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Tropical plant trait evolution and the consequences for savanna-forest transitions William A. Hoffmann North Carolina State University Collaborators Augusto Franco Sybil Gotsch M. Haridasan Lucas Silva Erika Geiger Davi Rossatto The Cerrado What determines tree cover in savannas? What determines the distribution of forest and savanna? •Sparse tree cover •Dense grass •Hot, dry, windy microclimate •Frequent fire •Dense tree cover •no grass •Cool, moist microclimate •Infrequent, mild fire Miconia pohliana (Savanna species) Miconia cuspidata (Forest species) Symplocos rhamnifolia (Savanna species) Symplocos mosenii (Forest species) Large areas of the tropics have climates in which either savanna or forest vegetation in possible Staver et al (2011) C4 grasses became abundant only in the past 8 million years Cerling et al 1997 Nature 389:153-158 Savanna trees and shrubs began to arise from forest ancestors approximately 10 million years ago. Simon et al 2009 There have been at least 115 independent origins of savanna trees or shrubs Black = forest taxa Red = savanna taxa Hoffmann (unpublished) Main questions • What selective pressures have shaped the evolution of tree species in savanna? • What are the consequences of savanna tree adaptations for ecosystem properties and vegetation dynamics? The multiple, independent origins of savanna lineages is ideal for comparative studies Black = forest taxa Red = savanna taxa 0.3 0.2 0.1 0.0 6 5 4 3 2 1 0 Savanna Forest 3.5 8 6 4 2 0 3.0 2.5 2.0 1.5 1.0 0.5 3 2 1 0 0.0 Savanna Forest 4 Seedling root:shoot ratio 10 Leaf Area Index Height at maturity (m) Savanna Forest Diameter growth rate (mm yr-1) Bark thickness:stem radius Savanna and forest species differ substantially in traits that have large implications for ecosystem structure and dynamics Savanna Forest Savanna Forest Note that this is a comparison of forest and savanna species. 53% of the evolutionary transitions from forest to savanna were associated with a shift to a smaller growth form. Question 2: What are the consequences of these adaptations for ecosystem properties and vegetation dynamics? Savanna and forest species survive fire equally well Post-fire survival (%) 100 80 60 40 20 0 Savanna species Forest species The fire trap (Bell 1984) Long time without fire Fire The fire trap (Bell 1984) Long time without fire Short time without fire Fire The fire trap (Bell 1984) Long time without fire Short time without fire Fire Fire The fire trap (Bell 1984) Long time without fire Short time without fire Fire The fire trap (Bell 1984) Long time without fire Short time without fire Fire Fire An analogous threshold exists for ecosystems An analogous threshold exists for ecosystems Short time without fire An analogous threshold exists for ecosystems Short time without fire Fire An analogous threshold exists for ecosystems Long time without fire Short time without fire Fire An analogous threshold exists for ecosystems Long time without fire Short time without fire Fire Fire Resource manipulation experiment 10m x 70m plots CONTROL NUTRIENTS WATER WATER + NUTRIENTS Tree growth is more strongly limited by nutrients than by water Growth rate (mm yr-1) 4 No added nutrients Added nutrients 3 2 1 0 Not irrigated Irrigated Water availability High-resource sites permit more rapid canopy closure Canopy cover High resource site Low resource site Time since fire (y) At what point is each thresholds reached? • How big must a tree be to avoid topkill? • How dense must the canopy be to substantially reduce flammability? Stem survival (%) 100 Low fire intensity 80 60 40 20 0 0.1 1 10 100 Bark thickness (mm) Hoffmann et al (2012) Ecology Letters Stem survival (%) 100 Low fire intensity 80 60 40 20 0 0.1 1 10 100 Bark thickness (mm) Hoffmann et al (2012) Ecology Letters A growing stem becomes fire resistant when its bark thickness exceeds 6 mm Stem survival (%) 100 Low fire intensity 80 60 40 5.9 mm 20 0 0.1 1 10 100 Bark thickness (mm) Hoffmann et al (2012) Ecology Letters A growing stem becomes fire resistant when its bark thickness exceeds 6 mm Stem survival (%) 100 Low fire intensity High fire intensity 80 60 40 9.1 mm 5.9 mm 20 0 0.1 1 10 100 Bark thickness (mm) Hoffmann et al (2012) Ecology Letters -30 30 10 -10 Distance (m) Microclimate •Wind speed •Relative humidity •Temperature Fuels •Mass •Moisture •Bulk density BehavePlus 5 (fire behavior model) Hoffmann et al (2012) Austral Ecology 55 Min. relative humidity (%) -1 (km hr ) Wind speed 2.5 2.0 1.5 1.0 0.5 45 40 35 0.0 28 27 26 25 24 23 22 21 20 Fine fuel moisture (%) Max air temp. (C) 50 -30 -20 -10 0 10 20 30 Distance (m) 18 16 14 12 10 8 6 -30 -20 -10 0 10 Distance (m) 20 30 0.20 0.15 0.10 0.05 0.00 -30 -20 -10 0 10 20 30 Distance (m) 400 (kW m-2) Fireline intensity 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 (Km hr-1) Rate of spread Flame length (m) Fire simulations with BehavePlus 300 200 100 0 -3 Fuel bulk density (kg m ) 30 20 10 0 -30 -20 -10 0 10 20 Distance along transect (m) 30 Flammability of savanna is determined primarily by the presence of grass Grass LAI 4 3 The canopy density at which grasses are excluded is a critical transition between savanna and forest. 2 1 0 0 1 2 3 Tree LAI 4 5 Thus we have two critical thresholds • A tree reaches a fire-resistance threshold when it accumulates a bark thickness of about 6 mm. • The ecosystem reaches a fire-suppression threshold when it attains a leaf area index of about 3. Bark thickness (cm) 10 As savanna trees grow, they accumulate bark thickness more quickly than forest species Savanna species Forest species 1 0.1 0.01 1 10 Stem diameter (cm) 100 Bark thickness (cm) 10 As savanna trees grow, they accumulate bark thickness more quickly than forest species Savanna species Forest species 1 Threshold bark thickness 0.1 0.01 1 10 Stem diameter (cm) 100 Bark thickness (cm) 10 As savanna trees grow, they accumulate bark thickness more quickly than forest species Savanna species Forest species 1 Threshold bark thickness 0.1 4.7 cm 0.01 1 10.2 cm 10 Stem diameter (cm) 100 Diameter growth rate (mm yr-1) Recall that forest species grow more quickly than savanna species when growing in the same environment 4 3 2 1 0 Savanna Forest Fire interval required to ensure <50% topkill Savanna species 8 years Forest species 14 years Expected total time under stochastic fire regime 5-year mean return interval 25 years 108 years 2-year mean return interval 510 years 32800 years Under a typical fire regime a forest tree has little chance of reaching maturity in savanna But, there is safety in numbers •Sparse tree cover •Dense grass •Hot, dry microclimate •Frequent fire •Dense tree cover •no grass •Cool, moist microclimate •Infrequent, mild fire Forest species have greater leaf area than savanna species when growing in the same environment 100 2 Leaf area (m ) 10 1 0.1 0.01 Savanna species Forest species 0.001 0.0001 0.1 1 Diameter (cm) 10 Forest species permit more rapid canopy closure 6 Site occupied by forest species Leaf area index 5 4 Site occupied by savanna species 3 2 1 0 0 5 10 15 20 25 30 Time (years) 35 40 45 50 Forest species permit more rapid canopy closure 6 Site occupied by forest species Leaf area index 5 4 3 Threshold canopy density Site occupied by savanna species 2 1 0 0 5 10 15 20 25 30 Time (years) 35 40 45 50 Forest species permit more rapid canopy closure 6 Site occupied by forest species Leaf area index 5 4 3 Threshold canopy density Site occupied by savanna species 2 1 0 0 5 10 15 20 25 30 Time (years) 35 40 45 50 -1 Diameter growth rate (mm yr ) 16 14 Forest species 12 10 8 6 Savanna species 4 2 0 -30 -20 -10 0 10 20 Distance along transect (m) 30 Conclusions •Many indepedent origins of savanna trees •Fire has exerted strong selection on traits •Nutrients, not water, limits tree growth in these mesic savannas. •Two critical thresholds govern savanna-forest dynamics: (1) bark thickness at which a stem becomes fire resistant. (2) canopy density at which grasses are excluded • Reaching a forest state requires forest species, but these are particularly constrained by the high frequency of fire Vegetation models should •Represent fire and its feedbacks with vegetation •Represent topkill and reprouting •Represent of savanna and forest tree functional types OR tradeoffs involving bark, carbohydrate storage, shade tolerance, canopy density. •Not assume that tree cover in mesic savannas are water limited. •Robustly simulate savannas over large areas in response to water deficits, multiple nutrient deficiencies, seasonal flooding, and physical soil constraints.