Fundamentals of Ecohydrology (Philippe CHOLER – CNRS...

4. Processes and models of dryland dynamics

Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)

Climatic constraints to plant growth in drylands

Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013) Source: Fensholt R., et al. (2012) Remote Sens. Environ., 121, 144-158.

4

Plant responses to precipitation in semi-arid grasslands

The Pulse-and-Reserve Paradigm

trigger(Precip.)

source(Soil Water –

Nutrients)

reserve(storage tissues

- seeds)growth pulse

(NPP)

loss

loss

feedback

storage

activation

Modified from : Noy-Meir I. (1973) Annu. Rev. Ecol. Syst., 4, 25-51.

4

Outline of part 4

• 4.1. Tree-grass coexistence in savannas (patch scale) • -> see computer lab session

• 4.2. The Grasslands – Shrublands dynamics (community scale)

• 4.3. Land use change and the water cycle (watershed scale)

• 4.4. Determinants of woody cover in Africa (continental scale)• -> reading discussion

• 4.5. Desertification and land degradation (global scale)• -> see Lecture 3

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4.1. Tree-grass coexistence in savannas

Introduction

• Savanna are tropical ecosystems that cover about 20% of the land surface of earth

• Patch mosaic of C4 grasses and C3 shrub/trees

• Savanna physiognomy controlled by multiple drivers (water, fire, herbivory).

• Tree increase as a major trend over the past century

• Tree-grass coexistence - Model of Non equilibrium dynamics.

• Ecohydrological perspective (Kalahari transect) Tree cover along rainfall gradient (ex. Africa)


4.1

Tree-grass coexistence in savannas


Deep rooting of trees

Superficial, dense rooting

of grasses

Source: D'Odorico P., et al. (2007) Journal of Geophysical Research-Biogeosciences, 112.

4.1

The Kalahari transect in Southern Africa


4.1

Soil moisture dynamics


c

i

c i > 0

Source: D'Odorico P., et al. (2007) Journal of Geophysical Research-Biogeosciences, 112.

4.1

Possible mechanisms & positive feedback


Enhanced soil infiltration capacity under tree canopy (Peff)

Shading reduces evaporative loss (E,T)

Higher rainfall interception (L) by canopy>

Tree cover

Higher soilmoisture)

Increasedcarrying

capacity for woody

biomass

Growth & recuitmentof woodyspecies

4.1

Advanced readings

• D'Odorico, P., et al. 2007. On soil moisture-vegetation feedbacks and their possible effects on the dynamics of dryland ecosystems. - Journal of Geophysical Research-Biogeosciences 112:

• Good, S. P. and Caylor, K. K. 2011. Climatological determinants of woody cover in Africa. - Proc. Natl. Acad. Sci. USA 108: 4902-4907.

• Hanan, N. P., et al. 2008. Do fires in savannas consume woody biomass? A comment on approaches to modeling savanna dynamics. - Am. Nat. 171: 851-856.

• Sankaran, M., et al. 2005. Determinants of woody cover in African savannas. - Nature 438: 846-849.


4.1

4.2. The Grasslands – Shrublands dynamics

The grassland biome

>3

source http://confluence.org/

4.2

Grasses in a nutshell

• excellent below ground competitors• -> reduces recruitment of trees

• dead biomass provides a highly flammable fuels• -> frequent fire increases mortality of young tree

• can support large herbivore densities• -> detrimental effects of grazers on trees (to be discussed)


Land cover of drylands


4.2

From steppes to shrubland in USA


tall grass Prairieshort grass Prairieshrubland deciduous forest

Rock Springs

4.2

Climate determinants of shrublands - grasslands


Summer precip. and dominance of grasslands vs. shrublands in the Western USA

Source: Ogle K. & Reynolds J.F. (2004) Oecologia, 141, 282-294.

4.2

Grasslands replaced by shrublands


Installation of C3 non-palatable woody shrubs and trees at the expense of C4 grasslands on the Colorado Plateau (USA)

Photos by William Henry Jackson in 1899 and H.E. Malde in 1977. Source: C. Allen, J. Betancourt, and T. Swetnam, USGS Biological Resources Division Southwestern U.S. LUHNA pilot project, 1997

1899 1977

44.2

Stages of shrub encroachment


Grasses outcompeted

Shrub dispersal and establishment

Soil erosion

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Source: D'Odorico P., et al. (2012) Ecohydrology, 5, 520-530.

