Linking above- and belowground phenology of hybrid walnut growing along a climatic gradient in temperate agroforestry systems
Background and aims
Plant phenology is a sensitive indicator of plant response to climate change. Observations of phenological events belowground for most ecosystems are difficult to obtain and very little is known about the relationship between tree shoot and root phenology. We examined the influence of environmental factors on fine root production and mortality in relation with shoot phenology in hybrid walnut trees (Juglans sp.) growing in three different climates (oceanic, continental and Mediterranean) along a latitudinal gradient in France.
Eight rhizotrons were installed at each site for 21 months to monitor tree root dynamics. Root elongation rate (RER), root initiation quantity (RIQ) and root mortality quantity (RMQ) were recorded frequently using a scanner and time-lapse camera. Leaf phenology and stem radial growth were also measured. Fine roots were classified by topological order and 0–1 mm, 1–2 mm and 2–5 mm diameter classes and fine root longevity and risk of mortality were calculated during different periods over the year.
Root growth was not synchronous with leaf phenology in any climate or either year, but was synchronous with stem growth during the late growing season. A distinct bimodal pattern of root growth was observed during the aerial growing season. Mean RER was driven by soil temperature measured in the month preceding root growth in the oceanic climate site only. However, mean RER was significantly correlated with mean soil water potential measured in the month preceding root growth at both Mediterranean (positive relationship) and oceanic (negative relationship) sites. Mean RIQ was significantly higher at both continental and Mediterranean sites compared to the oceanic site. Soil temperature was a driver of mean RIQ during the late growing season at continental and Mediterranean sites only. Mean RMQ increased significantly with decreasing soil water potential during the late aerial growing season at the continental site only. Mean root longevity at the continental site was significantly greater than for roots at the oceanic and Mediterranean sites. Roots in the 0–1 mm and 1–2 mm diameter classes lived for significantly shorter periods compared to those in the 2–5 mm diameter class. First order roots (i.e. the primary or parents roots) lived longer than lateral branch roots at the Mediterranean site only and first order roots in the 0–1 mm diameter class had 44.5% less risk of mortality than that of lateral roots for the same class of diameter.
We conclude that factors driving root RER were not the same between climates. Soil temperature was the best predictor of root initiation at continental and Mediterranean sites only, but drivers of root mortality remained largely undetermined.
KeywordsAgroforestry Rhizotron Root elongation Initiation Mortality Longevity
Soil water potential
Early growing season
Late growing season
Root elongation rate
Root initiation quantity
Root mortality quantity
Thanks are due to Jérôme Nespoulous, Luis Merino Martin and Merlin Ramel (INRA) for technical assistance, to Camille Béral (Agroof, France) for help finding field sites and to the farmers M. Queuille and M. Becue for letting us work in their agroforests.
Funding for a Ph.D. bursary was provided by Campus France and the Kurdish Institute, France (AM), la Fondation de France (YM) and fieldwork was funded by the FOARDAPT project, INRA metaprogram AAFCC (Adaptation of Agriculture and Forests to Climate Change), France.
Compliance with ethical standards
The authors declare that they have no competing interests.
Availability of data and materials
The datasets about root survivorship generated and/or analyzed during the current study are available in the [Zenodo] repository, “https://zenodo.org/record/842737#.WbrdOrJJaCj” . The other datasets generated and/or analyzed during the current study are available from the corresponding author on request.
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