[Show abstract][Hide abstract] ABSTRACT: Summary1. The occurrence and intensity of climate extremes, such as extremely warm years, are expected to continue to increase with increasing tropospheric radiative forcing caused by anthropogenic greenhouse gas emissions.2. Responses of terrestrial ecosystem processes and services – such as above-ground net primary productivity (ANPP) and maintenance of plant species diversity – to these extreme years for multiple years post-perturbation are poorly understood but can have significant feedback effects on net ecosystem CO2 uptake and ecosystem carbon sequestration.3. We exposed six 12 000-kg intact natural tallgrass prairie monoliths to an extremely warm year (+4 °C in 2003) in the second year of a 4-year study (2002–2005) using the EcoCELL whole-ecosystem controlled-environment, gas exchange facility. Six control monoliths were not warmed in the second year but were maintained under average field conditions. Natural diel and seasonal patterns in air temperature were maintained in both treatments throughout the study. Thus, with the exception of the second year in the ‘warmed’ treatment, we created 4 years of nearly identical climate in all EcoCELLs.4. Interannual ANPP (10 cm clipping height) responses of the entire plant community to the extreme year were largely determined by responses of the dominant C4 grasses. These included large decreases in ANPP in 2003 followed by complete recovery to levels observed in the control ecosystems in the year following warming. Species richness and productivity of the nitrogen-fixing plant functional group appeared to play a role in defining overall plant community ANPP, however, even though this richness and productivity could not explain the decrease in community ANPP observed in warmed ecosystems in the second year (2003) of the study or its recovery in the year after (2004). Surprisingly, very few of the 67 species present in plant communities during the 4-year study responded to the warm year at any time during or after the treatment.5. Synthesis. Results from this study indicate that as extreme climate years become more prevalent, their immediate and lagged impacts on collective ecosystem processes, such as whole-community ANPP, may be very pronounced, but effects on component ecosystem processes may be limited to the dominant plant functional group (ANPP).
[Show abstract][Hide abstract] ABSTRACT: Vegetation changes associated with climate shifts and anthropogenic disturbance can have major impacts on biogeochemical cycling and soils. Much of the Great Basin, U.S. is currently dominated by sagebrush (Artemisia tridentate (Rydb.) Boivin) ecosystems. Sagebrush ecosystems are increasingly influenced by pinyon (Pinus monophylla Torr. & Frém and Pinus edulis Engelm.) and juniper (Juniperus osteosperma Torr. and Juniperus occidentalis Hook.) expansion. Some scientists and policy makers believe that increasing woodland cover in the intermountain western U.S. offers the possibility of increased organic carbon (OC) storage on the landscape; however, little is currently known about the distribution of OC on these landscapes, or the role that nitrogen (N) plays in OC retention. We quantified the relationship between tree cover, belowground OC, and total below ground N in expansion woodlands at 13 sites in Utah, Oregon, Idaho, California, and Nevada, USA. One hundred and twenty nine soil cores were taken using a mechanically driven diamond tipped core drill to a depth of 90cm. Soil, coarse fragments, and coarse roots were analyzed for OC and total N. Woodland expansion influenced the vertical distribution of root OC by increasing 15–30cm root OC by 2.6Mgha−1 and root N by 0.04Mgha−1. Root OC and N increased through the entire profile by 3.8 and 0.06Mgha−1 respectively. Woodland expansion influenced the vertical distribution of soil OC by increasing surface soil (0–15cm) OC by 2.2Mgha−1. Woodland expansion also caused a 1.3Mgha−1 decrease in coarse fragment associated OC from 75–90cm. Our data suggests that woodland expansion into sagebrush ecosystems has limited potential to store additional belowground OC, and must be weighed against the risk of increased wildfire and exotic grass invasion.
Journal of Arid Environments - J ARID ENVIRON. 01/2011; 75(9):827-835.
