Dendrochronology of pedunculate oak (Quercus robur L.) in an old-growth pollarded woodland in northern Spain:: tree-ring growth responses to climate

Both time-independent and time-dependent responses of radial growth in pedunculate oak (Quercus robur L.) to climate were investigated in an old-growth woodland in the Cantabrian lowlands, northern Spain. This was done by using correlation analysis, bootstrapped response functions and Kalman filter analysis. Responses to climatic factors were explored in three different oak age-classes: young < 120 years, mature 170-225 years old, and old-growth trees 250-470 years. Time-independent climatic models showed age-specific responses. Between 55.2 to 66.8% of ring-width discovered variance was explained by weather conditions in the period 1940-1998. Radial growth of young oaks was limited by the temperatures in June of the year the ring was formed, while mature and old-growth oaks showed a negative response to winter and summer temperatures, and a positive one to summer precipitation. Time-dependent models revealed that the effect of several climatic variables on tree growth was not constant through time. This variability could be explained by physiological changes related to tree ageing, while it was unrelated to changes in other environmental factors. Overall, these results suggest that any assumption of an age-independent climate-growth relationship for oak in the studied locality is unreliable, and anticipate the probable responses of oaks growing in old-growth woodlands to global climatic changes.