Allen M. Solomon

Carbon pools and flux of global forest ecosystems

Forest systems cover more than 4.1 x 109 hectares of the Earths land area. Globally, forest vegetation and soils contain about 1146 petagrams of carbon, with approximately 37 percent of this carbon in low-latitude forests, 14 percent in mid-latitudes, and 49 percent at high latitudes. Over two-thirds of the carbon in forest ecosystems is contained in soils and associated peat deposits. In 1990, deforestation in the low latitudes emitted 1.6 � 0.4 petagrams of carbon per year, whereas forest area expansion and growth in mid- and high-latitude forest sequestered 0.7 � 0.2 petagrams of carbon per year, for a net flux to the atmosphere of 0.9 � 0.4 petagrams of carbon per year. Slowing deforestation, combined with an increase in forestation and other management measures to improve forest ecosystem productivity, could conserve or sequester significant quantities of carbon. Future forest carbon cycling trends attributable to losses and regrowth associated with global climate and land-use change are uncertain. Model projections and some results suggest that forests could be carbon sinks or sources in the future.

Tree Mortality in Gap Models: Application to Climate Change

Gap models are perhaps the most widely used class of individual-based tree models used in ecology and climate change research. However, most gap model emphasize, in terms of process detail, computer code, and validation effort, tree growth with little attention to the simulation of plant death or mortality. Mortality algorithms have been mostly limited to general relationships because of sparse data on the causal mechanisms of mortality. If gap models are to be used to explore community dynamics under changing climates, the limitations and shortcomings of these mortality algorithms must be identified and the simulation of mortality must be improved. In this paper, we review the treatment of mortality in gap models, evaluate the relationships used to represent mortality in the current generation of gap models, and then assess the prospects for making improvements, especially for applications involving global climate change. Three needs are identified to improve mortality simulations in gap models: (1) process-based empirical analyses are needed to create more climate-sensitive stochastic mortality functions, (2) fundamental research is required to quantify the biophysical relationships between mortality and plant dynamics, and (3) extensive field data are needed to quantify, parameterize, and validate existing and future gap model mortality functions.

EXPLAINING FOREST COMPOSITION AND BIOMASS ACROSS MULTIPLE BIOGEOGRAPHICAL REGIONS

Current scientific concerns regarding the impacts of global change include the responses of forest composition and biomass to rapid changes in climate, and forest gap models have often been used to address this issue. These models reflect the concept that forest composition and biomass in the absence of large-scale disturbance are explained by competition among species for light and other resources in canopy gaps formed when dominant trees die. Since their intiation 25 yr ago, a wide variety of gap models have been developed that are applicable to different forest ecosystems all over the world. Few gap models, however, have proved to be equally valid over a wide range of environmental conditions, a problem on which our work is focused. We previously developed a gap model that is capable of simulating forest composition and biomass in temperate forests of Europe and eastern North America based on a single model structure. In the present study, we extend the model to simulate individual tree species response to strong moisture seasonality and low temperature seasonality, and we modify the widespread parabolic temperature response function to mimic nonlinear increases in growth with increased temperature up to species-specific optimal values. The resulting gap model, ForClim V2.9, generates realistic projections of tree species composition and biomass across a complex gradient of temperature and moisture in the Pacific Northwest of the United States. The model is evaluated against measured basal area and stand structure data at three elevations of the H. J. Andrews LTER site, yielding satisfactory results. The very same model also provides improved estimates of species composition and stand biomass in eastern North America and central Europe, where it originated. This suggests that the model modifications we introduced are indeed generic. Temperate forests other than those we studied here are characterized by climates that are quite similar to the ones in the three study regions. Therefore we are confident that it is possible to explain forest composition and biomass of all major temperate forests by means of a single hypothesis as embodied in a forest gap model.

The global terrestrial carbon cycle

There is great uncertainty with regard to the future role of the terrestrial biosphere in the global carbon cycle. The uncertainty arises from both an inadequate understanding of current pools and fluxes as well as the potential effects of rising atmospheric concentrations of CO2 on natural ecosystems. Despite these limitations, a number of studies have estimated current and future patterns of terrestrial carbon storage. Future estimates focus on the effects of a climate change associated with a doubled atmospheric concentration of CO2. Available models for examining the dynamics of terrestrial carbon storage and the potential role of forest management and landuse practices on carbon conservation and sequestration are discussed.

