Relations entre phénologie de la croissance souterraine et aérienne de noyers hybrides en systèmes agroforestiers tempérés
Résumé
The study of phenology is primordial to understand tree response to climate change. Although many studies have examined shoot phenology, the difficulties in observing root system growth have resulted in a poor understanding of root phenology. As root system dynamics are considered as playing a major role in carbon cycling and sequestration, it is necessary to overcome methodological difficulties, so that root demography can be studied in the field and over several years. Our first objective was therefore to develop and evaluate methods for studying root system growth in the field. Studying mature hybrid walnut (Juglans L.), growing in agroforests, We showed that smartphone scanners are the best adapted tool for acquiring high quality images of tree roots growing in field rhizotrons. However, time-lapse cameras were good alternatives when a fully automated method was required (up to 4 months autonomy). Our second objective was to determine the main drivers of walnut hybrid root growth in field conditions. In particular, (i) We hypothesized that shoot and root phenologies were asynchronous, (ii) We evaluated the effect of environmental factors on root growth along a latitudinal gradient comprising three climates (Mediterranean, oceanic and continental) and (iii) We determined how phenology and environmental factors influenced root dynamics depending on root morphology and topology. Results show that flushes of fine root growth are not synchronized with budburst and leaf expansion, but are synchronized with stem and coarse root radial growth. Soil temperature was overall the main driver of root growth, and to a lesser extent, soil humidity, but which had contrasting effects on root growth. Root topological order had a major influence on root response to environmental variables, reflected in root elongation, production and longevity. Our third objective was to put more focus on the relationships between deep fine roots and shoot phenology of walnut trees (Juglans negra x Juglans regia L.) growing in Mediterranean alley-cropping system. Results show that fine root phenology was asynchronous with leaf phenology in all soil depths. Whereas, roots at the two upper soil layers only were synchronous with radial growth of both stem and coarse roots. Roots at deeper soil layers were independent of the climatic variables and flushed later than those at shallow soil layers reaching to maximal rate of growth in autumn and winter. Results will be highly useful not only for a better understanding of the role that root systems play in the carbon cycle, but also for tree ecophysiological models. Future studies should expand the knowledge gained here into a global understanding of the drivers of tree root initiation, growth and longevity, through the creation of databases and the use of meta-analyses.
Domaines
Sciences du Vivant [q-bio]
Origine : Fichiers produits par l'(les) auteur(s)
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