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Science Results (Summer 2008)
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Soil-dependent canopy-die back and plant mortality in two Mojave Desert shrubs


Hamerlynck, E.P.                 SouthwestWatershedResearchCenter

McAuliffe, J.R.                      DesertBotanical Garden


The desert regions of the Southwestern US have recently experienced unprecedented levels of drought, which has resulted in widespread, though patchy, mortality of adult plants.  This study showed how soil characteristics that affect seasonal soil hydrology affect the degree of branch and whole-plant mortality in the two dominant shrubs of the Mojave Desert, the drought-deciduous shrub white bursage (Ambrosia dumosa) and the evergreen creosotebush (Larrea tridentata).  Overall levels of branch mortality ("canopy die-back") and whole-plant mortality were greater in white bursage than in creosotebush.  Dead plants of both species tended to be smaller than surviving plants, especially in channel and hillslope locations, which indicate that juvenile plants were particularly vulnerable to drought in locations that receive additional water from runoff under normal climate conditions.  Canopy-die back was very pronounced in larger plants growing in young, weakly-developed soils, while smaller plants growing in older, well-developed soils showed much less branch and plant mortality.  We attributed these differences to the distribution of large rocks through the soil profile, which were more common in older soils.  Sub-surface rocks might create large numbers of favorable soil microsites, resulting in more water available throughout the soil, which would buffer against drought effects and also reduce the intensity of between- and within-species competition for water.



Ecophysiology of two SonoranDesert evergreen shrubs during extreme drought


Hamerlynck. E.P.                     SouthwestWatershedResearchCenter

Huxman, T.E.                           University of Arizona


This study compares how two evergreen SonoranDesert shrubs -- the broadleaved jojoba (Simmondisa chinensis) and creosotebush (Larrea tridentata), which has very small leaves -- cope with excessive light levels under conditions of unprecedented summer drought.  Even after near-normal July precipitation levels, the two plants had extremely low predawn water potentials, indicating that they were still experiencing severe water stress after experiencing over 11 months without measurable rainfall.  Chlorophyll fluorescence analysis showed that active creosote bush was better able to cope with high light levels than jojoba, and that the outer leaves of jojoba may act as a "sunscreen", allowing inner-canopy leaves to maintain physiological activity under prolonged drought conditions.  These findings show that leaf-level physiological differences accompanied differences in canopy structure in these long-lived desert shrubs.



Foliar D15N of Mojave Desert shrubs: relationship to soil hydrological characteristics


Hamerlynck, E.P.         SouthwestWatershedResearchCenter

McAuliffe, J.R.             DesertBotanical Garden



Desert shrubs are generally thought to act as "islands of fertility", serving as focal points for the accumulation of plant litter, dry deposition, and animal activity.  However, it is not clear how soil physical features that strongly determine soil hydrology affect the ability of long-lived desert shrubs to act as fertile islands.  In this study, we showed that younger soils with little or no soil horizon development supported dramatically larger creosotebush (Larrea tridentata) and white bursage (Ambrosia dumosa) plants, and had leaf stable nitrogen isotope ratios (?15N) highly enriched in the heavier 15N isotope,  suggesting these large plants utilized soil N from deeper in the soil profile, where it had likely undergone repeated microbial transformation, and were better traps for dry deposited nitrogen sources compared to plants growing in soils with strong surface and below-surface horizons that limited infiltration and percolation deeper into the soil profile.  Overall, this study shows the over-riding importance of soil hydrology - which is strongly determined by geological processes - has on plant ecological processes in desert systems.



Archiving and Distributing Three Long-Term Interconnected Geospatial Data Sets


Moran, M.S.                SouthwestWatershedResearchCenter

Hutchinson, B.             University of Arizona

Marsh, S.                     University of Arizona

McClaran, M.              University of Arizona

Olsson, A.                    University of Arizona


Scientists often come together for intensive field experiments to use satellite and aircraft imagery for studies of hydrological and ecological processes.  The result is a valuable data set of images and on-site measurements that extend over decades covering the experimental site.  There is a movement by government and academic institutions to preserve these long-term data for the benefit of both local scientists and the larger international body of students and researchers.  This report presents three case studies describing unit efforts to archive and distribute data over a multi-decadal period for a location in southeastern Arizona.  The challenges included interconnecting the data distribution systems and sustaining the archiving and distribution process.  The case studies present multiple approaches to meeting these challenges within the constraints of government and academic units.  Each of the example cases had impact in the form of publications, budget savings, outside recognition, student research, registered users, data downloads and new research.  Results should guide other local efforts to archive and preserve long-term scientific data and distribute them online.



