Sap flux-scaled canopy transpiration in species-rich and species-poor temperate deciduous forests

Transcription

1 Sap flux-scaled canopy transpiration in species-rich and species-poor temperate deciduous forests Tobias Gebauer Department of Plant Ecology, Georg-August-University of Göttingen

2 Why studying forest canopy transpiration along a diversity gradient? Forestry shifts from monospecific to mixed forest systems (Knoke et al. 2005). This could have profound consequences on energy and matter fluxes and diversity of other organism groups. Numerous studies have demonstrated a relationship between plant diversity and ecosystem functioning in grasslands, but forest ecosystems are poorly understood. Studies mostly done in simple structured systems (e.g. Fagus, Picea, Quercus); Mixed forests stands are investigated in lower numbers. Few did comparisons between stands (monospecific vs. two-species mixture). These studies could not clarify if monospecific or mixed forest stands transpire more.

3 Hypotheses Canopy transpiration (E c ) in the growing season is a function of tree diversity (due to niche partitioning in the soil and canopy in species-richer stands) Tree species identity exerts a major influence on the height and temporal variation of E c. (due to different phenological and physiological adaptations to climatic conditions)

4 1. Study Characterization of sapwood area and radial sap flux patterns of the co-occurring tree species Aims: Estimation of species-specific hydroactive xylem area Characterization of radial xylem sap flux patterns (Age determination of hydroactive sapwood)

5 Material & Methods (Radial xylem flux profiles) Xylem sap flux measurements (Granier-/ Constant-heating method) No sap flux Nadezhdina et al Lu et al Sap flux Nadezhdina et al J s 119 K 1.231

6 Example of diurnal courses of xylem sap flux density (J s ) in the outermost xylem of 5 tree species ( ) T. cordata A. pseudoplatanus C. betulus F. sylvatica F. excelsior 50 J s [g m -2 s -1 ] :00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 T. Gebauer, unpubl. data

7 Material & Methods (Radial xylem flux profiles) Determination of radial xylem sap flux patterns T. Gebauer Granier-type sensors at severals xylem depths: 0-2 cm, 2-4 cm, 4-6 cm, 6-8 cm (F. excelsior: 0-1 cm, 1-2 cm, 2-3 cm, 3-4 cm) 3 trees per species (DBH 25 to 50 cm)

8 Material & Methods (Radial xylem flux profiles) Estimation of hydroactive sapwood using a dye injection method Fraxinus excelsior (ring-porous) T. Gebauer Acer campestre (diffuse-porous) S. Haverstock S. Haverstock T. Gebauer

9 Sizes of sapwood area of seven co-occurring tree species F. sylvatica C. betulus T. cordata A. pseudoplatanus A. platanoides A. campestre All 6 diffuseporous species F. excelsior Gebauer, Horna & Leuschner Tree Physiol. 28:

10 Sizes of sapwood area of seven co-occurring tree species F. sylvatica C. betulus High proportions of sapwood area in diffuse-porous species: % Relative low proportion of sapwood area in ring-porous F. excelsior: 20 % T. cordata A. platanoides All 6 diffuseporous species A. pseudoplatanus A. campestre F. excelsior Gebauer, Horna & Leuschner Tree Physiol. 28:

11 Radial sap flux density patterns of seven co-occurring tree species Exponential decrease in sap flux from cambium to centre, especially for F. sylvatica and F. excelsior C. betulus, Acer and Tilia showed bellshaped curves peaking at 2 to 4 cm behind cambium Gebauer, Horna & Leuschner Tree Physiol. 28:

12 Ages of hydroactive xylem Tree species Diffuse-porous species n DBH (cm) Sapwood depth (cm) Number of annual rings in sapwood Mean annual ring width in the sapwood min. max. min. max. min. max. (mm ring -1 ) Fagus sylvatica (0.99) Carpinus betulus (0.71) Tilia cordata (0.88) Acer pseudoplatanus (0.74) Acer platanoides (0.61) Acer campestre (0.43) Ring-porous species Fraxinus excelsior (1.39) Gebauer, Horna & Leuschner Tree Physiol. 28: Number and width of annual rings were estimated by I. Schmidt

13 Ages of hydroactive xylem Tree species Diffuse-porous species n DBH (cm) Sapwood depth (cm) Number of annual rings in sapwood Mean annual ring width in the sapwood min. max. min. max. min. max. (mm ring -1 ) Fagus sylvatica (0.99) Carpinus betulus (0.71) Tilia cordata (0.88) Acer pseudoplatanus (0.74) Acer platanoides (0.61) Acer campestre (0.43) Ring-porous species Fraxinus excelsior (1.39) Gebauer, Horna & Leuschner Tree Physiol. 28: Number and width of annual rings were estimated by I. Schmidt

