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Slide1: 

Production and Biogeochemical Cycling in Ecosystems Dr. Jonathan Scurlock Environmental Sciences Division Oak Ridge National Laboratory UT General Ecology 250: Fall 2001 Humid Savanna NPP site - Lamto, Côte d’Ivoire photograph from the ORNL DAAC NPP archive

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Why is this important? Understanding the flow of major “life” elements and energy through ecosystems (C cycling, N cycling, ecological energetics) is essential today, and has many applications: we need to understand biospheric feedbacks to increased atmospheric CO2 humans have also doubled the global rate of atmospheric nitrogen fixation, with unknown consequences scaling of ecosystem energy, carbon and nutrient budgets from field study site, to region, to globe planetary science and management - we have to learn how to regulate Earth’s life system, for our own sustainability and maybe for future terraforming

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Plant production/productivity A fundamental measure, but also a vague one Net primary productivity (NPP) is a term used rather liberally for growth/carbon fixation which is available to other organisms. Unfortunately it cannot be directly measured, and it seems to mean different things to different users (physiologists, foresters, ecosystem modelers) A key variable for understanding global carbon dynamics Origins in Tansley’s ecosystem (1935) and IBP (1960s). Today’s global change research requires consistent data, but actual NPP field measurements are scattered and unevenly distributed across many vegetation types.

The ecosystem concept: 

The ecosystem concept Biological and physical components: Tansley recognized the importance of two-way interactions between organisms and their surroundings - i.e. a “system” Actually three components: (1) producers or autotrophs (mostly green plants); (2) consumers or heterotrophs (herbivores, carnivores and decomposers); (3) abiotic matter (inorganic surroundings and recycled organic residues) Thermodynamics: energy is conserved, but entropy increases with liberation of heat at successive trophic levels So what drives life?: input of Sun’s energy from outside our semi-closed system - i.e. primary production (photosynthesis) NPP = GPP - R NPP = B + D + [losses]

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Flows of energy and carbon Allocation of energy/biomass is important: some ecosystems, e.g. tall forests or crops, allocate most plant biomass above-ground. Others, e.g. grasslands, deserts, may allocate majority of production below-ground (roots and rhizomes). Important implications for measurement of NPP. Secondary production: growth and reproduction of herbivores (and higher trophic levels such as carnivores) limited by available food supply, i.e. limited by NPP and growth efficiency (metabolic losses)

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What feeds Bambi?

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Data for large herbivorous mammals in Africa (but same relationships would apply for insects, etc.)

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Consumers vary in their efficiency of secondary or tertiary production: (1) assimilation efficiency (energy absorbed/energy content of food) - usually more efficient in homeotherms (warm-blooded); (2) production efficiency (energy in growth/energy absorbed) - poikilotherms usually more efficient, since they use less energy for metabolism Decomposers: like primary producers, productivity and nutrient cycling are strongly influenced by climate (mainly temperature and moisture)

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Patterns in space and time NPP is a function of climate (e.g. precipitation and temperature) but other factors are important (not always the same ones), e.g. soil nutrients, inputs/outputs, fire, grazing, and human intervention (harvesting or clearing). NPP varies seasonally and from year to year, so long-term studies at the same site may be critically important NPP of oceans is severely limited by nutrients (N, P, Fe, etc.), so the most productive areas are usually near coasts (upwelling, estuarine inputs)

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WORLDWIDE NET PRIMARY PRODUCTIVITY greater solar resource, longer growing season in tropics and sub-tropics moderated by available soil moisture (rainfall) and sometimes by soil nutrients

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NPP as a function of climatic “driving” data (easily available worldwide) This is the basis of global NPP “models” which can predict patterns of carbon cycling, including places for which field data may not be available The “Miami” model of H. Lieth and G. Esser uses the minimum value of two such equations in which climate limits NPP

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Generalized nutrient cycle (here with figures for the terrestrial carbon cycle in billions of tonnes of carbon) 730 Gt 560 Gt 1500 Gt 1-2 Gt 120 Gt/yr gross (60 Gt/yr net) Fossil fuel +6 Gt/yr LUCC ~1.6 Gt/yr

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Carbon cycle illustrated in terms of the “real world” consumers!

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Nutrient cycles may have many complex pathways but it is possible to measure individual components (here, the N cycle) and then develop computer models with predictive ability e.g. CENTURY model (mainly C & N cycle) developed at Colorado State University

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Human perturbation of the global N cycle Natural N fixation: about 90-140 Mt per year (lightning about 10 Mt, the rest from microorganisms in soil and legume nodules) Anthropogenic N fixation: 140 Mt per year and growing! Mostly from N fertilizers (since 1940s), plus legume crops, fossil-fuel burning and land clearing Detailed effects are still unknown: some regional increase in NPP, some changes in species composition already observed

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The roles of fire & herbivory Both natural and anthropogenic fire play a key role in several biogeochemical cycles (C, N, etc.) fire frequency varies by biome (even occasional fires every 500 years in tropical forest!). Dendroecology shows impact of European settlement (from 1800-1880) on fire frequency in North America dendroecology also shows that major insect herbivory occurs in cycles. Large herbivores and/or livestock, sometimes together with fire, may be important at grassland/savanna/forest boundary

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Different biogeochemical cycles The carbon and nitrogen cycles operate in parallel in many ways, depending upon the C/N ratio of different types of organic matter The sulfur cycle (S) is similar, but the phosphorus cycle (P) has no atmospheric reservoir The S and N cycles are involved in acid rain (acid deposition) - first recognized in 1872 in Britain Other cycles may be largely anthropogenic in origin (heavy metals - Pb, Cd, Hg) or even 100% artificial (chlorinated hydrocarbons such as DDT)

Importance of long-term studies: 

Long-Term Ecological Research network http://lternet.edu/sites/ ORNL NPP database http://daac.ornl.gov/NPP/ Importance of long-term studies

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How do you measure NPP? (at least for grasslands) Monthly clipping of quadrats selected by randomized block design Determine live+dead matter above-ground and below-ground Calculate B and D Estimate decomposition losses (paired plots/litter bags) Sum over 12 months Grassland study site Montecillos, Mexico

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Savanna - Nairobi, Kenya NPP Study Site: Nairobi, Kenya Site description Monthly biomass Monthly climate Graphs Site photos Key references with summaries

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Precipitation Nairobi Study Site Modified Walter-Lieth Climate Diagram Temperature Precipitation Savanna - Nairobi, Kenya

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We need long-term multi-year studies in many different biomes worldwide Integrate new studies with historical measurements (previous studies, dendroecology, etc.) to understand biogeochemical processes such as photosynthetic productivity and nitrogen fixation over different temporal and spatial scales Data may then be used to develop and test models with predictive abilities Need to collect, compile and manage data Tropical forest - Barro Colorado, Panama

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TECO-NPP Using NPP data for model testing: comparison of > 200 field measurements...

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TECO-NPP .....with model values (here, VEMAP Phase I results for conterminous USA)