• Aubinet, M., and Coauthors, 2000: Estimates of the annual net carbon and water exchange of forests: The EUROFLUX methodology. Adv. Ecol. Res., 30 , 113175.

    • Search Google Scholar
    • Export Citation
  • Austin, T. A., and Coauthors, 2004: Water pulses and biogeochemical cycles in arid and semiarid ecosystems. Oecologia, 141 , 221235.

  • Baldocchi, D., , and Meyers T. , 1998: On using eco-physiological, micrometeorological and biogeochemical theory to evaluate carbon dioxide, water vapor and trace gas fluxes over vegetation: A perspective. Agric. For. Meteor., 90 , 125.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Blanken, P. D., and Coauthors, 1997: Energy balance and canopy conductance of a boreal aspen forest: Partitioning overstory and understory components. J. Geophys. Res., 102 , 2891528927.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Caldwell, M. M., 1985: Physiological ecology of North American plant communities. Cold Desert, B. F. Chabot and H. A. Mooney, Eds., Chapman and Hall, 198–212.

    • Search Google Scholar
    • Export Citation
  • Dobrowolski, J. P., , Caldwell M. M. , , and Richards J. H. , 1990: Basin hydrology and plant root systems. Plant Biology of the Basin and Range, C. B. Osmond, L. F. Pitelka, and G. M. Hidy, Eds., Ecological Studies, Vol. 80, Springer-Verlag, 243–292.

    • Search Google Scholar
    • Export Citation
  • Donovan, L., , and Ehleringer J. R. , 1994: Water stress and use of summer precipitation in a Great Basin shrub community. Funct. Ecol., 8 , 289297.

  • Ehleringer, J. R., , Schwinning S. , , and Gebauer R. , 1999: Water-use in arid land ecosystems. Plant Physiological Ecology, M. C. Press, J. D. Scholes, and M. G. Barker, Eds., Blackwell, 347–365.

    • Search Google Scholar
    • Export Citation
  • Emmerich, W. E., 2003: Carbon dioxide fluxes in a semiarid environment with high carbonate soils. Agric. For. Meteor., 116 , 91102.

  • Finnigan, J. J., , and Belcher S. E. , 2004: Flow over a hill covered with a plant canopy. Quart. J. Roy. Meteor. Soc., 130 , 129.

  • Flanagan, L. B., , Ehleringer J. R. , , and Marshall J. D. , 1992: Differential uptake of summer precipitation among co-occurring trees and shrubs in a pinyon-juniper woodland. Plant Cell Environ., 15 , 831836.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Flanagan, L. B., , Wever L. A. , , and Carlson P. J. , 2002: Seasonal and interannual variation in carbon dioxide exchange and carbon balance in a northern temperate grassland. Global Change Biol., 8 , 599615.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Goulden, M. L., , Daube B. C. , , Fan S. M. , , Sutton D. J. , , Bazzaz A. , , Munger J. W. , , and Wofsy S. C. , 1997: Physiological responses of a black spruce forest to weather. J. Geophys. Res., 102 , 2898728996.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hastings, S. J., , Oechel W. C. , , and Muhlia-Melo A. , 2005: Diurnal, seasonal and annual variation in the net ecosystem CO2 exchange of a desert shrub community (Sarcocaulescent) in Baja California, Mexico. Global Change Biol., 11 , 927939.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Huxman, T. E., , Cable J. M. , , Ignace D. D. , , Eilts J. A. , , English N. B. , , Weltzin J. , , and Williams D. G. , 2004a: Response of net ecosystem gas exchange to a simulated precipitation pulse in a semi-arid grassland: The role of native versus non-native grasses and soil texture. Oecologia, 141 , 295305.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Huxman, T. E., and Coauthors, 2004b: Precipitation pulses and carbon fluxes in semiarid and arid ecosystems. Oecologia, 141 , 254268.

  • Ivans, C. Y., , Leffler A. J. , , Spaulding U. , , Stark J. M. , , Ryel R. J. , , and Caldwell M. M. , 2003: Root responses and nitrogen acquisition by Artemisia tridentata and Agropyron desertorum following small summer rainfall events. Oecologia, 134 , 317324.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ivans, S., 2005: Response of water vapor and CO2 fluxes in semi-arid plant communities to variations in precipitation. Ph.D. dissertation, Utah State University, 119 pp.

  • Leffler, A. J., , and Caldwell M. M. , 2005: Shifts in depth of water extraction and photosynthetic capacity inferred from stable isotope proxies across an ecotone of Juniperus osteosperma (Utah juniper) and Artemisia tridentata (big sagebrush). J. Ecol., 93 , 783793.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Leffler, A. J., , Ryel R. J. , , Hipps L. , , Ivans S. , , and Caldwell M. M. , 2002: Carbon acquisition and water use in a northern Utah Juniperus osteosperma (Utah juniper) population. Tree Physiol., 22 , 12211230.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Loik, M., , Breshears D. D. , , Lauenorth W. K. , , and Belnap J. , 2004: A multi-scale perspective of water pulses in dryland ecosystems: Climatology and ecohydrology of the western USA. Oecologia, 141 , 269281.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lowe, T. M., 1999: Soil–vegetation relationships in a Juniper community on an Alluvian fan, western Utah. M.S. thesis, Department of Range Science, Utah State University, 180 pp.

  • Martens, S. N., , Breshears D. D. , , and Meyer C. W. , 2000: Spatial distributions of understory light along the grassland/forest continuum: Effects of cover, height, and spatial patterns of tree canopies. Ecol. Model., 126 , 7993.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Massman, W. J., , and Lee X. , 2002: Eddy covariance flux corrections and uncertainties in long-term studies of carbon and energy exchanges. Agric. For. Meteor., 113 , 121144.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ryel, R. J., , Leffler A. J. , , Peek M. S. , , Ivans C. Y. , , and Caldwell M. M. , 2004: Water conservation in Artemisia tridentata through redistribution of precipitation. Oecologia, 141 , 335345.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Saetre, P., , and Stark J. M. , 2005: Microbial dynamics and carbon and nitrogen cycling following re-wetting of soils beneath two semi-arid plant species. Oecologia, 142 , 247260.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Saigusa, N., , Oikawa T. , , and Liu S. , 1998: Seasonal variations of the exchange of CO2 and H2O between a grassland and the atmosphere: An experimental study. Agric. For. Meteor., 89 , 131139.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schwinning, S., , and Ehleringer J. R. , 2001: Water use trade-offs and optimal adaptations to pulse-driven ecosystems. J. Ecol., 89 , 464480.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schwinning, S., , and Sala O. E. , 2004: Hierarchy of responses to resource pulses in arid and semi-arid ecosystems. Oecologia, 141 , 211220.

