Abstract
Tree rings are valuable proxies of past climate that allow inferring past growth responses to climate variability and extreme events, which is only possible considering that the relationship between tree growth and environmental conditions is linear and stable over time. However, in the last decades, divergent growth patterns have been observed in trees from the same forest stand, while unprecedented growth convergence was observed between trees from distant locations. Here, we use a new approach that considers convergent and divergent event years in two populations of Pinus pinaster Aiton in an altitudinal and oceanic-continental gradient to investigate what is triggering divergence and convergence in tree growth. The two study sites are Tocha (TCH), a plantation on sand dunes at low altitude near the ocean, and Serra da Estrela (SdE), a mountain plantation located at 1,100 m altitude, 100 km away from the ocean. The analysis of the climatic conditions in convergent growth years revealed that positive convergent growth was related to above average precipitation in previous winter and that negative convergent growth was related to below average precipitation during the growing season. Divergent growth revealed a temperature signal with warmer temperatures in spring promoting growth in SdE and growth reduction in TCH. Convergent growth was associated with a regional climatic signal, reinforcing the importance of precipitation in the Mediterranean region, and divergent growth to site conditions, revealing local adaptation. The information gathered in this study gives valuable insights on the response of P. pinaster to extreme climatic events, allowing for more adjusted management strategies of Mediterranean pine forests.
Highlights
Dendroclimatology assumes that tree growth-climate responses are stable over time (Fritts, 1976)
Regarding Standardized Precipitation Evapotranspiration Index (SPEI), there was a positive correlation with previous September, October, and November and current June, July, and August in TCH, whereas in Serra da Estrela (SdE) there was a positive response from previous November to current July, except for May
The new method used in this study, which consisted in dividing event years in convergent and divergent, revealed that convergent growth, whether positive or negative, was triggered by identical climatic conditions in both sites
Summary
Dendroclimatology assumes that tree growth-climate responses are stable over time (Fritts, 1976). The divergence problem is characterized by an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings (D’Arrigo et al, 2008; Wilmking et al, 2020). It is defined as the weakening of temperature response in previous temperature-limited northern. Wilmking et al (2004) reported a divergence in trend in a study across Alaska, where some trees presented growth reduction while others increased growth in response to the recent temperature increase. Trees that showed growth reduction revealed a shift in climatic sensitivity, with increasing temperature inducing drought stress, and the limiting factor shifting from temperature to precipitation (Wilmking et al, 2004, 2005)
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