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Month Aug. Sept. Day 26 28 2 4 9 11 16 18 23 Topic Introduction The ecosystem concept Climate/soils Soils II Ecosystem energy balance Water cycling Carbon GPP/NPP NEP C,M&M 1 2 3 4 4 5 6 6 Gross Primary Production • Net photosynthesis at the ecosystem scale is GPP = Carbon Input to Ecosystems = Sum of Ps over space and time • Levels of control – Controls in individual leaves – Control by canopy processes • Controlling factors – Direct controls: CO2, light – Indirect controls: water, nutrients, temperature Do factors affecting leaf level Ps explain variation in GPP among ecosystems? Are the controls over leaf- and canopy-level Ps the same? Limitation Plants adjust components of Ps so physical and biochemical processes co-limit C fixation Plants adjust resource acquisition to maximize capture of the most limiting resource • CO2 • Water Acclimation versus adaptation! • Light • Nitrogen • [Temperature-modulator] Ecosystem GPP differs from leaf Ps Broader range of light levels where LUE is constant Canopy processes Multiple species Canopy processes • Leaves fill a 3-D space. Leaf area index (LAI) measured in m leaf/ m ground (LAI ≈ 3 in closed canopy forests) • Clumping canopy leaves, angled relative to sun, allows light to leaves lower down in canopy. Most GPP at top. • Canopy roughness allows CO2 to mix with free troposphere, reducing diffusional limitation. • When water is sufficient, moderate wind has a positive effect on Ps Controls over GPP • Ecosystem differences are determined primarily by LAI and by length of growing season • LAI is determined by soil resource supply (water, nutrients), but disturbances (fire, wind), herbivory, pathogens can further reduce LAI • Within ecosystems and growing seasons, controls are the environmental controls over Ps (1=PAR, CO2, 2 =H2O, N), similar to individual leaves. GPP estimated from satellites NDVI = (NIR-VIS) (NIR+VIS) NDVI: Normalized difference vegetation index NIR: % Near-infrared radiation reflected VIS: % Visible radiation reflected Leaf area reflects NIR (Hi lvs, big #) Chlorophyll absorbs VIS (Hi lvs, small #) 0 = no vegetation, ~1 = high productivity http://landweb.nascom.nasa.gov/animation/images/samp les/MOD13A2.NDVI.2003.004.8FS.gif Net Primary Productivity NPP = GPP - Respirationplant ≈50% of GPP is used for respiration Net Primary Productivity NPP = GPP - Respirationplant Three components of respiration: 1. Growth (~25+%) -- synthesis of cellulose, proteins, lipids, etc. 2. Ion uptake (~25+%) -- energy for nutrient uptake from soil and transport of ions across membranes (25-50% of root respiration) 3. Maintenance respiration (~50%) –repair of membranes, proteins, other Factors controlling respiration NPP = GPP - Respirationplant 1. Tissue N -- protein turnover; 6% replaced daily; 85% of maintence respiration 2. Temperature -- increased protein and membrane turnover at higher temperatures GPP and Rplant generally related, so NPP and GPP are proportional However, at hot temperatures, Ps may be maintained or inhibited, while Rplant increases. NPP/GPP decreases. What plants do with the carbon they do not respire… Component of NPP New biomass % NPP 40-70 Leaves and repro. Apical stem growth Secondary stem growth 10-30 0-10 0-30 New roots 30-40 Root secretions Root exudates Root transfers to mycorrhizae 20-40 10-30 10-30 Losses to herbivores and mortality Volatile emissions 1-40 0-5 NPP operationally defined Aboveground: Aboveground biomass increment* Belowground: Fine litterfall* Root biomass increment* Losses to consumers Volatile and leached organics (VOC, DOC) Dead roots* Losses to consumers Root exudates & export to symbionts Belowground NPP • Sequential coring – measure root mass in cores over fairly short time intervals (monthly), to minimize underestimates due to turnover • Ingrowth cores – measure growth of new roots into ‘empty’ soil, also at short time intervals • Minirhizotron observations – take frequent pictures of live, growing, and senescing roots using belowground cameras. Convert images (root length or area) to productivity (g m-2 d-1). Camera The minirhizotron technique for measuring root productivity Image of roots and root hairs (wheat grown in a sandy soil) Minrhizotron tube Example of software used to convert images to productivity (typically length of root per cm2 viewing area per day)… Additional challenge: convert these units to NPP (g m-2 ground area yr-1) Minirhizotron observations provide a window into belowground biology… Mg C ha-1 yr -1 Climate controls over NPP 25 20 l l l l l 15 10 5 0 0 2000 l 4000 6000 8000 Mean Annual Precipitation (mm) • At a global scale, NPP is strongly correlated with ppt and T • Water increases plant growth in drier ecosystems. Also increases decomposition and nutrient cycling. • In very wet ecosystems, ppt can limit NPP by decreasing light or nutrient availability Climate controls over NPP Mg C ha-1 yr -1 25 20 15 10 5 0 -15 -10 -5 l l l l 0 5 10 15 20 25 30 Mean Annual Temperature (ºC) • Temperature is related to growing season length • Temperature stimulates decomposition and nutrient cycling. 25 25 Mg C ha-1 yr -1 Mg C ha-1 yr -1 Climate controls over NPP 20 l l l l l 15 10 5 0 0 2000 l 4000 6000 20 15 10 5 8000 Mean Annual Precipitation (mm) 0 -15 -10 -5 l l l l 0 5 10 15 20 25 30 Mean Annual Temperature (ºC) In ecosystems where correlations suggest a strong climatic limitation of NPP, experiments and observations indicate that this is mediated primarily by climatic effects on belowground resources. Resource control • Proximate control over NPP is availability of resouces • Light, CO2, H2O, nutrients (N,P,K,Mg,Ca,…) • Many ecosystems increase NPP when N or P fertilizer is added. • Some ecosystems increase NPP when CO2 or H2O is added. • Where T has been manipulated, NPP doesn’t respond directly. Vitousek and Farrington 1997 Lecture ended here Leibig (1840’s) "Father of physiological ecology” studied mineral elements added to plant soil Law of the Minimum When the rate of a process is controlled by a number of separate factors, the rate is limited by the pace of the 'slowest' or most limiting factor. Limiting Factor The simplest idea is that growth is limited by a single resource. But, often more than one factor might increase NPP (e.g., water and nutrients) Multiple resource limitation 1. Plants adjust resource acquisition to maximize capture of most limiting resource 2. Changes in the environment alter the relative abundance of resources. Different factors limit NPP at different times 3. Plants exhibit mechanisms that increase the supply of the most limiting resource 4. Different resources limit different species in an ecosystem. Variation in NPP Global Potential Net Primary Productivity Mg C ha -1 yr-1 Biome Differences in NPP (Terrestrial) • Length of the growing season is the major factor that explains biome differences in NPP • Differences in leaf area account for most of the variation in biome NPP within a growing season • Leaf area, in turn, is determined by soil resources, climate, and time since disturbance • Disturbance substantially modifies the relationship between NPP and climate