  • Unland, H. E., , Houser P. R. , , Shuttleworth W. J. , , and Yang Z. L. , 1996: Surface flux measurement and modeling at a semi-arid Sonoran Desert site. Agric. For. Meteor., 82 , 119153.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Veenendaal, E. M., , Kolle O. , , and Lloyd J. , 2004: Seasonal variation in energy fluxes and carbon dioxide exchange for a broad-leaved semi-arid savanna (Mopane woodland) in southern Africa. Global Change Biol., 10 , 318328.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Webb, E. K., , Pearman G. I. , , and Leuning R. , 1980: Correction of flux measurements for density effects due to heat and water vapor transfer. Quart. J. Roy. Meteor. Soc., 106 , 85100.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wever, L. A., , Flanagan L. B. , , and Carlson P. J. , 2002: Seasonal and interannual variation in evapotranspiration, energy balance and surface conductance in a northern temperate grassland. Agric. For. Meteor., 112 , 3149.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Whitford, W. G., 2002: Ecology of Desert Systems. Academic Press, 327 pp.

  • Wilczak, J. M., , Oncley S. P. , , and Stage S. A. , 2001: Sonic anemometer tilt correction algorithms. Bound.-Layer Meteor., 99 , 127150.

  • Wilson, K. B., , and Baldocchi D. D. , 2000: Seasonal and interannual variability of energy fluxes over a broadleaved temperate deciduous forest in North America. Agric. For. Meteor., 100 , 118.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yan, S., , Wan C. , , Sosebee R. E. , , Wester D. B. , , Fish E. B. , , and Zartman R. E. , 2000: Responses of photosynthesis and water relations to rainfall in the desert shrub creosote bush (Larrea tridentata) as influenced by municipal biosolids. J. Arid Environ., 46 , 397412.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • View in gallery

    Friction velocity (u*) vs CO2 flux at the sagebrush site.

  • View in gallery

    Seasonal changes in soil moisture at the 0–8-cm depth for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage) during 2002 and 2003.

  • View in gallery

    Seasonal change in ET flux at the crested wheatgrass (CWG), juniper (jun), and sagebrush (sage) sites during 2002 and 2003.

  • View in gallery

    Seasonal change in daytime daily totals of CO2 flux for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage) during 2002 and 2003.

  • View in gallery

    (top) Air temperature (Tair) and saturation vapor pressure deficit (SVPD), (middle) net radiation (Rn), and (bottom) precipitation later in the spring of 2002 at the sagebrush site.

  • View in gallery

    (top) ET and (middle) CO2 flux responses to (bottom) rain later in the spring of 2002 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

  • View in gallery

    (top) ET and (middle) CO2 flux responses to (bottom) rain events in late summer of 2002 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

  • View in gallery

    (top) ET and (middle) CO2 responses to (bottom) rain events in early summer of 2003 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

  • View in gallery

    (top) ET and (middle) CO2 responses to (bottom) rain events in midsummer 2003 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

  • View in gallery

    (top) ET and (middle) CO2 responses to (bottom) rain events in the fall months of 2002 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 19 19 13
PDF Downloads 10 10 8

Response of Water Vapor and CO2 Fluxes in Semiarid Lands to Seasonal and Intermittent Precipitation Pulses

View More View Less
  • 1 Campbell Scientific, Inc., Logan, Utah
  • 2 Department of Plants, Soils, and Biometeorology, Utah State University, Logan, Utah
  • 3 Ecology Center, Utah State University, Logan, Utah, and Louisiana Tech University, Ruston, Louisiana
  • 4 Ecology Center, Utah State University, Logan, Utah
© Get Permissions
Full access

Abstract

Precipitation pulses are important in controlling ecological processes in semiarid ecosystems. The effects of seasonal and intermittent precipitation events on net water vapor and CO2 fluxes were determined for crested wheatgrass (Agropyron desertorum), juniper (Juniperus osteosperma), and sagebrush (Artemisia tridentata) ecosystems using eddy covariance measurements. The measurements were made at Rush Valley, Utah, in the northern Great Basin of the United States. Data were evaluated during the growing seasons of 2002 and 2003. Each of these communities responds to precipitation pulses in all seasons, but these responses vary among season and ecosystem, and differ for water vapor and CO2. The degree and direction of response (i.e., net uptake or efflux) depended upon the timing and amount of precipitation. In early spring, both evapotranspiration (ET) and CO2 fluxes responded only slightly to precipitation pulses because soils were already moist from snowmelt and spring rains. As soils dried later in the spring, ET response to rainfall increased. The summer season was very warm and dry in both years, and both water and CO2 fluxes were generally reduced as compared to fluxes in the spring. Water vapor fluxes increased during and immediately after periodic summer rain events at all sites, especially at juniper, followed by the sagebrush and crested wheatgrass sites. Net CO2 exchange changed significantly at the juniper and sagebrush sites but changed very little at the crested wheatgrass site due to senescence of this grass. However, in the wetter summer of 2003, the grass species maintained physiological activity and responded to rain events. In the fall of both years, responses of ET and CO2 fluxes to precipitation were very similar for all three communities, with only small changes, presumably due to significantly lower temperatures in the fall. This research documents the importance of the temporal distribution of rainfall on patterns of ET and CO2 fluxes and suggests that soil moisture and stand-level leaf area index (LAI) are critical factors governing ET and CO2 responses to precipitation in these communities.

