Stable isotope evidence of oak (Quercus rubra) susceptibility to disturbance events

Background/Question/Methods

       Decline events in North American forests may be promoted by climate change and associated disturbances.  A major oak (Quercus rubra) decline associated with increased activity of a wood-boring insect, recently observed in the Ozark Mountains of N. America, may have been promoted by a cycle of droughts beginning in the 1950s. We utilized tree-ring growth and stable isotopes to explore mechanisms governing tree survival during widespread decline.  Previous regional studies reveal growth patterns suggesting drought-induced susceptibility to decline in some trees, but carbon isotopes (δ¹³C) of tree-rings do not indicate greater water stress in dying trees.  We hypothesized that oxygen isotopes (δ¹⁸O) would help to elucidate tree water-use dynamics during stress and determine if changing responses to drought influenced susceptibility.  δ¹⁸O recorded in cellulose can reflect source water, atmospheric water demand, and biochemical fractionation; in conjunction with δ¹³C, it may elucidate patterns of tree response to drought. Our goals were to 1) examine δ¹⁸O in combination with δ¹³C from tree-rings to understand tree water and carbon dynamics in the decades prior to decline, 2) understand governing factors of differentially high and low numbers of infestations incurred among neighboring individuals and 3) determine if individual stress levels related to increased susceptibility to mortality following disturbances.

Results/Conclusions

            Declining trees experienced significantly reduced growth in the decades prior to the decline event; differences emerged after a significant drought in 1979, supporting the theory that drought disturbance was an influential factor in decline.  Consistent with prior research at the site but inconsistent with drought stress, tree-ring δ¹³C suggested no greater water stress in declining trees.  Oxygen isotopes are consistent with deeper water sources in declining trees at two of the three study sites. Correlations between δ¹³C and δ¹⁸O for apparently healthy trees revealed no significant trend (p=0.17, r= 0.34), but declining individuals exhibited a significant correlation between δ¹³C and δ¹⁸O (p=0.04, r=0.59).  The positive correlation between δ¹³C and δ¹⁸O in declining trees is consistent with stomatal control of photosynthesis in declining trees, while apparently healthy trees suggest that photosynthetic capacity may be a stronger regulator of their carbon capture.  We cannot explain the lack of enrichment in δ¹³C in declining trees, but δ¹³C and δ¹⁸O together suggest that tree responses to drought – rooting depth and/or stomatal control over photosynthesis – throughout the decades prior to a disturbance event are critical for determining survival during decline events that may occur decades later.