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Keywords:

  • Ambrosia dumosa;
  • FACE (free air CO2 enrichment);
  • Larrea tridentata;
  • Mojave desert;
  • plant community;
  • root production;
  • root standing crop;
  • root turnover

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
  • Experimental increases in atmospheric CO2 often increase root production over time, potentially increasing soil carbon (C) sequestration.
  • Effects of elevated atmospheric CO2 on fine root dynamics in a Mojave desert ecosystem were examined for the last 4.5 yr of a long-term (10-yr) free air CO2 enrichment (FACE) study at the Nevada desert FACE facility (NDFF). Sets of minirhizotron tubes were installed at the beginning of the NDFF experiment to characterize rooting dynamics of the dominant shrub Larrea tridentata, the codominant shrub Ambrosia dumosa and the plant community as a whole.
  • Although significant treatment effects occurred sporadically for some fine root measurements, differences were transitory and often in opposite directions during other time-periods. Nonetheless, earlier root growth under elevated CO2 helped sustain increased assimilation and shoot growth.
  • Overall CO2 treatment effects on fine root standing crop, production, loss, turnover, persistence and depth distribution were not significant for all sampling locations. These results were similar to those that occurred near the beginning of the NDFF experiment but unlike those in other ecosystems. Thus, increased C input into soils is unlikely to occur from fine root litter under elevated atmospheric CO2 in this arid ecosystem.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Over the last half century, evidence has mounted about the increase of global atmospheric CO2 and the resultant increase in photosynthesis, aboveground productivity and biomass accumulation in plants (Curtis & Wang, 1998; Kimball et al., 2002; Nowak et al., 2004a; Ainsworth & Long, 2005). Increased carbon (C) gain and shoot production with elevated CO2 is expected to enhance root production, and reviews of root production under experimental increases in atmospheric CO2 indicate increased belowground productivity in many studies (Rogers et al., 1994; Curtis & Wang, 1998; Kimball et al., 2002; de Graaff et al., 2006), especially at deeper depth (Iversen, 2010). However, not all belowground studies show increased root growth (Nowak et al., 2004a). For example, responses among forested ecosystems vary in both the magnitude of the response (25% increase in fine root production at the Duke free air CO2 enrichment (FACE) experiment, (Pritchard et al., 2008) vs 100% increase at Oak Ridge National Laboratory (ORNL) FACE experiment, (Iversen et al., 2008) as well as the direction of the response (these increases at Duke and ORNL FACE experiments vs up to 30% decrease at the Swiss web-FACE experiment, Bader et al., 2009). Furthermore, individual species within a community can either drive the community response (LeCain et al., 2006) or differ in sign from the community response (Anderson et al., 2010).

Deserts in particular are expected to be very sensitive to changes in atmospheric CO2 (Strain & Bazzaz, 1983; Melillo et al., 1993). Soil moisture is the limiting factor to plant productivity in the Mojave desert (Smith & Nowak, 1990) and other arid systems. Water-use efficiency generally improves under elevated CO2 (Kimball et al., 2002; Ainsworth & Long, 2005). Thus, elevated CO2 is expected to relieve water stress in desert ecosystems and ultimately increase productivity. Indeed, photosynthetic rates and aboveground productivity generally increased under elevated CO2 at the Nevada desert FACE facility (NDFF), although these elevated CO2 effects were more prominent during wetter periods (Huxman et al., 1998; Hamerlynck et al., 2000; Smith et al., 2000; Naumburg et al., 2003; Ellsworth et al., 2004; Housman et al., 2006).

Fine root production comprises up to 33% of global annual net primary productivity (Gill & Jackson, 2000) and likely influences the amount and rate of belowground C accumulation. Furthermore, fine roots are an ephemeral part of the root system and have relatively fast turnover, high metabolic activity and substantially higher nitrogen concentrations than roots of lower order (Pregitzer et al., 1997). Because deserts cover c. 40% of the terrestrial surface of Earth (Reynolds, 2001) and are the fifth largest pool of soil organic C (Jobbagy & Jackson, 2000), their belowground responses to increased atmospheric CO2 could affect regional or global C cycling.

In this study, we used minirhizotrons installed at the NDFF to measure standing crop, production, loss, turnover, persistence and depth distribution of fine roots for two codominant desert shrubs as well as for the community as a whole. Images of fine roots were collected from January 2003 through May 2007 and cover the last 4.5 yr of the NDFF experiment (i.e. 6–10 yr after the initiation of elevated CO2 treatment). Our study completes and complements measurements from the first 2 yr of the experiment (1998–99; Phillips et al., 2006). Because responses of root production to elevated CO2 may change through time (Day et al., 2006), long-term measurements are important to understand elevated CO2 effects on fine root dynamics and their potential to alter belowground soil C. Our hypotheses were that long term exposure to elevated CO2 results in: increased production of fine roots because increased C gain increases C allocation to roots; increased mean root persistence times (reduced turnover) because increased C gain during seasonal dry periods allows plants to maintain existing roots to a greater extent; an accumulation of fine root standing crop because of the increased production and reduced turnover; and altered root depth distribution, with more roots at deeper depths, partly because of increased production and decreased turnover of deep roots and partly because plants are expected to more completely utilize soil resources, especially at depth, in order to sustain increased growth.

Materials and Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Study site

The study was conducted at the NDFF on the US Department of Energy Nevada test site north of Las Vegas, Nevada (36°49′ N 115°45′ W, elevation 965 m). The NDFF used FACE technology to expose an undisturbed Mojave desert ecosystem to elevated atmospheric CO2. Biological and technical characteristics of this site were thoroughly described in Jordan et al. (1999). In brief, the Mojave desert is the driest region in North America, and our study site has mean annual precipitation of c. 140 mm. Between 1997 and 2007, which encompassed the period that the NDFF operated, maximum and minimum annual precipitations for the hydrologic year (October–September) were 328 mm (1997–98) and 47 mm (2001–02), respectively. Vegetation is characteristic of the northern Mojave desert and is codominated by the evergreen shrub Larrea tridentata (Sessé & Moc. ex DC.) Coville (creosote bush) and the drought-deciduous shrub Ambrosia dumosa (A. Gray) Payne (bur-sage). As in many desert ecosystems, vegetation is widely spaced, and shrubs and other perennial plants cover < 20% of the land surface. The area between shrubs and other perennial plants is called canopy interspace and is usually bare ground or covered by soil biological crust species, although annual grasses and forbs occur in canopy interspaces during springs with near-average or above-average precipitation.

Six NDFF plots were used in this study: three plots that were set to maintain atmospheric CO2 at 550 μmol mol−1 and three plots that circulated ambient air using the same FACE setup as elevated CO2 plots. From January 2003 when this minirhizotron study began to 1 July 2007 when the NDFF experiment ended, CO2 concentration of air above plots averaged 511 μmol mol−1 for the elevated treatment and 374 μmol mol−1 for the ambient treatment.