[Show abstract][Hide abstract] ABSTRACT: Vegetation changes associated with climate shifts and anthropogenic disturbance have major impacts on biogeochemical cycling. Much of the interior western United States currently is dominated by sagebrush (Artemisia tridentata Nutt.) ecosystems. At low to intermediate elevations, sagebrush ecosystems increasingly are influenced by cheatgrass (Bromus tectorum L.) invasion. Little currently is known about the distribution of belowground organic carbon (OC) on these changing landscapes, how annual grass invasion affects OC pools, or the role that nitrogen (N) plays in carbon (C) retention. As part of a Joint Fire Sciences-funded project called the Sagebrush Treatment Evaluation Project (SagesSTEP), we quantified the depth distribution of soil OC and N at seven sites experiencing cheatgrass invasion. We sampled plots that retained sagebrush, but represented a continuum of cheatgrass invasion into the understory. Eighty-four soil cores were taken using a mechanically driven diamond-tipped core drill to a depth of 90 cm, or until bedrock or a restrictive layer was encountered. Samples were taken in 15-cm increments, and soil, rocks, and roots were analyzed for OC and total N. We determined that cheatgrass influences the vertical distribution of OC and N within the soil profile and might result in decreased soil OC content below 60 cm. We also found that OC and total N associated with coarse fragments accounted for at least 10% of belowground pools. This emphasizes the need for researchers to quantify nutrients in deep soil horizons and coarse fragments. Los cambios en la vegetación asociados con cambios en el clima y disturbios antropogénicos tienen efectos superlativos sobre los ciclos biogeoquimicos. Gran parte del interior del oeste de los Estados Unidos está dominado por ecosistemas de Artemisia tridentata Nutt. En sitios de altitud baja e intermedia dichos ecosistemas están siendo influenciados de modo creciente por la invasión de Bromus tectorum L. Se sabe poco hoy en día sobre la distribución de carbono orgánico (CO) en el suelo en estos paisajes en estado de transición, sobre cómo la invasión de este pasto anual afecta los reservorios de CO, o el rol que juega el nitrógeno (N) en la retención de carbono (C). En el marco del Proyecto de Tratamiento y Evaluación de Artemisia (SageSTEP) financiado por el programa del Joint Fire Sciences, se cuantificó la profundidad y distribución de CO en el suelo en siete sitios que están siendo invadidos por B. tectorum. Se muestrearon parcelas que retuvieron Artemisia pero que representan un continuo de situaciones de invasión de B. tectorum del estrato herbáceo. Se tomaron 84 muestras de suelo hasta una profundidad de 90 cm o hasta encontrar la roca madre o alguna capa restrictiva utilizando un taladro barrenador con punta de diamante. Las muestras se tomaron en incrementos de 15 cm y se determinó el CO y N total del suelo, las rocas, y las raíces. Se determinó que B. tectorum afecta la distribución vertical de CO y N dentro del perfil edáfico pudiendo resultar en una disminución del CO del suelo a profundidades mayores a los 60 cm. También se encontró que el CO y el N total asociados a fragmentos de mayor tamaño constituyeron por lo menos un 10% de los reservorios totales del suelo. Estos resultados enfatizan la necesidad de que los investigadores cuantifiquen nutrientes en horizontes profundos y fragmentos de mayor tamaño.
