Carbon accumulation in Colorado ponderosa pine stands

Woody encroachment and thickening have occurred throughout the western United States and have been proposed as important processes in the US carbon (C) budget despite large uncertainty in the magnitude of these effects. In this study we investigated ponderosa pine encroachment near Boulder, Colorado. We reconstructed a time series of forest structure to estimate changes in C storage by the trees. Advantages of this technique include the ability to estimate changes in C stocks over time with a single series of measurements (i.e., no historical measurements), and the ability to calculate accumulation rate changes through time. Substantial variation occurred in the C amounts and accumulation rates among the three plots resulting from differences in slope, aspect, and soil conditions. Accumulation rates increased exponentially as trees increased in size and additional trees established within the plots, and were highly variable among plots (0.09–0.7 Mg C·ha–1·year–1 during 1980–2001). These rates were less than those used in studies of the US carbon budget, and only by assuming no mortality for the densest stand could we generate a projected rate in 2050 that was similar. Thus, time since the initiation of encroachment and rate of encroachment are variables that should be considered for accurately computing the continental C budget. Résumé : L’empiètement et la densification des forêts survenus partout dans l’ouest des États-Unis seraient, croit-on, des processus importants dans le bilan du carbone (C) aux États-Unis malgré une forte incertitude quant à l’ampleur de leurs effets. Dans cette étude, nous avons examiné l’empiètement par le pin ponderosa dans les environs de Boulder, au Colorado. Nous avons reconstitué des séries temporelles de la structure de la forêt pour estimer les changements dans le stockage du C par les arbres. Cette technique comporte plusieurs avantages, incluant la possibilité d’estimer les variations temporelles des stocks de C à l’aide d’une série unique de mesures (c.-à-d. sans mesures antérieures) et la capacité d’estimer les variations temporelles du taux d’accumulation. Les variations entre les trois parcelles dans la quantité de C et le taux d’accumulation étaient importantes à cause des différences dans la pente, l’orientation et les conditions du sol. Le taux d’accumulation a augmenté de façon exponentielle à mesure que les arbres augmentaient de taille et que de nouveaux arbres s’établissaient dans les parcelles, et il a varié fortement d’une parcelle à l’autre (0,09 à 0,7 Mg C·ha–1·an–1 entre 1980 et 2001). Ce taux est inférieur à ceux utilisés dans les études sur le bilan du C aux États-Unis, et seulement en assumant une absence de mortalité pour le peuplement le plus dense pouvions-nous prédire un taux comparable en 2050. Par conséquent, le temps écoulé depuis le début de l’empiètement et le taux d’empiètement sont des variables qui devraient être considérées pour calculer avec précision le bilan continental du C. [Traduit par la Rédaction] 1295 Hicke et al. Introduction Woody encroachment is the process by which woody plants expand into grasslands or meadows, and has been identified in ecosystems throughout the world (e.g., Archer et al. 2001). A related process called “woody thickening” (also referred to as “infilling” or “densification”) occurs when a forest increases in stem density (e.g., Covington and Moore 1994). Woody encroachment and thickening affect biodiversity and ecosystem function by changing species composition as well as stand age and size structure, and by altering carbon (C) and nutrient cycling. For example, mesquite encroachment over the last 100 years in central Texas has been found to increase soil and plant C and nitrogen stocks (Hibbard et al. 2001, Asner et al. 2003). In addition, woody encroachment and thickening have recently been identified as processes contributing to the proposed US carbon sink. Pacala et al. (2001) estimated that woody encroachment accounts for 0.12–0.13 Pg C·year–1 (1 Pg = 1015 g) or 17%–43% of the total estimated C sink of 0.3–0.7 Pg C·year–1. Pacala et al. (2001) relied on two modeling estimates of the contribution of woody encroachment to this proposed sink. The first was computed by a processbased ecosystem model, the Ecosystem Demographic model, and the second method was taken from Houghton et al. (2000), who used a bookkeeping model. Houghton et al. (2000) utilized values of burned and grazed area together with estimates of C accumulation rates resulting from fire exclusion to calculate the contributions of land-use change and fire exclusion to the US carbon budget. Accumulation rates in ponderosa pine forests were derived principally from Can. J. For. Res. 34: 1283–1295 (2004) doi: 10.1139/X04-011 © 2004 NRC Canada 1283 Received 8 May 2003. Accepted 17 December 2003. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 17 June 2004. J.A. Hicke1,2 and G.P. Asner. Department of Global Ecology, Carnegie Institution of Washington, Stanford, CA 94305, USA. R.L. Sherriff and T.T. Veblen. Department of Geography, University of Colorado, Boulder, CO 80309, USA. 1Corresponding author (e-mail: [email protected]). 