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Month Day Topic Aug. 26 Introduction 28 The ecosystem concept Sept. 2 Climate/soils 4 Soils II 9 Ecosystem energy balance 11 Water cycling 16 Carbon 18 GPP/NPP 23 NEP C,M&M 1 2 3 4 4 5 6 6 Characterizing functional differences among ecosystems… • Albdeo • Bowen ratio The water cycle Basic principle of water balance in ecosystems • Inputs – Mainly precipitation, some groundwater, fog • Outputs – Evaporation – Transpiration – Drainage Basic principle of water movement in ecosystems • Water moves along energy gradient – From high energy to low energy • What forces cause water to move? – Pressures • Gravity * Ecosystems differ in canopy storage Depends mainly on LAI Differs among species Infiltration • Water movement into soil • Depends largely on hydraulic conductivity – Texture – Aggregate structure – Macropores made by animals and roots Basic principle of water movement in ecosystems • Water moves along energy gradient – From high energy to low energy • What forces cause water to move? – Pressures • Gravity • Forces created by organisms – Osmotic gradients – Matric forces (adsorption) Water potential () * Water movement to root • Moves along water potential gradient* • Rate depends on hydraulic conductivity and path length • Depends on root volume Soil depth/volume exploited differs among ecosystems Transpiration is major driving force of water movement • Water moves in continuous column from film on soil particles to leaf cells • Moves upward because of strong cohesive forces among water molecules Plants have some water storage capacity Quite limited in most plants (2 hours in this graph) Most of water must come from soil (not plant storage) Water loss from leaves • Driving force is vapor pressure gradient – Depends on temperature and water vapor in bulk air • Stomata are major resistance – Stomatal conductance depends on • Soil moisture • Vapor pressure of air (in some species) Rate of water loss from leaves depends on water supply • Influences water potential gradient • Plants adjust stomatal conductance to match water supply Water-transport capacity of stems is balanced with LAI Water inputs to ecosystem determines water outputs • P + ∆S = ET + R – P = Precipitation – S = Storage – ET = Evapotranspiration (LE) – R = Runoff Controls over ET in moist soils • Ecosystem boundary layer conductance – Physical control over water loss – Depends on vegetation structure, temperature and wind – THE major vegetation control when soils are moist • Leaf surface conductance – Depends on stomatal conductance – Becomes increasingly important as soils dry Climatic effects on ET • Moisture content of air is major control in rough, well-mixed canopies • Net radiation is major control in smooth, poorly coupled canopies • These environmental controls are the same in wet and dry canopies Streamflow is the “leftovers” after soil storage and ET are met • Over long term, runoff depends on ppt and ET In moist ecosystems, ET is relatively insensitive to ppt. Precipitation directly regulates streamflow Jackson et al. 2005 Lecture ended here H/LE feedbacks in the Amazon Positive feedback to grass/fire Rooting depth ET/LE H Grass/fire + Humidity Air T ppt Consequences of deforestation • 25-40% of ppt in Amazon basin comes from within-basin ET • Dry air = decreased transport of ocean air to basin • Permanent reduction in ppt • Favors grasses/savannah vegetation/fire Albedo feedbacks in the arctic Positive feedback to migration of arctic treeline Trees Albedo (earlier spring) Warmer climate + Rnet H Air T Arctic climate forcing • Biome shift causes more heating of land surface • Comparison of climate forcing factors: Doubling of CO2: + 4.4 W/m2 Conversion of tundra to shrubland: + 9 W/m2 Conversion of tundra to forest: + 26 W/m2 Chapin et al. 2005