Corresponding author address: Sasha Ivans, Campbell Scientific, Inc., 815 West 1800 North, Logan, UT 84321-1784. Email: sasha@campbellsci.com

Abstract

Precipitation pulses are important in controlling ecological processes in semiarid ecosystems. The effects of seasonal and intermittent precipitation events on net water vapor and CO2 fluxes were determined for crested wheatgrass (Agropyron desertorum), juniper (Juniperus osteosperma), and sagebrush (Artemisia tridentata) ecosystems using eddy covariance measurements. The measurements were made at Rush Valley, Utah, in the northern Great Basin of the United States. Data were evaluated during the growing seasons of 2002 and 2003. Each of these communities responds to precipitation pulses in all seasons, but these responses vary among season and ecosystem, and differ for water vapor and CO2. The degree and direction of response (i.e., net uptake or efflux) depended upon the timing and amount of precipitation. In early spring, both evapotranspiration (ET) and CO2 fluxes responded only slightly to precipitation pulses because soils were already moist from snowmelt and spring rains. As soils dried later in the spring, ET response to rainfall increased. The summer season was very warm and dry in both years, and both water and CO2 fluxes were generally reduced as compared to fluxes in the spring. Water vapor fluxes increased during and immediately after periodic summer rain events at all sites, especially at juniper, followed by the sagebrush and crested wheatgrass sites. Net CO2 exchange changed significantly at the juniper and sagebrush sites but changed very little at the crested wheatgrass site due to senescence of this grass. However, in the wetter summer of 2003, the grass species maintained physiological activity and responded to rain events. In the fall of both years, responses of ET and CO2 fluxes to precipitation were very similar for all three communities, with only small changes, presumably due to significantly lower temperatures in the fall. This research documents the importance of the temporal distribution of rainfall on patterns of ET and CO2 fluxes and suggests that soil moisture and stand-level leaf area index (LAI) are critical factors governing ET and CO2 responses to precipitation in these communities.

Corresponding author address: Sasha Ivans, Campbell Scientific, Inc., 815 West 1800 North, Logan, UT 84321-1784. Email: sasha@campbellsci.com

1. Introduction

The Great Basin is an extensive region with hot, dry summers and cold winters (Caldwell 1985). Most of the annual precipitation in the Great Basin is in the form of snow, though intermittent rain events occur in the spring and summer (Dobrowolski et al. 1990). The timing and the magnitude of these precipitation pulses drive many ecological processes, such as availability of soil water for plants and soil microbes (e.g., Schwinning and Sala 2004; Saetre and Stark 2005). Most precipitation is received as small events of fewer than 5 mm, and the intervals between two rain events are often shorter than 10 days (Loik et al. 2004). Transformation of these precipitation pulses into available soil water for plants is complex and depends upon factors such as antecedent soil water content, interception, infiltration, runoff, evaporation, plant water use, hydraulic redistribution by plant roots, and percolation below the rooting zone (Loik et al. 2004). In addition, there is differential responsiveness of photosynthesis and microbial respiration to pulses of varying size (Huxman et al. 2004b). For example, recent studies suggest that small precipitation pulses that only recharge shallow soils affect only soil microfauna and microflora living very close to the soil surface (Schwinning and Sala 2004; Austin et al. 2004). In contrast, more water and deeper infiltration may be needed to affect the physiology, growth, and reproduction of higher plants. However, downward hydraulic redistribution of precipitation as small as 3 mm by plant roots has been documented (Ryel et al. 2004) and is thought to allow plants in arid environments to conserve water by limiting evaporation and competition by more shallow-rooted plants. In addition, small precipitation pulses may help plants survive or maintain leaf area, which can increase their capacity to respond to larger events (Yan et al. 2000).

Although several studies have investigated the role of episodic precipitation inputs on plant function and productivity (e.g., Ehleringer et al. 1999; Schwinning and Ehleringer 2001; Whitford 2002; Ivans et al. 2003; Huxman et al. 2004a), information is lacking on how net ecosystem exchanges of water and CO2 are controlled by changes in water status, especially in the Great Basin. Studies of CO2 and water fluxes in other regions suggest that flux responses are likely to vary among different species, soil substrates, and precipitation patterns. For example, Huxman et al. (2004a) found that 24 h following an experimental irrigation pulse, invasive species stands had greater evapotranspiration (ET) than native stands and that plots with invasive species accumulated between 5% and 33% of the carbon that plots with native species accumulated over a 15-day-pulse period. In addition, Aubinet et al. (2000) found that invasive species stands had greater ET rates immediately following a precipitation pulse than native stands, while maximum instantaneous net ecosystem exchange (NEE) increased for both species at roughly the same rate.

In this current study, the main objective is to examine the response of water vapor and CO2 exchanges to seasonal changes of precipitation and intermittent rain events in crested wheatgrass (Agropyron desertorum), sagebrush (Artemisia tridentata), and juniper (Juniperus osteosperma) ecosystems using eddy covariance measurements. Evaluation of these exchanges and their response to the intermittency of precipitation is important to understand the functioning of these plant communities and will increase our understanding of carbon and water fluxes in semiarid ecosystems.

2. Methods

a. Study site

Measurements were made over crested wheatgrass (Agropyron desertorum), juniper (Juniperus osteosperma), and sagebrush (Artemisia tridentata) in Rush Valley, Utah (40°17′N, 112°28′W, elevation 1600 m), during the spring, summer, and fall months of 2002 and 2003. Soils at the grass and shrub sites are a silt loam (Erda silt loam, a fine-loamy, mixed mesic Aridic Calcixeroll) with low shrink/swell capacity, whereas the juniper ecosystem is located on rocky silt-loam soils underlain by a hardpan at about 1 m (Tooele County, Utah, soil survey, 2 January 1993). The water table in the crested wheatgrass and sagebrush ecosystems is at 15–20 m with very dry soils existing between the rooting zone and the water table. In the juniper ecosystem, soils were at or near field capacity for water below the hardpan due to lateral subsurface flow (Lowe 1999). Juniper is able to access this water source when shallow soils become dry (Leffler et al. 2002). Eddy covariance stations were established in each of the three ecosystems. Upwind fetch was generally about 600 m for the juniper site and 500 m for the crested wheatgrass and sagebrush sites. Percent cover for each site was estimated from line transects and aerial photographs. While crested wheatgrass and sagebrush had fairly homogeneous distributions, juniper cover was highly variable. Crested wheatgrass height was ∼0.5 m with 44% cover, sagebrush height was ∼0.9 m with 49% cover, while juniper trees generally ranged from 0.5 to ∼4 m in height with 15%–55% cover (Leffler et al. 2002; Leffler and Caldwell 2005). Stand-level leaf area index (LAI) was 0.44 and 0.56 for crested wheatgrass and sagebrush and ranged from 1.4 to 5.5 for juniper (Ivans 2005).

b. Measurements of turbulence fluxes

Fluxes of sensible heat, latent heat, and CO2 were measured above the canopy using eddy covariance. The instruments included a CSAT3 3D sonic anemometer (Campbell Scientific, Inc., Logan, Utah) and a LiCor 7500 open-path water vapor and CO2 sensor. Sensors were sampled at 10 Hz, and 30-min block average fluxes were determined. Sensors were controlled and recorded with CR23X dataloggers in 2002 and until August 2003. CR5000 dataloggers were used for the remainder of 2003 (Campbell Scientific, Inc., Logan, Utah). Instruments were mounted 2.5 m above the soil surface at the crested wheatgrass and sagebrush sites, and 8.0 m above the soil surface at the juniper site. The planar fit method (Wilczak et al. 2001) was used to rotate the coordinate system for the sonic anemometer data. Correction to the initial flux values were made for high-frequency losses due to separation of sensors, path averaging, and sensor frequency response as described in Massman and Lee (2002), and correction for density effects, as described in Webb et al. (1980).

