Ecological Modelling, 42 (1988) 125-154 125 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands A GENERAL MODEL OF FOREST ECOSYSTEM PROCESSES FOR REGIONAL APPLICATIONS I. HYDROLOGIC BALANCE, CANOPY GAS EXCHANGE AND PRIMARY PRODUCTION PROCESSES STEVEN W. RUNNING and JOSEPH C. COUGHLAN School of Forestry, University of Montana, Missoula, MT 59812 (U.S.A.) (Accepted 18 November 1987) ABSTRACT Running, S.W. and Coughlan, J.C., 1988. A general model of forest ecosystem processes for regional applications. I. Hydrologic balance, canopy gas exchange and primary production processes. Ecol. Modelling, 42: 125-154. An ecosystem process model is described that calculates the carbon, water and nitrogen cycles through a forest ecosystem. The model, FOREST-BGC, treats canopy interception and evaporation, transpiration, photosynthesis, growth and maintenance respiration, carbon alloc- ation above and below-ground, litterfall, decomposition and nitrogen mineralization. The model uses leaf area index (LA0 to quantify the forest structure important for energy and mass exchange, and this represents a key simplification for regional scale applications. FOREST-BGC requires daily incoming short-wave radiation, air temperature, dew point, and precipitation as driving variables. The model was used to simulate the annual hydrologic balance and net primary production of a hypothetical forest stand in seven contrasting environments across North America for the year 1984. Hydrologic partitioning ranged from 14/86/0% for evaporation, transpiration and outflow, respectively, in Fairbanks, AK (annual precipitation of 313 mm) to 10/27/66% in Jacksonville, FL (annual ppt of 1244 mm), and these balances changed as LAI was increased from 3 to 9 in successive simulations. Net primary production (Nee) ranged from 0.0 t C ha -1 year -1 at Tucson, AZ, to 14.1 t C ha- year- ~ at Knoxville, TN and corresponded reasonably with observed values at each site. The sensitivity of ecosystem processes to varying LAI in different climates was substantial, and underscores the utility of parameterizing this model at regional scales in the future with forest LAI measurements derived from satellite imagery. INTRODUCTION Many of the most pressing ecological questions being asked today address ecosystem processes at regional to global scales. Concerns on the effects of global climate change, global CO 2 increase, regional air pollution, and 0304-3800/88/$03.50 © 1988 Elsevier Science Publishers B.V.126 regional shifts in vegetation cover on forest processes all require calculations of ecosystem processes at scales much larger than have previously been considered by ecosystem process models. Fundamental requirements in these ecological questions are the rates and controls of energy, carbon, water and nutrient exchange by vegetated surfaces, and the responses of these surfaces to some of the perturbations described above. Most ecosystem process models have been built from and used to simulate the dynamics of a study plot, often 10 m × 10 m or less in size. Some watershed models have been built to calculate hydrologic variables at larger scales, but they do not treat ecological processes. A number of global models have been developed, particularly for global carbon, but they are primarily static budgets that do not mechanistically treat processes, and are not driven by data for specific sites or conditions. We present here a new model that calculates key processes involved in the carbon, water and nitrogen cycles for forests. The model treats canopy interception and evaporation, transpiration, photosynthesis, growth and maintenance respiration, carbon allocation above and below-ground, litter- fall, decomposition and nitrogen mineralization mechanistically, but in a general way incorporating minimal species-specific data. The model repre- sents a conscious compromise between mechanistic detail and simplifying generality that will allow it to be implemented for regional-scale ecological research. Additionally, the model was designed to be driven ultimately by remote sensing inputs of surface climate and vegetation structure, in the framework of a geographic information system containing topographic and physical site characteristics (Peterson et al., 1985). Recent workshops on global ecological issues have identified leaf area index (LAI) as the most important single variable for measuring vegetation structure over large areas, and relating it to energy and mass exchange. (Wittwer, 1983; Botkin, 1986). LAI of natural vegetation has been successfully estimated from satellite resolution sensors (Asrar et al., 1984; Running et al., 1986). Consequently, this model is designed to be particularly sensitive to LAI, and LAI is used as the principal independent variable for calculating canopy interception, transpiration, res- piration, photosynthesis, carbon allocation and litterfall. This paper covers the logic and development of the model FOREST-BGC (BioGeochemical Cycles), and a first exercise of the model, simulating annual carbon and water balances for a forest under a range of climates. Because LAI is considered a critical variable, we will also test the sensitivity of the model to LAI across this climatic range. This paper will concentrate on the daily timestep part of the model: hydrologic balance, canopy gas exchanges and photosynthate partitioning to respiration and primary pro- duction (Fig. 1). A future paper will describe in detail above and below-DAILY PPT , ~EM APORA'~ION ~ t~ ',~1 K ') ME TEQROLOG|CAL DATA - AIR TEMPERATURE - RADIATION - PRECIPITATION SNOW J I~1 T;ANSP, RAT,ON I IrB~l :.IP.OTOSYNT"ES'S [ CARBON, NITROGEN - LEAF (LAI) | MAINTENANCE ~ - STEM - ROOT ] RESPIRATION[ L 7 t Cl ANNUAL ~ I - PHOTOSYNTHESIS - EVAPOTRANSPIRATION - RESPIRATION YEARLY OROWrrl I I Is Cl RESPIRAT,O~ ! .'t LEAF lh~t".. 7 ,El / I i~c! ~t" .... ~_\ - • t _ / .~1 STEM |'-- --I.----,-.b,| ..... I IAVAILABLEI6 CJ,¢~ 116 N I ~kOVE~ c - r~ t~ "~ % c l , ~ ROOT~ ,---]/'"F x J DECOMP. ] I RESPIRATION ,-I ~ -T |cl SOIL LITTER 118 "1 J~oNIT~O~EN I LO~ I~ I Fig. 1. A compartment