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@prefix dcterms: <http://purl.org/dc/terms/> . @prefix xsd: <http://www.w3.org/2001/XMLSchema#> . @prefix gcis: <http://data.globalchange.gov/gcis.owl#> . @prefix cito: <http://purl.org/spar/cito/> . @prefix biro: <http://purl.org/spar/biro/> . <https://data.globalchange.gov/report/second-state-carbon-cycle-report-soccr2-sustained-assessment-report/chapter/biogeochemical-effects-of-rising-atmospheric-carbon-dioxide/finding/key-message-17-2> dcterms:identifier "key-message-17-2"; gcis:findingNumber "17.2"^^xsd:string; gcis:findingStatement "While atmospheric CO<sub>2</sub> rises at approximately the same rate all over the globe, its non-climate effects on land vary depending on climate and dominant species. In terrestrial ecosystems, rising atmospheric CO<sub>2</sub> concentrations are expected to increase plant photosynthesis, growth, and water-use efficiency, though these effects are reduced when nutrients, drought, or other factors limit plant growth (<em>very high confidence, very likely</em>). Rising CO<sub>2</sub> would likely change carbon storage and influence terrestrial hydrology and biogeochemical cycling, but concomitant effects on vegetation composition and nutrient feedbacks are challenging to predict, making decadal forecasts uncertain."^^xsd:string; gcis:isFindingOf <https://data.globalchange.gov/report/second-state-carbon-cycle-report-soccr2-sustained-assessment-report/chapter/biogeochemical-effects-of-rising-atmospheric-carbon-dioxide>; gcis:isFindingOf <https://data.globalchange.gov/report/second-state-carbon-cycle-report-soccr2-sustained-assessment-report>; ## Properties of the finding: gcis:descriptionOfEvidenceBase "Research definitively shows that the bodies of marine and terrestrial organisms have incorporated CO<sub>2</sub> released from the burning of fossil fuels, based on the change in isotope ratios within their biological material (Fraile et al., 2016; Hilton et al., 2006; Suess 1955).<br><br> On land, the historical record of the impact of rising CO<sub>2</sub> is more complex. Physiological theory suggests that, as CO<sub>2</sub> rises, photosynthesis should increase. Using preserved plant specimens, isotopomer analysis appears to support this physiological prediction (Ehlers et al., 2015), though this is a novel technique. The effects of rising CO<sub>2</sub> on tree biomass over multiple decades may be inferred from tree-ring records, but they provide mixed results (Andreu-Hayles et al., 2011; Cole et al., 2009; Knapp and Soulé 2011; Koutavas 2013). Studies from a wide range of forest types across broad geographic regions have observed changes in the ratio of the C isotope to the C isotope (<em>δ</em>C), observations which imply trees have experienced increased water-use efficiency as CO<sub>2</sub> has risen over the last two centuries, but growth was not clearly stimulated by rising CO<sub>2</sub> (Peñuelas et al., 2011).<br><br> Rising CO<sub>2</sub> tends to make plants close their stomata and thus use water more efficiently. The primary enzyme responsible for CO<sub>2</sub> uptake, ribulose-1,5-bisphosphate carboxylase-oxygenase (RUBISCO), accounts for a substantial portion of every plant’s nitrogen requirement. As CO<sub>2</sub> rises, less RUBISCO is required for the same carbon gain, so plants become more efficient in nutrient use. These physiological effects play out differently in various types of plants and under diverse environmental conditions. Plants that lack a CO<sub>2</sub> concentration mechanism and pass a 3-carbon sugar molecule into the Benson-Calvin cycle (C<sub>3</sub> plants) are more likely to show an instantaneous photosynthetic response than plants with a CO<sub>2</sub> concentration mechanism like C<sub>4</sub> plants (that pass a 4-carbon sugar molecule to the Benson-Calvin cycle) or those that use crassulacean acid metabolism (CAM).<br><br> Twenty years of CO<sub>2</sub> enrichment experiments have shown that elevated CO<sub>2</sub> enhances photosynthetic carbon gain over the long term for certain ecosystem types but only over the short term for others (Leakey et al., 2009; Leuzinger et al., 2011; Norby and Zak 2011). Plant communities dominated by trees and grasses generally have shown greater stimulation of photosynthetic carbon uptake compared to that of legumes, shrubs, and nonleguminous C<sub>3</sub> crops (Ainsworth and Rogers 2007).<br><br> Net primary production (NPP) is calculated as either the balance between carbon gained through photosynthesis and lost through respiration or the sum of all growth over a year. NPP is enhanced by ~23% across a broad range of early successional forests in response to elevated CO<sub>2</sub> (Norby et al., 2005). These results are likely not indicative of all forests, and smaller responses have been observed in the limited number of studies carried out in old-growth temperate, boreal, and tropical forests (Hickler et al., 2008; Körner et al., 2005). Also clear is that the temporal pattern of NPP responses to elevated CO<sub>2</sub> differs among forests. For example, McCarthy et al. (2010) reported that NPP in coniferous forests was enhanced by 22% to 30% and sustained over 10 years of exposure to 550 parts per million (ppm) of CO<sub>2</sub>. In contrast, Norby et al. (2010) found that NPP was significantly enhanced for 6 years in hardwood forest plots exposed to 550 ppm CO<sub>2</sub> (compared with plots under current ambient CO<sub>2</sub>), after which time the enhancement of NPP under elevated CO<sub>2</sub> declined from 24% to 9%.