Net radiation was measured with a net radiometer (Kipp & Zonen, NR Lite) mounted at 2.0 m above the soil surface at the crested wheatgrass and sagebrush sites, and 10 m above the soil surface at the juniper site. Soil heat flux was determined with heat flow transducers and thermocouples (Hukseflux HFP01). The heat flux plates were buried at 8 cm and the thermocouples were placed to determine the average soil temperature in the layer between the plates and the surface (2 cm below the soil surface for the two upper probes and 6 cm for the two bottom probes. Soil moisture was measured in each plant community with CS616 water content reflectometers. Sensors were buried at an angle to cover a soil depth from 8 to 15 cm below the soil surface (Campbell Scientific, Inc., Logan, Utah). Precipitation was measured with a TE525MM tipping-bucket rain gauge (Texas Electronics), which was located centrally among the three study sites.

c. Nighttime CO2 flux

There was a problem recovering valid CO2 fluxes during the nighttime. Many Fluxnet sites observe that on nights of light wind, the sum of CO2 storage and the CO2 flux above the canopy is less than biological models of respiration predict (Finnigan and Belcher 2004). This can be caused by gravity-driven “katabatic” flow, or it can reflect turbulence values so small that the surface becomes decoupled from the atmosphere. Analyses of the friction velocity (u*) data from all three sites in both years showed small values during the night. This was associated with unrealistically small upward CO2 fluxes. Figure 1 shows nighttime CO2 flux versus friction velocity for a typical 5-day nighttime dataset at the sagebrush site in 2003. This includes a majority of very low wind nights, and only one night with higher winds. The results suggest that when u* was less than 0.20 m s−1 fluxes are too small, vary greatly, and sometimes appear to be slightly negative. Therefore, to get reliable flux values, friction velocities needed to be larger than 0.20 m s−1 at the sagebrush and juniper sites and higher than 0.3 m s−1 at the crested wheatgrass site. The larger required friction velocity at the crested wheatgrass site may be related to the short canopy, which resulted in less mixing of the air. Since low friction velocities were recorded more than 85% of the time at night, it was decided to leave out the nighttime data from any further investigations of the CO2 flux at all three research sites.

3. Results and discussion

a. Precipitation

Over the study period, the annual precipitation values at Rush Valley were 187 mm in 2002 and 248 mm in 2003. The historical climate data indicate the 30-yr-average annual precipitation at Vernon, Utah, ∼25 km from our site, is 243 mm, and that precipitation is generally evenly distributed throughout the year, with about 60 mm during each quarter of the year. However, 2002, and especially 2003, had uneven distributions of precipitation, which directly impacted biosphere–atmosphere exchanges. The summer of 2002 was much drier than the summer of 2003 and the fall of 2002 was much wetter than the fall of 2003 (Table 1).

b. Soil moisture

Seasonal variation of surface (0–8 cm) soil moisture changed as expected with the highest values in the spring and fall of both years and the lowest values during the summer months (Fig. 2). However, late spring and summer soil moisture were greater in 2003 than in 2002 due to several rain events. Thus, the two years provided a good contrast in seasonal changes of water availability.

Soils were drier at the juniper site in the spring months of both years compared to the sagebrush and crested wheatgrass sites. This may have resulted from differences in interception and evaporation due to differences in canopy cover and the distribution of understory light (Martens et al. 2000) as well as more coarse soils at the juniper site. During the summer of both years, soil moisture was lowest at the sagebrush site and highest at the juniper site. Again, these differences are likely due to interactions between canopy cover, shading, infiltration, evaporation, and plant water use. Although all of the species in this study are capable of utilizing shallow soil water during the summer (Ivans et al. 2003; Leffler et al. 2002), juniper responds to summer precipitation during some years but not others (Flanagan et al. 1992; Donovan and Ehleringer 1994; Leffler et al. 2002), apparently due to subtle differences in the timing and amount of rain (Leffler et al. 2002). During the fall, soil moisture was nearly equal at all three sites for both years.

c. Energy balance

By examining the energy balance closure it is possible to evaluate how well the eddy covariance data were estimated (Baldocchi and Meyers 1998; Saigusa et al. 1998; Wilson and Baldocchi 2000; Wever et al. 2002). The available energy is defined by net radiation, Rn, minus the soil heat flux, G. This is dissipated into turbulence fluxes of sensible heat, H, and latent heat, LE. The energy balance closure is defined as (H + LE)/ (RnG). Perfect measurements would result in a value of 1.0. In practice, values are lower than this, typically 0.8–0.9 is considered good. In the spring and summer seasons of both years (Table 2), crested wheatgrass had the best daytime closure compared to the sagebrush and juniper communities, which were very similar to each other. Values were greatest in the summer and lowest in the fall. The average closure value for all sites and seasons was 0.83. These values are typical of other studies (Goulden et al. 1997; Blanken et al. 1997; Aubinet et al. 2000; Wilson and Baldocchi 2000; Flanagan et al. 2002).

d. Seasonal changes of water vapor fluxes

The seasonal changes in ET for both years are documented in Fig. 3. Generally, the temporal changes in water vapor fluxes followed the changes of soil water. The greater values of soil moisture and ET in 2003 are consistent with greater precipitation in the late spring and summer. All sites had the highest values of ET in the spring of both years when soil water availability was greatest. Evapotranspiration fluxes significantly decreased in the summer of both years and increased periodically in the fall of both years in response to rain events. Juniper generally had larger ET values during the summer than the other communities, especially in the dry summer of 2002. This reflects greater deep soil water availability at the juniper site.

e. Seasonal changes of CO2 fluxes and comparison to other desert ecosystems

Recall that only the daytime [0600–1800 h, mountain standard time (MST)] CO2 flux values are discussed here, since turbulence at night was usually too small to recover reliable flux values. However, for our purposes of looking at how the CO2 fluxes respond to rain events, the daytime values are of great interest. The seasonal variations of daytime CO2 fluxes are shown for both years in Fig. 4. Note that negative flux values denote net ecosystem uptake of CO2.

