Doctoral candidate : Yoran Bornot
University / Institution : University of Lorraine
Contract duration : 2013-2016
Research topic — The effect of the nutritional status of trees on their resilience to drought.
Research team and supervising scientists —
Host research team: Equipe PhytoEcologie, UMR INRA-UL (1137) Écologie et Écophysiologie Forestières (EEF)
PhD supervisor: Nathalie Bréda
PhD co-supervisor: Stéphane Ponton
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Context – Work in dendroecology performed in our laboratory has shown that the depressive effect could contribute to the presence of nitrogen, and to a lesser extent phosphorus, sensitizing spruce, beech (Picard et al. 1999) or fir (Becker 1992) to effects of drought. These studies also stressed the positive impact of calcium on the resilience of these species to drought. More recently, analysis of diseases affecting Douglas trees as part of the ANR Dryade project, showed improved growth recovery of stands planted in soil with augmented trophic richness (Sergeant et al. 2012).
While liming and fertilization practices are fairly widely used, particularly in Northern Europe, their effects on the functioning of trees is still largely unknown. Therefore, it is virtually impossible to explain conflicting results observed between different species (Sikstrom 1997), between the effects of one added nutrient or the associative effects of several added nutrients (Picard et al. 1999), or further still of the effect of varying doses of fertilizers (Saarsalmi et al. 2012).
Furthermore, radial growth and the concentration of carbohydrate reserves in Oaks were reduced by sudden water deficit that occurred in 2003, inducing deaths directly related to the intensity of water deficit suffered and possibly to the trophic richness of the massifs studied. This transitional degradation, followed by recovery, of the crowns in a beech forest located in the Vosges was better after 2003 in a plot treated by liming compared to a control plot (Ponton et al. 2009). A thesis project recently completed by the team (Genet 2009, directed by N. Breda) showed that, contrary to the assumptions based on mortality rates observed post-2003, fertilization had no significant lasting effect on patterns of carbon in the studied context (beech and oak allocation, device FD pots INRA). Furthermore, it has been shown that Liming: (1) increases allocation to reproduction function (Dan & Leroy 1974), and (2) has lasting influence on communities of Ectomycorrhizae (Rineau et al. 2010) whose role in tree resistance to drought has also been proven (Parke et al. 1983, Leigh & Churin 1997).
Objectives and specific questions to be addressed – The objective will be to determine what functions of the tree are impacted in the long term by modified nutritional status which could lead to improved resilience, or could alternatively further expose them to high water stress events. This question will be asked for two species, the Sessile Oak (Quercus petraea) and the Pseudotsuga Menziesii (Douglas) FIR, and will look at water, carbon and nutritional functions a priori considered contrasting. Both species interest managers in terms of strategic adaptation to climate change, one course of action for improving resilience to drought hazards.
The experimental work will aim to validate or disprove several non-independent, elementary hypotheses. Taking an integrative approach, the next phase will seek to reconcile the mechanisms highlighted in a model for general understanding. This conceptualization will contribute to the collective effort of modelling launched by the LABEX ARBRE project in collaboration with the research unit ESE.
Hypotheses to be tested:
1. Changing the soil fertility changes carbon acquisition. When the fertility of the soil is improved, more than photosynthesis itself (Green & Mitchell 1992, Dreyer et al., 1994), it is the quantity (leaf surface) and the quality (specific mass, nitrogen content) of foliage that are modified.
2. Changing the soil fertility improves the acquisition of hydro resources by changing the distribution and operation of the roots. When the fertility of the soil is improved:
2.1. the vertical pattern of distribution of fine roots is not modified (Faber et al. 1995, Genet 2009),
2.2. the efficiency of water aquisition and total hydraulic conductance are enhanced through mycorrhizal. The community of fungal symbionts and their enzymatic functions are modified (Rineau & Garbaye 2009, Rineau et al. 2010) allowing the tree, in the event of water stress, access to a larger reservoir of water and nutrients (Tobar et al. 1994 Courty et al. 2007, Rennenberg et al. 2009).
