Forest carbon allocation as a determinant of net primary productivity. Ivan Janssens

Transcription

1 Forest carbon allocation as a determinant of net primary productivity Ivan Janssens

2 4. Take home messages: 1. Variation in forest GPP* is predominantly climate-controlled, In contrast: variation in NPP**/GPP depends on nutrient availability, not on climate 2. Understanding / R-S monitoring of tropical forest NPP is not possible when only looking at climate and standing biomass *GPP = sum of sugars produced during photosynthesis **NPP = sum of sugars converted to biomass (not respired)

3 1. Forest gross & net primary productivity GPP = sum of sugars produced during photosynthesis NPP = sum of sugars converted to biomass (not respired)

4 1.1 Monitoring by remote sensing

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8 Remote sensing is ideal because it is global and has high temporal resolution. But are the basic assumptions correct? Is it OK to estimate NPP from GPP using only climate and standing biomass as scalars of respiration? We compiled a large database of measured forest C pools and fluxes (> 500 forest sites).

9 Productivity (g m -2 a -1 ) Productivity Field observations of Forest NPP and GPP Tropical forests: more light + more precip + temperature closer to optimum: Higher GPP, but not higher NPP 4000 GPP NPP GPP NPP Latitude Mean annual temperature

10 Mean GPP: 840 g C m - ² a -1 Mean NPP: 288 NPP - to - GPP ratio Temperate Tropical Boreal Mean annual temperature

11 Mean GPP: 840 g C m - ² a -1 Mean NPP: 288 NPP - to - GPP ratio NPP increases more than GPP Boreal Temperate NPP increases less than GPP Tropical Mean annual temperature

12 No current model is able to reproduce this global pattern What is the underlying mechanism?

13 NPP - to - GPP ratio 2. Reason for the low NPP to GPP ratio in tropical (and boreal) forests? Mean annual temperature

14 1. Boreal forests

15 Boreal forests : decomposition = very slow, nutrients locked up in huge soil organic matter pools Nitrogen = main macronutrient = most limiting factor for growth, not temperature

16 FLAKALIDEN FLAKALIDEN fertilization experiment (Sune Linder)

17 FLAKALIDEN fertilization experiment (Sune Linder)

18 Boreal forests : growth is not temperaturelimited but nitrogen-limited; Of course: global warming will stimulate decomposition, enhance nutrient cycling and thus accelerate growth

19 2. Tropical forests

20 (Jon Lloyd) Tropical forests: very fast nutrient cycling But also: often severe weathering, hence strong nutrient limitation on old soils (Central & East-Amazonia)

21 Quesada, Biogeosciences 2009

22 Tropical forests: typically: not N - but P limited (Quesada et al 2009; Cleveland et al., EL, 2011) P content Stem volume increment Quesada, Biogeosciences 2009

23 3. Temperate forests

24 Temperate forests: often young soils (less severely weathered) + high atmospheric N deposition = often nutrient-richer than boreal and tropical forests Galloway et al., 2004

25 NPP - to - GPP ratio N-limited to nutrient rich NPP increases more than GPP Temperate nutrient rich to P-limited NPP increases less than GPP Boreal Tropical Mean annual temperature

26 NPP to GPP ratio Fertility is key! Averaged across all climate zones, NPP/GPP is higher in fertile than in infertile forests Fertile forest convert 60% of GPP into biomass Infertile forests less than 40% 26

27 Climate zone, GPP NPP NPP / GPP nutrient availability (g C m -2 y -1 ) (g C m -2 y -1 ) Boreal, low 911 (184) 355 (124) 0.39 (0.10) Temperate, low 1320 (718) 565 (264) 0.43 (0.05) Tropical, low 2985 (591) 1233 (315) 0.41 (0.11) Boreal, medium 803 (204) 390 (112) 0.49 (0.10) Temperate, medium 1328 (372) 659 (208) 0.50 (0.11) Temperate, high 1724 (408) 1008 (354) 0.58 (0.13) Higher NPP/GPP in temperate forests due to more common fertile soils Within forests of similar fertility, NPP to GPP ratio is not lower in tropical forests than in temperate forests Within temperate (boreal) zone: more fertile = small effect on GPP, but large on NPP (faster growth in fertile stands not due to higher GPP)

28 Underlying Mechanism? More growth because of reduced respiration?

29 Underlying Mechanism? Reduced respiration? Probably not. More likely: Nutrient-poor = less growth because of higher need for C investment in nutrient acquisition : root symbionts & root exudation

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31 Could the unaccounted allocation to symbionts & exudation really amount to > 20% of GPP?? Phillips & Fahey 2007: fertilization reduces rhizosphere microbial activity by 40-50% Högberg M. et al. 2010: fertilization reduces C allocation from roots to soil biota by 60% Hobbie E. et al. 2006: fertilization reduces root exudation and symbionts by 22% of GPP Treseder 2004: P fertilization reduces mycorrhizal abundance by 32%

32 3. Take home messages: 1. Variation in forest GPP is predominantly climatecontrolled, not fertility-controlled. In contrast: variation in NPP/GPP depends on nutrient availability, not on climate 2. Understanding / R-S monitoring of tropical forest net primary productivity is not possible when only looking at climate and standing biomass

33 Thank you 33

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35 1. Forests may play a key role in the future carbon cycle

36 Vulnerability of the Carbon Cycle in the 21 st Century Hot Spots of the Carbon-Climate-Human System Land Permafrost HL Peatlands T Peatlands Veg.-Fire/LUC Oceans CH 4 Hydrates Biological Pump Solubility Pump Global Carbon Project 2005

37 Sara Vicca et al. (under review)

38 Total Carbon Stocks in the Terrestrial Biosphere Total: 2190 Gt C Wetlands 7% Agriculture 9% Tropical Forests 20% Tundra 8% Temperate Forests 7% Desert 5% Temperate Grasslands 10% Tropical Savannas 8% Boreal Forests 26% Source: Dixon et al. 1994, Schlesinger 1998

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40 RESULTS : Overall mean effects Results

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43 Effects of N deposition on NEP

44 CUE 0,9 0,8 0,7 0,6 0,5 0,4 0,3 low fertility medium fertility high fertility 0,2 0,1 0 The NPP/GPP ratio varies with site fertility Reason = higher C allocation to mycorrhizae & exudation in nutrient^poor forests 0 0,5 1 1,5 2 2,5 3 3,5

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