14 Ages of hydroactive xylem Tree species Diffuse-porous species n DBH (cm) Sapwood depth (cm) Number of annual rings in sapwood Mean annual ring width in the sapwood min. max. min. max. min. max. (mm ring -1 ) Fagus sylvatica (0.99) Carpinus betulus (0.71) Tilia cordata (0.88) Acer pseudoplatanus (0.74) Acer platanoides (0.61) Acer campestre (0.43) Ring-porous species Fraxinus excelsior (1.39) Gebauer, Horna & Leuschner Tree Physiol. 28: Number and width of annual rings were estimated by I. Schmidt

15 Conclusion (Radial xylem flux profiles) F. excelsior (ring-porous) had a lower sapwood area than all diffuse-porous species (21 % vs. 70 to 90 % of the trunk is sapwood area). Radial sap flux patterns differ considerably among the diffuse-porous species. Sap flux highest near cambium for F. sylvatica and F. excelsior with exponential decrease towards sapwood-heartwood boundary. C. betulus, Acer and Tilia showed bell-shaped curve peaking at 2 to 4 cm behind cambium. For F. excelsior the sapwood is still hydroactice for up to 30 years; in diffuse-porous species for 100 to 170 years.

16 2. Study Canopy transpiration along a tree diversity gradient Aims: Comparison of canopy transpiration of broad-leaved forest stands which vary in tree species diversity (low, moderate and high diversity: 1 to >5 tree species). Is species diversity or species identity influencing canopy transpiration?

17 Study stands DL1a DL2c DL3a Number of trees sampled per species was in relation to the species composition in the stand and within a species the tree size distribution (stem diameter classes) was considered: DL1a 8 trees: Fagus (8) DL2c 16 trees: Fagus (8), Tilia (3), Fraxinus (5) Maps by K.M. Daenner DL3a 20 trees: Fagus (3), Tilia (8), Fraxinus (3), Carpinus (3), Acer (3) Fagus sylvatica Tilia sp. Fraxinus excelsior Carpinus betulus Acer sp. other tree species Leuschner, Gebauer & Horna. in revision. Ecosystems

18 Up-scaling from sensor to stand level Step 1: T. cordata A. pseudoplatanus C. betulus F. sylvatica F. excelsior T. Gebauer, unpubl. data 50 J s 119 K J s [g m -2 s -1 ] :00 06:00 12:00 18:00 00:00 Step 2: Step 3:

19 Up-scaling from sensor to stand level Step 1: T. cordata A. pseudoplatanus C. betulus F. sylvatica F. excelsior T. Gebauer, unpubl. data 50 J s 119 K J s [g m -2 s -1 ] :00 06:00 12:00 18:00 00:00 Step 2: Gebauer et al J st n n1 J s B S ( x A S i ) W ( x i ) x Step 3:

20 Up-scaling from sensor to stand level Step 1: T. cordata A. pseudoplatanus C. betulus F. sylvatica F. excelsior T. Gebauer, unpubl. data 50 J s 119 K J s [g m -2 s -1 ] :00 06:00 12:00 18:00 00:00 Step 2: Gebauer et al J st n n1 J s B S ( x A S i ) W ( x i ) x Step 3: E cj A A Sj G 1 m m m1 J st DL1 DL2 DL3

21 Climatic conditions in 2005 and was a wetter year than 2006 (610 vs. 518 mm precipitation, lower vpd) REW min max min Leuschner, Gebauer & Horna. in revision. Ecosystems (Relative extractable soil water = Actual extractable soil water / Maximum extractable soil water); 0.4 x REW = point where soil water stress occurs (stomatal closure) (Granier et al. 1999, Bernier et al. 2002)

22 Canopy transpiration 2005 and 2006 Leuschner, Gebauer & Horna. in revision. Ecosystems mm mm 97.3 mm mm mm mm 2005: DL3 shows 30 % higher canopy transpiration than F. sylvatica dominated DL1 and DL2 (at relative low vpd values) 2006: No differences in annual totals of canopy transpiration between DLs, but Differences in seasonal pattern: DL3 higher canopy transpiration till end of June earlier soil water depletion; F. sylvatica dominated stands show more conservative water use

23 Atmospheric control of canopy transpiration Single regressions Gebauer, Horna & Leuschner. Submitted.

24 Conclusion (Canopy transpiration along a tree diversity gradient) Annual total of canopy transpiration was not effected by tree species diversity. Interannual variability in canopy transpiration is dependent on susceptibility of tree species to changing climatic conditions (e.g. relative high transpiration of Tilia at low vpd, decreased at higher vpd) Species with relatively high water use (e.g. Tilia) may exhaust soil water early in summer, thus increasing drought stress.

25 Take Home Message Tree species diversity had no directional effect on the annual total of E c Species identity and functional traits (e.g. ring- or diffuse-porous, size of sapwood, radial xylem flux pattern, stomatal regulation) of the species are important for the height and temporal variation of E c A higher diversity depending on associated species identity may reduce ecosystem stability.

26 THANK YOU VERY MUCH for your attention! Financed by the German Research Foundation (DFG) T. Gebauer