Pour mieux affirmer ses missions, le Cemagref devient Irstea Thomas Bourdier, Thomas Cordonnier, Georges Kunstler & Benoit Courbaud 9th IUFRO International Conference on Uneven ‐ aged Silviculture June 19 th 2014 Size inequality reduces productivity in pure forest stands

2 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 jobs Total: 58 Mm 3 / year: Timber wood 22 Mm 3 / year Wood for industry 12 Mm 3 / year Energy wood 24 Mm 3 / year 9th IUFRO Conference June 19 th 2014

3 Background EFFECT 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 diversity More ambiguous: some postive effects (Hardiman et al. 2011, Lei et al. 2009) and no effect (Long & Shaw, 2010) 9th IUFRO Conference June 19 th 2014

4 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 9th IUFRO Conference June 19 th 2014

5 Shade tolerant species Light demanding species Pure stands Uneven-agedEven-aged same density and mean diameter 9th IUFRO Conference June 19 th 2014 Hypotheses EFFECT OF SIZE HETEROGENEITY ON PRODUCTION Negative effect of size heterogeneity on stand production? Effect decreases with shade tolerance?

6 Material & methods THE FRENCH NATIONAL FOREST INVENTORY Data from 2006 to plots available Dendrometric and ecological data Georeferenced: climatic data can enhanced the plot information Pure stands >80% pure (basal area) 10 species studied Stand descriptionFlora inventory Soil characteristicsTree measurement Species Scots pine sessile oak Peduncul ate oak maritime pine Beech Downy oak Silver fir Aleppo pine Norway Spruce Larch Nb plots th IUFRO Conference June 19 th 2014

7 Material & methods SIZE 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 x 9th IUFRO Conference June 19 th 2014

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

9 Material & methods SHADE 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) 9th IUFRO Conference June 19 th 2014

10 Material & methods STAND PRODUCTIVITY: MODEL SELECTION 9th IUFRO Conference June 19 th 2014

11 Effect of size heterogeneity on the production of pure forest stands Significant negative effect for 7 out of the 10 species studied 9th IUFRO Conference June 19 th 2014

12 Interaction between shade tolerance and size heterogeneity No clear interaction between species shade tolerance and Gini effect No difference between deciduous and conifer trees 9th IUFRO Conference June 19 th 2014

13 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 Conference June 19 th 2014

14 Experiments with a forest dynamics model SAMSARA 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 9th IUFRO Conference June 19 th 2014

15 Simulation results 9th IUFRO Conference June 19 th 2014

16 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? 9th IUFRO Conference June 19 th 2014

17 Thank you for your attention Questions??

18 Effect of slope on the effect of Gini

19 Model selection

20 Thank you for your attention Questions??

21 Shade tolerant species Light demanding species Pure stands Mixed stands Uneven-agedEven-aged same density and mean diameter -- 9th IUFRO Conference June 19 th 2014 Hypotheses EFFECT OF SIZE HETEROGENEITY ON PRODUCTION