Evergreen tree dynamics in tropical savanna Lindsay Hutley and friends
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 ?
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
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
Savanna climate - monsoonal Howard Springs 1700 mm y-1
Seasonality – Leaf Area Index Dry season
Vegetation response to climate Tree water use Prediction ??
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
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)
Deep drainage Macropores in laterite Tree roots at 4 m
Volumetric soil water content Sub-soil Upper soil
Soil water balance – end of dry season S = soil water store (mm) Dry season tree water use (~0.9 mm d-1) =
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
Tree clearing and hydrology Depth profile - soil moisture content (m3/m3) Uncleared 5-60 mm drainage Cleared 50-200 mm drainage
Tree clearing and carbon Chen, Hutley, Eamus (2005) Loss of SOC ~ 2 t C y-1 post clearing
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
TNT Tower Network of the NT Hydrological cycle - conceptual model Moisture inputs Catchment processes Outflow measurements
Current study area • Additional sites required to cover range of land types, soil types, climate gradient
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.
Daly River towers – part of TNT Cleared native pasture – 5 yo Daly uncleared Cleared improved pasture – 25 yo
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
Scaling heat pulse measurestree water use v size • Combine with plot surveys • Tree water use in mm d-1
Eucalypt savanna evapotranspiration Eo Total ET Eu/s Etree
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)