Landscape Model Case Study Jabowa Assessment of Atmospheric CO2 Increases on Forests

JABOWA, and its subsequent versions including JABOWA-II, is a generalized model of the reproduction, growth, and mortality of trees in mixed-species forests in response to environmental conditions. It is among the first multispecies computer simulations of terrestrial ecosystems developed, and it has undergone extensive use and modification during the 30 years since it originated.

JABOWA was originally designed to be used for the Hubbard Brook Forest in northeastern North America; however, its underlying concepts are general, so that any nonhydrophytic tree species could be used for the simulation. The model enables the user to examine forest responses to a variety of abiotic (e.g., temperature, elevation, soil moisture-holding capacity) and biotic (e.g., competition for light) variables. The user can determine the kind and number of tree species; current versions allow for up to 45 species. Processes affecting growth and mortality in the chosen species take place independently within a series of grid areas (10m x 10m is the default value, which is user-adjustable in early versions of the model).

The model functions by setting a number of environmental conditions which, in turn, determine the shape of the trees, their growth, reproduction, and mortality. These outcomes define community endpoints including stand density, species diversity, and composition (Figure 13). The basic growth function is a species-specific growth curve (determined empirically for a tree growing under optimum conditions) with dependence on

Figure 13 Structure of JABOWA forest landscape model. Copyright (© 2002) Ecological Modeling in Risk Assessment: Chemical Effects on Populations, Ecosystems, and Landscapes, by Pastorok RA, Bartell SM, Ferson S, and Ginzburg LR (eds.). Reproduced by permission of Routledge/Taylor & Francis Group, LLC.

Figure 13 Structure of JABOWA forest landscape model. Copyright (© 2002) Ecological Modeling in Risk Assessment: Chemical Effects on Populations, Ecosystems, and Landscapes, by Pastorok RA, Bartell SM, Ferson S, and Ginzburg LR (eds.). Reproduced by permission of Routledge/Taylor & Francis Group, LLC.

canopy-level solar radiation; the temperature index which simulates effects of monthly mean temperatures on photosynthesis rates; a soil quality index which simulates the effect of soil structure on tree growth; and, in more recent versions of the model, soil nitrogen, depth of the water table, depth of soil, and other soil characteristics that also influence growth.

The growth function is modified by a coefficient accounting for competition between trees within the same grid plot as a function of tree density, and for the effects of the existing environment on each species. Species-specific recruitment depends on species density and a coefficient defining seed production for each species. Mortality is also modeled to affect competition for light by altering the number of tree stems on a plot. Annual tree mortality is determined for each age class and is modeled stochastically with two algorithms: the first algorithm is for healthy trees, in which it is assumed that 2 % of the individuals of the species will, on average, reach the maximum known age for that species. The second algorithm is for trees that grow poorly, with a user-determined minimum growth; a second stochastic function assumes that such a tree will, on average, survive only 10 years unless growth rises above the user-set minimum.

At the landscape scale, grids are defined by state variables determining the number of saplings within a stand based on shade, elevation, soil type, soil capacity, percentage rock in soil, and monthly temperature and precipitation.

An example of applying the JABOWA landscape model was a study conducted in the United States by researchers at Indiana University and the Richard Stockton College of New Jersey to assess the response of southern Great Lakes forests to a doubling of atmospheric CO2. For this study, JABOWA-II was embedded within a geographic information system (GIS) in order to examine temporal and spatial changes in forest condition in response to a changed climate relative to growth under baseline climate conditions.

Data providing the inputs for the model included:

• type of land use: urban, agricultural, forest, water, wetland, etc.;

• tree species composition: species name and diameter at breast height (dbh);

• thermal data: minimum- and maximum-growing degree-days used to define a thermal response curve in order to estimate tree growth increment;

• nutrient concentrations: available nitrogen, as a function of climate, litter quality, and soil texture (JABOWA-II does not simulate nutrient dynamics; therefore, available nitrogen levels remained static throughout the modeling period);

• baseline climate conditions: average monthly temperature and precipitation across the region, spatially interpolated from 1180 weather stations.

