Comparing the Performance of Forest gap Models in North America

Forest gap models have a long history in the study of forest dynamics, including predicting long-term succession patterns and assessing the potential impacts of climate change and air pollution on forest structure and composition. In most applications, existing models are adapted for the specific question at hand and little effort is devoted to evaluating alternative formulations for key processes, although this has the potential to significantly influence model behavior. In the present study, we explore the implications of alternative formulations for selected ecological processes via the comparison of several gap models. Baseline predictions of forest biomass, composition and size structure generated by several gap models are compared to each other and to measured data at boreal and temperate sites in North America. The models ForClim and LINKAGES v2.0 were compared based on simulations of a temperate forest site in Tennessee, whereas FORSKA-2V, BOREALIS and ForClim were compared at four boreal forest sites in central and eastern Canada. Results for present-day conditions were evaluated on their success in predicting forest cover, species composition, total biomass and stand density, and allocation of biomass among species. In addition, the sensitivity of each model to climatic changes was investigated using a suite of six climate change scenarios involving temperature and precipitation. In the temperate forest simulations, both ForClim and LINKAGES v2.0 predicted mixed mesophytic forests dominated by oak species, which is expected for this region of Tennessee. The models differed in their predictions of species composition as well as with respect to the simulated rates of succession. Simulated forest dynamics under the changed climates were qualitatively similar between the two models, although aboveground biomass and species composition in ForClim was more sensitive to drought than in LINKAGES v2.0. Under a warmer climate, the modeled effects of temperature on tree growth in LINKAGES v2.0 led to the unrealistic loss of several key species. In the boreal forest simulations, ForClim predicted significant forest growth at only the most mesic site, and failed to predict a realistic species composition. In contrast, FORSKA-2V and BOREALIS were successful in simulating forest cover, general species composition, and biomass at most sites. In the climate change scenarios, ForClim was highly sensitive, whereas the other two models exhibited sensitivity only at the drier central Canadian sites. Although the studied sites differ strongly with respect to both the climatic regime and the set of dominating species, a unifying feature emerged from these simulation exercises. The major differences in model behavior were brought about by differences in the internal representations of the seasonal water balance, and they point to an important limitation in some gap model formulations for assessing climate change impacts.     

Instantaneous global ozone balance including observed nitrogen dioxide

The catalytic destruction of stratospheric ozone by the oxides of nitrogen is believed to be an important part of the global ozone balance. The lack of sufficient measurements of NO _x concentrations has impeded efforts to quantify this process. Recent measurements of stratospheric nitrogen dioxide from ground-based stations as well as aircraft and balloons have provided a first approximation to a global distribution of NO₂ vertical columns at sunset. These observed vertical columns have been translated into time-dependent vertical NO₂ profiles by means of a one-dimensional atmospheric photochemical model. Using recent observations of air temperature and ozone along with this information, the independent instantaneous (one second) rates of ozone production from oxygen photolysis P(O₃), of ozone destruction from pure oxygen species (Chapman reactions) L(O _x ), and of ozone destruction by nitrogen oxides L(NO _x ) were estimated over the three-dimensional atmosphere. These quantities are displayed as zonal average contour maps, summed over various latitude zones, summed over various altitude bands, and integrated globally between 15 and 45 km. Although the global summation between 15 and 45 km by no means tells the complete story, these numbers are of some interest, and the relative values are: P(O₃), 100; L(O _x ), 15; L(NO _x ), 45±15. It is to be emphasized that this relative NO _x contribution to the integrated ozone balance is not a measure of the sensitivity of ozone to possible perturbations of stratospheric NO _x ; recent model results must be examined for current estimates of this sensitivity.     

New concepts in ecological risk assessment: where do we go from here?

Through the use of safety factors, the use of single-species test data has been adequate for use in protective hazard assessments and criteria setting but, because hazard quotients do not consider the presence of multiple species each with a particular sensitivity or the interactions that can occur between these species in a functioning community, they are ill-suited to environmental risk assessment. Significant functional redundancy occurs in most ecosystems but this is poorly considered in single-species tests conducted under laboratory conditions. A significant advance in effects assessment was the use of the microcosm as a unit within which to test interacting populations of organisms. The microcosm has allowed the measurement of the environmental effect measures such as the NOAEC(community) under laboratory or field conditions and the application of this and other similarly derived measures to ecological risk assessment (ERA). More recently, distributions of single-species laboratory test data have been used for criteria setting and, combined with distributions of exposure concentrations, for risk assessment. Distributions of species sensitivity values have been used in an a priori way for setting environmental quality criteria such as the final acute value (FAV) derived for water quality criteria. Similar distributional approaches have been combined with modeled or measured concentrations to produce estimates of the joint probability of a single species being affected or that a proportion of organisms in a community will be impacted in a posteriori risk assessments. These techniques have not been widely applied for risk assessment of dredged materials, however, with appropriate consideration of bioavailability and spatial and nature of the data these techniques can be applied to soils and sediments.     

