Carbon Inputs to Ecosystems 5 Photosynthesis by plants provides the carbon address carbon inputs to ecosystems through and energy that drive most biological pro- photosynthesis in this chapter and the carbon cesses in ecosystems. This chapter describes losses from plants and ecosystems in Chaps. 6 the controls over carbon input to ecosystems. and 7, respectively. The balance of these processes governs the patterns of carbon accumulation and loss in ecosystems and the carbon distribution Introduction between the land, atmosphere, and ocean. The energy fixed by photosynthesis directly supports plant growth and produces organic A Focal Issue matter that is consumed by animals and soil microbes. The carbon derived from photosynthe- Carbon and water exchange through pores sis makes up about half of the organic matter on (stomata) in the leaf surface governs the effi- Earth; hydrogen and oxygen account for most of ciency with which increasingly scarce water the rest. Human activities have radically modified resources support food production for a grow- the rate at which carbon enters the terrestrial bio- ing human population. Open stomata (Fig. 5.1) sphere by changing most of the controls over this maximize carbon gain and productivity when process. We have increased the quantity of atmo- water is abundant, but at the cost of substantial spheric CO2 by 35% to which terrestrial plants are water loss. Partial closure of stomata under dry exposed. At regional and global scales, we have conditions reduces carbon gain but increases the altered the availability of water and nutrients, the efficiency with which water supports plant growth. major soil resources that determine the capacity What constrains the capacity of the biosphere to of plants to use atmospheric CO2. Finally, through gain carbon? Where and in what seasons does changes in land cover and the introduction and most photosynthesis occur? How do plants regu- extinction of species, we have changed the late the balance between carbon gain and water regional distribution of the carbon-fixing potential loss? Application of current understanding of the of the terrestrial biosphere. Because of the central controls over tradeoffs between carbon gain and role that carbon plays in the climate system (see water loss could reduce the likelihood of a “train Chap. 2), the biosphere, and society, it is critical wreck” resulting from current trends in increasing that we understand the factors that regulate food demands and declining availability of fresh- its cycling through plants and ecosystems. We water to support agricultural production. F.S. Chapin, III et al., Principles of Terrestrial Ecosystem Ecology, 123 DOI 10.1007/978-1-4419-9504-9_5, © Springer Science+Business Media, LLC 2011 124 5 Carbon Inputs to Ecosystems Fig. 5.1 Surface of a Tradescantia virginiana leaf with opening influences both the maximum rate and the effi- open stomatal pores. Selection for plants that differ in sto- ciency with which plants use water to gain carbon. matal density and physiological regulation of stomatal Photograph courtesy of Peter Franks Carbon is the main element that plants reduce Overview of Carbon Inputs with energy derived from the sun. Carbon and to Ecosystems energy are therefore tightly linked as they enter, move through, and leave ecosystems. Photosynthesis is the process by which most Photosynthesis uses light energy (i.e., radiation carbon and chemical energy enter ecosystems. in the visible portion of the spectrum) to reduce The proximate controls over photosynthesis at CO2 and produce carbon-containing organic the cellular or leaf level are the availability of compounds. This organic carbon and its associ- photosynthetic reactants such as light energy ated energy are then transferred among compo- and CO2; temperature, which governs reaction nents within the ecosystem and are eventually rates; and the availability of nitrogen, which is released to the atmosphere by respiration or required to produce photosynthetic enzymes. combustion. Photosynthesis at the scale of ecosystems is The energy content of organic matter varies termed gross primary production (GPP). Like among carbon compounds, but for whole tissues, photosynthesis by individual cells or leaves, GPP it is relatively constant at about 20 kJ g−1 of ash- varies diurnally and seasonally in response to free dry mass (Golley 1961; Larcher 2003; variations in light, temperature, and nitrogen sup- Fig. 5.3). The carbon concentration of organic ply. Differences among ecosystems in annual matter is also variable but averages about 45% of GPP, however, are determined primarily by the dry weight in herbaceous tissues and 50% in wood quantity of photosynthetic tissue and the duration (Gower et al. 1999; Sterner and Elser 2002). Both of its activity (Fig. 5.2). These, in turn, depend on the carbon and energy contents of organic matter the availability of soil resources (water and nutri- are greatest in materials such as seeds and animal ents), climate, and time since disturbance. In this fat that have high lipid content and are lowest in chapter, we explore the mechanisms