Minirhizotron measurements

Minirhizotron tube installation (Phillips et al., 2000) occurred July–September 1997 and consisted of 28 polycarbonate tubes in each plot with each tube at a 30° angle from vertical. Tube locations were chosen to sample individual shrubs of the codominant species Larrea and Ambrosia and to sample the plant community. Four Larrea and four Ambrosia shrubs were randomly selected from the population of plants in each plot, and two minirhizotron tubes were placed beneath each shrub. One tube sampled the center of a plant’s canopy with the bottom of the tube placed at 1.0 m depth directly below the central crown of the plant. The second tube sampled the edge of a plant’s canopy and was placed parallel to the first tube with the bottom of the second tube directly under the edge of the shrub canopy. To sample the plant community, four tubes were installed at 2 m spacing along three systematically placed transects. One transect was along the plot radius oriented due south, and the other two transects were located at 120° from south. Owing to low overall perennial plant cover, transects mostly sample canopy interspace locations (Jordan et al., 1999). Portions of tubes extending out of the ground were covered by insulated, plastic end caps that prevented light infiltration and temperature fluctuations around the top of tubes. More details about installation and orientation of minirhizotron tubes are given in Phillips et al. (2000, 2006).

Minirhizotron imaging restarted in January 2003, which was 3 yr after the previous termination of minirhizotron measurements (Phillips et al., 2006), > 5 yr after minirhizotron tube installation, and almost 6 yr after onset of CO2 fumigation. An indexed camera handle provided image capture from the same location on each sampling date, which allowed individual roots to be tracked and their fate determined (Johnson & Meyer, 1998). On each measurement date, 23 images were collected per tube starting at 4 cm below the soil surface and descending at 4-cm depth intervals. A total of 53 imaging sessions occurred between January 2003 and May 2007. Our normal sampling interval was 4 wk. Additional details about collecting and processing minirhizotron images are given in the Supporting Information, Methods S1.

Fine root segments are the basic unit of root data in this study. We define root segments as continuous pieces of unbranched roots that are clearly visible in the image and whose length and diameter can be delineated. Digital images were manually traced using the image analysis program rootracker (David Tremmel, Duke University, Durham, NC, USA). Individual roots were traced, or ‘digitized’, by placing points along the root image that defined its length and diameter. Root-length data was expressed as rooting intensity, that is, root length per image area (m m−2). In addition, morphological characteristics of each root, such as color, presence of root hairs, apparent root order and other visible characteristics, were recorded.

Because each root segment was uniquely identified by the rootracker software, total fine root standing crop at each imaging session and fine root production and loss between sequential pairs of imaging sessions were calculated. Fine root standing crop was simply the sum of all fine root segments that were present on a particular sampling date. Fine root production was the sum of all fine root segments that appeared or that increased in length between two sampling dates. Fine root loss was the sum of all fine root segments that were lost between two sampling dates. Net change in fine roots was the difference between production and loss. These production, loss and net change measurements provide absolute measurements of changes in fine roots for discrete time-intervals.

We also calculated an annual fine root production index and an annual fine root turnover index from the sum of fine root production or loss over a calendar year divided by mean standing crop for that year. These annual indices provide measurements of changes in fine roots on an annual basis relative to the size of the fine root system, that is the increase or decrease in fine roots as a fraction of the average size of the fine root system. For example, an ecosystem with large root systems may produce more fine roots than one with small root systems, but both ecosystems may have similar annual production indices, that is the relative production of roots are similar. These annual indices also indicate if long-term production and loss of fine roots are in equilibrium for an ecosystem: at equilibrium, the annual production index should equal the annual turnover index. Finally, annual indices are needed to compare our results with those of the earlier NDFF study (Phillips et al., 2006) as well as other studies in the literature.

Owing to ambiguity in definition of root death, especially regarding loss of physiological functioning, a root was classified as ‘lost’ when it became ‘very faint or discontinuous with indistinct edges, shriveled to a fraction of its previous width or completely disappeared and did not reappear in future sessions’ (Comas et al., 2000). Clearly, our classification of lost did not reflect duration of root function, but rather indicated when a root had clearly moved from the fine root pool to the soil organic matter pool. Because one focus of our study was ecosystem carbon dynamics, our method to classify loss was appropriate. However, we recognize that some C and nutrients likely were released into soil by exudation or leaching and were incorporated into microbial biomass before our classification as lost.

Statistical analyses

Root standing crop, production and loss  Standing crop, production, and loss of fine root intensity for each tube to a depth of 1 m were analysed with analysis of variance (ANOVA) using the GLIMMIX procedure in SAS 9.2 (SAS Institute, Inc., Cary, NC, USA). Plot means for each tube location on each sampling date were used in the ANOVAs: four tubes were averaged to calculate each plot mean for Ambrosia canopy center, Ambrosia canopy edge, Larrea canopy center and Larrea canopy edge locations, and plot means for transects were calculated from 12 transect tubes in each plot (three transects with four minirhizotron tubes per transect). Each shrub species was analysed separately with nested, repeated measures ANOVA, where CO2 treatment (elevated, ambient) was the main plot effect, canopy location (center, edge) was a nested effect and image session (date) was a repeated measures effect. Transect data were analysed with a repeated measures ANOVA, where CO2 treatment (elevated, ambient) was the main plot effect and image session (date) was a repeated measures effect. Because we were interested in overall treatment differences as well as treatment differences on individual sampling dates, we included tests for treatment effects sliced by date in all ANOVAs. The BoxCox procedure in SAS 9.2 was used as needed to transform data to meet ANOVA assumptions of a normal distribution and homogeneity of variance.

Root persistence  Differences in root longevity caused by treatment factors and measured root characteristics (root diameter, coloration, time of appearance and depth in the soil profile) were investigated using survival analyses. These root characteristics were identified in other studies as being significant in persistence models (Wells & Eissenstat, 2001; Peek et al., 2005). Analysis of mean longevity used the LIFETEST procedure in SAS 9.2 (Allison, 1995). Any root segments that were present at the first imaging session as well as any root segments that were present during the study and were still present at the end of the last session were considered ‘right censored’, that is, these roots persisted for a period of time greater than the length of the study. In addition, factors that contributed to loss of roots were investigated using a Cox proportional hazard regression to model persistence as a function of root color, diameter and depth (Wells & Eissenstat, 2001).