2Present address: Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523-1499, USA. ecosystem modeling (Keane et al. 1990, Covington and Moore 1994). Houghton et al. (2000) estimated a conterminous US carbon sink due to fire exclusion of 0.24 Pg C·year–1, with about half resulting from woody encroachment (used in the Pacala et al. (2001) study) and half from woody thickening. In North America, woody encroachment is associated with several types of ecosystems, including mesquite shrublands in arid and semi-arid rangelands throughout the southwestern US (e.g., Archer 1994, Archer et al. 2001), pinyon-juniper woodlands in the West (e.g., Burkhardt and Tisdale 1976), aspen stands in the northern Great Plains (Köchy and Wilson 2001), and conifer forests, especially ponderosa pine and Douglas-fir across western North America (Steinauer and Bragg 1987, Mast et al. 1997, Hessburg et al. 1999, Turner and Krannitz 2001). Causes of woody encroachment and thickening vary among the different ecosystems and possibly by location. Fire suppression has occurred since the 1920s in the US (Houghton et al. 2000). Increases in cattle grazing over the past 100 years have affected woody plant invasion (Miller and Wigand 1994, Mast et al. 1998). Climate is a third process that could contribute to woody encroachment and thickening by influencing the establishment of seedlings during favorable growing conditions (Savage and Swetnam 1990, Miller and Wigand 1994, Mast et al. 1998, Swetnam and Betancourt 1998). Other factors such as CO2 fertilization, nitrogen deposition, and exotic species invasions may also play important roles (Miller and Wigand 1994, Köchy and Wilson 2001). The co-occurrence and coupling of these factors, their wide spatial variation, and lack of historical observations challenge our understanding of the controls over woody encroachment and thickening and make projections highly uncertain. In conifer forests, fire is an important driver of forest structure, although different regions have different fire frequencies. Ponderosa pine forests in the pre-settlement Southwest were thought to be maintained as open, park-like woodlands by frequent, low intensity surface fires, but the past century of fire exclusion has resulted in more densely populated ponderosa pine stands (Cooper 1960, Covington and Moore 1994, Fulé et al. 1997). In contrast, other western US conifer forests, and even other ponderosa pine forests, have longer fire return times with naturally occurring standreplacing fires, or a mixture of longer and shorter return intervals (Romme and Despain 1989, Shinneman and Baker 1997, Kaufmann et al. 2000, Veblen et al. 2000, Veblen 2003). In this paper we estimated C accumulation resulting from ponderosa pine encroachment using plot-level measures of tree age and diameter. Plot-scale conifer production in the Cascade Mountains was calculated by Graumlich et al. (1989) from dendrochronological measurements. These authors compared production with CO2 and climate data to determine that temperatures increased growth in this area. Similarly, we reconstructed the temporal behavior of stand age and size for three plots on different slope aspects at the grassland-forest ecotone near Boulder, Colorado, USA. This area has experienced pine encroachment during the last 150 years through higher elevation forest expansion into lower elevation grasslands. Tree establishment dates together with diameter measurements and allometric equations allowed us to calculate a time series of C accumulation for each tree. This method provided a means of calculating C storage without relying on previous studies, an advantage for locations lacking historical information. Summing the C accumulation across each plot resulted in a time series of accumulation associated with trees on a per area basis. We did not address changes in soil C that are likely to affect the total C stocks of the plots (Jackson et al. 2002). We extend the work of past studies by utilizing field measurements to estimate time series of C at the tree level, and by focusing on C in addition to forest structural variables such as basal area. Thus, we can compare our results with ecosystem modeling studies used to calculate the impacts of encroachment and thickening on the continental-scale C budget. Since we studied only one site, our intent was not to characterize accumulation rates across the western US, but rather investigate results at a location experiencing documented change. Materials and methods Study site Our study site is located near Boulder, Colorado (39.94°N, 105.28°W) at an elevation of 2000 m. The area is at the montane forest–grassland ecotone, which occurs at similar elevations along a north–south line at the eastern base of the Rocky Mountains in northern Colorado. Vegetation of the montane zone varies from open park-like stands of ponderosa pine (Pinus ponderosa Dougl. ex Laws var. scopulorum Engelm.) at the forest–grassland ecotone to denser stands mixed with Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco) in more mesic sites and north-facing slopes. Mean temperatures reported by the nearby Boulder station range from 25 °C in July to –10 °C in January. Annual precipitation averages 395 mm, with the majority occurring in spring. The site encompasses a north–south and an east–west ridge (Fig. 1). The original purpose of this field study was to characterize changes in stand structures and fire regimes in the northern Colorado Front Range along the environmental gradient from the lower ecotone to the upper limits of the ponderosa pine distribution; this site was the lowest elevation location of several identified for study. Because the fire regime study had somewhat different objectives than the study discussed in this paper, not all data useful for quantifying C accumulation were measured (e.g., tree heights). However, as demonstrated later, our results and conclusions are not sensitive to the assumptions made about the missing data. Three plots were located on west-, east-, and north-facing aspects (hereafter “W plot”, “E plot”, and “N plot”, respectively) to study differences in forest properties. The area was varied among plots to allow 60–80 trees to be sampled within each plot (Table 1). The W plot was located in a large meadow (Fig. 1) and had considerably higher herbaceous cover compared with the other two plots. The N and E plots were located higher on the ridge with steeper slopes than the W plot. Numerous rocky outcroppings were observed in the E plot. In this region, the lowest elevation ponderosa pine stands were characterized by historically high fire frequencies (e.g., often 8–10 year return intervals to the same small stand or © 2004 NRC Canada 1284 Can. J. For. Res. Vol. 34, 2004