The Sulawesi Throughfall Displacement Experiment

Transcription

1 The Sulawesi Throughfall Displacement Experiment Are tall trees more sensitive to prolonged drought in tropical perhumid forests? Overall aim was to simulate a prolonged drought period as they occur during natural El Niño Southern Oscillation (ENSO) events Bernhard Schuldt, Viviana Horna and Christoph Leuschner Plant Ecology, Albrecht von Haller Institute for Plant Sciences, Georg August University of Göttingen 1 Bernhard Schuldt KLIFF FZW 21

2 The El Niño Southern Oscillation (ENSO) phenomenon Study area modified from Ashok and Yamagata 29 Warm surface water and air are pushed to the west (e.g. Indonesia) by prevailing winds The west is warmer and wetter Easterly winds weaken, causing warmer than normal sea surface temperatures The west gets drier 2 Bernhard Schuldt KLIFF FZW 21

3 The El Niño Southern Oscillation (ENSO) phenomenon ENSO events are causing plant-available soil moisture to stay below a critical threshold level for a prolonged period An ENSO event occurs every 3-8 years Droughts observed between 183 and 1953 were to 93 % associated with the ENSO phenomenon Climate Change Scenarios South-East Asia could experience a shift in precipitation patterns leading to more frequent and/or more severe droughts in the future (Cox et al. 24, Christensen et al. 27, Newbery and Lingenfelder 28, Sheffield and Wood 28) 3 Bernhard Schuldt KLIFF FZW 21

4 The Sulawesi Throughfall Displacement Experiment (STDE) Pono Valley Toro Location Central Sulawesi, Indonesia (15 m.a.s.l) Pre-montane per-humid primary rainforest 4 Bernhard Schuldt KLIFF 21

5 Study design of the STD Experiment in the Pono valley Plot m 16 m 155 m 15 m 145 m 14 m 2 m m 13 m 175 m Plot 5 m 2 17 m 165 m 1 16 m 155 m 15 m 145 m 14 m 135 m Plot 4 m 1 Plot 3 Plot Plot 1 1 m 1 4 m m 5 Bernhard Schuldt KLIFF FZW 21 G. Moser In total, 48 m 2 were covered with bamboo panels 135 m 13 m

6 What happens when trees are exposed to a prolonged drought period? o stomatal closure o decreased photosynthesis o leaf loss o cavitation-inducing xylem tensions causing embolisms in water conducting vessels and loss of hydraulic conductivity 6 Bernhard Schuldt KLIFF FWZ 21

7 Soil-Plant-Atmosphere-Continuum (SPAC) Water in soil is held under tension Trees must exert greater tension than soil to extract water As soil dries, leaf water potential must be more negative to maintain gradient When difference in Ψ between soil and leaves gets too large, water column in the xylem will break, cutting off the water flow Cavitation Direct relation between cavitation risk and vessel diameter Salleo and Lo Gullo 1989, Hargrave et al. 1994, Tyree et al. 1994, Davis et al. 1999, Pockman and Sperry 2, Martinez-Vilalta et al. 22, Tyree and Zimmermann 22, Pittermann and Sperry 23, Wheeler et al. 25, Hacke et al. 29, Cai and Tyree 21 7 Bernhard Schuldt KLIFF FZW 21

8 How do tropical trees respond to a prolonged drought period? 1997/98 El Nino related severe two years long drought Amazonian throughfall exclusion experiment Van Nieuwstadt et al. 25, Journal of Ecology 93(1): Tree mortality after 2 years drought highest in large trees Slik 24, Oecologia 141(1): Mortality significantly higher in large diameter trees Nepstad et al. 27, Ecology 88(9): Mortality rates increased 4.5-fold in large trees (DBH > 3 cm) Lianas more vulnerable to droughtinduced mortality than trees or palms da Costa et al. 21, New Phytologist 187(3): Mortality rates increased 3-fold in large trees (DBH > 3 cm) 8 Bernhard Schuldt KLIFF FZW 21

9 Why are tall tropical trees stronger affected by a prolonged drought? A. Microclimatic conditions The driving force behind measured sap flow (XFD) is the atmospheric Vapour Pressure Deficit of the Air (VPD) -2 D(kPa),R(MJm ) XFD (g m s ) Global radiation (MJ m -2) VPD (kpa) XFD C. acuminatissima Time (h) 9 Bernhard Schuldt KLIFF FZW 21

10 A. Microclimatic conditions: A gradient of changing VPD with height Relative VPD Vertical position (m) Upper canopy exposed to double as high VPD values compared to the forest ground 1 Bernhard Schuldt KLIFF FZW 21

11 A. Microclimatic conditions: Influence on water consumption Tree individual basis Species level Four tree height classes -2-1 XFD (g cm d ) p<.1 r 2 =.38 p<.1 r 2 =. 9 A B C D p<.1 r 2 = Tree height (m) Tree height (m) Tree height (m) High VPD High water consumption (here xylem flux density) 11 Bernhard Schuldt KLIFF FZW 21