[Show abstract][Hide abstract] ABSTRACT: Vegetation changes associated with climate shifts and anthropogenic disturbance can have major impacts on biogeochemical cycling and soils. Much of the Great Basin, U.S. is currently dominated by sagebrush (Artemisia tridentate ssp. (Rydb.) Boivin) ecosystems. At intermediate elevations, sagebrush ecosystems are increasingly influenced by pinyon (Pinus monophylla Torr. & Frém and Pinus edulis Engelm.) and juniper (Juniperus osteosperma Torr. and Juniperus occidentalis Hook.) expansion. Some scientists and policy makers believe that increasing woodland cover in the intermountain western U.S. offers the possibility of increased carbon (C) storage on the landscape; however, little is currently known about the distribution of C on these landscapes, or the role that nitrogen (N) plays in carbon retention. As part of a Joint Fire Sciences funded project called the Sagebrush Treatment Evaluation Project (SageSTEP), we quantified the spatial distribution of soil C and N in expansion woodlands at 12 sites in Utah, Oregon, Idaho, California, and Nevada. The sites span a geographic range of more than 800 km, and represent conditions that vary considerably in elevation, topography, soils, and climate. Soils were derived from carbonate and igneous rocks. Each site contained at least three core plots, and within each core plot we sampled three sub-plots which represent a different phase of woodland expansion into sagebrush systems. Phase I (shrub-dominated stands), Phase II (shrubs and trees share dominance), and Phase III (tree-dominated stands). Soil cores were taken using a mechanically driven diamond tipped core drill to a depth of 90 cm, or until bedrock or a restrictive layer was encountered. Samples were taken in 15 cm increments, dried, sieved to 2 mm, and roots were separated from rock by flotation. Soil, rocks, and roots were analyzed for total C and N, and data was expressed on a mass per unit area basis. Carbonate-derived soils contained more inorganic and organic C than igneous-derived soils. Phase of woodland expansion had significant affects on the total mass of root C and N, and influenced the vertical distribution of soil organic C. Multiple linear regressions suggests that tree cover can help explain some of the variance associated with the mass of belowground organic C after considering the mass of belowground N, parent material, and depth to bedrock. Our data suggests that woodland expansion into sagebrush ecosystems has limited potential to store significant amounts of belowground C, and must be weighed against the risk of increased wildfire and exotic grass invasion.
[Show abstract][Hide abstract] ABSTRACT: Quantifying root biomass is critical to an estimation and understanding of ecosystem net primary production, biomass partitioning, and belowground competition. We compared 2 methods for determining root biomass: a new soil-coring technique and traditional excavation of quantitative pits. We conducted the study in an existing Joint Fire Sciences demonstration area in the central Great Basin. This area is representative of a shrub (sagebrush) ecosystem exhibiting tree (pinyon and juniper) encroachment. The demonstration area had a prescribed burn implemented 4 years prior to our study, and we sampled both control and burned plots. The samples were stratified across 3 microsites (interspace, under shrub, and under tree) and 4 soil depths (0-8, 8-23, 23-38, and 38-52 cm) to determine the effects of plant life form and burning on root biomass. We found that estimates of total root biomass were similar between quantitative pits and our soil cores. However, cores tended to show a more even distribution of root biomass across all microsites and depths than did pits. Overall, results indicated that root biomass differs significantly among microsites and soil depths and that the amount of root biomass at a given depth differs among microsites. Burning reduced root biomass in our study by 23% and altered the spatial distribution of root mass.
[Show abstract][Hide abstract] ABSTRACT: Terrestrial ecosystems control carbon dioxide fluxes to and from the atmosphere through photosynthesis and respiration, a balance between net primary productivity and heterotrophic respiration, that determines whether an ecosystem is sequestering carbon or releasing it to the atmosphere. Global and site-specific data sets have demonstrated that climate and climate variability influence biogeochemical processes that determine net ecosystem carbon dioxide exchange (NEE) at multiple timescales. Experimental data necessary to quantify impacts of a single climate variable, such as temperature anomalies, on NEE and carbon sequestration of ecosystems at interannual timescales have been lacking. This derives from an inability of field studies to avoid the confounding effects of natural intra-annual and interannual variability in temperature and precipitation. Here we present results from a four-year study using replicate 12,000-kg intact tallgrass prairie monoliths located in four 184-m(3) enclosed lysimeters. We exposed 6 of 12 monoliths to an anomalously warm year in the second year of the study and continuously quantified rates of ecosystem processes, including NEE. We find that warming decreases NEE in both the extreme year and the following year by inducing drought that suppresses net primary productivity in the extreme year and by stimulating heterotrophic respiration of soil biota in the subsequent year. Our data indicate that two years are required for NEE in the previously warmed experimental ecosystems to recover to levels measured in the control ecosystems. This time lag caused net ecosystem carbon sequestration in previously warmed ecosystems to be decreased threefold over the study period, compared with control ecosystems. Our findings suggest that more frequent anomalously warm years, a possible consequence of increasing anthropogenic carbon dioxide levels, may lead to a sustained decrease in carbon dioxide uptake by terrestrial ecosystems.