Depth distribution  To determine if elevated CO2 affected depth distribution of fine roots, we examined fine root standing crop, production and loss for three intervals: 0–0.25, 0.25–0.75 and 0.75–1.0 m soil depth. These depth intervals corresponded both to typical soil water recharge depths and to typical root distribution profiles. Our measurements at the NDFF and nearby study areas (Yoder & Nowak, 1999; Nowak et al., 2004b; see Fig. S1) indicated that soil water recharge reliably occurred during spring to 0.25 m, but maximum depth of soil water recharge rarely exceeded 0.75 m. Global analyses of root distributions showed that c. 50% of roots in desert ecosystems were in the top 0.25 m of soil, c. 35% in the 0.25–0.75 m depth and the remaining c. 15% below 0.75 m (Jackson et al., 1996). Minirhizotron measurements at individual depths that corresponded to these depth increments were summed and then analysed as described earlier for the entire 1 m depth. Production and loss data are discussed and presented in the text; standing crop data are summarized in the text but only presented in Figs S2–S4.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Root characteristics

In this study, 25 932 root segments were followed from date of production or first observation until either they were ‘lost’ or the study ended. A total of 3070 repeated image locations were used, which results in c. 8.3 roots per image. Roots were very fine in diameter: > 97% of observed roots had diameters < 0.40 mm and > 73% had diameters < 0.20 mm (mean = 0.164 mm, median = 0.147, maximum = 1.22, minimum = 0.01) (Fig. 1a). Mean root diameter and distribution of root diameter classes were similar among treatments. Root color characteristics were also similar over all treatments. On average, 48% of roots were white, 37% were brown and 11% were tan (Fig. 1b). Few roots were classified as other colors.

image

Figure 1. Diameters (a) and colors (b) of fine roots observed along minirhizotron tubes in ambient CO2 (unshaded) and elevated CO2 (shaded) plots at the Nevada desert free-air CO2 enrichment (FACE) facility from January 2003 to May 2007. Minirhizotron tubes were located near Ambrosia dumosa plants (diagonal hatching), near Larrea tridentata plants (no hatching) and systematically placed throughout the plant community (cross-hatching). ‘Other’ root colors include tan, transparent, grey, dark colored, blue and peach.

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Fine root standing crop

During 4.5 yr of study, standing crop of fine roots slowly changed owing to seasonal dynamics of root production and loss at all sampling locations (near A. dumosa plants, near L. tridentata plants, and for the plant community as a whole, Fig. 2), but CO2 treatment differences were not significant over the entire period of study (Table 1). Fine roots rapidly accumulated during the first 6 months of the study (January–July 2003) and then standing crop was relatively stable during the next year (until July 2004) with small but nonsignificant differences between CO2 treatments. Fine root standing crop decreased from July 2004 to January 2005, with the decrease in standing crop for ambient CO2 plots greater than that for elevated CO2 plots such that significant differences between CO2 treatments occurred for the plant community and for Ambrosia in December 2004 and January 2005, respectively (Fig. 2; Treatment × Date interaction term, Table 1). For January–March 2005, fine root standing crop increased rapidly, with significant differences between CO2 treatments largely maintained for Ambrosia and the plant community. Standing crop peaked at all sampling locations in late March 2005 for elevated CO2 treatments but c. 6 wk later in late April–early May 2005 for ambient CO2 treatments. By late May 2005, and for the next 2 yr until the study ended in May 2007, fine root standing crop slowly declined and differences between CO2 treatments were very small.

image

Figure 2. Time-series of fine root (< 1 mm diameter) standing crop totaled to 1 m soil depth for ambient (open circles) and elevated (closed circles) CO2 treatments for Ambrosia dumosa (a), Larrea tridentata (b), and the plant community (c). Values are fine root intensity, that is, root length per image viewing area (m m−2). For Ambrosia and Larrea on each sampling date, data at two individual microsites for each plant (under the center of the plant and at the edge of the plant’s canopy) were averaged to estimate root intensity for each plant, and then data for the four plants in each plot were averaged to obtain a plot mean. Data shown are the treatment means and standard errors derived from the three plots for each treatment. For the community on each date, data from the 12 minirhizotron tubes in each plot were averaged to obtain plot means; as with Ambrosia and Larrea, data shown are treatment means and standard errors derived from the three plots for each treatment. Asterisks indicate a significant difference (P < 0.05) between ambient and elevated CO2 treatments on an individual sampling date. (d) Cumulative precipitation (mm) for each hydrologic year (October of one year to September of the next year).

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Table 1.   ANOVA results for fine root standing crop
EffectNum dfDen dfAmbrosiaLarrea
P-valueP-value
  1. Data from individual Ambrosia dumosa and Larrea tridentata plants (a) or from transects systematically placed throughout the plot to sample the entire plant community (b). Bold numbers indicate a significant (P < 0.05) effect. Den, Denominator; Num, Numerator.

(a) Individual plants
 CO2 treatment140.6850.870
 Microsite140.6540.406
 Treatment × Microsite140.5130.110
 Date52416< 0.001< 0.001
 Treatment × Date52416< 0.001< 0.001
 Microsite × Date524160.970< 0.001
 Treatment × Microsite × Date524161.0000.999
EffectNum dfDen df P-value
(b) Community transects
 CO2 treatment14 0.345
 Date52208 < 0.001
 Treatment × Date52208 0.073

Fine root production and loss

Overall CO2 treatment effects were not significant (Table 2). Even when fine root production was summed over the entire 4.5-yr period, fine root production under elevated CO2 for each sampling location (mean ± SE: Ambrosia 149 ± 20; Larrea 206 ± 38; community 204 ± 16) were not significantly different from those under ambient CO2 (Ambrosia 146 ± 7; Larrea 223 ± 8; community 169 ± 3). Similarly, fine root loss summed over the entire 4.5-yr period under elevated CO2 for each sampling location (Ambrosia 171 ± 21; Larrea 246 ± 48; community 219 ± 21) was not significantly different from that under ambient CO2 (Ambrosia 158 ± 16; Larrea 233 ± 13; community 183 ± 11).

Table 2.   ANOVA results for production, loss, and net change in fine root intensity
EffectNum dfDen dfProductionLossNet change
 P-value 
  1. Data for each half-year from individual Ambrosia dumosa plants, from individual Larrea tridentata plants, and from transects systematically placed throughout the plot to sample the entire plant community. Bold numbers indicate a significant (P < 0.05) effect. Den, Denominator; Num, Numerator.

