Tree acclimation to wind : how trees filters the chronic winds to acclimate to high winds

Transcription

1 Tree acclimation to wind : how trees filters the chronic winds to acclimate to high winds Vivien Bonnesoeur, Thiéry Constant, Meriem Fournier, Bruno Moulia 1

2 Introduction Tree acclimation is a key La factor montagne, for wind 16 sept risk 2015modeling Increasing level of exposure (forest edge, elevations...) decreasing levels of risk 2

3 2 version of GALES to calculate the critical wind speed : Roughness method TMC method = based on the acclimation of tree dimensions to chronic wind load Slope =Tc(kg) D 1,32 H or D 1,3 3 (m3) 3

4 A very promising method but it needs further validation : only one study influence of wind climate? influence of tree growth (species, age)? Objectives of this talk : Confirm the genericity of the method thigmomorphogenesis = driver of the acclimation Focus on wind-induced strain M max =a D 3 u 2 ε max = M max E D = a u2 3 E Strain regime uniform among trees of different dimensions Trees can sense wind-induced strain which trigger growth responses = thigmomorphogenesis Mecanism for the regulation of strain regime of trees 4

5 Introduction Even in a even-aged stand there is variability in tree size. contrasted social class Different priority? Dominant = Large crown exposed to sun and wind. Large stem 1) To protect from the wind 2) Light foraging Suppressed = sheltered from the wind, high competition for light. Small stem 1) Light foraging 2) To protect from the wind suppressed dominant Hypothesis : Dominant and suppressed trees experience the same strain regime 5

6 Materials & methods stand 6

7 Materials & methods The stand Beech stand Even aged stand from a natural regeneration Age : years Hauteur dominante = 15 m Well acclimated to the wind (never thinned) 7

8 Materials & methods The stand 8

9 Materials & methods The stand 15 pairs of dominant-suppressed trees 9

10 Materials & methods The stand 15 pairs of dominant-suppressed trees Circumference at BH (mm) Height (m) Crown depth (m) Suppressed dominant Suppressed dominant Suppressed dominant 10

11 Materials & methods The wind 2D sonic anemometers : At local canopy top (12m), At the dominant height of the stand (15m) Synthetic descriptors over 30min periods Wind speed Mean, Maximum, Standard error, Most frequent wind direction 11

12 Materials & methods Wind induced longitudinal strain Strain gauges glued on homemade transducer Synthetic descriptors over periods of 30 min Maximum Mean Standard error Averaged on the stem cross section Sampling rate = 8Hz 12 12

13 Materials & methods Statistical analysis Large variability of the strain regime over a year: Comparison of the strain regime between trees during contrasted situations: phenology : Leaf-off Leaf-on Marcescence Wind direction West South-West South (prevailing winds) North-East Other directions Wind rose for

14 Materials & methods Statistical analysis Random sampling with replacement of the wind speed Number Maximal wind speed (m/s) Maximal wind speed (m/s) Linear mixed effect model log (ε i,t )=a+a i +(b+b i ) log(u t )+e i,t 14

15 15 Maximal strain over 30 min (%) Results S-SW NE Auther directions L e af - o n M ar ce sc e nc e L e af - of f Maximal wind speed over 30 min m/s)

16 Discussion Hypothesis: Dominant and suppressed trees experience the same strain regime, whatever the period of the year They acclimate to winds in the same way 16

17 Discussion Hypothesis: Dominant and suppressed trees experience the same strain regime, whatever the period of the year They acclimate to winds in the same way However, Strain regime presents variations over the year Maximal strain over 30 min (%) 17 Maximal wind speed over 30 min m/s)

18 Discussion Confirm the generality of the TMC method. 1:6 range of stem strenght trees are spread over 1 ha but no evident spatial effect in the residuals >1500 hours/tree Phenology influence more the tree behaviour than wind direction Extrapolation to Critical wind speed? b ε max =a u max Influence of the exponent b : 1,4-2,4 CWS ( m/s) 18

