|Science Results (Summer 2008)|
Soil-dependent canopy-die back and plant mortality in two
The desert regions of the
This study compares how two evergreen
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.
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
Pierson, F.B. USDA-ARS
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
The Federal government of the
Verhoest Niko, E.C.
Álvarez-Mozos, J. Public
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.
Drake, B. Smithsonian Environmental
Meyers, T. NOAA/ARL
Paw U, K.
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
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.
Richardson, A. D.
Drake, B. Smithsonian Environmental Research Lab
Foster, D. Harvard
Hadley, J. Harvard
Meyers, T. NOAA/ARL
Munger, J.W., Harvard
Paw U, K.T.
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