Ambrosia dumosa
 CO2 treatment140.0950.7410.623
 Microsite140.2080.5800.817
 Treatment × Microsite140.9170.2490.963
 Time period864< 0.001< 0.001< 0.001
 Treatment × Time8640.8620.012< 0.001
 Microsite × Time8640.9170.9490.341
 Treatment × Microsite × Time8640.9680.2870.588
Larrea tridentata
 CO2 treatment140.6140.8440.853
 Microsite140.7410.8100.775
 Treatment × Microsite140.2810.2180.777
 Time period864< 0.001< 0.001< 0.001
 Treatment × Time8640.2690.2040.025
 Microsite × Time8640.3870.9970.811
 Treatment × Microsite × Time8640.8370.9540.694
Community
 CO2 treatment140.1200.3760.728
 Time period832< 0.001< 0.001< 0.001
 Treatment × Time8320.2480.2250.003

Although CO2 treatments were not significant over the entire period, significant differences between CO2 treatments for fine root production, loss and net change occurred during specific time-intervals (significant Treatment × Time interactions, Table 2), especially during July–December 2004 and January–June 2005 (Fig. 3). More rapid declines in Ambrosia and community fine root standing crop at the end of 2004 for ambient CO2 plots discussed earlier (Fig. 2) was reflected in significantly greater net loss of fine roots during July–December 2004 for ambient CO2 plots (Fig. 3, triangles). For Ambrosia, significantly greater net loss for ambient CO2 during July–December 2004 was primarily caused by significantly greater rates of fine root loss in ambient CO2 plots, but for the plant community, significantly greater net loss for ambient CO2 was primarily caused by smaller fine root production under ambient CO2. During January–June 2005, significant differences in fine root standing crop in both Ambrosia and the plant community (Fig. 2) gradually eroded because net gain in fine roots was significantly greater for ambient CO2 (Fig. 3). For both Ambrosia and the plant community, greater net gain for ambient CO2 during January–June 2005 largely resulted from significantly smaller fine root loss under ambient CO2. Larrea also had significant differences between treatments for net change in fine roots during July–December 2004 and January–June 2005 (Fig. 3), but although fine root standing crop in elevated CO2 tended to be greater for Larrea during this period (Fig. 2), treatment differences in standing crop were not significant (Table 1).

image

Figure 3. Effects of elevated (closed bars) and ambient (open bars) CO2 treatments on production and loss of fine root intensity during 6-month time increments for Ambrosia dumosa (a), Larrea tridentata (b) and the plant community (c), totaled to 1 m soil depth. Asterisks indicate a significant difference (P < 0.05) between ambient and elevated CO2 treatments for an individual period. For periods when the net change was significantly different between treatments, mean and standard errors for the net change in fine roots are shown as closed triangles for elevated and open triangles for ambient CO2 treatments. For Ambrosia and Larrea during each period, data at two individual microsites for each plant (under the center of the plant and at the edge of the plant’s canopy) were averaged to estimate root intensity for each plant, and then data for the four plants in each plot were averaged to obtain a plot mean. For the community on each date, data from the 12 minirhizotron tubes in each plot were averaged to obtain plot means. For all locations, data shown are treatment means and standard errors derived from the three plots for each treatment.

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Seasonal patterns of fine root production occurred for all sampling locations but not for fine root loss (significant Time effect, Table 2). For all sampling locations, fine root production during January–June in 2003, 2005 and 2006 were significantly greater than that during the July–December period in that year (Fig. 3, Table 2). In 2004, production during January–June was significantly greater than that for July–December for Ambrosia but not for Larrea or the plant community. Interestingly, peak production occurred c. 4 wk earlier in elevated CO2 plots compared with ambient for all microsites during the wetter years 2003 and 2005 (Table 3), but were largely at the same time in the drier years 2004 and 2006. By contrast, loss of fine roots did not have strong seasonal patterns but rather gradually increased to a maximum during the wet 2004–05 hydrologic year, then gradually decreased (Fig. 3).

Table 3.   Annual precipitation for hydrologic years (October of preceding year to September of current year) at the Nevada desert free-air CO2 enrichment (FACE) facility during the study and periods of peak fine root production for minirhizotron sampling locations
Hydrologic yearAnnual precipitation (mm)Period of peak fine root production
Ambrosia dumosaLarrea tridentataCommunity
Elevated CO2Ambient CO2Elevated CO2Ambient CO2Elevated CO2Ambient CO2
200314929 March–19 April19 April–21 May29 March–19 April19 April–21 May29 March–19 April29 March–19 April
200412322 February–4 April4 April–17 April22 February–4 April22 February–4 April17 April–13 May17 April–13 May
20052426 January–12 February12 February–10 March6 January–12 February12 February–10 March6 January–12 February10 March–26 March
200611331 March–28 April31 March–28 April31 March–28 April31 March–28 April31 March–28 April31 March–28 April

We also calculated annual fine root production and turnover indices, that is total production or loss over a 1-yr period divided by mean standing crop for that same year (Table 4). Significant differences between CO2 treatments only occurred for annual fine root turnover of Ambrosia and Larrea in 2004; all other individual treatment–sampling location combinations were not significant within each year. Although annual root turnover in 2004 for the ambient CO2 treatments was greater than that for elevated CO2 treatments, the converse occurred in 2005 (although means were not significantly different in 2005). Because overall treatment effects were not significant and because one treatment was not consistently greater than the other over all years of study, the significant differences in 2004 result from larger absolute rates of root loss during July–December 2004 for ambient CO2 treatments (Fig. 3) coupled with smaller root standing crops during that same period (Fig. 2). Annual indices also significantly varied among years. Annual production indices in wetter years (2003 and 2005) were significantly greater than those in dryer years (2004 and 2006). For root turnover, the annual turnover index in 2003 was smaller than in the other 3 yr.

Table 4.   Annual fine root production and turnover indices for each combination of sampling location and treatment for each year of the study
Sampling locationCO2 treatment2003200420052006
  1. Annual production index is calculated from the total fine root production over a 1-yr period divided by the mean standing crop over that same year; annual turnover index is calculated from total root loss over a 1-yr period divided by mean standing crop over that same year. Note that these annual indices are unitless because units are the same in the numerator and denominator. Means and standard errors are given for each full year of measurement. Bold numbers indicate a significant difference between CO2 treatments within a sampling location for an individual time period; ANOVA results in the Supporting Information, Table S1(a,b).

Annual production index
 Ambrosia dumosaAmbient0.50 ± 0.050.16 ± 0.040.61 ± 0.100.12 ± 0.04
Elevated0.52 ± 0.040.15 ± 0.010.43 ± 0.060.14 ± 0.03
 Larrea tridentataAmbient0.54 ± 0.040.19 ± 0.010.63 ± 0.050.07 ± 0.02
Elevated0.53 ± 0.060.15 ± 0.010.57 ± 0.090.09 ± 0.03
 CommunityAmbient0.60 ± 0.040.14 ± 0.020.51 ± 0.040.13 ± 0.02
Elevated0.63 ± 0.070.18 ± 0.040.50 ± 0.030.16 ± 0.04
Annual turnover index
 Ambrosia dumosaAmbient0.08 ± 0.010.48 ± 0.100.43 ± 0.120.51 ± 0.07
Elevated0.08 ± 0.020.24 ± 0.060.72 ± 0.170.48 ± 0.14
 Larrea tridentataAmbient0.08 ± 0.010.43 ± 0.080.47 ± 0.040.54 ± 0.05
Elevated0.06 ± 0.010.26 ± 0.090.71 ± 0.150.52 ± 0.07
 CommunityAmbient0.07 ± 0.020.38 ± 0.060.55 ± 0.110.55 ± 0.06
Elevated0.11 ± 0.040.28 ± 0.060.75 ± 0.020.50 ± 0.12

