330 BOTKIN, JANAK, AND WALLIS 



height and diameter, between total leaf weight and diameter, between rate of 

 photosynthesis and available light, and between relative growth and a measure of 

 climate; a range of soil-moisture conditions within which the species can grow; 

 and the number of saplings that can enter the stand under shaded, open, or very 

 open conditions. The abiotic environment is defined by elevation, soil depth, soil 

 moisture-holding capacity, percentage of rock in the soil, a set of average 

 monthly temperatures and precipitation records from a nearby weather station, 

 and by a value for the annual insolation above the forest canopy. 



Direct competition among individuals is restricted to competition for light 

 (taller trees shade smaller ones, and species with more leaves for a given diameter 

 shade smaller competitors more than other species; under shaded conditions, 

 photosynthesis is higher for shade-tolerant species than intolerant ones, and vice 

 versa). Species strategy is also invoked by species-specific survival probabilities 

 and by differential addition of new saplings in relation to light at the forest 

 floor. Because the annual probability of survival of an individual is related to the 

 maximum known lifetime of its species, individuals of long-lived species have a 

 better chance of survival in any one year than those with short maximum lives. 



Two basic kinds of species strategies are defined in the simulation, those of 

 shade-intolerant and shade-tolerant trees. The former strategy is to capitalize on 

 catastrophe, to provide many young individuals soon after a clearing occurs in a 

 forest, to grow quickly when the light intensity is high, and to produce viable 

 seeds in a comparatively short time. This strategy is to get in and get out quickly 

 and to sacrifice durability and individual persistence for speed. The shade- 

 tolerant strategy is characterized by the ability to grow at all ages in 

 comparatively deep shade, to live a long time, but to grow slowly even when 

 light conditions are high. This strategy sacrifices productivity for persistence and 

 capitalizes on the older stages of a forest. Species with this strategy are 

 characteristic of the climax stage of a forest. There are intermediate strategies, of 

 course, such as that of yellow birch, but the dichotomy is drawn here to help 

 clarify the reader's understanding of the simulation. 



The program is written entirely in FORTRAN IV, using only standard 

 library routines and a good uniform random-number generator. A complete 

 listing of the source deck is available from the authors, and a flow chart for the 

 main program, called JABOWA, is given in Fig. 1. The program has been 

 successfully operated under the IBM time-sharing system (TSS) release 7, the 

 Cambridge Monitor System (CMS), and in batch modes. Prospective users with 

 similar facilities should have no trouble using the simulator. 



For each plot year of simulation, three major subroutines are called: 

 subroutine GROW, which deterministically provides the annual growth in- 

 crement for each tree; subroutine BIRTH, which stochastically adds new 

 saplings; and subroutine KILL, which stochastically decides which trees die. 

 Certain features of the simulator are the result of constraints imposed by its 

 initial application to the Hubbard Brook Ecosystem Study. 



