savannas and legumes
TRANSCRIPT
Savannas and legumes
Caroline Lehmann1, Abigail Cabrelli1, Kyle Dexter1,2 and Toby Pennington2
1. School of GeoSciences, University of Edinburgh 2. Royal Botanic Garden Edinburgh
Distribution of tropical savannas
Lehmann et al 2011; Lehmann et al., unpublished
Regional floristic differences in woody species
Australia: Myrtaceae Africa: Mimosaceae, Caesalpiniaceae Combretaceae India & Asia: Combretaceae Dipterocarpaceae South America: Mix of dominance & Patterns of dominance different
Dominant woody plant families separated over long periods
Cosmopolitan dominant grass genera/species
Themeda triandra Morales, 2014
Dispersal limitation?
Legume biogeography
• Focus on forests – not open savannas but see Hogberg 1986
• Old and diverse lineages – opportunity for dispersal
• Legume lineages in the savanna biome achieved pantropical distributions prior the origin of savannas via Neogene dispersal
• Why isn’t there more commonality in the woody savanna flora? Eucalyptus…?
Bruneau et al 2008
DU
PA
RQ
UE
TIO
IDE
AE
Dim
orp
han
dra
e
CE
RC
IDIO
IDE
AE
DE
TA
RIO
IDE
AE
PA
PIL
ION
OID
EA
E
Mim
oso
idea
Cassie
ae
Caesalp
inia
e
Pelt
op
ho
reae
DIA
LIO
IDE
AE
Um
tiza g
rad
e
Nitrogen
fixation
Deep homology/precursor trait
Lack of functional equivalence among
lineages
What is the relationship between legume dominance, biome and environment?
• Abundance of N-fixing legumes will be linked to environments where fixation provides a competitive advantage (not random). Ie. a biological mechanism to (re)grow rapidly or compensate for N losses.
• Distribution and abundance of non-fixing legumes will be a product of dispersal history. Or, whats so special about Brachystegia?
- 1304 plots (savanna/open and forest/closed) - 5678 woody plant species - 723 legume species - 408 nodulating species - 315 non-nodulating species
- Assigned genera based on work by Janet Sprent
Proportion of Species Proportion of Stems Proportion of Biomass
Asia Africa
S. Am.
Aust.
Species, stems or biomass? Biome or continent?
Asia Africa
S. Am.
Aust. Asia
Africa
S. Am.
Aust.
All Legumes
Nodulating
Non Nodulating
Functional differences?
Legume dominance declines with increasing rainfall
Only a few genera are ever super-dominant: • Acacia • Senegalia + Vachellia • Brachystegia
• Dominate in species poor savannas
Nodulating legumes dominate dry & disturbed regions
Total Exchangeable Bases
< 14.1 > 14.1
Continent
Africa S. America
Asia Australia
Rainfall
Continent < 1044 > 1044
Biome
S. America
Savanna
Africa, Australia, Asia
Continent
Africa Australia, Asia
Forest
• 43% deviance explained
Fire return times across legume dominated sites
N loss? Rapid rates of (re)growth? Herbivory?
Non-nodulating legumes Only relevant explanatory variable - continent
Continent
Continent
Africa S. America
Continent
Asia Australia
• Less deviance explained • Only 28% DE
Higher dominance of non-nodulating legumes than expected across African savannas and forests
• Far fewer in Australia • Competitive exclusion? • Other persistence related plant traits?
Vegetative reproduction?
Distribution of tropical savannas
Lehmann et al 2011; Lehmann et al., unpublished
Andropogoneae grasses
Conclusions and future directions
• Nodulating legumes dominate regions not dissimilar in environment to C4 grasses - are these innovations to succeed in stressful environments?
• Nodulation provides an ability to grow fast when needed – post rain, post fire • Most successful in open canopy environments (Links with forest work) • Flip side – shade intolerant due to carbon economy of fixation?
• How can we explain the success of non-nodulating legumes (e.g., in mesic African savannas)?
• Test for competitive exclusion using dated phylogenies (did legumes get there first in Africa, and last in Australia?)
• Dispersal-based legume biogeography suggests all lineages arrive everywhere; must be about whether lineages can establish on arrival – generation time and life history traits – keys to success in grasses!
• Establishment limitation! Not dispersal limitation….
