Polycyclism, a fundamental tree growth process, decline with recent climate change: the example of Pinus halepensis Mill. in Mediterranean France

Polycyclism, the ability for a plant to produce several flushes in the same growing season, is a key process of plant development. Polycyclism frequency is likely to change with the anticipated climate trend, expected to impact plant growth over the next century. However, polycyclism processes are not well described in the literature, and an important lack of knowledge prevents any possible prediction for the twenty-first century. Aleppo pine is a good model to study polycyclism: it is known to produce up to four annual flushes in one growing season. In this study, we used architectural analysis to describe and reconstruct polycyclism processes, periodicity and frequency on Aleppo pine in a Mediterranean site for the last 15 years. We also assessed relationships between polycyclism frequency and climate. Since 1995, climate was far hotter and drier than normal in South-eastern France: polycyclism was significantly reduced, particularly after 2003 heat-wave, which delayed effect remains till 2008, exacerbated by repeated droughts. Morphologically, polycyclism is primarily influenced by twig vigour, status (principal/secondary and strong axes) and position (low, middle or top crown). Climatically, it depends on summer temperatures of the current and preceding year, rainfall of the first half of preceding year and winter rainfall. Previous year abundant rainfall combined with colder temperatures and high rainfall in spring or at the end of summer of the current year increase tricyclism frequency. Polycyclism is likely to decrease significantly in the twenty-first century due to a hotter and drier climate.