Ecohydrology_Lecture4_F11

Ecohydrology_Lecture4_F11 - Arid Ecohydrology Ecohydrology...

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Arid Ecohydrology Ecohydrology Fall 2011
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Ecosystem RUE All terrestrial ecosystems use MAP to create NPP, and are therefore, at some level, water dependent Differential sensitivity to variance in MAP NPP:MAP defines rain use efficiency Sensitivity effectively measures the strength of water limitation vis-à- vis nutrient or evolutionary constraints Huxman et al. (2004) - Nature
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Convergent RUEmax? RUEmax generally occurs at low water availability There is a convergence of RUEmax across all biomes RUEmax is close to RUEmean for arid sites RUEmax is really low compared to RUEmean for humid sites
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Two Predictions 1 – NPP will be affected a lot if PPT is driven below the historic minimum 2 – Removal of other resource limitations will allow RUE to approach RUEmax
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Ergo Water limitation imposes a common constraint on NPP across biomes Ecosystems have the same RUEmax despite large differences in ppt, physiology, phytogenetic origin and climate history Altering resource limitation underscores the relevance of biogeochemistry on NPP (i.e., compared with species)
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Inhibited Deep Drainage Over Millenia In dry areas (<500 mm) there is a nearly ubiquitous pore-water Cl- peak at 5-15 m below grade Mass balance of Cl yields 10,000-15,000 years of accumulation (without major downward flux event) Consistent with ~ 1 mm/yr deep drainage Seyfried et al. (2005) - Ecology
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But… Upward water potential gradient Water is moving out of the soil Ergo, no downward movement Seyfried et al. (2005) - Ecology
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Strong Climatic Control Cl inventory suggests 7,000-8,000 years of accumulation at a site with 360 mm MAP Inventory is 1,000 years at 400 mm MAP at nearby site
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Ecohydrologic Mechanism Walvoord et al. (2002) propose a conceptual model to reconcile these attributes Low water potentials at the base of the root zone were established at the end of the Pleistocene (dramatically increased dryness) Potentials (< -1 MPa) were maintained continuously during the Holocene Any deep wetting event would reset the chloride concentrations Slow upward movement of water above ca. 20 m
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Vegetation is Essential Plants are required to impose and maintain the low potentials Two key attributes: Deep roots (2-3 m) of sufficient density to capture all downward percolating water Able to maintain low water potentials at depth continuously The demand for continuous maintenance has enormous implications for contemporary vegetation management Clearing vegetation can result in huge fluxes of Cl (and nitrate!) to the deep groundwater
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Deep Roots Root density decreases exponentially with depth, though slower in arid biomes Max rooting depths ~ 5 m, almost all co-dominant shrubs ~ 1.8 m Root depths increased with MAP and soil coarseness Maintaining root potentials demands cavitation avoidance
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Ecohydrology_Lecture4_F11 - Arid Ecohydrology Ecohydrology...

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