More to tree rings than meets the eye

Tree rings record past environmental conditions, as well as the tree’s physiological response to those conditions. Although ring widths are easily measured indicators of annual tree growth, they are limited in what they can tell us about more detailed aspects of tree physiology. Fortunately, we can use stable isotopes to help us infer the tree’s leaf-level physiology in a given year. Leaves sample C and O from the atmosphere and H2O from the soil for photosynthesis, and thereby produce sugars. Some of these sugars are then converted to cellulose and laid down in the cell walls of the xylem (which makes up the wood each growing season). So, environmental conditions such as drought or extreme temperatures, that affect how the tree sampled C and O that year, are forever captured in the annual tree ring. Therefore, we are able to use isotopic ratios of C and O of tree ring cellulose (δ13C, δ18O) to reconstruct environmental growing conditions (such as the CO2 in the atmosphere and H2O from the soil). We can also use these ratios to understand the tree’s physiology: for example, δ13C is an indication of how open the stomata were during the growing season, and reflects the amount of carbon fixed in photosynthesis compared to the amount of water lost in transpiration.

Recently in Tree Physiology, Hartl-Meier et al. (2014) used tree ring widths and tree ring δ13C and δ18O to compare climate responses of spruce, larch, and beech, which have a range of drought tolerances, at three sites with varying soil moisture availability. δ13C and δ18O help paint a more complete picture of the species’ physiological responses to climate and these parameters showed a higher sensitivity to climate than the tree ring width signal. Cloud cover, which had the strongest correlations with δ13C and δ18O, was linked to climatic factors that influence stomatal behavior. The uniform sensitivity between climate and δ13C and δ18O was linked to differing hydraulic strategies across the species related to rooting depth, growing season length, stomatal behavior, and growth among the evergreen and deciduous species. Studies such as Hartl-Meier et al. (2014) show us the benefit of combining stable isotope analyses with traditional tree ring research, to generate a more nuanced picture of plant-environment interactions, and make it clear that tree cores can offer much more information about past climates and tree physiology than initially meets the eye.

Figure 1 from Hartl-Meier et al. 2014 describing the study sites
Figure 1 from Hartl-Meier et al. 2014 describing the study sites

Hartl-Meier C., Zang C., Buntgen U., Esper J., Rothe A., Gottlein A., Dirnbock T. & Treydte K. (2014). Uniform climate sensitivity in tree-ring stable isotopes across species and sites in a mid-latitude temperate forest, Tree Physiology, DOI: http://dx.doi.org/10.1093/treephys/tpu096