Ecohydrology_Lecture5_F11

Ecohydrology_Lecture5_F11 - Alternative Stable States and...

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Alternative Stable States and Patterned Landscapes Ecohydrology Fall 2011
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Alt. Stable State – Desert Streams Wetlands used to be a major component of desert streams (Sycamore Creek, AZ) Reduced to zero over the late 19th and 20th C. Recently become reestablished with controls on grazing
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Cienegas In-stream herbaceous wetlands Dominated by perennials Historically sites of heavy grazing (density reduction) Herbaceous patches were small and rare in 2000 Covered 20% of channel area in 2005 Coincided with grazing release Pollen records indicate dependence on climate
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Cienegas
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Post-Flood Gravel Wash
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Well-Established Cienegas
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Desert Streams (Sycamore Creek) Extreme disturbance regime Low inter-event flow limits moisture for riparian plants Plants act to stabilize sediments, and this effect is density dependent
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Basic Mechanism Extreme events scour all vegetation During smaller events, wetland plants control sediment dynamics Density dependent stabilization Ergo, dense vegetation persists through intermediate floods and dry periods, as do areas without G – veg growth S – vegetation mortality Ks – channel sediment stability w/o veg Cs – per capita stabilization of sediment rs – scour vegetation mortality V – vegetation Q – flood frequency
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Plant-Controlled Feedbacks Increased plant density changes: Flow velocity (lower) Fine sediment retention/deposition (higher) Sediment hypoxia (higher) Increased P availability Vegetative dispersal (higher) Sediment water holding capacity (higher) In systems with feedbacks, remember to consider reciprocal causality E.g. Plants and moisture holding capacity
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Hypotheses Density dependent relationship between vegetation and flood scour effects creates alternative equilibria at intermediate flood intensity Prediction 1: Aboveground biomass will be negatively correlated with vegetation removal by floods (necessary, but sufficient?) Prediction 2: Vegetation abundance will diverge to a bimodal distribution in response to floods
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Empirical Support Water and veg cover over time at 26 sites Major Resetting Flood
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Prediction 1 – Density Dependent Loss Per capita flood mortality decreases with increasing biomass.
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Ecohydrology_Lecture5_F11 - Alternative Stable States and...

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