Czech Republic in 1995

After agreement with the National Radiation Protection Institute (SURO), the Department of Biomonitoring provided archived samples of spruce bark collected in 1995 and 2010 for determining of ¹³⁷Cs, ²³⁹Pu, ^239,240Pu and ⁹⁰Sr activities.  The distribution of these radionuclides is related to the fallout after the Chernobyl accident and the pre-Chernobyl period of atmospheric nuclear weapons tests. We determined the aggregated transfer factors for bark ¹³⁷Cs activities ([7], Fig. 6). We evaluated the specific activity of these radionuclides in [7] and [8].

[7] Suchara I. et al. (2011): Retrospective determination of ¹³⁷Cs distribution in spruce bark and bark aggregated transfer factor in forests on the scale of the Czech Republic ten years after the Chernobyl accident.- Sci. Total Environ. 409 (10): 1927-1934.

[8] Hölgyle Z. et al. (2013): Retrospective determination of ²³⁸Pu, ^239,240Pu and ⁹⁰Sr activities in the outer bark of Norway spruce (Picea abies (L.) Karst.) collected at various sites in the Czech Republic.- J. Radioanal. Nucl. Chem. 297: 19–25.