Size inequality reduces productivity in pure forest stands

1. Size inequality reduces productivity in pure forest stands Thomas Bourdier, Thomas Cordonnier, Georges Kunstler & Benoit Courbaud 9th IUFRO International Conference on Uneven‐aged SilvicultureJune 19th2014

2. Context 9th IUFRO Conference Growinginterest in understanding the links betweenforest structure and ecosystemservices (Gamfeldtet al. 2013) • Ecosysteme services =the benefits people obtain from ecosystems(MEA,2005) • Link withforest management: Which structures and speciesmixingshouldwefavour 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: • Timberwood 22 Mm3 / year • Wood for industry 12 Mm3 / year • Energywood 24 Mm3 / year June 19th 2014

3. Background 9th IUFRO Conference Effect of Structure on production Effect of speciesdiversity • Increased production : • whenlowproductivity(Belote et al. 2011, Paquette & Messier 2011, Toigotin prep.) • Mixing of treeswithdifferentfunctionalcharacteristics(Kelty1992, Pretzch 2005, Zhang et al. 2012, Vilàet al. 2013) • No effect or decreased production • In mature forests(Long & Shaw 2010, Hardimanet al. 2011) • Stands dominated by beech(Jacob et al. 2010) Effect of size diversity • More ambiguous: somepostiveeffects(Hardimanet al. 2011, Lei et al. 2009) and no effect(Long & Shaw, 2010) June 19th 2014

4. Aim of the study Quantify the marginal effect of size heterogeneity on the production of forest stands with regard to the functionalcharacteristics of the species Betterqualify and quantify the effect of size heterogeneity in pure forest stands of variousspecies Determinate if speciesfunctionalcharacteristics have an effect on stand production 9th IUFRO Conference June 19th 2014

5. Hypotheses Uneven-aged Even-aged Shadetolerantspecies Pure stands Light demandingspecies samedensity and meandiameter EFFECT OF SIZE HETEROGENEITY ON PRODUCTION Negativeeffect of size heterogeneity on stand production? Effectdecreaseswithshadetolerance? 9th IUFRO Conference June 19th 2014

6. Material & methods Stand description Flora inventory Soilcharacteristics Treemeasurement The 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 9th IUFRO Conference June 19th 2014

7. Material & methods Size heterogeneityindex: The gini coefficient Index derivedfrom the economy • Used in ecologysince the 80’s (Weiner1985, Geber 1989, Knox et al. 1989) • Better performance thanother 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 perfectequality to 1 maximum theoriticalinequality • In practice 0,1 to 0,7 Gini = 2 x 9th IUFRO Conference June 19th 2014

8. GINI Gini=0.35 Gini=0.2 Gini=0.7 Gini=0.55

9. Material & methods 9th IUFRO Conference Shadetolerance indexes Shadetolerance = capacity for growth in the shade(Niinemets & Valladares, 2006) Wetested 2 indices: Ellenberg’sindicator values (1991) • From 3 to 9 for woodyspecies • Developpedfor central Europe • Characterize species’ potential to grow in the understory Niinemets & Valladares (2006): • Index confinedbetween1 (veryintolerant) and 5 (verytolerant) • Available for manyspecies (806 worldwide) Differencesbetweendeciduousand evergreenspecies? (Lusk et al. 2008) June 19th 2014

10. Material & methods STAND PRODUCTIVITY: Model selection • G=basal area • Dq= meanquadraticdiameter • Gini= Gini coefficient α:site effect • WB= water budget • sgdd= sum of degreedays Model selectionfor eachspecies to retainonly the influent variables Whatis the effect of the Gini coefficient on the forest stand production? 9th IUFRO Conference June 19th 2014

11. Effect of size heterogeneity on the production of pure forest stands Significantnegativeeffect for 7 out of the 10 speciesstudied 9th IUFRO Conference June 19th 2014

12. Interaction betweenshadetolerance and size heterogeneity No clear interaction betweenspeciesshadetoleranceand Gini effect No differencebetweendeciduous and conifertrees 9th IUFRO Conference June 19th 2014

13. Effect of size heterogeneity on the production of pure forest stands For mostspecies, stands withlow size heterogeneity are more productive than stands with high size heterogeneity • Effect of size heterogeneitycould not berelated to shadetolerance • No effect of other traits (wooddensity, SLA) 2 Hypotheticalmecanismsinvestigated: • Light interceptionefficiencyreduced by heterogeneity? • Conversion efficiencyreduced for largesttrees? 9th IUFRO Conference June 19th 2014

14. Experimentswith a forestdynamicsmodel 9th IUFRO Conference Samsara 2 3 species: • Norwayspruce • Silverfir • Europeanbeech FNI plots Growthmodelled for 10 years Variables: • Production • Light interception • Light conversion rate= plot production / plot intercepted light June 19th 2014 Courbaud et al.submitted

15. Simulation results 9th IUFRO Conference Negativeeffect of Gini on: Production (-0,556***) Light interception (-0,257***) Light conversion rate (-0,299***) June 19th 2014

16. Conclusion and perspectives 9th IUFRO Conference Convergence betweenempiricalresults and simulations An increase of size heterogeneitydecreases: • Efficiency in stand light interception • Efficiency in light conversion in growth No cleardifferencesbetweenspecies Staticresults: forestdynamicmay affect ourresults Size heterogeneityeffectisinfluenced by groundslope? Effect on volume instead of basal area? June 19th 2014

17. Questions?? Thankyou for your attention

18. Effect of slope on the effect of Gini

19. Model selection

20. Questions?? Thankyou for your attention

21. Hypotheses Uneven-aged Even-aged Shadetolerantspecies Pure stands Light demandingspecies -- EFFECT OF SIZE HETEROGENEITY ON PRODUCTION 9th IUFRO Conference June 19th 2014 Mixed stands samedensity and meandiameter