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Evergreen tree dynamics in tropical savanna

This talk explores the dynamics of evergreen trees in tropical savannas, focusing on Australian savannas dominated by evergreen species. It discusses how these trees survive in a strongly seasonal climate and the impacts of tree removal from the system. The talk also examines the water use of savanna trees and their adaptation to the wet-dry tropical climate.

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Evergreen tree dynamics in tropical savanna

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  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 Springsmesic tropical savanna Overstorey LAI Wet to dry 0.6 - 1 Eucalyptus dominated Soils – red earths Understorey LAI Wet to dry 0.2 - 1.4 Sarga dominated Frequently burnt Rainfall 1700 mm BA 10-12 m2 ha-1 Stems ha-1 700

  5. Wet-dry climate and rainfall

  6. Climate and soil/groundwater

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

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

  15. SOIL PROFILE from Kimber (1974) A Sand Loamy Sand B 100 Sandy Clay Loam B/C Duricrust 200 C Sandy clay 300 5 m rooting depth WATER TABLE (April/May)

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

  17. Volumetric soil water content Sub-soil Upper soil

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

  19. 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: 0 - 0.5 m (wet) • trees: 0 - 5 m (wet and dry) • competition with grasses limited or avoided

  20. Impact of clearing ?

  21. Impacts of land use change

  22. Tree clearing and hydrology Depth profile - soil moisture content (m3/m3) Uncleared 5-60 mm drainage Cleared 50-200 mm drainage

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

  24. Conclusions • Fluxes of carbon and water rapid in tropical ecosystem • Hydrological change after 5 years following clearing • 50-100 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

  25. Questions ?

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

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

  28. 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.

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

  30. Savanna vegetation and climate • Evergreen trees dominates savanna vegetation • Adaptation to long dry season • Zero rainfall, Epan 10-12 mm d-1 • Deep rooted • Use of deep soil reserves and groundwater likely

  31. Scaling heat pulse measurestree water use v size • Combine with plot surveys • Tree water use in mm d-1

  32. Eucalypt savanna evapotranspiration Eo Total ET Eu/s Etree

  33. Use of groundwater - conclusions • Impact of tree removal • increase deep drainage by 50-150 mm • significant amount of water • impacts on stream flow and water table ?? • Offset by increased grass growth and soil evaporation (limited)

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