Sources of radiogenic helium in a clay till aquitard and its use to evaluate the timing of geologic events

Concentrations of dissolved gases (He, Ne, Ar and N₂) were measured in pore waters collected from a Pleistocene (0-80 m depth) and Cretaceous (80-156 m depth) aged, clay-rich, aquitard system using both a conventional copper-tube method and in situ diffusion samplers. Both methods yielded similar results. He data revealed a well-defined increase in concentration with depth, from 1.7 × 10⁻⁸ cm³ He STP g⁻¹ water near the water table (at 2.3 m below ground surface) to 6.9 × 10⁻⁶ cm³ He STP g⁻¹ water at ∼80 m below ground surface (B.G.) The concentration at depth greatly exceeded values for atmospheric solubility at in situ temperatures and could not be attributed to steady-state production from α-decay of U and Th within the aquitard system. Trends in He isotope ratios and concentrations suggest accumulation as a result of both release of residual ⁴He from within the Pleistocene clay-rich till (ca. 87%) and an ascending radiogenic helium flux from underlying Cretaceous-age sediments (ca. 13%). One-dimensional transport modeling of the He data suggests that between 15 ka and 25 ka were required to produce the observed profile. This time frame agrees with studies utilizing isotope tracers (i.e., D/H and ¹⁴C-DIC) to estimate the timing of deposition for the till at this site. Our results demonstrate the utility of using helium, a nonreactive tracer, for delineating hydrogeologic processes in slow permeability geologic strata, and also support the contention that thick clay-rich surficial aquitards can provide long-term containment of contaminants, including hazardous wastes.     

Secular cooling of the Earth as a source of intraplate stress

As a result of secular cooling and contraction of the Earth, an increasingly extensional horizontal stress is imposed on the central portions of the plates. While the rate of increase of this stress is small in most situations, it is enhanced for large plates and during any episodes of accelerated global cooling. This source of stress may have contributed to the rifting and breakup of long-lived supercontinental plates.     

Global CO2 emissions trading: Early lessons from the U.S. acid rain program

The U.S. Environmental Protection Agency is implementing a program of SO₂ emission allowance trading as part of the Acid Rain Program authorized by the Clean Air Act Amendments of 1990. Electric utilities may use allowance trading as part of their compliance strategy to meet SO₂ emission reduction requirements, which begin in 1995. In the interest of a free market in emission credits, some utilities began trading in 1992. A strict but essential requirement for continuous-emissions monitoring was developed to support the trading program. This program is being widely watched and will be evaluated as part of an effort to determine if market concepts can be successfully extended to other environmental issues. One such issue is greenhouse gas emissions and their link with global warming and climate change. This paper focuses on the early lessons learned, issues, and challenges involved in going from a domestic electric utility SO₂ emissions trading program to inter-industry, inter-gas and international as well as national emissions trading and offsets programs. Prominent among these issues are CO₂ allowance allocations, equity, emissions monitoring, enforcement, and cost-effectiveness.Paper presented at the90th Annual Meeting of the Association of American Geographers, San Francisco, April 2, 1994, at the Air & Waste Management Association International Specialty Conference onGlobal Climate Change, Tempe, Arizona, April 8, 1994, and at a Cornell University Center for the Environment seminar on global climate change, April 25, 1994. I thank Michael Grubb, Joe Kruger, Elliot Lieberman, Brian McLean, Renee Rico, Richard Schuler and two anonymous referees for helpful comments on previous drafts of this paper. The views and opinions expressed herein are those of the author alone, and do not necessarily represent the policies of the U.S. Environmental Protection Agency.     

Groundwater study using remote sensing and geographic information systems (GIS) in the central highlands of Eritrea

Remote sensing, evaluation of digital elevation models (DEM), geographic information systems (GIS) and fieldwork techniques were combined to study the groundwater conditions in Eritrea. Remote sensing data were interpreted to produce lithological and lineament maps. DEM was used for lineament and geomorphologic mapping. Field studies permitted the study of structures and correlated them with lineament interpretations. Hydrogeological setting of springs and wells were investigated in the field, from well logs and pumping test data. All thematic layers were integrated and analysed in a GIS. Results show that groundwater occurrence is controlled by lithology, structures and landforms. Highest yields occur in basaltic rocks and are due to primary and secondary porosities. High yielding wells and springs are often related to large lineaments, lineament intersections and corresponding structural features. In metamorphic and igneous intrusive rocks with rugged landforms, groundwater occurs mainly in drainage channels with valley fill deposits. Zones of very good groundwater potential are characteristic for basaltic layers overlying lateritized crystalline rocks, flat topography with dense lineaments and structurally controlled drainage channels with valley fill deposits. The overall results demonstrate that the use of remote sensing and GIS provide potentially powerful tools to study groundwater resources and design a suitable exploration plan.An erratum to this article can be found at     