Similar to what was observed for water vapor, CO2 uptake was generally higher in 2003 compared to 2002. The highest values of net CO2 uptake were measured during the spring of both years and the lowest values occurred in the summer when available water was limited and temperatures were very high.

Except for early spring, juniper generally had the largest negative CO2 fluxes, with sagebrush intermediate and crested wheatgrass the least. In early spring, net CO2 uptake was similar for the crested wheatgrass and juniper ecosystems. This is somewhat surprising given that peak leaf-level CO2 assimilation rates for juniper are much smaller (∼2.8 μmol CO2 m−2 s−1) than crested wheatgrass (∼24 μmol CO2 m−2 s−1) or sagebrush (11 μmol CO2 m−2 s−1) (data from concurrent study). However, estimates of stand-level assimilation rates during the spring of 2000 (based on stand-level LAI and leaf gas exchange measurements) were 0.47 and 0.26 mg CO2 m−2 s−1 for the crested wheatgrass and sagebrush communities, respectively, and ranged from 0.13 to 0.51 (mean = 0.30) mg CO2 m−2 s−1 for the juniper community (currently unpublished data). This broad range in values for the juniper community reflects the heterogeneity of LAI within this ecosystem. These stand-level estimates based on leaf gas exchange compare very well with our eddy covariance data (although the data were collected in different years). For example, eddy covariance estimates of CO2 NEE at the juniper site from April to June 2002 varied from 0.05 to 0.61 mg m−2 s−1 (mean = 0.28). These data suggest that stand-level LAI plays a considerable role in net ecosystem uptake of CO2. Greater deep soil water availability at the juniper site may also have contributed to greater net CO2 uptake at this site compared to the sagebrush and crested wheatgrass sites.

The greatest daily amplitude of NEE of CO2 was observed for each site during the late spring when conditions were most favorable for plant growth. Peak daytime NEE at the juniper site was −8.8 μmol CO2 m−2 s−1, and peak daytime NEE values at the sagebrush and crested wheatgrass sites were −7.0 and −6.5 μmol CO2 m−2 s−1, respectively. These values are within the range of values reported for other desert ecosystems. For example, the maximum NEE values during the wet season for a sarcocaulescent shrub community in Baja California, Mexico, were −5.9 μmol CO2 m−2 s−1 (Hastings et al. 2005), and Emmerich (2003) reports peak uptake levels of −9 μmol CO2 m−2 s−1 for a desert shrub community in Arizona. However, in the same study, a desert grassland exhibited a maximum daytime NEE of −22.6 CO2 μmol m−2 s−1 (Emmerich 2003). Interpretation of the data from the Emmerich study, however, is confounded by the simultaneous release of inorganic-derived carbon at both study sites. Veenendaal et al. (2004) report peak net daytime uptake rates of −10 μmol CO2 m−2 s−1 for a semiarid woodland in southern Africa.

In this current study, the lowest maximum amplitude in CO2 NEE values occurred in the late summer of 2002 and ranged from −4.5 μmol CO2 m−2 s−1 for the juniper site to −2 and −1.8 μmol CO2 m−2 s−1 for the crested wheatgrass and sagebrush sites, respectively. These values are higher than those reported for a Sonoran Desert shrub site in July and August where the maximum daily uptake following a rain event was −0.8 μmol m−2 s−1 (Unland et al. 1996). In part, different plant functional types, species, percent cover, and soil properties may explain these differences. For example, in the Unland et al. study, a considerable portion of the vegetation was cacti and agaves with crassulacean acid metabolism (CAM). There were no CAM species present in this current study.

f. Response to short-term precipitation pulses

Because ET and CO2 responses to intermittent precipitation pulses were similar for both 2002 and 2003, most of the results will be presented for 2002. Data for late spring 2002 indicate that, as expected, there were significant decreases in net radiation (Rn) and saturation vapor pressure deficit (SVPD) during rain events (Fig. 5) due to increases in cloud cover and air humidity. However, each of these variables quickly returned to prerainfall values at the end of the rain event (Fig. 5), which indicates that changes in ET and CO2 fluxes were due to precipitation and not other environmental changes. Similar findings were observed for all of the rain events described in this paper. Because soil moisture was much greater during the early spring relative to late spring (Fig. 2), we consider early and late spring separately in order to evaluate the role of soil antecedent moisture. The results for early spring are not shown since there was very small, insignificant response of ET and CO2 in all three communities to a few intermittent rain events totaling less than 1 mm in 60 h.

Evapotranspiration responses to rain events were much stronger in the late spring as shown in Fig. 6. A series of small events totaling 9.1 mm on day of year (DOY) 117–118 resulted in larger water vapor fluxes and more negative CO2 fluxes for several days. The soil moisture (Fig. 2) also responded in a significant way to these events. The ET flux at the juniper site changed from 0.14 mm h−1 on DOY 116 to 0.55 mm h−1 36 h later, which was the peak of the response. The sagebrush site had a smaller response, increasing from about 0.1 mm h−1 on DOY 116 to 0.3 mm h−1 on DOY 118. The lowest response occurred at the crested wheatgrass site, but the values still doubled. The largest increases are associated with larger leaf area values, suggesting that the total water vapor fluxes were closely linked to the amount of transpiring foliage.

CO2 fluxes also responded to this rain event, but the responses were of a smaller relative magnitude than water vapor. Net CO2 uptake increased only slightly at the sagebrush and crested wheatgrass sites, and more substantially at the juniper site. The crested wheatgrass site exhibited a temporary change from net CO2 uptake during the day to net CO2 efflux on DOY 117 when a rain pulse occurred. This indicates that CO2 displacement and ecosystem respiration during this time was greater than ecosystem CO2 assimilation. These differences are also consistent with the LAI values in each ecosystem. The lower LAI and larger areas of exposed soil in the sagebrush and crested wheatgrass ecosystems likely resulted in proportionally more soil respiration increase due to the wetted soil. Hence, the increase in net CO2 uptake would be smaller in those communities. Juniper had a larger LAI, and was able to increase net CO2 uptake by larger values. The effect of this size of rain pulse lasted for about a week in all three plant communities. This is similar to results reported by Unland et al. (1996), where transpiration was enhanced for about 10 days after rain events in the Sonoran Desert.

