Seasonal variation of LAI in the footprint of a flux measurement tower. Studiedag 17 maart 2011 Starters in het bosonderzoek

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1 24/02/10 09/04/10 05/05/10 23/06/10 07/07/10 12/08/10 22/09/10 08/10/10 08/12/10 1 Studiedag 17 maart 2011 Starters in het bosonderzoek Seasonal variation of LAI in the footprint of a flux measurement tower Šimpraga M., Verbeeck H., Heinesch B., Soubie R., Jonckheere I., Laffineur Q., Aubinet M., Vincke C., Steppe K.

2 what is LAI? Leaf area index (LAI) = m² leaf/m² ground why LAI? to study interception of light and precipitation sensible and latent heat fluxes forest productivity atmospheric deposition to the forest modelling studies of carbon, water, nutrient cycle modelling of biogenic volatile organic compounds (BVOC) fluxes 3 Content Part A. LAI theory and methodology I. theory II. experimental forest and methodology Part B. Results and discussion I. LAI seasonal behaviour II. strengths and weaknesses of each technique III. conclusions which method to use? 4

3 LAI theory Beer s law (Monsi & Saeki) Io reference level o Light extinction law I=radiation intensity at a certain depth of the vegetation I/Io = e -k(β).lai β Legend: I=transmitted sunlight Io=incident sunlight β=penetration angle k=extinction coefficient LAI=leaf area index LAI= 1 ln I/Io k β ( ) 5 LAI theory Small k ( )= Radiation more evenly distributed, better PAR efficiency Big k ( )= Radiation absorbed in upper part of leaf, lower PAR efficiency Zea mays β=32.5 k=0.9 Helianthus annuus k indicates degree of decrease of light & depends on the leaf orientation 6

4 Vielsalm experimental forest 7 1. DATA ACQUISITION 1.same time 2.same place 3.same direction LAI methodology MARK SAMPLING POINTS TAKE MEASUREMENTS 2. DATA ANALYSIS DATA ANALYSIS LAI=FAI Leaf area index=foliage area index PAI=VAI plant area index=vegetation area index 8

5 Hemispherical photography (HP) Fish eye lens, camera and tripod Field of View β Radius ~ 90 -β WinScanopy software 9 LAI circular sensors ~ g(β) for 5 β = HP LAI-2000 algorithm 10

6 Intercepted PAR (photosynthetically active radiation) 10 PAR sensors in beech subplot One PAR sensor on top of tower Data logger (Li-cor, Inc., USA) Frequency: 5-min intervals 11 NDVI (Normalized Difference Vegetation Index) R / FR ratio = ζ-waarde Tower-based home-made NDVI sensor COSINUS HEAD DOWNWARDS measuring light quality 12

7 Seasonal LAI 7 6 Leaf production Leaf maturity Leaf senescence 5 LAI (m 2 m -2 ) Day of the year (DOY) 13 Seasonal NDVI and CO (canopy openess) 1.0 NDVI typical trapezoidal pattern (Kodani et al. 2005) NDVI (-) CO (%) Day of the year (DOY) Day of the year (DOY) Canopy more open, no leaves, just trunks Light regime is richer with 730nm then from 660nm= signal for spring growth 14

8 saturation NDVI (-) HP LAI Intercepted PAR LAI-2200 Correlation analysis CO (%) 0.2 R²=0.85 HP R²=0.82 Intercepted PAR R²=0.91 LAI LAI (m² m -2 ) 60 HP LAI 50 Intercepted PAR LAI R²=0.98 HP R²=0.87 Intercepted PAR R²=0.67 LAI LAI (m² m -2 ) CO (%) R²= NDVI (-) Strengths and weaknesses of each technique HP LAI-2200 PAR NDVI + - permanent record many algorithms LAI PAR + other canopy structural parameters stable robust LAI PAR info automatic stable LAI PAR info cheaper automatic stable seasonality manual very expensive additional post-processing labour-intensive sensitive to weather conditions manual direct labour-intensive expensive sensitive to weather conditions less spatially represented interesting for comparing purposes additional post-processing additional post-processing no LAI 16

9 Comparison to other LAI seasonality Year 2000 From Aubinet et al. (2002) VAI evolution in beech subplot during vegetation season 17 Conclusions LAI, NDVI and CO seasonality with all techniques clear relation between LAI, CO and NVDI is observed depending on research scope, certain available method is advised 18

10 Summary We learned: Some canopy structural parameters Some explanations for the observed difference between 4 techniques Several fundamental questions were addressed: How does LAI vary over season (for deciduous subplot)? Strengths and weaknesses of each technique 19 Questions? Thank you for your attention BELSPO Prof. dr. em. Raoul Lemeur Philip Deman Geert Favyts Michele Yernaux Henri Chopin Stephanie Goffin 20 BELSPO-sponsored research