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

Abstract

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.

Climate Change Impact on Tree Architectural Development and Leaf Area

“…The resulting increase in summer and spring water stress may reduce tree growth in Mediterranean areas [32][33][34][35]. Raising temperature may also lead to phenological lags [36], particularly in the beginning and end of the growth season, with direct consequences on some primary growth processes and architectural development such as polycyclisme and branching rates [37,38].…”

“…This was systematically done for each parameter using the difference measured at equal cambial age for branches of respectively top vs middle and middle vs base of the crown, for the period 1995-2000 considered as accidentfree (figure 8, method in references [12,37]). …”

“…As illustrated by figure 10, the number of growth cycles within a year present huge interannual variations that are directly linked with climatic conditions [37]. Polycyclism can be viewed as a fast way to improve foraging abilities when growth conditions are favourable.…”

Pith: a new criterion for monitoring the architecture in Mediterranean pines

“…Although there were frequent exceptions for individual trees and years, this decrease could reflect the fact that GUs are preformed in the bud. The first GUs of an AS elongate at the beginning of spring Girard et al 2011). The preformation of these GUs occurs from the middle of the autumn of the previous year to winter .…”

“…Barthélémy and Caraglio 2007: they should not be confused with prolepsis (branches develop right after the end of their organogenesis, then after the end of the elongation of their mother axis) and syllepsis (branches develop simultaneously with their mother axis). Depending on the species, the GU or the AS can be identified by external markers such as shorter internodes, scars from bud scales or branching (Thabeet et al 2009;Girard et al 2011;Taugourdeau et al 2012). These markers can be used to mark out and date GUs and ASs and then allow the retrospective reconstruction of tree development in a nondestructive way.…”