There were significant responses of ET to precipitation pulses in the summer, as shown in Fig. 7. A few short but very intense rain events (DOY 249–251) caused increases in water vapor fluxes at all sites. The rain caused a significant increase in CO2 uptake from values of −2.3 to −4.5 μmol CO2 m−2 s−1 at the juniper site (Fig. 7). However, this flux response did not persist for longer than 2–3 days. A smaller increase in net uptake of CO2 occurred at the sagebrush site, changing from about −0.5 to −1.8 μmol CO2 m−2 s−1. In contrast to juniper and sagebrush, the crested wheatgrass site experienced CO2 efflux in response to the event (Fig. 7). The CO2 efflux increased from 1.1 μmol CO2 m−2 s−1 on DOY 248 to about 5.7 μmol CO2 m−2 s−1 on DOY 252. These responses are consistent with senescence of crested wheatgrass at this time, which resulted in primarily microbial responses and physical displacement of CO2 by water. Huxman et al. (2004a) reported a large efflux of CO2 from grassland plots during the night and day following a large irrigation pulse (39 mm) and attributed this to substantial physical displacement of CO2 by water as well as upregulation of respiration. The delay in peak CO2 efflux exhibited at the crested wheatgrass site in this study suggests that the physical displacement of CO2 may have contributed less to the observed efflux than changes in respiration.

In the summer of 2003, there were two major rain events that significantly influenced the fluxes. Rain on DOY 174 totaled about 20 mm in 12 h and greatly increased ET at both the sagebrush and crested wheatgrass sites (Fig. 8). Unfortunately, the flux system at the juniper site was down, so data were not available. Before the rain event, ET peaked at about 0.13 mm h−1 at the crested wheatgrass site and at about 0.1 mm h−1 at the sagebrush site. By 36 h later, ET reached 0.3 mm h−1 at the crested wheatgrass site and 0.25 mm h−1 at the sagebrush site. On DOY 176 the juniper site was again online, and results show it had smaller increases in water vapor flux than the other two sites.

The net CO2 exchanges also changed after this event. Both the crested wheatgrass and sagebrush sites changed from net uptake before the largest rain event to net efflux during and immediately after the rain event. The efflux was initially greater at the sagebrush site, which is consistent with greater labile C pools at this site (Saetre and Stark 2005). However, at the sagebrush site, the CO2 flux returned to uptake by DOY 175, though the uptake was smaller than before, suggesting more intense ecosystem respiration. At the crested wheatgrass site, CO2 efflux continued and peaked after the last rain events. Finally on DOY 179, the crested wheatgrass site showed some small net CO2 uptake. In this wetter year, there was still green vegetation at the crested wheatgrass site, compared to 2002, where all vegetation was senescent. Resumption of data collection at the juniper site on DOY 176 indicates the greatest CO2 uptake from this site.

An event of about 12 mm occurred late on DOY 206, and significantly changed ET and CO2 fluxes (Fig. 9). Evapotranspiration increased from near 0.1 mm h−1 before the rain to ∼0.5 mm h−1 during and immediately after the rain at all sites. This increase, however, lasted only for one day after the rain event, and then fluxes fell back to prerainfall values. Instantaneous water vapor responses to rainfall could be explained by soil evaporation during and immediately following the rain event.

Figure 9 indicates that for one day there was net CO2 efflux from all sites. The response at the crested wheatgrass site was slower, possibly due to more labile carbon pools at the other sites (Saetre and Stark 2005). Within two days, all sites exhibited net uptake of CO2. The sagebrush site experienced the largest response, increasing peak CO2 flux from −4.1 to −6.8 μmol CO2 m−2 s−1. This effect lasted for about 5 days. In contrast, juniper experienced a decrease in CO2 uptake from −2.5 μmol CO2 m−2 s−1 before to −1.1 μmol CO2 m−2 s−1 after the rain event. This decrease in net CO2 uptake at the juniper site suggests a larger response in ecosystem respiration relative to ecosystem assimilation of CO2. Because leaf-level CO2 assimilation rates for juniper are much smaller than sagebrush or crested wheatgrass, juniper was not able to have as large of a short-term response in CO2 assimilation. However, juniper has more consistent assimilation values throughout the year than sagebrush and especially crested wheatgrass. In crested wheatgrass, CO2 uptake in response to this midsummer rain peaked at about −2.7 μmol CO2 m−2 s−1, and the effect lasted about 5 days. However, this level of CO2 uptake continued through much of the summer.

Responses of ET and CO2 fluxes to fall rain events are presented in Fig. 10. There were two rain events totaling ∼15 mm that caused changes in water vapor and CO2 flux in all three communities with corresponding significant changes in soil moisture (Fig. 2). Increased fluxes lasted for ∼4 days at all three sites. After the rain event, the highest values of water vapor flux occurred at the juniper site, followed by sagebrush and crested wheatgrass.

The same event caused the daytime net CO2 flux to more than triple at the juniper site, from −0.9 μmol CO2 m−2 s−1 (DOY 312) to −3.4 μmol CO2 m−2 s−1 (DOY 313) (Fig. 10). At the sagebrush site, the net CO2 flux went from −0.9 μmol CO2 m−2 s−1 on DOY 312 to −4.1 μmol CO2 m−2 s−1 on DOY 314 (Fig. 10). Crested wheatgrass experienced the smallest change in net CO2 flux, but this flux changed its sign to negative, which is very important because it indicates that after this rain event net ecosystem exchange of CO2 at the crested wheatgrass site changed from efflux to uptake.

4. Summary and conclusions

We have documented responses of water vapor and net CO2 fluxes to intermittent rain events for perennial grass, shrub, and tree communities in the Great Basin of northern Utah. The largest changes in ET and CO2 fluxes resulted from seasonal-scale changes in soil moisture. Fluxes were greatest in the spring when soil moisture was large, and lowest in the summer when soil water is very small. Evapotranspiration and CO2 fluxes increased again in the fall when soil moisture recharge occurred. This seasonal behavior and its dependence on soil moisture followed a pattern that one might have expected.

Each of these species responds to precipitation pulses in all seasons, but these responses vary among season and ecosystem, and differ for water vapor and CO2. In early spring, ET was not sensitive to precipitation pulses since soils were already wet. As soils dried later in the spring, ET response to rainfall increased.

In contrast to ET, CO2 fluxes were more sensitive to spring precipitation with upregulation of assimilation and respiration. The degree and direction of response (i.e., uptake or efflux) depended upon the timing and amount of precipitation. The sagebrush ecosystem, however, generally exhibited faster upregulation of net efflux, which is consistent with more labile carbon pools at this site (Saetre and Stark 2005).