Molecular signature and sources of biochemical recalcitrance of organic C in Amazonian Dark Earths

Amazonian Dark Earths (ADE) are a unique type of soils developed through intense anthropogenic activities that transformed the original soils into Anthrosols throughout the Brazilian Amazon Basin. We conducted a comparative molecular-level investigation of soil organic C (SOC) speciation in ADE (ages between 600 and 8700 years B.P.) and adjacent soils using ultraviolet photo-oxidation coupled with ¹³C cross polarization-magic angle spinning nuclear magnetic resonance (CP-MAS NMR), synchrotron-based Fourier transform infrared-attenuated total reflectance (Sr-FTIR-ATR) and C (1s) near edge X-ray absorption fine structure (NEXAFS) spectroscopy to obtain deeper insights into the structural chemistry and sources of refractory organic C compounds in ADE. Our results show that the functional group chemistry of SOC in ADE was considerably different from adjacent soils. The SOC in ADE was enriched with: (i) aromatic-C structures mostly from H- and C-substituted aryl-C, (ii) O-rich organic C forms from carboxylic-C, aldehyde-C, ketonic-C and quinine-C, and (iii) diverse group of refractory aliphatic-C moieties. The SOC in adjacent soils was predominantly composed of O-alkyl-C and methoxyl-C/N-alkyl-C structures and elements of labile aliphatic-C functionalities. Our study suggests that the inherent molecular structures of organic C due to selective accumulation of highly refractory aryl-C structures seems to be the key factor for the biochemical recalcitrance and stability of SOC in ADE. Anthropogenic enrichment with charred carbonaceous residues from biomass-derived black C (BC) is presumed to be the precursor of these recalcitrant polyaromatic structures. Our results also highlight the complementary role that might be played by organic C compounds composed of O-containing organic C moieties and aliphatic-C structures that persisted for millennia in these anthropic soils as additional or secondary sources of chemical recalcitrance of SOC in ADE. These organic C compounds could be the products of: (i) primary recalcitrant biomolecules from non-BC sources or (ii) secondary processes involving microbial mediated oxidative or extracellular neoformation reactions of SOC from BC and non-BC sources; and stabilized through physical inaccessibility to decomposers due to sorption onto the surface or into porous structures of BC particles, selective preservation or through intermolecular interactions involving clay and BC particles.     

Pacific tropical–extratropical thermocline water mass exchanges in the NCAR Coupled Climate System Model v.3

In this study we document how model biases in extratropical surface wind and precipitation, due to ocean–atmosphere coupling, are communicated to the equatorial Pacific thermocline through Pacific Subtropical Cell (STC) pathways. We compare the simulation of climate mean Pacific Subtropical Cells (STCs) in the NCAR Community Climate System Model version 3 (CCSM3) to observations and to an uncoupled ocean simulation (the ocean component of the CCSM3 forced by observed wind stress and surface fluxes). We use two versions of the CCSM3 with atmospheric resolution of 2.8° (T42) and 1.4° (T85) to investigate whether the climate mean STCs are sensitive to the resolution of the atmospheric model.Since STCs provide water that maintains the equatorial thermocline, we first document biases in equatorial temperature and salinity fields. We then investigate to what extent these biases are due to the simulation of extratropical–tropical water mass exchanges in the coupled models. We demonstrate that the coupled models’ cold and fresh bias in the equatorial thermocline is due to the subduction of significantly fresher and colder water in the South Pacific. This freshening is due to too much precipitation in the South Pacific Convergence Zone. Lagrangian trajectories of water that flows to the equatorial thermocline are calculated to demonstrate that the anomalously large potential vorticity barriers in the coupled simulations in both the North and South Pacific prevent water in the lower thermocline from reaching the equator. The equatorial thermocline is shown to be primarily maintained by water that subducts in the subtropical South Pacific in both the coupled and uncoupled simulations. It is shown that the zonally integrated transport convergence at the equator in the subsurface branch of the climate mean STCs is well simulated in the uncoupled ocean model. However, coupling reduces the net equatorward pycnocline transport by 4 Sv at 9°S and 1 Sv at 9°N. An increase in the atmospheric resolution from T42 to T85 results in more realistic equatorial trades and off-equatorial convergence zones.