Transcript Evergreen tree dynamics in tropical savanna Lindsay Hutley and friends Talk Outline • Evergreen savanna trees species – Australian savannas dominated by evergreen tree species – All.

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
Understorey LAI
Wet to dry 0.2 - 1.4
Sarga dominated
Frequently burnt
Soils – red earths
Rainfall 1700 mm
BA 10-12 m2 ha-1
Stems ha-1 700
Monthly rainfall (mm)
Wet-dry climate and rainfall
1000
800
600
400
200
0
'94 '95 '96 '97 '98 '99 '00 '01
Year
Climate and soil/groundwater
0
Water level (m BGL)
-2
-4
-6
-8
-10
-12
Savanna climate - monsoonal
450
11
Howard Springs
400
9
300
250
8
1700 mm y-1
200
7
RF
Sun Hr
150
100
6
5
50
Dec
Nov
Oct
Sep
Aug
Jul
Jun
May
Apr
Mar
4
Feb
0
Jan
Rainfall (mm)
350
Sunshine hours (day-1)
10
Seasonality – Leaf Area Index
2.0
Understorey
Overstorey
LAI
1.5
1.0
Dry season
0.5
0.0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov Dec
Seasonality - tree increment
Tree increment (mm mon -1)
0.8
0.6
0.4
0.2
0
-0.2
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Seasonality – fine root turnover
-1
Growth rate (kg m mon )
0.8
-2
0.6
0.4
0.2
0.0
Dec Jan Mar May Jun Aug Oct Nov Jan
Monthly rainfall (mm)
Wet-dry climate and rainfall
1000
800
600
400
200
0
'94 '95 '96 '97 '98 '99 '00 '01
Year
3
Tree water use
-1
Tree transpiration (mm d )
Vegetation response to climate
Prediction ??
2
1
0
'94
'95
'96
'97 '98
Year
'99
'00
'01
Tree transpiration (mm d-1)
Tree water use and leaf photosynthesis
- aseasonal response
1.2
Amax
15-18 umol m2 s-1
Amax
13-16 umol m2 s-1
0.8
Tree water use
0.4
0.0
Jan
Mar
May
Jul
Sep
Nov
How is this possible ?
Root distribution of
savanna vegetation
SOIL PROFILE
A
Sand
B
Loamy
Sand
from Kimber (1974)
100
B/C Sandy
Clay Loam
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
450
0.45
rf
20
0.40
0.35
Sub-soil
70
290
400
350
300
0.30
250
0.25
200
0.20
150
0.15
100
0.10
Upper soil
0.05
0.00
Oct-98
50
0
Dec-98
Mar-99
May-99
Jul-99
Sep-99
rainfall (mm)
3
-3
volumetric water content (m m )
0.50
Soil water balance – end of dry season
Depth (cm)
20
S (mm d-1)
50
70
0
0
120
0
290
0.15
350
0.54
0.01
S = soil water store
(mm)
-1
over 4m
0.69 mm d
over 5m
1.15 mm d-1
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)
Moisture content (m3/m3)
0
0.025
0.05
0.075
0.1
0.125
0.15
0.175
0.2
0.225
0.25
0
Native A
Native B
5 y.o. A
5 y.o. B
Depth below surface (cm)
20
40
60
10am, 13 September 2005
80
100
120
140
Uncleared
Cleared
5-60 mm drainage
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
measurements
Catchment processes
Outflow
• Current study area
• Additional sites
required to cover range
of land types, soil
types, climate gradient
Project 4.1 objectives
1.
2.
3.
4.
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
60
Q (l/day)
50
Y=0.0748x1.8168
r2=0.80
•Combine with plot surveys
40
•Tree water use in mm d-1
30
20
10
0
0
10
20
DBH (cm)
30
40
E. tetrodonta
E. miniata
Eucalypt savanna evapotranspiration
Evaporation (mm d-1)
8
Eo
6
4
Total ET
Eu/s
2
Etree
0
Jan
Mar
May
Jul
Sep
Nov
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)