The warm and dry summer conditions generally resulted in smaller values of NEE for CO2. However, the juniper ecosystem was able to maintain larger net CO2 uptake and water use than the sagebrush and crested wheatgrass ecosystems, which is consistent with greater stand-level LAI and deep water availability for this community. Intermittent rain events during the summer resulted in doubled net downward flux of CO2 for a few days after the rain event. The water vapor fluxes also increased significantly in response to rain events.

The results of this study also emphasize that the temporal distribution of precipitation is as important as the seasonal total in governing the responses of the ecosystems to rain pulses. Even though total summer precipitation for 2002 was greater than in 2003, rain was very intermittent in 2002. In 2003, more even distribution of rain maintained soil moisture levels and allowed all three plant communities to stay physiologically active. Hence, the crested wheatgrass site exhibited net CO2 uptake after summer rain events in 2003, but not in 2002, when this grass was senescent due to summer drought. Changes in the water and carbon balances of these communities in response to precipitation events depend upon the history of precipitation at the site, which is consistent with other studies (e.g., Huxman et al. 2004b).

The increase in net CO2 uptake after rain events in the fall indicates that all three ecosystems were still physiologically able to respond. However, the crested wheatgrass site did not appear to respond greatly to fall precipitation events.

This study clearly shows the magnitude of CO2 and ET responses to precipitation pulses among different plant ecosystems and seasons. However, more research is needed to understand the factors that control carbon and water use in this system. For example, some of the differences we observed among our communities were likely due to species-specific interactions with soil surfaces that may have altered the manner in which pulses of precipitation were translated into biological activity (Huxman et al. 2004a). Huxman et al. (2004a) also indicate that along with plant and microbial respiration, the physical displacement of soil CO2 by water strongly controls whole-ecosystem carbon exchange during precipitation pulses. Hence, in order to better understand the relative roles of soil microbial respiration, CO2 displacement by water, and autotrophic activity, it will be necessary to study not only plant ecophysiological, canopy structural, and microbial responses to variation in precipitation, but also soil gas exchange characteristics and the influence of different soil surfaces on these responses. In addition, the nighttime values for respiration, and their response to precipitation, need to be documented for these systems, likely using a combination of models and measurements.

Acknowledgments

We thank D. Johnson for allowing studies to be conducted on his property. This study was funded by a grant from the National Science Foundation (DEB-9807097) and the Utah Agricultural Experiment Station. Comments by three anonymous reviewers helped to improve the quality of this manuscript.

REFERENCES

  • Aubinet, M., and Coauthors, 2000: Estimates of the annual net carbon and water exchange of forests: The EUROFLUX methodology. Adv. Ecol. Res., 30 , 113175.

    • Search Google Scholar
    • Export Citation
  • Austin, T. A., and Coauthors, 2004: Water pulses and biogeochemical cycles in arid and semiarid ecosystems. Oecologia, 141 , 221235.

  • Baldocchi, D., , and Meyers T. , 1998: On using eco-physiological, micrometeorological and biogeochemical theory to evaluate carbon dioxide, water vapor and trace gas fluxes over vegetation: A perspective. Agric. For. Meteor., 90 , 125.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Blanken, P. D., and Coauthors, 1997: Energy balance and canopy conductance of a boreal aspen forest: Partitioning overstory and understory components. J. Geophys. Res., 102 , 2891528927.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Caldwell, M. M., 1985: Physiological ecology of North American plant communities. Cold Desert, B. F. Chabot and H. A. Mooney, Eds., Chapman and Hall, 198–212.

    • Search Google Scholar
    • Export Citation
  • Dobrowolski, J. P., , Caldwell M. M. , , and Richards J. H. , 1990: Basin hydrology and plant root systems. Plant Biology of the Basin and Range, C. B. Osmond, L. F. Pitelka, and G. M. Hidy, Eds., Ecological Studies, Vol. 80, Springer-Verlag, 243–292.

    • Search Google Scholar
    • Export Citation
  • Donovan, L., , and Ehleringer J. R. , 1994: Water stress and use of summer precipitation in a Great Basin shrub community. Funct. Ecol., 8 , 289297.

  • Ehleringer, J. R., , Schwinning S. , , and Gebauer R. , 1999: Water-use in arid land ecosystems. Plant Physiological Ecology, M. C. Press, J. D. Scholes, and M. G. Barker, Eds., Blackwell, 347–365.

    • Search Google Scholar
    • Export Citation
  • Emmerich, W. E., 2003: Carbon dioxide fluxes in a semiarid environment with high carbonate soils. Agric. For. Meteor., 116 , 91102.

  • Finnigan, J. J., , and Belcher S. E. , 2004: Flow over a hill covered with a plant canopy. Quart. J. Roy. Meteor. Soc., 130 , 129.

  • Flanagan, L. B., , Ehleringer J. R. , , and Marshall J. D. , 1992: Differential uptake of summer precipitation among co-occurring trees and shrubs in a pinyon-juniper woodland. Plant Cell Environ., 15 , 831836.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Flanagan, L. B., , Wever L. A. , , and Carlson P. J. , 2002: Seasonal and interannual variation in carbon dioxide exchange and carbon balance in a northern temperate grassland. Global Change Biol., 8 , 599615.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Goulden, M. L., , Daube B. C. , , Fan S. M. , , Sutton D. J. , , Bazzaz A. , , Munger J. W. , , and Wofsy S. C. , 1997: Physiological responses of a black spruce forest to weather. J. Geophys. Res., 102 , 2898728996.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hastings, S. J., , Oechel W. C. , , and Muhlia-Melo A. , 2005: Diurnal, seasonal and annual variation in the net ecosystem CO2 exchange of a desert shrub community (Sarcocaulescent) in Baja California, Mexico. Global Change Biol., 11 , 927939.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Huxman, T. E., , Cable J. M. , , Ignace D. D. , , Eilts J. A. , , English N. B. , , Weltzin J. , , and Williams D. G. , 2004a: Response of net ecosystem gas exchange to a simulated precipitation pulse in a semi-arid grassland: The role of native versus non-native grasses and soil texture. Oecologia, 141 , 295305.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Huxman, T. E., and Coauthors, 2004b: Precipitation pulses and carbon fluxes in semiarid and arid ecosystems. Oecologia, 141 , 254268.