Depth distribution

Significant CO2 treatment differences occurred only below 0.25 m soil depth for Ambrosia and Larrea (Figs 4, 5). Significant differences between CO2 treatments for fine root production, loss and net change for the 0.25–0.75 and 0.75–1.0 m depth intervals of Ambrosia (Fig. 4) were similar to those over the 1-m soil profile (Fig. 3a). During July–December 2004, fine root loss of Ambrosia was significantly greater under ambient CO2 for the 0.25–0.75 and 0.75–1.0 m depth intervals (Fig. 4), which resulted in greater net loss under ambient CO2. By contrast, fine root loss during January–June 2005 was significantly greater under elevated CO2 for these two deeper depth intervals, which contributed to smaller net gain under elevated CO2. For Larrea during January–June 2005 (Fig. 5), net change in fine roots was significantly greater under ambient CO2 for the 0.25–0.75 and 0.75–1.0 m depth intervals. Both greater production and smaller loss under ambient CO2 contributed towards greater net gain under ambient CO2, although treatment differences in fine root production and loss were not significantly different between CO2 treatments for the 0.25–0.75 depth interval. Interestingly, significantly greater net loss of fine roots over the 1-m soil profile for Larrea during July–December 2004 (Fig. 3b) was not reflected in significantly different production, loss, or net change at any of the three depth intervals (Fig. 5), although fine root loss for all three depth intervals was consistently greater under ambient CO2 for Larrea during this period.

image

Figure 4. Effects of elevated (closed bars) and ambient (open bars) CO2 treatments on production and loss of fine root intensity of Ambrosia dumosa during 6-month increments for three depth intervals. Asterisks indicate a significant difference (P < 0.05) between ambient and elevated CO2 treatments for an individual period. For periods when the net change was significantly different between treatments, mean and standard errors for the net change in fine roots are shown as closed triangles for elevated and open triangles for ambient CO2 treatments. Data at two individual microsites for each plant (under the center of the plant and at the edge of the plant’s canopy) were averaged to estimate root intensity for each plant, and then data for the four plants in each plot were averaged to obtain a plot mean; shown are treatment means and standard errors derived from the three plots for each treatment.

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image

Figure 5. Effects of elevated (closed bars) and ambient (open bars) CO2 treatments on production and loss of fine root intensity of Larrea tridentata during 6-month time increments for three depth intervals. Asterisks indicate a significant difference (P < 0.05) between ambient and elevated CO2 treatments for an individual period. For periods when the net change was significantly different between treatments, mean and standard errors for the net change in fine roots are shown as closed triangles for elevated and open triangles for ambient CO2 treatments. Data at two individual microsites for each plant (under the center of the plant and at the edge of the plant’s canopy) were averaged to estimate root intensity for each plant, and then data for the four plants in each plot were averaged to obtain a plot mean; shown are treatment means and standard errors derived from the three plots for each treatment.

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For the plant community, significant CO2 treatment effects occurred at all three depth intervals (Fig. 6). During January–June 2005: fine root loss was significantly greater under elevated CO2 at all three depth intervals; fine root production was significantly greater under elevated CO2 for the top 0.25 m of soil but significantly greater under ambient CO2 for the 0.75–1.0 m depth interval; and thus net gain was significantly greater under ambient CO2 only for the 0.25–0.75 and 0.75–1.0 m depth intervals. For the 0–0.25 depth interval: significantly greater fine root production under elevated CO2 occurred during both the January–June 2005 and July–December 2005 periods, but significantly greater root production occurred under ambient CO2 during July–December 2006; fine root loss was significantly greater under elevated CO2 throughout 2005 and during July–December 2006; and net loss of fine roots was significantly smaller under ambient CO2 during July–December 2005 and 2006. This smaller net loss of fine roots in the top 0.25 m of soil during July–December 2006 was balanced by significantly a greater net loss under ambient CO2 for the 0.75–1.0 m soil depth (Fig. 6), which resulted in nearly identical fine root loss over the 1-m soil profile during this time period (Fig. 3c).

image

Figure 6. Effects of elevated (closed bars) and ambient (open bars) CO2 treatments on production and loss of fine root intensity of the plant community in 6-month time increments for three depth intervals. Asterisks indicate a significant difference (P < 0.05) between ambient and elevated CO2 treatments for an individual period. For periods when the net change was significantly different between treatments, mean and standard errors for the net change in fine roots are shown as closed triangles for elevated and open triangles for ambient CO2 treatments. Data from 12 minirhizotron tubes in each plot were averaged to obtain plot means; shown are treatment means and standard errors derived from the three plots for each treatment.

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For completeness, we also include a brief summary of depth distribution for fine root standing crop. Significant CO2 treatment differences in fine root standing crop over the 1-m soil profile were largely caused by significant differences in fine root standing crop for a specific depth interval. For Ambrosia, significant differences between the ambient and elevated CO2 treatments occurred only for the mid-depth interval (0.25–0.75 m soil depth) and only during December 2004 and January 2005 (Fig. S2), which were the same dates that CO2 effects occurred for the 1-m soil profile (Fig. 2a). For Larrea, fine root standing crop was not significantly different between CO2 treatments on all sampling dates for all three depth intervals (Fig. S3), a result that also occurred for the 1-m soil profile (Fig. 2b). For the plant community, significant differences between ambient and elevated CO2 treatments occurred only for the shallow-depth interval (0–0.25 m soil depth) (Fig. S4), but unlike the 1-m soil profile (Fig. 2c), these significant differences in the top 0.25 m of soil started in March 2005 and largely persisted for over 1.5 yr until October 2006.

Root persistence

Many fine roots persisted for very long period during our study. For example, 1756 of 25 932 observed roots were present at the first session and were still present without significant degradation to indicate ‘loss’ at the end of the study, that is 6.8% were never lost during > 4 yr of observation. An additional 9718 (37.5%) roots persisted longer than we were able to observe them (i.e. either produced before the start of the study in 2003 but lost during the study, or produced during the study but still present at the end of the study) and are considered right-censored. The remaining 14458 (55.8%) were uncensored (i.e. produced and lost during the study). Analysis of log–log survivor plots and comparison of log-likelihood values under different survivor functions suggest that root survivor function is consistent with the Weibull distribution, which was used for all persistence analyses (Allison, 1995). Estimated longevities (Table 5) of fine roots were uniformly long, with 75% of fine roots persisting for > 1 yr and 50% for ≥ 2 yr. However, note that these values likely are underestimates because of large percentages of right-censored observations.