19 influence of tree growth (species, age)? Objectives of this talk : Confirm the genericity of the method thigmomorphogenesis = driver of the acclimation 19

20 Introduction «Trees acclimate to the chronic winds» is rather vague Complexity of the wind regime Wind spectrum in Europe 20

21 Introduction 1 day Thermal winds Convectives cells 21

22 Introduction : perception des déformations des arbres en forêt Le régime de déformation d'un arbre est complexe et riche en information sur son environnement : Vents chroniques 4 days in Europe Forcing of the surface winds by geostrophic winds regime depending on synoptic scale events : low and high pressure area... 22

23 Hypothesis Only the strongest winds are sensed by trees Suppressed and dominant trees acclimate to winds in the same way : they have the same thigmomorphogenetic responses 23

24 Materials & methods Radial growth Automatic point dendrometer Sampling rate = 1 point/ 30min 24 24

25 Materials & methods Radial growth 25 Croissance journalière (rouge) = croissance du point le plus bas de la journée 25

26 Materials & methods Strain regime G G 5 strain regime of different intensity (6 trees / treatement) : Nat S Nat S G : Guying Nat S : natural strain Nat S + S=0,015 % : natural strain + imposed strain of 0,015 % daily wind speed peak Nat S + S=0,015 % Nat S + S=0,015 % Nat S + S=0,04 % : natural strain + imposed strain of 0,04 % wind speed peak of a mean synopticscale events (1 / week) Nat S + S=0,15 % : natural strain + imposed strain of 0,15 % wind speed peak of an extreme synoptic-scale events (1 / year) Nat S + S=0,04 % Nat S + S=0,15 % 26 time maximal wind speed (m.s -1 ) Nat S + S=0,04 % Nat S + S=0,15 %

27 Matériels et méthodes Radial growth R(t)= t M e 1 e exp ( (t 0 t ) +1 ) t t f M t t 0 Non linear mixed effect model (Gompertz growth curve) : Effect of thigmomorphogenesis on t 0, M and t? 27

28 a b b b b Cumulative growth (mm) a b b b b a a a a a G Nat S 28 Nat S + S=0.015 % Nat S + S=0.04 % Nat S + S=0.15 % Results Between treatements M t 0 T Nat S+S=0,15 % S<0,15 % t f t 0 M t

29 Time Results Between trees of contrasted social status Dominant trees Suppressed trees Nat S + S=0,15 % +0,9mm +76 % Nat S + S=0,15 % S<0,15 % +0,7mm +156 % S<0,15 % 29

30 Discussion How are sensed the chronic winds? The daily wind speed peaks are filtered out Only the strongest wind speed are sensed by the trees Thigmomorphogenesis triggered by surface winds forced by geostrophic wind: Relevant information for acclimation to wind since such kind of wind 30 can be responsible for damaging storm

31 Discussion How do trees from contrasted social status sense the wind? absolute same radial response require stronger changes in the biomass allocation for suppressed tree 31

32 Conclusion Uniform strain regime between trees, regardless the phenology or wind directions validity of TMC basis in a different context extrapolation of such a method for critical wind speed? Strain sensing induces growth that drive strain regulation between trees focusing on wind surface forced by synotpic winds (not on annual mean wind speed) absolute growth response similar among the trees! thigmomorphogenesis could be involved in the determinism of social status Basis for a dynamical tree growth model wind dependent duration of the acclimation? 32

33 Conclusion dd dt =Pot 2 +Th 2 f [ε (t ) ε 0 ] dm dt =Pot 1 +Th 1 f [ε (t ) ε 0 ] ε (t )= a M (t ) U b (t ) D 3 (t ) Tree growth (diameter, dd dt =Pot 2 +Th 2 [exp[ ( ε 0 D3 (t ) a M f (t ) λ b)k/b]] 33