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Wood formation in rhododendrons at the stress line: take a shrub to the limit


Academic year: 2021

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HAL Id: hal-01844912


Submitted on 5 Jun 2020

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Wood formation in rhododendrons at the stress line:

take a shrub to the limit

Loïc Francon, Thierry Ameglio, Erwan Roussel, Irène Till Bottraud, Guillaume Charrier, Christophe Corona

To cite this version:

Loïc Francon, Thierry Ameglio, Erwan Roussel, Irène Till Bottraud, Guillaume Charrier, et al.. Wood formation in rhododendrons at the stress line: take a shrub to the limit. Colloque Wood formation and tree adaptation to climate, Le Stadium. FRA., May 2018, Orléans, France. pp.A0. �hal-01844912�









-0,2 0 0,2


Rhododendron ferrugineum Taillefer 2400m

Bootstrapped correlation fonctions

snowmet timing


Correlation coefficient

Jun Jul Aug Sep Oct Nov Dec JAN FEB MAR APR MAY JUN JUL AUG SEP Pdec-MAY TMAY-JUL snowmelt timing


-0,2 0 0,2


Rhododendron ferrugineum Taillefer sal 1800


Correlation coefficient

Correlations with minimum (1), mean (2), maximum (3) monthly temperature precipitations (4) and snowmelt timing.

Significant r-values in highlighted colors (p<0.05)

5 1 2 3 4

-15 -101015202530354045-505

-175 -150 -125 -100 -75 -50 -25 0 25 50 75 100

July August Sept.

Oct. 2016 Nov. Dec. Jan. 2017 Feb. March April May June Oct.

Near-ground temperature

records (°C)Diameters (µm)

Rhododendron ind. 43 Rhododendron ind. 46 Rhododendron ind. 45

4/1/17 4/2/17 4/3/17 4/4/17 4/5/17 4/6/17 4/7/17 4/8/17 4/9/17 4/10/17 4/11/17 4/12/17 4/13/17 4/14/17 4/15/17 4/16/17 4/17/17 4/18/17 4/19/17 4/20/17 4/21/17 4/22/17 4/23/17 4/24/17 4/25/17 4/26/17 4/27/17 4/28/17 4/29/17 4/30/17 5/1/17

-15 -10 -5 0 5 10 15 20 25 30

-175 -150 -125 -100 -75 -50 -25 0 25 50

10/9/16 10/10/16 10/11/16 10/12/16 10/13/16 10/14/16 10/15/16 10/16/16 10/17/16 10/18/16 10/19/16 10/20/16 10/21/16 10/22/16 10/23/16 10/24/16 10/25/16 10/26/16 10/27/16 10/28/16 10/29/16 10/30/16 10/31/16 11/1/16 75

-10 -5 0 5 10 15 20

-175 -150 -125 -100 -75 -50 -25 0 25 50 75

100 6/10/17 6/11/17 6/12/17 6/13/17 6/14/17 6/15/17 6/16/17 6/17/17 6/18/17 6/19/17 6/20/17 6/21/17 6/22/17 6/23/17 6/24/17 6/25/17 6/26/17 6/27/17 6/28/17 6/29/17 6/30/17 7/1/17 7/2/17

-5 0 5 10 15 20 25 30 35 40 45

-125 -100 -75 -50 -25 0 -5

0 5 10 15 20 25 30 35

-75 -50 -25 0 25 50 75 100 125

150 July 2016 August Sept.

Near-ground temperature records (°C)

Diameter of 1800m asl. site rhododendron individual stem (µm)

June 2017: Heat wave for the rhodoendrons - Globally warm temperatures

- Summer rain the 16th, the 26th and wet weather at the end of the month

After a period directly afetr snowmelt when all indivi-

duals (Rf45 and Rf46) seem to lose water, they seem to re- cover in June.

Rf46 is growing (cambial innitiation about the 19th) whereas Rf45 does not make wood.

In this study, electronic micro-dendrometers were used to monitor the variations in stem diameter of 11 Rhododendron ferrugineum (Rf) individuals located at 1800,

2100 and 2500m asl in the Taillefer Massif (French Alps) and at 1700m asl at Vicdessos (French Pyrenees). In addition, ground-level temperature was recorded for each individual.

