1. Evergreen tree dynamics in tropical savanna
Lindsay Hutley and friends

2. Talk Outline Evergreen savanna trees species
Australian savannas dominated by evergreen tree species
All other savannas of the world dominated by deciduous woody species
How do evergreen species survive in a strongly seasonal climate?
Impacts tree removal from system ?

3. Australian tropical savanna
Savanna - trees (C3) and grass (C4)
Open-forest/woodland savanna of the wet-dry tropics
25% of Australia, approximately 2 million sq km
Mining
Tourism
Pastoralism
Aboriginal land management

4. Howard Springs mesic tropical savanna
Overstorey LAI
Wet to dry
Eucalyptus dominated
Soils – red earths
Understorey LAI
Wet to dry
Sarga dominated
Frequently burnt
Rainfall 1700 mm
BA m2 ha-1
Stems ha-1 700

5. Wet-dry climate and rainfall

6. Climate and soil/groundwater
Wet-dry climate reflected in patterns seen here, with strong seasonal signal in groundwater levels - about a 10 m amplitude
Note the increasing height of min water levels over time, not being matched by increases in level peaks

7. Savanna climate - monsoonal
Howard Springs
1700 mm y-1

8. Seasonality – Leaf Area Index
Dry season

9. Seasonality - tree increment

10. Seasonality – fine root turnover

11. Wet-dry climate and rainfall

12. Vegetation response to climate
Tree water use
Prediction ??

13. Tree water use and leaf photosynthesis - aseasonal response
Amax
15-18 umol m2 s-1
13-16 umol m2 s-1
Tree water use

14. How is this possible ? Root distribution of savanna vegetation

16. WATER TABLE (April/May)
SOIL PROFILE
from Kimber (1974) A
Sand
Loamy
Sand B
100
Sandy
Clay Loam
B/C
Duricrust
200 C
Sandy clay
Laterite - derived from Latin word ‘later’ meaning brick
Typical of tropical regions,
Higly weathered and leached profile
Duricrust/ferecrete - accumulation zone
Water table well within root zone of trees
While deep rooting, low root biomass below duricrust
300
5 m rooting depth
WATER TABLE (April/May)

17. Macropores in laterite
Deep drainage
Macropores in laterite
Tree roots at 4 m

18. Volumetric soil water content
Sub-soil
Upper soil

19. Soil water balance – end of dry season
S = soil water store
(mm)
Dry season
tree water use
(~0.9 mm d-1) =

20. Features of savanna water use carbon allocation
Dual root systems – maximise carbon and water uptake in seasonal climate
Wet season, 0-1 m depth
Surface fine roots – water and nutrient uptake
Stem increment possible
Dry season, 2-5 m depth
No surface soil moisture, limited nutrient availability, no stem growth possible
Account for dry season ET using soil water balance
Trees using up to 5 m of soil for dry season water requirements
Sub-soil water storage critical
Photosynthesis maintained
Carbon partitioned into maintenance of deep roots, storage in lignotuber and reproduction
Partitioning of soil water usage
grasses: m (wet)
trees: m (wet and dry)
competition with grasses limited or avoided

21. Impact of clearing ?

22. Impacts of land use change

23. Tree clearing and hydrology
Depth profile - soil moisture content (m3/m3)
Uncleared
5-60 mm drainage
Cleared
mm drainage

24. Tree clearing and carbon
Chen, Hutley, Eamus (2005)
Loss of SOC ~ 2 t C y-1 post clearing

25. Conclusions Fluxes of carbon and water rapid in tropical ecosystem
Hydrological change after 5 years following clearing
years in temperate systems
Carbon turnover rapid, ~5 years (Chen et al. 2003)
Carbon gain can be rapid - NBP 2-4 t C ha-1 y-1, Beringer et al 2007)
Carbon loss can be rapid – 2 t C ha-1 y-1 in soil alone
Clearing impact is likely to be significant

26. Questions ?

27. Hydrological cycle - conceptual model
TNT
Tower Network of the NT
Moisture inputs
Catchment processes
Outflow
measurements

28. Current study area
Additional sites required to cover range of land types, soil types, climate gradient

29. Project 4.1 objectives
Determine the fate of rainfall falling on catchments, and partition this into evapotranspiration, recharge and surface runoff.
Investigate historical patterns of surface water availability, particularly as they relate to persistence of dry season water holes and changes in inundation extent during the wet season.
Develop simple models that can be used to predict changes in surface water and groundwater availability that might result from changes in land use or climate change.
Assess the suitability of surface water – groundwater models for water resource management.

30. Daly River towers – part of TNT
Cleared native pasture – 5 yo
Daly uncleared
Cleared improved pasture – 25 yo

31. Savanna vegetation and climate
Evergreen trees dominates savanna vegetation
Adaptation to long dry season
Zero rainfall, Epan mm d-1
Deep rooted
Use of deep soil reserves and groundwater likely

32. Scaling heat pulse measures tree water use v size
Combine with plot surveys
Tree water use in mm d-1

33. Eucalypt savanna evapotranspirationEo
Total ET
Eu/s
Generate data for water use over the wet-dry cycle
Separated into components of savanna vegetation
Etree

34. Use of groundwater - conclusions
Impact of tree removal
increase deep drainage by mm
significant amount of water
impacts on stream flow and water table ??
Offset by increased grass growth and soil evaporation (limited)