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Thomas Bourdier, Thomas Cordonnier, Georges Kunstler & Benoit Courbaud

9th IUFRO International Conference on Uneven‐aged Silviculture

June 19th 2014

Size inequality reduces productivity in pure forest stands

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Context

Growing interest in understanding the links between forest structure and ecosystem services (Gamfeldt et al. 2013)

• Ecosysteme services = the benefits people obtain from ecosystems (MEA,2005)

• Link with forest management: Which structures and species mixing should we favour to maintain high level of ecosystem services?

Production: ecosystem service with high economic value (France)• Revenue: 40 billions € and 230 000 jobs• Total: 58 Mm3 / year:

• Timber wood 22 Mm3 / year• Wood for industry 12 Mm3 / year• Energy wood 24 Mm3 / year

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June 19th 2014

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BackgroundEFFECT OF STRUCTURE ON PRODUCTION

Effect of species diversity • Increased production :

• when low productivity (Belote et al. 2011, Paquette & Messier 2011, Toigot in prep.)

• Mixing of trees with different functional characteristics (Kelty 1992, Pretzch 2005, Zhang et al. 2012, Vilà et al. 2013)

• No effect or decreased production• In mature forests (Long & Shaw 2010, Hardiman et al. 2011)

• Stands dominated by beech (Jacob et al. 2010)

Effect of size diversityMore ambiguous: some postive effects (Hardiman et al. 2011, Lei et al. 2009) and no effect (Long & Shaw, 2010)

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Aim of the study

Quantify the marginal effect of size heterogeneity on the production of forest stands with regard to the functional characteristics of the species

• Better qualify and quantify the effect of size heterogeneity in pure forest stands of various species

• Determinate if species functional characteristics have an effect on stand production

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Shade tolerant species

Light demanding species

Pure stands

Uneven-agedEven-aged

same density and mean diameter

9th IUFRO Conference

June 19th 2014

HypothesesEFFECT OF SIZE HETEROGENEITY ON PRODUCTION

Negative effect of size heterogeneity on stand production?

Effect decreases with shade tolerance?

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Material & methodsTHE FRENCH NATIONAL FOREST INVENTORY

Data from 2006 to 2011• 44 000 plots available• Dendrometric and ecological data• Georeferenced: climatic data can

enhanced the plot information

Pure stands• >80% pure (basal area)• 10 species studied

Stand description Flora inventory

Soil characteristics Tree measurement

Species Scots pine

sessile oak

Pedunculate oak

maritimepine Beech Downy

oakSilver

firAleppo

pineNorwaySpruce Larch

Nb plots 1142 1082 870 860 813 667 421 252 232 152

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June 19th 2014

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Material & methodsSIZE HETEROGENEITY INDEX: THE GINI COEFFICIENT

Index derived from the economy• Used in ecology since the 80’s (Weiner 1985, Geber 1989, Knox et al. 1989)

• Better performance than other indices (Lexerod and Eid 2006, Valbuena et al. 2011)

• Increasingly used: (Duduman, 2011; Klopcic and Boncina, 2011, Valbuena et al. 2011)

• Can be used with several species and in all forest stands

• Range from 0 perfect equality to

1 maximum theoritical inequality• In practice 0,1 to 0,7

• Gini = 2 x9th IUFRO

Conference

June 19th 2014

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GINI

Gini=0.2 Gini=0.35

Gini=0.55 Gini=0.7

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Material & methodsSHADE TOLERANCE INDEXES

Shade tolerance = capacity for growth in the shade (Niinemets & Valladares, 2006)

We tested 2 indices:• Ellenberg’s indicator values (1991)

• From 3 to 9 for woody species• Developped for central Europe• Characterize species’ potential to grow in the understory

• Niinemets & Valladares (2006):• Index confined between 1 (very intolerant) and 5 (very tolerant)• Available for many species (806 worldwide)

Differences between deciduous and evergreen species? (Lusk et al. 2008)

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Material & methodsSTAND PRODUCTIVITY: MODEL SELECTION

G=basal area

Dq= mean quadratic diameter

Gini= Gini coefficient

α: site effectWB= water budget

sgdd= sum of degree days

Model selection for each species to retain only the influent variables

What is the effect of the Gini coefficient on the forest stand production?

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Effect of size heterogeneity on the production of pure forest stands

Significant negative effect for 7 out of the 10 species studied

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June 19th 2014

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Interaction between shade tolerance and size heterogeneity

No clear interaction between species shade tolerance and Gini effect

No difference between deciduous and conifer trees

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June 19th 2014

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Effect of size heterogeneity on the production of pure forest stands For most species, stands with low size heterogeneity are more productive than stands with high size heterogeneity

Effect of size heterogeneity could not be related to shade tolerance• No effect of other traits (wood density, SLA)

2 Hypothetical mecanisms investigated:• Light interception efficiency reduced by heterogeneity?• Conversion efficiency reduced for largest trees? 9th IUFRO

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June 19th 2014

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Experiments with a forest dynamics modelSAMSARA 2

3 species:• Norway spruce• Silver fir• European beech

FNI plots

Growth modelled

for 10 years

Variables:• Production• Light interception• Light conversion rate= plot production / plot intercepted light

Courbaud et al. submitted

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June 19th 2014

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Simulation results

Negative effect of Gini on:• Production (-0,556***)• Light interception (-0,257***)• Light conversion rate (-0,299***)

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Conclusion and perspectives

Convergence between empirical results and simulations

An increase of size heterogeneity decreases:• Efficiency in stand light interception• Efficiency in light conversion in growth

No clear differences between species

Static results: forest dynamic may affect our results

Size heterogeneity effect is influenced by ground slope?

Effect on volume instead of basal area?

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June 19th 2014

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Thank you for your attention

Questions??

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Effect of slope on the effect of Gini

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Model selection

Models with Gini

Models without Gini

Test

PinSyl 1751 1757 True **CheSes 931 947 True ***ChePed 769 798 True ***PinMar 720 791 True ***Het 969 990 True ***ChePub 822 820 False 0,656Sap 517 519 True *PinAle 351 353 True *Epi 230 231 True *Mel 240 238 False 0,684

AIC comparisonAnovaSpecies

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Thank you for your attention

Questions??

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Shade tolerant species

Light demanding species

Pure stands

Mixed stands

Uneven-agedEven-aged

same density and mean diameter

--9th IUFRO Conference

June 19th 2014

HypothesesEFFECT OF SIZE HETEROGENEITY ON PRODUCTION