Regeneration origin affects radial growth patterns preceding oak decline and death - insights from tree-ring δ13C and δ18O

Oak trees (Quercus spp.) develop a taproot system which extends several meters deep and enables them to survive periods of water deficit that occur during their long lifespan. Regeneration forestry practices disturb the natural development and proportion of oak root systems by repeated undercutting of the taproots of seedlings in nurseries, or the repeated cutting of stems in coppice systems. Thus, regeneration practices may affect tree growth and susceptibility to environmental stresses, including increasingly frequent and severe droughts. To test this hypothesis, tree-ring dendroclimatological and stable isotope data were used to assess the physiological responses of acorn-sown, planted, and coppiced Quercus robur trees of different health classes (healthy, defoliated and recently dead) to interannual climatic variations over the last 41 years. Annual basal area increment (BAI) was negatively and positively related with delta C-13 and delta O-18, respectively; although the intensity of these relations depended on tree regeneration origin and current vigor. Compared to healthy oaks, dead acorn-sown oaks exhibited significantly higher BAI, and lower delta C-13 and intrinsic water use efficiency (iWUE) during drought periods, but not during non-drought periods; in contrast, dead coppiced oaks showed lower BAI, delta C-13, iWUE and delta O-18 irrespective of climatic conditions. Acorn-sown and planted oaks growing fast at early stages of regeneration exhibited sudden decreases in BAI before dying following a severe drought period. In comparison, growth decline preceding the death of coppiced stems was progressive, suggesting that long-term C limitations to increasingly weakened trees underline drought-induced decline and eventual dieback. Different climate sensitivity of Q. robur trees of different regeneration origin suggests that regeneration forestry practices should be taken into account when adapting forest management plans to climate change.