Species-specific responses of tree ring and leaf stable isotope signals in isohydric and anisohydric trees to drought

Eastern US forests, like many globally, have experienced a significant increase in temperature and summer drought. Recently, it has been suggested to classify tree's water use strategy in response to drought along the spectrum of isohydric to anisohydric species depending on their leaf-level hydraulic regulation. The differences in water use strategy lead to differences in internal leaf CO2 concentrations (Ci). Changes in Ci from stomatal conductance (gs) response to drought and changes in carbon assimilation rates (A) contribute to the tree's intrinsic water use efficiency (iWUE), which is the ratio of A to gs. Changes in iWUE are recorded in 13C/12C (d13C) ratios of stem wood in annual tree rings. Further information from the 18O/16O ratio (d18O) of wood is hypothesized to qualitatively separate the impact of A or gs using the dual-isotope method (Scheideggar et al. 2000). However, recent studies have questioned the applicability of the dual-isotope approach in cases of severe drought. In this study, we will use 3 years (2011-2013) of bulk leaf samples and tree ring cellulose from three isohydric and two anisohydric species in Morgan-Monroe State Forest to examine how the iWUE of each tree species responds to drought in d13C and d18O. To examine dual-isotope approach applied to tree ring measurements in a mechanistic way, we will compare the temporal changes of bulk leaf isotope measurements and leaf gas exchange measurements from an infrared gas analyzer. We will further use the annual dual-isotope signals in leaves and tree rings to test the coupling between leaf and tree ring signals. We hypothesize that (1) the iWUE of isohydric species will respond more sensitively to the severe drought in 2012 than the anisohydric species, and (2) the dual-isotope approach may be more applicable for isohydric species since isotope signals are mainly controlled by the stomata, not the leaf's complicated downstream process. This study will show that oxygen and carbon isotope signals may be more faithfully recorded in some species compared to others. This study will further enable us to explain the reason of uncertain dual-isotope approach and coupling of tree ring and leaf isotope signals by the species-specific physiological mechanisms.