Tree-ring elemental concentrations in oak do not necessarily passively record changes in bioavailability

Proxy records of pollution are necessary when direct instrumental monitoring of the environment is not available. Longevity, fixed location and visible annual growth rings make trees attractive as potential sentinels of environmental change. Chemical variations observed across tree-rings have led to the suggestion they can potentially be used to generate chronologies of changes in biogeochemical cycles (Watmough, 1997, Padilla and Anderson, 2002, Witte et al., 2004). For tree-ring elemental concentrations to be effective proxies for elemental bioavailability, trees must be passive monitors and the physiological effects on uptake of elements should be predictable. There must be minimal elemental mobility across growth rings and the elemental concentrations in the tree-ring must be proportional to bioavailability (Momoshima et al., 1995, Watmough, 1997), which may not always be the case (Watmough and Hutchinson, 2002). Failure to meet these conditions means that tree-ring elemental concentrations are not passive monitors and developing chronologies of environmental change based on tree-rings becomes difficult.

There has been considerable debate as to whether elemental concentrations of tree-rings passively record bioavailability in their environment or whether concentration trends are dominated by tree physiology (Chun and Hui-yi, 1992, Smith and Shortle, 1996, Garbe-Schönberg et al., 1997, Watmough, 1999, Watmough and Hutchinson, 2003). The effect of stress on tree-ring elemental concentrations has not been systematically studied, but could help assess whether tree-ring chemistry passively records environmental conditions. This study examines the relationship between stress and tree-ring elemental concentration of nutrients (Ca, Mg and Mn), non-nutrients that are chemically similar to Ca (Ba, Sr) and the anthropogenic pollutant Pb. Despite chemical similarity, these elements have been shown to display some dissimilar behavior in tree-rings including differences in radial mobility and uptake (Cutter and Guyette, 1993). If an environmental stressor affects geochemical cycling, similar behavior should be observed in both nutrients and chemically similar non-nutrients. Dissimilar response would indicate that environmental stress is affecting physiological functioning of the tree, which in turn differentially affects uptake and distribution of elements in the tree. Conversely, a lack of stress-related variation in uptake would be consistent with trees passively recording changes in their geochemical environment. Lead was analyzed as an element with an approximately known long-term pollution history in North America, with an increasing input function during the twentieth century that has been recorded in tree-rings with some success (Watmough, 1997, Watmough, 1999, Watmough et al., 1998).

The carbon isotopic composition of cellulose in tree-rings is affected by both the δ¹³C value of atmospheric CO₂ and physiological functioning of the tree (Farquhar et al., 1982, O'Leary et al., 1992, Lajtha and Marshall, 1994, Panek and Waring, 1997). Atmospheric CO₂ concentration has been increasing while its δ¹³C value has been decreasing since 1750, due primarily to the combustion of fossil fuels (Freyer, 1979, Freyer and Belacy, 1983, Leavitt and Long, 1989, O'Leary et al., 1992, Lajtha and Marshall, 1994, February and Stock, 1999). Farquhar et al. (1982) modeled the δ¹³C value of plant tissue with respect to atmospheric CO₂ for C3 plants and found the plant δ¹³C values were controlled by photosynthetic pathway, diffusion related fractionation, and the ratio of the partial pressure of CO₂ inside the leaf to atmospheric CO₂. When stressed, decreases in stomatal aperture can occur (Kozlowski and Pallardy, 1997). This decreases isotopic fractionation resulting in higher tree-ring δ¹³C values (Freyer, 1979, Martin and Sutherland, 1990, O'Leary et al., 1992, Lajtha and Marshall, 1994, Panek and Waring, 1997, Sakata and Suzuki, 2000, Savard et al., 2002). As a result, positive deviations in tree-ring δ¹³C values from the long-term trend of decreasing δ¹³C values of atmospheric CO₂ were used indicate periods of stress.

This study is divided into three parts: (1) an examination of changes in tree-ring elemental concentration during periods of stress, (2) an examination of the relationship among tree-ring concentrations, site characteristics and soil leachate concentrations, and (3) an assessment of the cohesiveness of site-normalized long-term non-nutrient elemental trends in tree-rings from sites with multiple trees analyzed. Together these data are used to assess whether tree-ring elemental concentrations passively record changes in their geochemical environment.

We characterize periods of stress in individual red oak (Quercus rubra) and white oak (Quercus alba) based on increases in δ¹³C values in cellulose of tree-rings from trees at sites proximal and distal to urban areas in Ontario, Canada. Tree-ring elemental concentrations during these periods of stress are compared to long-term trends to assess the extent to which variations in tree-ring chemistry may be physiological responses to stress. Tree-ring elemental concentrations were also considered with respect to soil characteristics such as soil pH and leachable concentrations of each element. Comparing tree-ring elemental concentrations to soil characteristics examines the extent to which recent tree-ring chemistries reflect soil characteristics and potentially may be reconstructed using historic tree-ring chemical compositions. In addition, the normalized long-term trends in elemental concentrations from multiple red oaks at three sites across Southern Ontario were compared to examine whether the normalization would be informative in developing site-specific and regional chronologies.