4.2

Herbivory and producer–decomposer feedbacks

Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013) Source: Bardgett R.D., et al. (2005) Trends in Ecology and Evolution, 20, 634-641.

Dominant grasses benefit from herbivory through positive feedbacks between herbivores, plants, and soil biota. Colonization by later successional plants producing poorer litter quality is

prevented

4.2

Where all the prairies have gone ?


Source : George Catlin / BIBLIOTHÈQUE ET ARCHIVES Canada / C-100022

4.2

Large wild herbivores replaced by cattles


BISON -> CATTLEextensive grazing -> intensive grazing

4.2

Land use changes as the primary driver


Recruitment of Ponderosa Pines in semi-arid grasslands (Utah)

CO2 concentration

4

Source: Archer S., et al. (1995) Clim. Change, 29, 91-99.

4.2

Local drivers (patch scale)


Effect of grazing intensification by livestcok

Preferential utilization of palatable grasses

Soil compaction. Changes in soil nutrient

cycling

Fire regime changes(regulation by herders,

reduced fuel load)

Dispersal of woody plants

Reduced belowground Compettition by grasses

Reduced fuel load

44.2

Global drivers


CO2 fertilization effect

Benefitting C3 shrubs and trees

Increased belowground carbohydrate storage, and survival ?

Climate change

Glacial periods : low CO2, cold -> C4 grasslandsInterglacial periods: high CO2, warm -> C3 shrubs/trees

Increased temperatures

Limitation of frost damage to shrubs ?

44.2

Examples of positive feedbacks


Shrubencroachment

Small mammals(rodents)

Grass comsunption

ReducedBelowgroundcompetition

Shrubencroachment

Reduced fuel load (grass)

Decreasing fireintensity and

frequency

Increasedsurvival of

seedlings and sapling

Fire regimeRodents

44.2

Examples of positive feedbacks


Shrubencroachment

Reduced grasscover

Increased soilerosion,

decrerasedwater infiltration

ReducedBelowground

competition by grasses

Reduced grasscover,

increased baresoil

Increased nearsurface

temperature

Increasedsurvival of

seedlings and sapling

Shrubencroachment

Soil erosion Energy balance

44.2

Conclusion on Woody plant encroachment

• A global phenomena, at the expense of grasslands

• Main drivers include overgrazing and reduction in fire frequency, and possibly increases in greenhouse gases

• Abruptness and irreversibility of the shift suggest positive feedbacks

• Impact on the ecohydrological functioning of watersheds is largelyunknown (increase in surface runoff ? Decline in ground water storage ?)


44.2

Advanced readings

• Archer, S., et al. 1995. Mechanisms of shrubland expansion – land-use, climate or CO2 ?- Clim. Change 29: 91-99.

• D'Odorico, P., et al. 2012. A synthetic review of feedbacks and drivers of shrub encroachment in arid grasslands. -Ecohydrology 5: 520-530.

• Knapp, A. K., et al. 2008. Shrub encroachment in North American grasslands: shifts in growth form dominance rapidly alters control of ecosystem carbon inputs. - Global Change Biol. 14: 615-623.

• Scott, R. L., et al. 2006. Ecohydrological impacts of woody-plant encroachment: seasonal patterns of water and carbon dioxide exchange within a semiarid riparian environment. - Global Change Biol. 12: 311-324.


44.2

Advanced readings

• Bai, Z. G., et al. 2008. Proxy global assessment of land degradation. - Soil Use and Management 24: 223-234.

• D'Odorico, P., et al. 2013. Global desertification: Drivers and feedbacks. - Advances in Water Resources 51: 326-344

• Fensholt, R. and Rasmussen, K. 2011. Analysis of trends in the Sahelian 'rain-use efficiency' using GIMMS NDVI, RFE and GPCP rainfall data. - Remote Sens. Environ. 115: 438-451.

• Fensholt, R., et al. 2012. Greenness in semi-arid areas across the globe 1981-2007 - an Earth Observing Satellite based analysis of trends and drivers. - Remote Sens. Environ. 121: 144-158.

• Wessels, K. J., et al. 2012. Limits to detectability of land degradation by trend analysis of vegetation index data. - Remote Sens. Environ. 125: 10-22.


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