Assessing the response of grassland, shrubland and a mixed grassshrub ecotone to precipitation varia

1
Assessing the response of grassland, shrubland
and a mixed grass-shrub ecotone to precipitation
variability.
W.T. Pockman and E.E. Small
Thanks to J. Balulis, E. Bhark, J. Boulanger, J.
Elliott, O. Hopkins, S. Kurc, M. Liter, J.
Martinez, D. McGhee, A. Phillips, J. Phillips, E.
Posdamer, M. Rawlings, S. White, D. Yin, C. Zack,
X. Zhao, et al. M. Friggens, C. Hickman, T.
Koontz, S. Munson, C. Roberts, H. Simpson and K.
Wetherill. T. Kieft, D. Peters, J. Herrick, J.
Gosz, and R. Parmenter
2
Vegetation change in the southwestern US shrub
invasion into grasslands
Larrea tridentada Creosotebush
Bouteloua eriopoda Black Grama
3
Shrubland and grassland
Shrubland 28 canopy
Grassland (54 canopy)
4
Ecotone Grass Coverage 43.91 Shrub Coverage
8.70 Interspace Coverage 47.39
5
How does climate variability influence vegetation
changes?
6
Why might grasses and shrubs respond differently
to climate extremes?
1) Infiltration is greater beneath canopies in
grassland and shrubland 2) Canopy-interspace
infiltration contrast is greater in shrubland
?water islands are more intense and further
apart Implications for vegetation transition
plant growth and establishment is limited in
shrubland interspaces
7
Model of drought response in grassland
Grassland
Decreased productivity High mortality
8
Model of drought response in grassland
Grassland
Decreased productivity High mortality
9
Model of drought response in shrubland
Shrubland
Decreased productivity Limited mortality
10
Sevilleta LTER 5 Points
Grass
Mixed grass-shrub
Shrub
1 km
11
Using rainfall manipulation to assess ecosystem
responses to climate
Environment
Treatment
Ecotone 3 blocks, separated by 40 70 m Grass
and Shrub 3 drought plots under one large
shelter
12
Trenching
13
Drought plot preparation
14
Installing soil moisture probes
15
Control (or water addition) plot
need photo here
16
Completed Drought Plots
17
(No Transcript)
18
Treatments

• Scenarios tested using 70 yr. rainfall record
  from Socorro (30 km).
• Drought (goal)
• ON OFF ON OFF OFF
• once rain is detected, roof remains in place
  until midnight
• During OFF, big storms endanger the treatment.
• roof moves into position after 10 20 mm rain.
• Water addition (2003 w/ NIGEC funding)
• install and instrument final 6 plots (grass and
  shrub)
• 7 storms added per year
• amount added calculated to maintain 50 control
• treated water used to minimize effects on soil
  chem.

19
Measurements

• Hydrology
• soil moisture (5 80 cm, water content and
  potential, gt360 sensors comprising 3 methods)
• infiltration, runoff (real time)
• Plant function
• water potentials, gas exchange
• periodic monitoring and intensive during dry-down
• productivity
• species cover and abundance
• Nutrient Cycling
• N min potential, available and organic N
• Organic C, microbial biomass and respiration

20
Deeper infiltration under canopy than in
interspaces
5 cm
25 cm
canopy
interspace
21
Plant response to 15 mm storm
22

• Surface reflectance grass and shrub sites,
  October 2002.
• Grass higher reflectance in visible and
  near-infrared after drought
• Grass and shrub showed different response to
  drought.

Grass site
Shrub site
preliminary data from Xiaobing Zhao (NMT EPSCOR)
23
Additional Measurements

• Crypto-biotic crust sampling (J. Belnap)
• Mini-rhizotron tubes (M. Allen)
• Others??
• limitation maintain integrity of soil surface
  and vegetation.

24
Assessing the accumulated ecosystem response to
drought and wet periods

• Responses to similar sized storm events with
  increased treatment period.
• Temporal changes in plant functional responses to
  moisture pulses and hydrologic responses of the
  plant/soil system used to assess the accumulated
  ecosystem response

25
the end