Post-doctoral researcher: Henri Cuny
Contract period: October 2013 – September 2014
Research topic: Influence of climate on wood formation dynamics
Research team and supervising scientists:
Research team: LERFob (Research of Forest and Wood Resources) INRA-Nancy Centre, France.
Supervisors: Cyrille Rathgeber, Meriem Fournier
Context– A remarkable aspect of xylogenesis is its plasticity, i.e. its capacity to generate various growth forms in response to environmental changes. The environmental factors, by influencing the dynamics (when, how long and how fast) of xylogenesis processes (division, cell radial enlargement and secondary cell-wall formation), leave permanent imprints in the wood formed. Thus, the structure of an annual ring represents a natural archive of environmental changes, with the tree-ring width, wood density, and cell anatomical features providing valuable bio-indicators used to reconstruct past environmental conditions in general and past climate in particular. Such retrospective analyses, however, still lack a precise understanding of the detailed mechanisms by which environmental changes influence tree-ring structure. Such information is crucial, because it is the only way to ensure the reliability of past-climatic reconstructions and climate change impact assessments. So, a more detailed mechanistic understanding of the influence of the environmental influence on xylogenesis is needed, which implies to evaluate the influence of climatic factors not only on the final tree-ring structure, but also on the processes responsible for the making of this structure.
Understanding the influence of climate on xylem cell differentiation is difficult, because the growth processes involved are coupled with other physiological processes (e.g., photosynthesis, respiration, plant nutrition, water uptake, carbon allocation), which are themselves subject to environmental influences. Thus, the influence of environmental factors on xylem differentiation may either be direct, through physical inhibition or stimulation of xylogenesis processes, or indirect, e.g. mediated through the synthesis and transport of growth substrates and/or regulators. Moreover, environmental influence is supposed to be specific to each process, according to the nature of the physiological events involved. The new xylem cells produced by division in the cambial zone enter in the enlargement phase where they undergo a marked increase in radial diameter through wall loosening, osmotic-driven water uptake, and deposition of new primary wall material. The basic element for cell enlargement is water, which exerts the pressure to extend the wall, and plays on the wall extensibility itself. So, cell expansion has long-term been depicted as one of the plant processes most sensitive to water stress, if not the most sensitive of all. Following enlargement, cells enter the wall formation phase, where they build a rigid, waterproof, and multi-layered secondary cell wall composed of cellulose, hemicellulose, and lignin. The secondary walls involved high-energy costs and represent the bulk of biomass allocation in trees. So, it should depend on the quantity of carbohydrates produced by photosynthesis, for which light is the indispensable energy source.
Temperate and boreal regions are characterized by regular seasonal cycles of environmental factors. In parallel, typical changes of cell dimensions occurred along conifer tree-rings, from the large thin-walled cells of earlywood mainly devoted to water transport, produced at the beginning of the season, to the narrow thick-walled cells of latewood that provide biomechanical strength, produced toward the end of the season. So, another way to study the influence of environmental factors on wood formation is to observe, in situ, how the seasonal cycles of environmental factors influence wood formation dynamics and the resulting tree-ring structure. Based on the idea that cell enlargement primarily depends on water, we expect that the changes in the kinetic of cell enlargement and resulting diameter along the ring are mainly related to changes in soil water content during the season. Considering that the formation of secondary wall depends on the availability of carbohydrates, we rather expect that the changes in the kinetic of wall deposition and resulting amount of wall deposited in the cells are primarily influenced by changing light conditions during the season.
Objectives and specific questions to be addressed – In this post-doctoral project, our objective was to understand the detailed mechanisms by which seasonal cycles of environmental factors (temperature, water and light) influence tree growth, wood formation dynamics and tree-ring structure. To do that, leaf phenology, tree growth and wood formation dynamics were monitored over 4 years (2007-2010) for more than 45 trees belonging to three conifer species (Norway spruce, Scots pine, and Silver fir) and grown in three mixed stands along an altitudinal gradient (from 350 to 650 m ASL) in the Vosges Mountains (France). In parallel, environmental conditions (e.g. air temperature, soil water content, and solar radiation) were monitored in parallel by two pairs of weather stations (one inside and one outside the forest) for each stand. For each xylem cell along the ring, the kinetics (duration and rate) of each developmental phase (enlargement and wall thickening) and its final result (cell radial diameter and wall thickness) will be related to the climatic factors occurring during the corresponding time-window.
Within this context, we are seeking to achieve the following:
- Statistical analysis of the data collected during the last seven years and the publication of the results in top plant science journals.
Methodological approaches –
- Producing a tree-ring chronology for each species and studied stand. This task involves the elaboration of cross-dated tree-ring series using existing material, but also possibly field and lab work.
- Data mining and elaboration of a clean database of climatic data for comparison with wood formation data.
- Data mining and elaboration of a clean database of automatic dendrometer data for feeding a global analysis in the framework of a European project.
- Completing the wood formation database for the bottom site and the last year.
- (Possible) Investigating the relationship between primary and secondary growth (and also leaf phenology).