12 Stability of Rainforest Margins in Indonesia Why are tall tropical trees stronger affected by a prolonged drought? A. Microclimatic conditions Increasing VPD with forest height Higher Xylem flux densities B. Xylem path length of tall trees Vessels do taper with increasing xylem path length o Water stress rises with height due to increasing hydraulic resistance Tall trees need large vessels at the o To cope herewith, vessels are tapering with height trunk base Vessel diameter (µm) Tree height (m) 12 Bernhard Schuldt KLIFF FZW 21

13 B. Vessel anatomy and tree height Mean vessel diameter (µm) Trunk p<.1 r 2 = Twig p>.1 r 2 = Tree height (m) Tree height (m) The tallest trees have the largest vessels 13 Bernhard Schuldt KLIFF FZW 21

14 Why are tall tropical trees stronger affected by a prolonged drought? Compared to the understory, tall tropical trees are exposed to a higher vapor pressure deficit have the highest xylem flux densities at the trunk base have to cope with a long flow path possess the largest vessels in the trunk wood Expected to have the highest cavitation risk 14 Bernhard Schuldt KLIFF FZW 21

15 Soil Moisture of Control (above) and Roof (below) plots within two years 15 Bernhard Schuldt KLIFF FZW 21 Total amount of rainfall during the experiment: 6331 mm

16 Soil water parameters of Control and Roof plots within two years Rainfall (mm d -1 ) Volumetric water content VWC. -.5 m VWC.5-3. m Soil matric potential Ψ.-.5m (MPa) Ψ. -.5 m (MPa) Control Roof Relative extractable water REW. -.5 m REW.5-3. m /27 9/27 12/27 3/28 6/28 9/28 12/28 3/29 6/29 9/29 16 Bernhard Schuldt KLIFF FZW 21

17 Vertical root distribution within the forest stand -2 cm 91 % -2 cm 75 % Soil depth (cm) a) 2 4 b) Coarse root biomass (g L -1 ) Soil depth (cm) Fine root biomass (g L -1 ) Hertel et al. (29) Forest Ecology and Management 258(9): Bernhard Schuldt KLIFF FZW 21

18 A. Were tall trees reacting like assumed in our experiment? 2 P. firma 33.8 m Roof opening Roof Xylem flux density (g cm d ) P. excelsa 35.9 m Roof C. acuminatissima 41. m Roof Bernhard Schuldt KLIFF FZW 21

19 B. Were tall trees reacting like assumed in our experiment? 12 P. firma 12.8 m Roof opening Roof Xylem flux density (g cm d ) P. excelsa 15. m Roof C. acuminatissima 17.3 m Roof Bernhard Schuldt KLIFF FZW 21

20 C. Were tall trees reacting like assumed in our experiment? Xylem sap flux density (g cm -2 d -1 ) Drought response of four tree individuals of the upper canopy species C. acuminatissima after two years of desiccation 43.9 m 17.6 m 41. m 17.3 m 2 % 4 % Roof opening 3/28 6/28 9/28 12/28 3/29 6/29 9/29 2 Bernhard Schuldt KLIFF FZW 21

21 Drought response of the upper canopy species C. acuminatissima after two years of desiccation A. Twig hydraulic properties from upper sun-lit crown (white bars) and lower shade crown (black bars) Leaf specific conductivity Vessel-lumen specific conductivity LSC (1 kg m MPa s ) A A B B a a b b Control Roof k s (kg m MPa s ) A A B B a a a a Control Roof Decreased empirically determined hydraulic conductivity 21 Bernhard Schuldt KLIFF FZW 21

22 Drought response of the upper canopy species C. acuminatissima after two years of desiccation B. Leaf morphology from upper sun-lit crown (white bars) and lower shade crown (black bars) Mean leaf size Specific leaf number 2 A L (cm ) A a A B B b a b spec -2 n L (n mm ) A A B A a a a a Control Roof Control Roof Increased single leaf size and decreased total leaf number 22 Bernhard Schuldt KLIFF FZW 21

23 Drought response of the upper canopy species C. acuminatissima after two years of desiccation C. Trunk wood anatomy Outermost first centimetre analyzed Significant decrease in mean vessel diameter by 3% According to the Hagen-Poiseuille law, this results in an decrease in hydraulic conductivity per vessel by 12 % 23 Bernhard Schuldt KLIFF FZW 21

24 Observation After 24 month of desiccation, we found significant tree physiological and morphological reactions, but much less pronounced than previously expected Explanations Conclusions We did not manage to sufficiently dry out the deeper soil layers Rooting depth from the large trees unknown We only manipulated the soil water content. Throughout the experiment we had much lower VPD values compared to a real ENSO event During a real strong drought event tall trees should be most affected But under a moderate drought event, which mainly affects the upper soil layers, small trees might be most affected 24 Bernhard Schuldt KLIFF FZW 21

25 Thank you very much for your attention 25 Bernhard Schuldt KLIFF FZW 21 Funding by DFG is gratefully acknowledged