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 Springs mesic 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

Wet-dry climate and rainfall

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

Savanna climate - monsoonal Howard Springs 1700 mm y-1

Seasonality – Leaf Area Index Dry season

Seasonality - tree increment

Seasonality – fine root turnover

Wet-dry climate and rainfall

Vegetation response to climate Tree water use Prediction ??

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

How is this possible ? Root distribution of savanna vegetation

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)

Macropores in laterite 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

Impact of clearing ?

Impacts of land use change

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

Questions ?

Hydrological cycle - conceptual model TNT Tower Network of the NT 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 measures tree water use v size Combine with plot surveys Tree water use in mm d-1

Eucalypt savanna evapotranspiration Eo Total ET Eu/s Generate data for water use over the wet-dry cycle Separated into components of savanna vegetation 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)