4. Processes and models of dryland dynamics. Fundamentals of Ecohydrology (Philippe CHOLER CNRS France). Wuwei (09/2013)

Transcription

1 4. Processes and models of dryland dynamics

2 4 Climatic constraints to plant growth in drylands Source: Fensholt R., et al. (2012) Remote Sens. Environ., 121,

3 4 The Pulse-and-Reserve Paradigm trigger (Precip.) growth pulse (NPP) activation storage reserve (storage tissues - seeds) loss source (Soil Water Nutrients) feedback loss Plant responses to precipitation in semi-arid grasslands Modified from : Noy-Meir I. (1973) Annu. Rev. Ecol. Syst., 4,

4 4 Outline of part 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

5 4.1. Tree-grass coexistence in savannas

6 4.1 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)

7 4.1 Tree-grass coexistence in savannas Superficial, dense rooting of grasses Deep rooting of trees Source: D'Odorico P., et al. (2007) Journal of Geophysical Research-Biogeosciences, 112.

8 4.1 The Kalahari transect in Southern Africa

9 4.1 Soil moisture dynamics c i > 0 c i Source: D'Odorico P., et al. (2007) Journal of Geophysical Research-Biogeosciences, 112.

10 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 Growth & recuitment of woody species Tree cover Increased carrying capacity for woody biomass Higher soil moisture)

11 4.1 Advanced readings D'Odorico, P., et al 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 Climatological determinants of woody cover in Africa. - Proc. Natl. Acad. Sci. USA 108: Hanan, N. P., et al Do fires in savannas consume woody biomass? A comment on approaches to modeling savanna dynamics. - Am. Nat. 171: Sankaran, M., et al Determinants of woody cover in African savannas. - Nature 438:

12 4.2. The Grasslands Shrublands dynamics

13 4.2 The grassland biome >3 source

14 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)

15 4.2 Land cover of drylands

16 4.2 From steppes to shrubland in USA Rock Springs shrubland short grass Prairie tall grass Prairie deciduous forest

17 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,

18 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 Source: C. Allen, J. Betancourt, and T. Swetnam, USGS Biological Resources Division Southwestern U.S. LUHNA pilot project, 1997

19 4.2 Stages of shrub encroachment Shrub dispersal and establishment Grasses outcompeted Soil erosion Source: D'Odorico P., et al. (2012) Ecohydrology, 5,

20 4.2 Herbivory and producer decomposer feedbacks 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 Source: Bardgett R.D., et al. (2005) Trends in Ecology and Evolution, 20,

21 4.2 Where all the prairies have gone? Source : George Catlin / BIBLIOTHÈQUE ET ARCHIVES Canada / C

22 4.2 Large wild herbivores replaced by cattles BISON -> CATTLE extensive grazing -> intensive grazing

23 4.2 Land use changes as the primary driver Recruitment of Ponderosa Pines in semi-arid grasslands (Utah) CO2 concentration Source: Archer S., et al. (1995) Clim. Change, 29,

24 4.2 Local drivers (patch scale) Effect of grazing intensification by livestcok Soil compaction. Changes in soil nutrient cycling Preferential utilization of palatable grasses Fire regime changes (regulation by herders, reduced fuel load) Dispersal of woody plants Reduced belowground Compettition by grasses Reduced fuel load

25 4.2 Global drivers Climate change CO 2 fertilization effect Benefitting C3 shrubs and trees Increased temperatures Limitation of frost damage to shrubs? Increased belowground carbohydrate storage, and survival? Glacial periods : low CO2, cold -> C4 grasslands Interglacial periods: high CO2, warm -> C3 shrubs/trees

26 4.2 Examples of positive feedbacks Reduced Belowground competition Shrub encroachment Increased survival of seedlings and sapling Shrub encroachment Grass comsunption Small mammals (rodents) Decreasing fire intensity and frequency Reduced fuel load (grass) Rodents Fire regime

27 4.2 Examples of positive feedbacks Reduced Belowground competition by grasses Shrub encroachment Shrub encroachment Reduced grass cover, increased bare soil Increased soil erosion, decrerased water infiltration Reduced grass cover Increased survival of seedlings and sapling Increased near surface temperature Soil erosion Energy balance

28 4.2 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 largely unknown (increase in surface runoff? Decline in ground water storage?)

29 4.2 Advanced readings Archer, S., et al Mechanisms of shrubland expansion land-use, climate or CO 2?- Clim. Change 29: D'Odorico, P., et al A synthetic review of feedbacks and drivers of shrub encroachment in arid grasslands. - Ecohydrology 5: Knapp, A. K., et al Shrub encroachment in North American grasslands: shifts in growth form dominance rapidly alters control of ecosystem carbon inputs. - Global Change Biol. 14: Scott, R. L., et al Ecohydrological impacts of woody-plant encroachment: seasonal patterns of water and carbon dioxide exchange within a semiarid riparian environment. - Global Change Biol. 12:

30 Advanced readings Bai, Z. G., et al Proxy global assessment of land degradation. - Soil Use and Management 24: D'Odorico, P., et al Global desertification: Drivers and feedbacks. - Advances in Water Resources 51: Fensholt, R. and Rasmussen, K Analysis of trends in the Sahelian 'rain-use efficiency' using GIMMS NDVI, RFE and GPCP rainfall data. - Remote Sens. Environ. 115: Fensholt, R., et al Greenness in semi-arid areas across the globe an Earth Observing Satellite based analysis of trends and drivers. - Remote Sens. Environ. 121: Wessels, K. J., et al Limits to detectability of land degradation by trend analysis of vegetation index data. - Remote Sens. Environ. 125: