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Carbon dynamics of pristine and hydrologically modified fens in the southern Ro

Carbon dynamics of pristine and hydrologically modified fens in the southern Rocky Mountains Rodney A. Chimner and David J. Cooper Abstract: We measured water table levels, above- and below-ground plant production, and CO2 and CH4 emissions for five fens in Rocky Mountain National Park, Colorado, to determine whether a water diversion project was adversely affecting carbon cycling. Two fens were located beneath the water diversion, and three fens were located in an adjacent pristine watershed. The diversion lowered water table levels in one fen, while the other fen was not hydrologically modified. Total NPP (net primary production) for all sites ranged from 130 to 316 g C·m–2·year–1, with a mean of 217 g C·m–2·year–1, and belowground NPP accounted for ~60% of the total. Maximum CO2 emissions for pristine fens ranged between 170 and 273 mg CO2-C·m–2·h–1, with annual emissions of 230–388 g CO2-C·m–2·year–1. However, the hydrologically modified fen had maximum CO2 emissions of 457 mg CO2-C·m–2·h–1 and had an annual flux of 573 g CO2-C·m–2·year–1. Maximum CH4 emissions ranged from 3 to 25 mg CH4-C·m–2·h–1, with annual emissions of 9–61 g CH4-C·m–2·year–1. The water diversion structure lowered water tables, increased CO2, decreased CH4 and NPP, and resulted in the site likely becoming a net source of carbon. Key words: peatlands, fens, CO2, CH4, hydrology, Rocky Mountains, Rocky Mountain National Park, plant production. Résumé : Afin de déterminer si un projet de diversion de l’eau affecte le cyclage du carbone, les auteurs ont mesuré les niveaux de la nappe phréatique, la production végétale hypogée et épigée, ainsi que les émissions de CO2 et de CH4 , dans cinq tourbières basses du Rocky Mountain National Park, au Colorado. Deux tourbières basses étaient localisées en aval de la diversion de l’eau, et trois tourbières basses se retrouvaient dans un bassin versant vierge adjacent. La diversion a abaissé les niveaux de la nappe phréatique dans une des tourbières basses, alors que l’autre tourbière n’a pas été affectée par les modifications hydrologiques. Pour l’ensemble des sites, le NPP total va de 130 à 316 g C·m–2·an–1, avec une moyenne de 217 g C·m–2·an–1, alors que le NPP hypogé compte pour ~60% du total. Les émissions maximum de CO2 dans les tourbières basses vierges vont de 170 à 273 mg CO2-C·m–2·h–1, et des émissions annuelles de 230 à 388 g CO2-C·m–2·an–1. Cependant, la tourbière dont l’hydrologie a été modifiée montre des émis- sions maximum de CO2 de 457 mg CO2-C·m–2·h–1 et un flux annuel de 573 g CO2-C·m–2·an–1. Les émissions maxi- mum de CH4 vont de 3 à 25 CH4-C·m–2·h–1, avec des émissions annuelles de 9 à 61 g CH4-C·m–2·an–1. La structure de diversion de l’eau a abaissé les niveaux de la nappe phréatique, augmenté le CO2, diminué le CH4 et le NPP, et a vraisemblablement conduit le site à devenir une source nette de carbone. Mots clés : tourbières, tourbières basses, CO2, CH4, hydrologie, Rocky Mountain National Park, productivité végétale. [Traduit par la Rédaction] Chimner and Cooper 491 Introduction Peatland ecosystems cover an estimated 4 million km2 of the earth, primarily on low-gradient landscapes in cool boreal regions (Maltby and Proctor 1996). They are also abundant in mountain ranges in maritime climate regions such as western Canada (Banner et al. 1988), Norway (Moen 1985), New Zealand (Wardle 1991), and Japan (Iwakuma 1996). Continental interior regions, such as the Rocky Moun- tains in the western U.S.A, support fewer peatlands due to low atmospheric humidity and summer rainfall (Chadde et al. 1998). However, peatlands in the Rocky Mountains support endemic and widely disjunct taxa and unique communities (Cooper and Andrus 1994; Cooper 1996; Cooper and San- derson 1997), yet the hydrologic regimes and carbon dynam- ics that support them are poorly known. Peat accumulates where anaerobic soil conditions, created by perennially high water tables, inhibit organic matter de- composition (Clymo 1984). Natural or anthropogenic water table declines can increase decomposition rates and carbon gas efflux from soils (Moore and Knowles 1989; Silvola et al. 1996a; Nykänen et al. 1998) and decrease net primary production (Weltzin et al. 2001). Because annual carbon storage may be a small fraction of gross primary production, slight changes in production or decomposition rates can shift a peatland from gaining to losing peat on an annual basis (Francez and Vasander 1995). Organic soil development is driven by a positive feedback between plant production, microbially mediated organic matter decomposition rates, and the perennially saturated environment (Wilson and Agnew 1992). Fens are tied to par- ticular groundwater flow systems and bogs to particular pre- Can. J. Bot. 81: 477–491 (2003) doi: 10.1139/B03-043 © 2003 NRC Canada 477 Received 13 March 2002. Published on the NRC Research Press Web site at http://canjbot.nrc.ca on 30 May 2003. R.A. Chimner1, 2 and D.J. Cooper. Graduate Degree Program in Ecology and Department of Earth Resources, Colorado State University, Ft. Collins, CO 80523, U.S.A. 1Corresponding author (e-mail: rchimner@nrel.colostate.edu). 2Present address: Natural Resources Ecology Laboratory, Colorado State University, Ft. Collins, CO 80523-1499, U.S.A. cipitation and humidity patterns, which makes them both extremely susceptible to hydrologic changes (Siegel 1988). All peatlands in the southern Rocky Mountains are hydro- logically tied to local and regional groundwater flow sys- tems that discharge at the base of mountain slopes, alluvial fans, and glacial moraines, making them fens (Cooper and Andrus 1994). On a global scale, ditching is the most widespread means of converting peatlands to forestry, agriculture, or mining uses (Rubec et al. 1988; Gorham 1991). Ditching alters wa- ter table depths, vegetation composition, primary production, and trace gas emissions and can convert peatlands from net sinks to sources of carbon (Francez and Vasander 1995; Komulainen et al. 1999). Watershed-scale hydrologic changes, such as diversion ditches and groundwater pump- ing, may deplete aquifers and are an additional threat to groundwater-driven fens. However, little is known about the indirect effects of these larger scale hydrologic changes. A large human population with extensive irrigated agricul- tural and urban and suburban landscapes has developed in a corridor just east of the Colorado Rocky Mountain front and uses water far in excess of that produced by the South Platte and Arkansas River watersheds that supply the area (Baron et al. 1998). To overcome this deficit, numerous streams in western Colorado have been dammed or diverted and their water transferred in ditches and pipelines to eastern Colo- rado. Some very large water projects have been constructed in areas that subsequently became national parks, and signif- icant questions remain about the sustainability of the ecosys- tems that they affect. For example, Rocky Mountain National Park (RMNP) was established in 1915, long after settlers had constructed reservoirs and water diversion ditches that affect many of its streams and wetlands (Cooper et al. 1998; Woods 2000). The largest water project in RMNP is the Grand Ditch, constructed beginning in the late 1800s to intercept and divert the flow of 11 streams that naturally flowed from the Never Summer Mountain Range to the Colorado River over the Continental Divide into the South Platte River drainage. On an annual basis, ~30% of the Colorado River flow is diverted to the South Platte, and flows are depleted by up to 50% during the summer (Woods 2000). The hydrologic and ecological effects of water diversions and dams on low-elevation riparian ecosystems in the west- ern United States have been well documented (Patten 1998). However, there is little information on the ecological im- pacts of dams and diversions on high-elevation wetlands despite the fact that almost all headwater streams in the upper Colorado River Basin are hydrologically modified (Petsch 1985). In this work, we analyze the hydrologic re- gime, plant production, and CO2 and CH4 emissions for three pristine fens and two fens that are potentially affected by the Grand Ditch diversion. Our goal was to understand fen carbon cycling in relation to water table dynamics and determine how sensitive fens were to hydrologic changes in their watershed. Study areas We worked in the Colorado River headwaters region of RMNP (Fig. 1). The climate is continental with cool sum- mers, frequent thunderstorms, and cold snowy winters. Up to 80% of the annual precipitation falls as snow (Hauer et al. 1997). Mean annual precipitation at the Phantom Canyon SNOTEL station, located 5 km from the study sites, is 65 cm with a mean annual temperature of 0.8°C. Spring snowmelt recharges hillslope and valley bottom aquifers. Dry summers with low humidity limits peatland develop- ment to sites supplied with groundwater throughout the sum- mer (Cooper 1990). The Grand Ditch is 26 km in length and intercepts and diverts 11 streams as they flow from high basins of the Never Summer Range toward the Colorado River valley bottom. These streams have historically supported shallow groundwater flow systems in the mountain front lateral mo- raine and valley bottom alluvial fans. The ditch operates only in the snow-free season and diverts an average of 20 million m3·year–1 from the Colorado River watershed to the Cache la Poudre River watershed (Woods 2000), which is tributary to the South Platte River. Ditches completely divert the uploads/Geographie/ carbon-dynamics-of-pristine-and-hydrologically-modified-fens-in-the-southern-rocky-mountains.pdf