Table 5.   Estimates of survival for roots by treatment and sampling location using LIFETEST procedure in SAS
Sampling locationCO2 treatmentNumber of roots% censored75% survival (95% CI)50% survival (95% CI)
  1. Percentage censored indicates per cent of individual roots that were right censored (i.e. roots that were either present at the beginning of the study or present at the end of the study) of the total number of roots for each sampling location × treatment combination. Per cent survival is the number of estimated days until the percentage of roots surviving is 75% or 50% of the original population; numbers in parentheses are lower and upper 95% confidence intervals for the estimate.

Ambrosia dumosaAmbient316348.7440 (420, 457)803 (761, 869)
Elevated319645.8427 (406, 446)791 (756, 819)
Larrea tridentataAmbient405544.0410 (385, 426)723 (659, 754)
Elevated435343.0412 (398, 433)750 (734, 771)
CommunityAmbient514844.0440 (414, 446)759 (735, 787)
Elevated601742.4377 (356, 390)723 (711, 737)

The hazard over time, or the chance that a root was lost, was not significantly different from zero for CO2 treatment effects for both species and for the plant community (Table 6). Thus, elevated CO2 did not influence the probability of root persistence. However, different fine root characteristics influenced root loss, as indicated by rejecting the null hypothesis that estimated values for each parameter are significantly different from zero (Table 6). For root diameter and root depth, estimated per cent change in hazard for each one unit increase in value of the parameter averaged 19% and 1%, respectively, over all treatment-sampling location combinations. Because units of diameter are 0.1 mm, the probability of fine root loss is decreased by c. 19% for every 0.1 mm increase in root diameter. For depth, which is in centimeters, each 1 cm increase in soil depth decreases risk of fine root loss by c. 1%. Root color also had a significant effect on risk of root loss. Because > 95% of the roots were brown, tan or white, we only included these categories of roots in the hazard analysis. For the categorical variable root color, results are interpreted as relative risk between categories after accounting for all other parameters, that is brown fine roots have 20–32% decrease in loss vs white roots and tan roots have 18–32% decrease in loss vs white roots. For both Ambrosia and Larrea, roots produced in autumn have c. 28% decrease in loss vs those produced in spring; for the plant community as a whole, roots produced in autumn still persisted longer than those produced in spring, but the difference was not as large as that for the two shrub species.

Table 6.   Proportional hazards regressions of root loss using PHREG procedure in SAS
ParameterEstimate ± SEP-valueHazard ratioPercentage decrease
  1. For each parameter, maximum likelihood estimate of the parameter with standard error is given along with P-value from the 1 degree of freedom χ2 test of the null hypothesis that the parameter values equal zero, the hazard ratio, and percentage decrease in root loss with a 1-unit increase in the parameter or between indicated categories for CO2 treatment, root color and season when root produced parameters. Percentage decrease is calculated from hazard ratio as indicated by Allison (1995) and Wells & Eissenstat (2001). Bold numbers indicate a significant (P < 0.05) relationship between root persistence and the corresponding parameter. ns, nonsignificant.

Ambrosia dumosa (n = 3686; 21.6% censored)
 Treatment: elevated–ambient−0.060 ± 0.0380.1110.942ns
 Diameter (0.1 mm)−0.252 ± 0.022< 0.0010.77722.3
 Depth (cm)−0.013 ± 0.001< 0.0010.9881.2
 Color: white–brown−0.273 ± 0.042< 0.0010.76123.9
 Color: white–tan−0.375 ± 0.066< 0.0010.68731.3
 Season: autumn–spring−0.315 ± 0.050< 0.0010.72927.1
Larrea tridentata (n = 5266; 19.2% censored)
 Treatment: elevated–ambient−0.044 ± 0.0310.1500.956ns
 Diameter (0.1 mm)−0.155 ± 0.018< 0.0010.85714.3
 Depth (cm)−0.010 ± 0.001< 0.0010.9901.0
 Color: white–brown−0.384 ± 0.038< 0.0010.68131.9
 Color: white–tan−0.205 ± 0.061< 0.0010.81518.5
 Season: autumn–spring−0.334 ± 0.044< 0.0010.71628.4
Community (n = 6611; 17.7% censored)
 Treatment: elevated–ambient−0.040 ± 0.0080.1450.961ns
 Diameter (0.1 mm)−0.234 ± 0.017< 0.0010.79120.9
 Depth (cm)−0.010 ± 0.001< 0.0010.9901.0
 Color: white–brown−0.219 ± 0.031< 0.0010.80419.6
 Color: white–tan−0.389 ± 0.049< 0.0010.67832.2
 Season: autumn–spring−0.136 ± 0.035< 0.0010.87312.7

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Elevated CO2 effects

Overall CO2 effects on fine root standing crop, production, and loss were not significant in our study. Thus, over long periods, we reject our first and third hypotheses: elevated CO2 did not increase fine roots for either the two dominant Mojave desert shrubs (A. dumosa, L. tridentata) or for the plant community. These results in an arid ecosystem differ from those of most FACE experiments in agricultural and mesic ecosystems. Elevated CO2 typically increased root biomass for agricultural species grown under FACE experiments, with average root biomass increases of 25%, 47% and 64% for clover, grass crops and cotton, respectively (Kimball et al., 2002). Increases in fine roots often were observed in forest FACE experiments (Norby et al., 2004; Iversen et al., 2008; Pregitzer et al., 2008; Pritchard et al., 2008; Jackson et al., 2009; Liberloo et al., 2009) and in a grazed pasture FACE experiment (Allard et al., 2005). Many open-top chamber studies in grasslands also show greater fine roots under elevated CO2 (Pendall et al., 2004; Sindhøj et al., 2004; Milchunas et al., 2005; LeCain et al., 2006; Volder et al., 2007; Anderson et al., 2010). However, increased fine roots under elevated CO2 were not uniform or did not occur in some FACE studies. For California annual grasslands, increased fine roots under elevated CO2 only occurred at the end of the growing season, presumably owing to increased soil water under elevated CO2 (Higgins et al., 2002). For FACE experiments in mature deciduous forests (Bader et al., 2009) or at treeline (Handa et al., 2008), elevated CO2 did not affect fine roots or even suppressed them.

Although the overall CO2 effect was not significant, significant CO2 effects on root standing crop occurred on individual sampling dates during winter and spring of 2004–05 (Fig. 2). However, these CO2 treatment differences in standing crop need to be interpreted cautiously. Fine roots persist for long periods in desert soils (Table 5), and the extent that fine roots maintain physiological function over a several-year period is unknown, but unlikely. Thus, greater fine root standing crop does not necessarily indicate greater access to soil resources. Furthermore, differences in standing crop were transitory and were related more to differences in fine root loss than production (Fig. 3). For example, significantly greater standing crop under elevated CO2 for Ambrosia in December 2004 (Fig. 2a) occurred largely because of less root loss under elevated CO2 during July–December 2004 (Fig. 3a), and differences in standing crop became nonsignificant during spring 2005 largely because root loss under elevated CO2 was significantly greater than ambient during January–June 2005. Thus, these transient CO2 effects are not consistent with the mechanism underlying the first and third hypotheses, that is, greater fine root production under elevated CO2.