A new Splash and Sheet Erosion Equation for Rangelands    


Wei, H.                        University of Arizona

Nearing, M.A.              SouthwestWatershedResearchCenter

Stone, J.J.                    SouthwestWatershedResearchCenter

Guertin, D.P.                University of Arizona

Spaeth, K.                    University of Arizona

Pierson, F.B.                USDA-ARS Boise, ID

Nichols, M.H.              SouthwestWatershedResearchCenter

Moffet, C.A.                USDA-ARS Dubois, ID


Erosion rates predicted from models for rangelands have the potential to be important quantitative indicators for rangeland health and for assessing conservation practice effects by government agencies, rangeland managers, conservationists and rangeland scientists. Process-based erosion models are preferred to empirical models because of their ability to capture the physical erosion processes in the field. However, the equations of existing process-based erosion models were all developed from experiments carried out in cropland fields, which make them unsuitable for rangeland applications because of differences in soils and plants species.   Rangelands also have relatively greater spatial heterogeneity than croplands, which make modeling the processes a bit more problematic. The splash and sheet erosion equation is one of the core relationships needed for rangeland soils. It is very important because splash and sheet erosion is the dominant soil loss process on most rangelands with good vegetation cover. In this paper we developed a new splash and sheet erosion equation based on an extensive rangeland database from rainfall simulator experiments, which covers a wide range of rangeland soils and plant systems from across the western United States. Our proposed equation quantifies the splash and sheet erosion rate as a combined function of rainfall intensity and runoff rate, and is the first one that has been developed from rangeland experiments.  The new equation should enable improved estimation of water erosion on rangelands in the western U.S. and perhaps on other rangelands of the world.



Assessing the benefits of conservation practices in western rangelands of the United States: the CEAP rangelands project


Nearing, Mark A.                     SouthwestWatershedResearchCenter


The Federal government of the United States expended in the order of $20 billion primarily in direct payments to Farmers and Ranchers between 2002 and 2007 for the designated purpose of implementing farm and ranch conservation practices. These payments came under a number of programs, and were authorized by the Farm Security and Rural Investment Act (2002), also known as the 2002 Farm Bill, and the amount allocated represents a substantial increase in funding for conservation programs in the United States.  This paper summarized USDA's efforts to assess the impacts of the application of conservation programs on soil and water conservation in western US rangelands.



On the soil roughness parameterization problem in soil moisture retrieval of bare surfaces from Synthetic Aperture Radar


Verhoest Niko, E.C.                 Ghent UniversityBelgium

Lievens, H.                               Ghent UniversityBelgium

Wagner, W.                             ViennaUniversity of Technology

?lvarez-Mozos, J.                    Public University of Navarre, Los Tejos Pamplona, Spain

Moran, M.S.                            SouthwestWatershedResearchCenter

Mattia, F.                                Consiglo Nazionale delle Ricerche (CNR)


Regional maps of soil moisture are critical for predicting floods, managing high tides, and scheduling crop irrigations. It is possible to map soil moisture over large areas of the Earth using orbiting satellite-based sensors.  This is complicated by differences in surface roughness that affect the radar signal and interfere with the ability to accurately determine soil moisture.  This review (citing 200 previous studies) summarizes the possibilities for mapping soil roughness and for minimizing the error in soil moisture mapping.  Suggestions were made to use multiple radar images or different radar configurations over a single site to normalize the radar signal for roughness and improve the accuracy of soil moisture estimates.  This review describes the state-of-the-science in soil moisture mapping and provides a direction forward to overcome limitations and improve map products. 


Estimation of Net Ecosystem Carbon Exchange for the Conterminous United States by Combining MODIS and AmeriFlux Data