  • Ivans, C. Y., , Leffler A. J. , , Spaulding U. , , Stark J. M. , , Ryel R. J. , , and Caldwell M. M. , 2003: Root responses and nitrogen acquisition by Artemisia tridentata and Agropyron desertorum following small summer rainfall events. Oecologia, 134 , 317324.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ivans, S., 2005: Response of water vapor and CO2 fluxes in semi-arid plant communities to variations in precipitation. Ph.D. dissertation, Utah State University, 119 pp.

  • Leffler, A. J., , and Caldwell M. M. , 2005: Shifts in depth of water extraction and photosynthetic capacity inferred from stable isotope proxies across an ecotone of Juniperus osteosperma (Utah juniper) and Artemisia tridentata (big sagebrush). J. Ecol., 93 , 783793.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Leffler, A. J., , Ryel R. J. , , Hipps L. , , Ivans S. , , and Caldwell M. M. , 2002: Carbon acquisition and water use in a northern Utah Juniperus osteosperma (Utah juniper) population. Tree Physiol., 22 , 12211230.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Loik, M., , Breshears D. D. , , Lauenorth W. K. , , and Belnap J. , 2004: A multi-scale perspective of water pulses in dryland ecosystems: Climatology and ecohydrology of the western USA. Oecologia, 141 , 269281.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lowe, T. M., 1999: Soil–vegetation relationships in a Juniper community on an Alluvian fan, western Utah. M.S. thesis, Department of Range Science, Utah State University, 180 pp.

  • Martens, S. N., , Breshears D. D. , , and Meyer C. W. , 2000: Spatial distributions of understory light along the grassland/forest continuum: Effects of cover, height, and spatial patterns of tree canopies. Ecol. Model., 126 , 7993.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Massman, W. J., , and Lee X. , 2002: Eddy covariance flux corrections and uncertainties in long-term studies of carbon and energy exchanges. Agric. For. Meteor., 113 , 121144.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ryel, R. J., , Leffler A. J. , , Peek M. S. , , Ivans C. Y. , , and Caldwell M. M. , 2004: Water conservation in Artemisia tridentata through redistribution of precipitation. Oecologia, 141 , 335345.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Saetre, P., , and Stark J. M. , 2005: Microbial dynamics and carbon and nitrogen cycling following re-wetting of soils beneath two semi-arid plant species. Oecologia, 142 , 247260.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Saigusa, N., , Oikawa T. , , and Liu S. , 1998: Seasonal variations of the exchange of CO2 and H2O between a grassland and the atmosphere: An experimental study. Agric. For. Meteor., 89 , 131139.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schwinning, S., , and Ehleringer J. R. , 2001: Water use trade-offs and optimal adaptations to pulse-driven ecosystems. J. Ecol., 89 , 464480.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Schwinning, S., , and Sala O. E. , 2004: Hierarchy of responses to resource pulses in arid and semi-arid ecosystems. Oecologia, 141 , 211220.

  • Unland, H. E., , Houser P. R. , , Shuttleworth W. J. , , and Yang Z. L. , 1996: Surface flux measurement and modeling at a semi-arid Sonoran Desert site. Agric. For. Meteor., 82 , 119153.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Veenendaal, E. M., , Kolle O. , , and Lloyd J. , 2004: Seasonal variation in energy fluxes and carbon dioxide exchange for a broad-leaved semi-arid savanna (Mopane woodland) in southern Africa. Global Change Biol., 10 , 318328.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Webb, E. K., , Pearman G. I. , , and Leuning R. , 1980: Correction of flux measurements for density effects due to heat and water vapor transfer. Quart. J. Roy. Meteor. Soc., 106 , 85100.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wever, L. A., , Flanagan L. B. , , and Carlson P. J. , 2002: Seasonal and interannual variation in evapotranspiration, energy balance and surface conductance in a northern temperate grassland. Agric. For. Meteor., 112 , 3149.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Whitford, W. G., 2002: Ecology of Desert Systems. Academic Press, 327 pp.

  • Wilczak, J. M., , Oncley S. P. , , and Stage S. A. , 2001: Sonic anemometer tilt correction algorithms. Bound.-Layer Meteor., 99 , 127150.

  • Wilson, K. B., , and Baldocchi D. D. , 2000: Seasonal and interannual variability of energy fluxes over a broadleaved temperate deciduous forest in North America. Agric. For. Meteor., 100 , 118.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yan, S., , Wan C. , , Sosebee R. E. , , Wester D. B. , , Fish E. B. , , and Zartman R. E. , 2000: Responses of photosynthesis and water relations to rainfall in the desert shrub creosote bush (Larrea tridentata) as influenced by municipal biosolids. J. Arid Environ., 46 , 397412.

    • Crossref
    • Search Google Scholar
    • Export Citation
Fig. 1.
Fig. 1.

Friction velocity (u*) vs CO2 flux at the sagebrush site.

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Fig. 2.
Fig. 2.

Seasonal changes in soil moisture at the 0–8-cm depth for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage) during 2002 and 2003.

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Fig. 3.
Fig. 3.

Seasonal change in ET flux at the crested wheatgrass (CWG), juniper (jun), and sagebrush (sage) sites during 2002 and 2003.

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Fig. 4.
Fig. 4.

Seasonal change in daytime daily totals of CO2 flux for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage) during 2002 and 2003.

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Fig. 5.
Fig. 5.

(top) Air temperature (Tair) and saturation vapor pressure deficit (SVPD), (middle) net radiation (Rn), and (bottom) precipitation later in the spring of 2002 at the sagebrush site.

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Fig. 6.
Fig. 6.

(top) ET and (middle) CO2 flux responses to (bottom) rain later in the spring of 2002 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Fig. 7.
Fig. 7.

(top) ET and (middle) CO2 flux responses to (bottom) rain events in late summer of 2002 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Fig. 8.
Fig. 8.

(top) ET and (middle) CO2 responses to (bottom) rain events in early summer of 2003 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Fig. 9.
Fig. 9.

(top) ET and (middle) CO2 responses to (bottom) rain events in midsummer 2003 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Fig. 10.
Fig. 10.

(top) ET and (middle) CO2 responses to (bottom) rain events in the fall months of 2002 for crested wheatgrass (CWG), juniper (jun), and sagebrush (sage).

Citation: Journal of Hydrometeorology 7, 5; 10.1175/JHM545.1

Table 1.

Seasonal distribution of precipitation with 30-yr normal values in Rush Valley, Utah.

Table 1.
Table 2.

Daytime energy balance closure at all three sites for both years during spring months (Apr–Jun), summer months (Jul–Sep), and fall months (Oct–Dec).

Table 2.
Save