We also reject the second hypothesis that fine root persistence increases and root turnover decreases under elevated CO2. Fine root persistence was similar between CO2 treatments for all microsites (Table 5), and the probability of root loss was not significantly affected by CO2 treatment (Table 6). Although annual root turnover was significantly lower under elevated CO2 for Ambrosia and Larrea in 1 yr (Table 4), root turnover under ambient CO2 (0.38 ± 0.06) was nearly identical to that under elevated CO2 (0.39 ± 0.07) when averaged over all sampling locations and all 4.5 yr of measurements. These results from the Mojave desert differ from those in some, but not all, elevated CO2 studies. For example, decreased fine root life spans under elevated CO2 were reported in both grasslands (Allard et al., 2005; Anderson et al., 2010) and forests (Pritchard et al., 2008). Increased annual root turnover under elevated CO2 has been reported (Liberloo et al., 2009), but turnover was not significantly affected in other ecosystems (Higgins et al., 2002).

Lastly, in contrast to forested ecosystems (Iversen, 2010), we also reject the fourth hypothesis that elevated CO2 shifts root distribution to deeper within the soil profile in the Mojave desert. The only significant CO2 treatment effect on fine root production below 0.25 m soil depth occurred for the 0.75–1.0 m depth interval during January–June 2005 (Figs 4–6), but increased root production occurred under ambient, rather than elevated, CO2 at all sampling locations. Our results also indicate general lack of fine root production below 0.75 m soil depth, which may seem surprising given the known ability of Ambrosia and Larrea to produce roots to 5 m soil depth (Hartle et al., 2006) as well as greater maximum rooting depths generally found in deserts (Jackson et al., 1996; Jobbagy & Jackson, 2000). However, January–June 2005 was the only period during the 4.5 yr of study that soil water was consistently available below 0.65 m soil depth (Fig. S1). Because percolation of precipitation to deep within the soil profile is rare in the Mojave desert, we suspect that potential shifts to deeper fine roots under elevated CO2 is at least as rare. Furthermore, the propensity of desert ecosystems to extract all available water from the soil profile (Anderson et al., 1987, 1993; Yoder & Nowak, 1999), even under elevated CO2 (Nowak et al., 2004b), may ultimately constrain fine root production.

Two unanticipated effects of elevated CO2 on Mojave desert vegetation, earlier peaks in fine root production (Table 3) and greater amount of fine root production in shallow soils for the plant community during an above-average precipitation year (Fig. 6), appear to be ecologically important. Growth of both annuals (Smith et al., 2000) and perennials (Smith et al., 2000; Housman et al., 2006) are accelerated under elevated CO2, especially in wetter years. Increased C assimilation in these wetter years (Huxman et al., 1998; Hamerlynck et al., 2000; Naumburg et al., 2003; Housman et al., 2006), coupled with earlier fine root growth (Table 3), help provide C, water and nutrient resources needed to sustain earlier growth. Lower water content of shallow soils under elevated CO2 during spring 2005 (Fig. S1) is consistent with this concept that earlier shoot growth is sustained by greater extraction of water from soil by greater amounts of fine roots earlier in the growing season.

The CO2 effects on fine root dynamics in our current study from 2003 to 2007 were similar in most regards to those reported by Phillips et al. (2006) for 1998–99 at our site. For Ambrosia and Larrea, Phillips et al. (2006) found no significant overall CO2 treatment effects nor any significant CO2 × time interactions on fine root standing crop, production, and loss. Our results (Tables 1, 2) also show no overall CO2 effect, although greater standing crop under elevated CO2 occurred for Ambrosia for a short period in early 2005 (Fig. 2) and production and loss for both species were occasionally significantly different between CO2 treatments (Fig. 3). For the plant community as a whole, fine root standing crop under elevated CO2 was significantly less than ambient CO2 during 1998–99 (Phillips et al., 2006), but was significantly greater for c. 6 months in late 2004 and early 2005 in our study (Fig. 2). For annual root turnover, Phillips et al. (2006) found no significant differences between CO2 treatments for Ambrosia and Larrea but significantly lower under elevated CO2 for the community. In our study, significant CO2 treatment effects occurred in one of 4 yr of measurement for Ambrosia and Larrea, but no significant differences occurred for the community. Increased root biomass has been reported for Larrea seedlings grown in controlled environments under elevated CO2 (Obrist & Arnone, 2003; Clark et al., 2010), but positive effects of elevated CO2 decreased through time (Obrist & Arnone, 2003). Responses of three Mojave desert grasses to elevated CO2 also were examined in controlled environment experiments (Yoder et al., 2000): only a C3 perennial tussock grass had significant increase in root biomass under elevated CO2, whereas the root biomass of a C3 invasive annual grass and of a C4 perennial tussock grass were not significantly affected by elevated CO2. Thus, taken as a whole, results indicate that significant effects of elevated CO2 on root system dynamics can occur in some Mojave desert species, but CO2 effects are rare and transient.

Fine root dynamics

As in many other ecosystems, fine root production in the Mojave desert was strongly seasonal, peaking between February and May with peak production both greater and earlier in wet years. Similar to shortgrass steppe ecosystems (Milchunas et al., 2005), fine root loss was not as strongly influenced by time of year or precipitation as production, and survival and proportional hazard analyses indicate that roots typically are slowly lost through time. By contrast, mesic forest (Norby et al., 2004) and grassland (Sindhøj et al., 2004) ecosystems have seasonal losses of fine roots that start near the end of the growing season.

Very long persistence of fine roots in the Mojave desert also differs from those in other ecosystems. In our study, 50% of fine roots persisted for two or more years, and 7% of fine roots persisted for > 4 yr. In comparison, the longest time to 50% mortality that was reported in a review of annual crops, perennial herbaceous crops, woody fruit crops and forest communities was just short of 1 yr, with results from 22 of 34 studies indicating 50% mortality in < 100 d (Eissenstat & Yanai, 1997). In more recent publications, 50% of fine roots survived only 2–3 months for deciduous (Black et al., 1998; Wells & Eissenstat, 2001) and evergreen (Black et al., 1998; Johnson et al., 2000) trees and for grapes (Anderson et al., 2003). For three grass and shrub species in a shrub steppe ecosystem, 50% of fine roots typically survived for 3–6 months (Peek et al., 2005). However, not all trees have fine roots with short life spans (Eissenstat et al., 2000; Pritchard et al., 2008; Strand et al.; 2008), and even grasslands (Milchunas et al., 2005) have fine roots whose life spans approach 2 yr. Many factors affect fine root longevity, including root (root diameter, root order, etc.) and environmental (temperature, soil water content, etc.) characteristics (Eissenstat & Yanai, 1997; Black et al., 1998; Johnson et al., 2000; Wells & Eissenstat, 2001; Anderson et al., 2003; Peek et al., 2005). Long persistence of roots in the Mojave desert does not appear to be related to root diameter or root order characteristics: root longevity typically decreases with decreased root diameter and order, but root diameters in our study were as small as or smaller than those in most other studies and most fine roots in our study appeared to be first-order roots. Because fine root persistence through winter can be very low (Wells & Eissenstat, 2001), the lack of severe winters with cold air temperatures and freezing soils in the Mojave desert may partially account for long persistence of fine roots in our study. We also suspect that slow decomposition rates in extremely dry desert soils delays loss of fine roots to the soil organic matter pool. Thus, fine roots persist for long periods in the Mojave desert even though fine roots are not physiologically functional for the entire time they are present.