Xiao, J.                        PurdueUniversity

Zhuang, Q.                   PurdueUniversity

Baldocchi, D.               University of CaliforniaBerkeley

Law, B.                        OregonStateUniversity

Richardson, A.             University of New Hampshire

Chen, J.                        University of Toledo

Starr, G.                       University of Alabama

Noormets, A.               North CarolinaStateUniversity

Ma, S.                         University of CaliforniaBerkeley

Verma, S.                     University of Nebraska

Wharton, S.                  University of CaliforniaDavis

Wofsy, S.                     HarvardUniversity

Bolstad, P.                   University of Minnesota

Burns, S.                      University of Colorado

Cook, D.                      Argonne National Lab

Curtis, P.                      OhioStateUniversity

Drake, B.                     Smithsonian Environmental ResearchCenter

Falk, M.                       University of CaliforniaDavis

Fischer, M.                   LawrenceBerkeley National Lab

Foster, D.                     HarvardUniversity

Gu, L.                          Oak Ridge National Lab

Hadley, J.                     HarvardUniversity

Hollinger, D.                 USDAForest Service

Katul, G.                      DukeUniversity

Litvak, M.                    University of New Mexico

Martin, T.                     University of Florida

Matamala, R.                Argonne National Lab

McNulty, S.                 USDAForest Service

Meyers, T.                   NOAA/ARL

Monson, R.                  University of Colorado

Munger, J.                    HarvardUniversity

Oechel, W.                   San DiegoStateUniversity

Paw U, K.                    University of CaliforniaDavis

Schmid, H.                   IndianaUniversity

Scott, R.L.                   SouthwestWatershedResearchCenter

Sun, G.                        USDAForest Service

Suyker, A.                    University of Nebraska

Torn, M.                      LawrenceBerkeley National Lab


In the face of climate change, in part caused by increases in atmospheric carbon dioxide due to human activities, it is important to properly account for amount of carbon dioxide that is released or taken in by Earth's biosphere.  This study used a combination of satellite measurements and measurements of carbon dioxide exchange over a large diversity of ecosystems to develop a relationship that was used to estimate the exchange of carbon dioxide between the atmosphere and land surface for every eight days in 2005 over the conterminous United States. The relationship between the satellite data and the carbon dioxide measurements produced fairly accurate results when compared with measurements and captured the expected patterns of the exchange both in time and space over the U.S. This study demonstrates that this approach can produce results that are accurate enough to be used to test new predictive tools that are being developed to quantify the Earth's terrestrial carbon budgets.



The effect of drought on the water and carbon dioxide exchange of a woody-plant-encroached semiarid grassland


Scott, R.L.                   SouthwestWatershedResearchCenter

Jenerette, G.                 University of CaliforniaRiverside

Potts, D.                       BuffaloState College

Huxman, T.                  University of Arizona


The increase in the amount of woody plants (shrubs and trees) into grassland ecosystems is one of the most pervasive changes in land cover in the southwestern US and around the world. It is important to understand how this change in vegetation will affect water and nutrient cycling of ecosystems in order to predict the outcomes of this change on society. We measured energy, water and carbon dioxide exchange between a woody-plant-encroached grassland and the atmosphere over a four-year period and determined how the amount of precipitation controlled these exchanges. In contrast to the current paradigm that woody plant encroachment might result in more ecosystem carbon sequestration and to the many recent results showing that various semiarid ecosystems were a sink for carbon dioxide, we found that this ecosystem was a source, which was likely a consequence of the decade-long drought that was on-going over the study period. These results highlight a complex relationship between vegetation change and climatic variation in precipitation that likely influences the carbon sequestration potential of water-limited landscapes.



A Continuous Measure of Gross Primary Productivity for the Conterminous U.S. Derived from MODIS and AmeriFlux Data


Xiao, J.                                    PurdueUniversity

Zhuang, Q.                               PurdueUniversity

Law, B.                                    OregonStateUniversity

Chen, J.                                    University of Toledo

Baldocchi, D.                           University of CaliforniaBerkeley

Cook, D.                                  Argonne National Lab

Oren, R.                                   DukeUniversity

Richardson, A. D.                     University of New Hampshire

Wharton, S.                              University of CaliforniaDavis

Ma, S.                                      University of CaliforniaBerkeley

Martin, T.                                 University of Florida

Verma, S.                                 University of Nebraska

Suyker, A.                                University of Nebraska

Scott, R.L.                               SouthwestWatershedResearchCenter

Monson, R.                              University of Colorado

Litvak, M.                                University of New Mexico

Hollinger, D.                             USDAForest Service

Sun, G.                                    USDAForest Service

Bolstad, P.                               University of Minnesota

Burns, S.                                  University of Colorado

Curtis, P.                                  OhioStateUniversity

Drake, B.                                 Smithsonian  Environmental Research Lab

Falk, M.                                   University of CaliforniaDavis

Fischer, M.                               LawrenceBerkeley National Lab

Foster, D.                                 Harvard                                   

Gu, L.                                      Oak Ridge National Lab

Hadley, J.                                 Harvard

Katul, G.                                  DukeUniversity

Matamala, R.                            Argonne National Lab

McNulty, S.                             USDAForest Service

Meyers, T.                               NOAA/ARL

Munger, J.W.,                          Harvard

Noormets, A.                           North CarolinaStateUniversity

Oechel, W.C.                           San DiegoStateUniversity

Paw U, K.T.                            University of CaliforniaDavis

Schmid, H.P.                            IndianaUniversity

Starr, G.                                   University of Alabama

Torn, M.S.                               LawrenceBerkeley National Lab

Wofsy, S.C.                            Harvard


The quantification of carbon fluxes between the terrestrial biosphere and the atmosphere is of scientific importance and also relevant to climate-policy making. This study used a combination of satellite measurements and measurements of carbon dioxide exchange over a large diversity of ecosystems to develop a relationship that was used to estimate the uptake of carbon dioxide by terrestrial ecosystems for every eight days in 2005 and 2006 over the conterminous United States. The relationship between the satellite data and the carbon dioxide measurements produced fairly accurate results when compared with measurements and captured the expected patterns of the exchange both in time and space. The total carbon dioxide uptake was estimated to be between 6.91 and 7.33 Pg C yr-1 for the conterminous U.S. over the period 2001-2006. Extreme climate events (e.g., drought) and disturbances (e.g., wildfires and hurricanes) had substantial impacts on carbon uptake. These continuous estimates provide an independent dataset that is useful for examining the spatiotemporal patterns of carbon uptake and the impacts of climate variability, disturbances, and management practices on ecosystem carbon fluxes.