Using these data sets, site files were generated from the average conditions existing in each 4 km cell for input into the JABOWA-II model. An Oregon State University general circulation model (OSUGCM) was used to generate a changed climate scenario, hypothesized as a doubling of CO2 over an 80-year modeling period that produced linear changes in temperature and precipitation. Baseline and climate-changed conditions were iterated separately and forest growth outputs were compiled at 10-year intervals as total basal area for each species. The outputs ofthese two scenarios were analyzed in three ways: (1) nonspatial, specific responses to the changed climate in terms of total basal area of trees; (2) basal area-weighted population centroids were calculated to investigate changes in individual species' population distributions; and (3) maps were produced for species groups and regionally dominant species to assess spatial patterns of growth response for the larger forest community, across a landscape (e.g., Figure 14).

Relative to baseline forest conditions, the climate change scenario predicted large decreases in total basal area of northern conifers and northern deciduous species (>99% decrease), and population centroid shifts were primarily toward the northeast. These results were in agreement with other studies examining effects of climate change. In contrast with other studies, only slight increases in intermediate and southern species (which represent c. 90% of the region's basal area) were predicted from this model. The authors attributed this finding to the ability of their model to consider a high degree of spatial resolution, realism of modeled forest conditions, and accuracy in spatial configuration within the land-use matrix. Some important limitations of the model were highlighted in this application as well. These limitations included the inability to simulate nutrient dynamics, whereas changes in nutrient levels over time are a potentially important outcome of climate change; the inability to simulate nonlinear responses such as variable growth responses across age classes; and the inability of JABOWA-II, which allocates annual growth increments, to account for longer-term climatic episodes like multiyear droughts and their effects on forest communities.

JABOWA meets several criteria of a useful and relevant environmental model for terrestrial forest ecosystems. The model combines mechanistic functions and site-specific empirical relationships that accurately describe forest processes. The model can simulate a variety of ecologically relevant endpoints, including tree biomass, forest productivity, or effects of toxic chemicals. It is quite flexible; the parameters of each species are based on information for its entire range, and it has been applied to many types of forests over a range of environmental conditions in North America, Siberia, Eastern Europe, and Costa Rica. JABOWA incorporates uncertainty in the form of stochastic functions for mortality and reproduction. It also

1981 Baseline climate

2060 Baseline climate

1981 Baseline climate

2060 Baseline climate

Basal area (m-2 km-2) 1-100 tm 100-250 250-500 M 500-1000 = 1000-10 000

Population centroids £ 1981 baseline climate 2060 baseline climate A, 2060 changed climate Impact vector

Basal area (m-2 km-2) 1-100 tm 100-250 250-500 M 500-1000 = 1000-10 000

Population centroids £ 1981 baseline climate 2060 baseline climate A, 2060 changed climate Impact vector

2060 Changed climate

Kilometers 0

Northern deciduous

Albers equal area projection

Figure 14 Results from application of JABOWA forest landscape model to assess effects of increasing atmospheric CO2. Reprinted from Ehman JL, Fan W, Randolph JC, Southworth J, and Welch NT (2002) An integrated GIS and modeling approach for assessing the transient response of forests of the southern Great Lakes region to a doubled CO2 climate. Forest Ecology and Management 155: 237-255, with permission from Elsevier.

includes a statistical analysis module that reports the mean, variance, and 95% confidence intervals for each year the user requests output. This model also has a long history of development and use; its structure has served as the basis for many generations of forest-gap models. One potential drawback to the use of this model in a regulatory context is the fact that it has no regulatory status, and it does not appear to have been used in this context.

population models, so that users can select among models with and without density dependence, age/stage structure, and other characteristics. Model-building program such as STELLA, ModelMaker, and MATLAB can be used to build ecotoxicological models. However, substantial effort is needed for testing and 'debugging' a user-built model to ensure calculations are done correctly. For the novice modeler, using available models may be best.

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