Production and loss of fine roots in shrub sampling locations typically was similar to that in the community as a whole except for spring 2005 when Larrea produced and lost greater amounts of fine roots. Phillips et al. (2006) also observed similar root production and loss among the three sampling locations. Given that > 80% of the ground surface is bare soil at the NDFF (Jordan et al., 1999) and that the majority of the community transect minirhizotron tubes were located in areas with bare soil, it may seem surprising that fine root production in areas without perennial plants (and often few annual plants) that is sampled by the community transects would have similar amounts of fine roots as that under perennial, dominant shrubs. However, shrubs in the Mojave desert have horizontal rooting that typically exceed 1 m and can extend beyond 5 m (Hartle et al., 2006), and fine root growth of some shrubs can be as great or greater in the interspace as under the shrub canopy (Wilcox et al., 2004). Thus, although much of the desert surface is bare soil interspace between plants, roots of surrounding plants readily explore and extract resources from these unvegetated interspaces.

Our results differ from earlier studies at the NDFF in that our initial fine root standing crop (January 2003) was higher than the final standing crop in January 2000 reported by Phillips et al. (2006). Phillips et al. (2006) reported a net increase of c. 25 m m−2 in fine root standing crop for all sampling locations over 2 yr. Given the 3-yr gap between the end of the Phillips et al. study and the beginning of our study coupled with the numeric (but not significant) difference in fine roots observed on the top of tubes (Phillips et al., 2006) and on the side of tubes (this study) (see Supporting Information, Methods S1 for details), our initial standing crops of c. 75 m m−2 would be consistent with a gradual accumulation of fine roots through time, as observed by Phillips et al. and by us during the first 6 months of our study (Figs 2, 3). Largely because standing crop was greater in our study than in Phillips et al. (2006), our estimates of annual root turnover are much smaller than theirs: their smallest mean annual root turnover (2.33) is over three times greater than the greatest mean root turnover in our study (0.75). Because absolute production and loss rates are similar between the two studies, annual production and turnover indices must be interpreted with caution in ecosystems where fine roots persist for periods of time > 1 yr. Furthermore, although Johnson et al. (2001) suggest that it may take 6–12 months for fine roots to recover from tube installation, equilibration may take significantly longer, as suggested by Strand et al. (2008).

Implications for carbon cycling

Carbon dynamics of ecosystems are influenced by root loss (Eissenstat et al., 2000; Strand et al., 2008): root decomposition increases C input to soil organic matter. In our study, fine root loss reached maximum values for both CO2 treatments in late 2004 and early 2005 (Fig. 3). These large losses of roots, which indicates movement from the fine root pool to the soil organic matter pool, also corresponds with a period of above-average precipitation (Table 3). Soils at the end of summer in 2004 were very dry (Fig. S1), but a series of storms beginning in mid-October 2004 and continuing into May 2005 kept soils relatively wet from late 2004 to spring 2005, providing warm and wet soil conditions ideal for microbial transformation of fine roots into soil organic matter. Once initiated, transformation of fine roots into soil organic matter continued over the next 2 yr even though soils were relatively dry during much of this period. Thus, the rate of soil C input from decomposition of fine roots in the Mojave desert was temporally variable. However, total C input from fine root litter over long periods should be similar between ambient and elevated CO2 treatments: minirhizotron measurements of root length per area of tube sidewall (Phillips et al., 2006; this study) as well as measurements of specific root length and fine root C (Clark et al., 2010) were not significantly affected by elevated CO2. Thus, differences in nutrient cycling (Jin & Evans, 2007) and microbial communities (Jin & Evans, 2010) between ambient and elevated CO2 at our Mojave desert study area indicate that soil microbial factors may be more important determinants of C balance below ground than input of fine root litter.

Conclusions

Contrary to observations in many other ecosystems, fine root growth in the Mojave desert is not increased by elevated CO2 over the long term; neither is root growth shifted to deeper depths. However, transitory effects of elevated CO2 do occur, and earlier and greater amounts of root growth under elevated CO2, especially in the top 0.25 m of soil, likely helped sustain increased assimilation and shoot growth during wetter years by providing greater access to soil resources earlier in the growing season. Nonetheless, these transitory effects also result in more rapid depletion of soil water under elevated CO2 in this desert ecosystem. Thus, the limited ability of precipitation to wet the soil profile ultimately constrains fine root production, even under elevated CO2.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

These studies were supported in part by the US Department of Energy Office of Science, Biological and Environmental Research (DE-FG02-03ER63650 and DE-FG02-03ER63651) and the Nevada Agricultural Experiment Station. Additional support for the Nevada desert FACE facility was provided by DOE National Nuclear Security Administration/Nevada Operations Office and Brookhaven National Laboratory. We thank: Iker Aranjuelo, Naomi Clark, Michael Fears, Eric Hoskins, Jacob Landmesser and Andrew Young for help with image collection and processing; Don Phillips, Mark Johnson and Dave Tingey for help setting up the minirhizotron system and for guidance with image processing; Lynn Fenstermaker and Eric Knight for logistical support at the NDFF; three anonymous reviewers for constructive comments that improved the manuscript; and Dave Evans and Stan Smith for > 10 yr of collaborations.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Methods S1 Minirhizotron sampling and processing details – additional details about capturing and processing root images collected by the minirhizotron camera system as well as quality control efforts to minimize and document two potential sources of measurement error, errors in calibrating the camera system and in variation among people who digitized images.

Fig. S1 Additional information on soil water content in portions of the soil profile that most closely match those used in fine-root analyses.

Figs S2–S4 Fine root standing crop at three soil depth intervals for the three sampling locations (near shrubs of Ambrosia dumosa, and of Larrea tridentata, plus systematically through the plant community) at the Nevada desert FACE (free-air CO2 enrichment) facility (NDFF).

Table S1 Complete ANOVA results for analysis of annual fine root production and turnover indices

Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing material) should be directed to the New Phytologist Central Office.

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