The results presented here are focused on the 2500m site. They cover the period October 2016 - October 2017.


- High-latitude and high-altitude ecosystems are adapted to high level of en- vironmental constraints and are therefore amongst the most sensitive to on- going climate change. Numerous studies demonstrated that recent global warming leads to an increase in shrubs cover and productivity at the Arctic and Alpine treelines since the 1980s (ie ‘shrubification’).

- Dendroecological studies along an altitudinal gradient in the Taillefer massif (French Alps) reveal that R. ferrugineum growth was significantly and positively associated with May temperature and negatively correlated with winter precipitations at 2500m asl.

- Daily-resolved and homogeneized series of temperature, precipitation and snow cover depth from the SAFRAN-CROCUS datasets - for the period 1959-2016 - further confirm that both climatic parameters are the main drivers for snowmelt and subsequent growing season length at this altitude.

- However, the dendroecological analyses that approximate relationships between tree rings and climate by a time-invariant function consider the under- lying processes as a black-box (Guiot et al., 2014). In this context, the monito- ring approach developed here is complementary to the correlation function analyses to document the ecophysiologixcal response of rhododendrons to meteorological/climatic fluctuations.

At 1800m., during the growing season (summer 2016), we observe a typical growth curve. Growth, at this altitude stops by end August. This shows the po- tential of microdendrometers to track daily variations of diameter, growth and

cambial innitiation of dwarf shrubs.

2- A first try at 1800m in summer : the potential of microdendrometers on dwarf shrubs

3- One-year hourly monitoring of three rhododendrons shrubs stem diameter variations at 2500m asl.

Raw curves of diameters variations and temperatures are strongly snowpack-dependants. The gaps in the middle or at the end of the records corres- ponds to (1) loss of the signal or (2) abberant values due mechanical moving of the sensor.

In conclusion, our study demonstrates the com- plementarity of dendroecological and ecophy- siological approaches to assess the response of shrub growth to (micro)climatic variability in alpine environments. The monitoring of dwarf shrubs at high altitude/latitude yet remains a technical and scientific challenge and further investigations are needed.

From a technical point of view, the main challenges are related to the improvement of the sensor reliability - electronic components and batteries - in extreme humidity and temperature conditions. Amongst the 11 microdendrometers ins-

talled at the different sites, only 5 operated for the winter, 4 ran out of battery and two lost the data before the end of the winter.

From a scientific point of view, we expect that our approach will permit to better assess the role of microclimatic fluctua- tions on the ecophysiology of alpine and arctic dwarf shrubs in the continuum ground-snowpack-plant-atmosphere.

Loïc Francon (1), Thierry Améglio (2), Erwan Roussel (1), Irène Till Bottraud (1), Guillaume Charrier (2) and Christophe Corona (1)

(1) Université Clermont Auvergne, CNRS, GEOLAB, F-63000 CLERMONT-FERRAND, France (2) Université Clermont Auvergne, INRA, PIAF, F-63000 CLERMONT-FERRAND, France

contact: loic.francon@uca.fr

1- Results of the dendroecological approach: Climate-growth relationships (1960-2016)

October 2016: Very contrasted conditions

- No snowcover --> freeze/thaw cycles (beggining of the month) High amplitude contractions (125µm)

- First snowfalls (13th and 14th) Rehydration of the stems

- Warm temperatures during the day do not allow snowcover per- sistance (Only until the 19th). No snowcover --> freeze/thaw

cycles (19th - 24th)

Moderate contractions

- End October: positive temperatures (days and nights) Transpiration for all individuals

April 2017: The week without snow

- Warmer temperature and shallow snowpack lead to an excep- tionnally early snowmelt. Individuals have to endure a prolonged period without protection against frost modulated by their microto- pographic position. Rf46 undergoes the longest period.

High amplitude contractions are observed for Rf46 and Rf45- Sunny days and probably frozen soil

Transpiration during the day -> Rf45 and Rf46 lose water.

- Snowfalls the 27th. Rehydration of all stems

Wood formation and tree adaptation to climate May 23, 2018 - May 25, 2018

Wood formation in rhododendrons at the stress line:

Take a shrub to the limit


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