behind these tissues with high concentrations of minerals or causal relationships. organic acids. Because of the relative constancy Biochemistry of Photosynthesis 125 LONG-TERM SHORT-TERM CONTROLS CONTROLS STATE Interactive Direct FACTORS controls controls Plant BIOTA functional Leaf area types TIME Soil Nitrogen resources Season GPP PARENT length MATERIAL Temperature CLIMATE Light CO2 Fig. 5.2 The major factors governing temporal and Thickness of the arrows indicates the strength of the direct spatial variation in gross primary production (GPP) in and indirect effects. The factors that account for most of ecosystems. These controls range from proximate con- the variation among ecosystems in GPP are leaf area and trols, which determine the diurnal and seasonal variations length of the photosynthetic season, which are ultimately in GPP, to the interactive controls and state factors, which determined by the interacting effects of soil resources, are the ultimate causes of ecosystem differences in GPP. climate, vegetation, and disturbance regime of the carbon and energy contents of organic Needle-leaved matter, carbon, energy, and biomass have been 28 tree Broad-leaved used interchangeably as currencies of the carbon tree and energy dynamics of ecosystems. The pre- ) 26 1 Herb − ferred units differ among subfields of ecology, g depending on the processes that are of greatest 24 interest or are measured most directly. Production studies, for example, typically focus on biomass, 22 trophic studies on energy, and gas exchange stud- ies on carbon. Energy content (kJ 20 18 Biochemistry of Photosynthesis Leaf Stem Seed Root/ The biochemistry of photosynthesis governs rhizome the environmental controls over carbon inputs Fig. 5.3 Energy content of major tissues in conifer trees, to ecosystems. Photosynthesis involves two major broad-leaved trees, and broad-leaved herbs. Compounds groups of reactions: The light-harvesting reac- that contribute to a high energy content include lipids tions (or light-dependent reactions) transform light (seeds), terpenes and resins (conifers), proteins (leaves), and lignin (woody tissues). Values are expressed per gram energy into temporary forms of chemical energy of ash-free dry mass. Data from Larcher (2003) (ATP and NADPH; Lambers et al. 2008). The 126 5 Carbon Inputs to Ecosystems Light Light-harvesting Starch reactions e chl Sugar export NADP + 2 3C sugars H O H + O2 2 Carbon-fixation reactions oid oid ylak ylak NADPH CO2 Th Th RuBP O2 ATP (5C sugar) H+ Photo- respiration 2 2C compounds ADP Thylakoid 2C Stroma export O2 Fig. 5.4 A chloroplast, showing the location of the major provide the energy to synthesize ribulose-bisphosphate photosynthetic reactions. The light-harvesting reactions (RuBP), which reacts either with CO2 to produce sugars and occur in the thylakoid membranes; chlorophyll (chl) absorbs starch (carbon-fixation reactions of photosynthesis) or with visible light and funnels the energy to reaction centers, O2 to produce two-carbon intermediates (photorespiration) + where water inside the thylakoid is split to H and O2, and and ultimately CO2. Through either carbon fixation or pho- resulting electrons are passed down an electron-transport torespiration, ADP and NADP are regenerated to become chain in the thylakoid membrane, ultimately to NADP, pro- reactants in the production of additional ATP and NADPH. ducing NADPH. During this process, protons move across The net effect of photosynthesis is to convert light energy the thylakoid membrane to the stroma, and the proton (H+) into chemical energy (sugars and starches) that is available gradient drives the synthesis of ATP. ATP and NADPH to support plant growth and maintenance carbon-fixation reactions (or light-independent potential efficiency of photosynthesis in convert- reactions, sometimes called the dark reaction) ing solar radiation into chemical energy. use the products of the light-harvesting reactions The carbon-fixation reactions of photosynthe- to convert CO2 into sugars, a more permanent sis use the chemical energy (ATP and NADPH) form of chemical energy that can be stored, from the light-harvesting reactions to reduce CO2 transported, or metabolized. Both groups of to sugars. The rate-limiting step in the carbon- reactions occur simultaneously in the light in fixation reactions is the reaction of a five-carbon chloroplasts, which are organelles inside pho- sugar (ribulose-bisphosphate [RuBP]) with CO2 to tosynthetic cells (Fig. 5.4). In the light-harvest- form two three-carbon organic acids (phospho- ing reactions, chlorophyll (a light-absorbing glycerate), which are then reduced using ATP and pigment) captures energy from visible light. NADPH from the light reactions to form three- Absorbed radiation is converted to chemical carbon sugars (glyceraldehyde 3-phosphate). The energy (NADPH and ATP), and oxygen is pro- initial attachment of CO2 to a carbon skeleton is duced as a waste product. Visible radiation catalyzed by the enzyme ribulose-bisphosphate accounts for 40% of incoming solar radiation carboxylase-oxygenase (Rubisco). The rate of this (see Chap.