3. Changing the soil fertility changes patterns of carbon allocation [decrease in C allocation to roots (Ericsson 1995, Bakker et al. 1999, Litton et al. 2007 Epron et al. 2011), reduces the turnover of fine roots (Bakker, 1999), modifies the allometric relationships of trees (leaf area/sapwood), increases (Wargo 2002) or decreases (Goodsman et al., 2010) storage reserves in roots and increases reproductive allocation functioning (Dan & Leroy 1974).
Originality of the project – The effects of liming on tree growth and their soil were clearly identified by early collaborative work carried out between our team, and the research units Biogeochemistry of Forest Ecosystems (BEF) and Tree-Microbe Interaction research unit (UMR). However, the ecophysiological processes involved in improving resilience to drought through the nutritional state of the tree have been little studied to take into account the different compartments; soil, rhizosphere, tree. This represents a margin of significant progress which was identified by our teams and which serves as the basis for a ‘structural’ project (QLSPIMS) funded by LABEX ARBRE.
The originality of this project lies in that we have chosen adult trees for long term study. We are resolute in taking into account the ecophysiological mechanisms triggered by changes in nutritional status over long periods and which are sustainable over time. Responses to short term study will be addressed in a companion project also under the LABEX ARBRE.
Scientific and socioeconomic issues – French forests are often installed on soils that agriculture has abandoned because of their low fertility levels or constraint for their agricultural development (topography, stoniness, logging). While the forest itself allows for enrichment of organic matter through the annual litter fall and decomposition, logging operations can contribute over the long term to lower mineral fertility of the soil by storing and then exporting a portion of the mineral elements into woody biomass. Recent economic recovery together with increasing forest waste (branches, bark) contributing to the production of biomass energy and shortening rotations reducing exposure to climatic hazards are increasing this phenomenon (Ranger et al. (2002). To correct mineral deficiencies or nutritional imbalances induced by management and nitrogen deposition, fertilizer or liming operations tend to be more widely used in French forests, especially on acidic soils. Reuse of ashes from industrial boiler plants are also predicted.
Furthermore, expected climate changes will expose forests to summer drought conditions more frequent and more extreme than seen in the past. The effect on the level of soil fertility and on the resistance or resilience of adult trees exposed to episodes of severe water stress, is not currently known. Different studies have shown that nutritional imbalances could be a contributing factor to making forest stands on acid soil vulnerable to decline (Zoettl et al. 1989, Huettl et al. 1990, Le Goaster et al. 1990). Moreover, observations of experimental fertilization and liming practices show the specific sensibilities of trees to drought, relative or according to the applied nutrient treatment. Managers anticipate strategies for adaptation to climate change that would combine shortened rotations, increase exports and compensation by amendments. The study of this interaction between mineral nutrition and water supply is understood today as being a milestone towards understanding the resilience of forest stands.
Available equipment, material and methodological approaches – The thesis work will use several fertilization/liming application plans put into place by INRA, the Technological Institute of Forest Cellulose Bois (FCBA), or the National Forestry Office (ONF) that will compare trees grown in different fertility conditions. The selection of these devices will also contribute to building a water budget to address years of severe drought that have occurred since beginning the treatment.
The hypotheses 2.1 and 2.2 will be tested by the quantitative description of the vertical distribution of fine roots (levies, endoscopy, maps on front of soil pits) as well as by monitoring transpiration (SAP flow) and summer leaf water levels. The 2.2 hypothesis. will be also assessed through levels of morphotypes on root tips, and possibly dissection for enzyme tests (collaborative).
Measurements taken directly from key parameters will test hypotheses 1 (LAI, mass per unit area) and 3 (carbohydrate and nitrogen compounds in the trunk and roots).
Resilience to drought will be better understood through this retrospective analysis of annual radial growth (dendrochronology) following episodes of severe water stress. A retrospective estimate of the level of water stress will be ascertained daily through water balance calculations. A master II course held early in the thesis, planned for the first half of 2013, will identify the most appropriate devices.
In addition to dendrochronological analysis, dendroisotopical (d13C, d18O, d15N) approaches and dendrochemical (Ca, Mg, Mn) appreoachees will also be activiated to characterize the long-term dynamics of tree response to fertilization (mainly addressing hypotheses 1.1 and 2.1).