MODELING DYNAMIC VEGETATION RESPONSE TO RAPID CLIMATE CHANGE USING BIOCLIMATIC CLASSIFICATION

Modeling potential global redistribution of terrestrial vegetation frequently is based on bioclimatic classifications which relate static regional vegetation zones (biomes) to a set of static climate parameters. The equilibrium character of the relationships limits our confidence in their application to scenarios of rapidly changing climate. Such assessments could be improved if vegetation migration and succession would be incorporated as response variables in model simulations. We developed the model MOVE (Migration Of VEgetation), to simulate the geographical implications of different rates of plant extirpation and in-migration. We used the model to study the potential impact on terrestrial carbon stocks of climate shifts hypothesized from a doubling of atmospheric greenhouse gas concentration. The model indicates that the terrestrial vegetation and soil could release carbon; the amount of this carbon pulse depends on the rate of migration relative to the rate of climate change. New temperate and boreal biomes, not found on the landscape today, increase rapidly in area during the first 100 years of simulated response to climate change. Their presence for several centuries and their gradual disappearance after the climate ceases to change adds uncertainty in calculating future terrestrial carbon fluxes.

Simulating the Role of Climate Change and Species Immigration in Forest Succession

Forest succession in its traditional sense implies two important features that resist direct examination. First, classical definitions of forest succession generally connote directional changes in species composition and in community structure through time. Following a major disturbance, a directional succession of tree species in our temperate forests may involve 200 (Oosting 1942), 400 (McAndrews 1976), or even 1000 years (Franklin and Hemstrom Chapter 14). There is hardly the opportunity to observe the direction, let alone the species succession, that occurs within our lifetime.

The Interaction of Climate and Land Use in Future Terrestrial Carbon Storage and Release

The processes controlling total carbon (C) storage and release from the terrestrial biosphere are still poorly quantified. We conclude from analysis of paleodata and climate-biome model output that terrestrial C exchanges since the last glacial maximum (LGM) were dominated by slow processes of C sequestration in soils, possibly modified by C starvation and reduced water use efficiency of trees during the LGM. Human intrusion into the C cycle was immeasurably small. These processes produced an averaged C sink in the terrestrial biosphere on the order of 0.05 Pg yr-1 during the past 10,000 years.

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.

Workshop summary statement: Terrestrial bioshperic carbon fluxesquantification of sinks and sources of CO2

Understanding the role of terrestrial ecosystems in the global carbon (C) cycle has become increasingly important as policymakers consider options to address the issues associated with global change, particularly climate change. Sound scientific theories are critical in predicting how these systems may respond in the future, both to climate change and human actions.In March 1993, 60 scientists from 13 nations gathered in Bad Harzburg, Germany, to develop a state-of-the-science assessment of the present and likely future C fluxes associated with the major components of the earths terrestrial biosphere. In the process, particular emphasis was placed on the potential for improving C sinks and managing long-term C sequestration.The majority of the weeks work was conducted in eight working groups which independently considered a particular biome or subject area. The working groups considered: the Global Carbon Cycle; Boreal Forests and Tundra; Temperate Forests; Tropical Forests; Grasslands, Savannas and Deserts; Land and Water Interface Zones; Agroecosystems; and Biomass Management. This paper presents a brief overview of their major conclusions and findings. In addition, Table 1 brings together the best estimates from each group as to the current magnitude and estimated future direction of changes in the terrestrial C fluxes.

Boreal forest carbon stocks and wood supply: past, present and future responses to changing climate, agriculture and species availability

Abstract The paper assesses the role in boreal forest growth played by environment. It examines past changes in climate coupled with glaciation, and future changes in climate coupled with agricultural land use and tree species availability. The objective was to define and evaluate potential future changes in wood supply and global carbon stocks. Calculations were based on a standard static vegetation model (BIOME 1.1) driven by the most recent climate change scenarios from three coupled ocean-atmosphere general circulation models (GCMs). The results indicated that boreal terrestrial carbon stocks increased greatly following the retreat of continental ice sheets, before which boreal forests covered only about a third the amount of land they cover now. Carbon stocks and wood supplies in boreal forests were also projected to increase if vegetation stabilized under all three future climate scenarios (6–15%). However, the opposite response occurred with the addition of expected constraints on forest growth, provided by the lags in immigration of tree species suitable for warmed climate. This transient depauperate condition reduced wood supplies considerably (4–6%). Inclusion of present and future agricultural land uses permitted by a warming climate forced carbon stocks and wood supplies to decline even more (10–20%). The decline in boreal carbon stocks is the equivalent of 1–2.6 Pg year −1 emitted to the atmosphere (rather than the 1–2 Pg year −1 global modelers hypothesize is currently being taken up by vegetation from the atmosphere), during the time greenhouse gases are expected to double in concentration.