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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 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 evapotranspiration
Eo 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)

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