<br><r> Plants balance carbon gain and water loss. Stomatal conductance is depressed at elevated CO<sub>2</sub>, so plants may reduce water loss without reducing carbon gain. This physiological effect has been observed at the leaf and canopy scales (Keenan et al., 2013; Leakey et al., 2009; Peñuelas et al., 2011) and represents the major mechanism leading to observations of decreased canopy evapotranspiration under elevated CO<sub>2</sub>. For the hydrological cycle, this mechanism results in increased soil moisture. Even plants with CO<sub>2</sub> concentration mechanisms (i.e., C<sub>4</sub> and CAM plants) may experience increased water-use efficiency without any direct stimulation in photosynthesis (Leakey et al., 2009). Under drought conditions, elevated CO<sub>2</sub> may not directly stimulate photosynthesis in C<sub>4</sub> plants but can indirectly increase carbon gain by increasing water-use efficiency.<br><br> Physiological theory and experimental evidence indicate that rising CO<sub>2</sub> increases the photosynthetic temperature optimum (Long 1991) because of the decreasing relative solubility of CO<sub>2</sub> versus oxygen at higher temperatures (Jordan and Ogren 1984). These results imply that biomes that experience high temperatures may experience disproportionately enhanced photosynthesis and growth. Interannual variation in the increased growth of Lobolly pine trees was disproportionately enhanced by experimentally elevated CO<sub>2</sub> in warmer years (Moore et al., 2006).</p> Plant growth is not limited by CO<sub>2</sub> alone (Körner 2015). If, for example, another environmental factor limits growth, then experimentally increasing CO<sub>2</sub> has reduced effects on photosynthesis and growth (Ainsworth and Rogers 2007). This outcome is called “sink limitation.” Research suggests that nitrogen limitation may be one mechanism leading to declining NPP responses to elevated CO<sub>2</sub> in some ecosystems (Norby et al., 2010).<br><br> Nitrogen is sequestered in long-lived biomass and soil pools and may not be readily available to plants under some conditions. In this case, nitrogen limitation inhibits increases in plant production associated with elevated CO<sub>2</sub>, an effect which is referred to as a negative feedback. In systems where nitrogen supply was sufficient, CO<sub>2</sub> fertilization effects on NPP persisted (Drake et al., 2011; Finzi et al., 2006). Nevertheless, elevated CO<sub>2</sub> also increases photosynthetic nitrogen-use efficiency, defined as the net amount of CO<sub>2</sub> assimilated per unit of leaf nitrogen (Ainsworth and Rogers 2007; Bader et al., 2010; Leakey et al., 2009).<br><br> Elevated atmospheric CO<sub>2</sub> experiments have demonstrated that seed yield can be increased (LaDeau and Clark 2001, 2006). In some crop species, increased seed production was accompanied by reduced quality (Ainsworth et al., 2002), but this was not observed in tree species (Way et al., 2010). Species show different growth responses to rising CO<sub>2</sub> (Dawes et al., 2011), and dominant plants may have an advantage with rising CO<sub>2</sub> (McDonald et al., 2002; Moore et al., 2006), leading to changes in forest structure."^^xsd:string; gcis:newInformationAndRemainingUncertainties "Unclear is whether rising CO<sub>2</sub> will lead to larger standing biomass and carbon storage or simply faster cycling of carbon (Norby and Zak 2011). While instantaneous and annual fluxes of carbon are well studied in the Free-Air CO<sub>2</sub> Enrichment (FACE) literature, the allocation of carbon to different pools varies between experiments (DeLucia et al., 2005), and enhancement of multidecadal carbon stocks (e.g., woody biomass and soil organic matter) is not well studied (Leuzinger and Hattenschwiler 2013; Norby and Zak 2011). Plant growth is increased by CO<sub>2</sub>, but gross plant respiration is also stimulated (Leakey et al., 2009). Root growth and the incorporation of organic material below ground are observed in response to elevated CO<sub>2</sub> but so too is enhanced soil respiration fueled by releases of carbon from root systems (Drake et al., 2011; Hoosbeek et al., 2007; Jackson et al., 2009; Lagomarsino et al., 2013; Selsted et al., 2012). Increased carbon supply from plants can lead to enhanced activity of soil fauna and more rapid cycling of carbon, rather than increased carbon storage in soils (Phillips et al., 2012; van Groenigen et al., 2011, 2014). Observed changes in soil carbon were small over the timescale of the FACE studies (3 to 16 years), and thus firm conclusions remain elusive (Luo et al., 2011). In general, large effects of rising CO<sub>2</sub> on carbon storage in soils are not expected (Schlesinger and Lichter 2001).<br><br> <p>The long-term effects of rising CO<sub>2</sub> are uncertain because there is only one whole-ecosystem study (i.e., of a salt marsh) that extends to 20 years. Instantaneous physiological responses to CO<sub>2</sub> (Farquhar et al., 1980) typically are modified by feedbacks in system-level studies (Leakey et al., 2009; Norby and Zak 2011). Long-term records from tree-ring analyses are limited to reconstructions of aboveground growth. These studies rarely account for changes in carbon allocation strategies (DeLucia et al., 2005; Norby et al., 2010) caused by rising CO<sub>2</sub> or changes in nutrient limitation (Finzi et al., 2006; McCarthy et al., 2010; Zhu et al., 2016) or belowground carbon storage (Drake et al., 2011; Phillips et al., 2012; van Groenigen et al., 2014)."^^xsd:string; a gcis:Finding . ## This finding cites the following entities: <https://data.globalchange.gov/report/second-state-carbon-cycle-report-soccr2-sustained-assessment-report/chapter/biogeochemical-effects-of-rising-atmospheric-carbon-dioxide/finding/key-message-17-2> prov:wasDerivedFrom <https://data.globalchange.gov/report/second-state-carbon-cycle-report-soccr2-sustained-assessment-report/chapter/preface/figure/figurep-4>.