Analysis of factors regulating ecosystemdevelopment on Mauna Loa using the Century model

We used the Century model to evaluateenvironmental controls over ecosystem developmentduring the first 3500 y of primary succession onpahoehoe (i.e., relatively smooth, solid) lava flowsof wet, windward Mauna Loa, Hawaii. The Century modelis a generalized ecosystem model that simulatescarbon, nitrogen and phosphorus dynamics forplant-soil systems. Preliminary results indicated theneed to modify the model to include the effects ofsoil C accumulation on soil water storage anddrainage. The modified model was parameterized tosimulate observed values of aboveground productivity,biomass and soil element pools on a 3400-y-old site at700 m elevation. Testing the model parameters at 1660m elevation indicated that N inputs were lower andsoil water drainage rates were slower at the higherelevation. We applied the modified and fullyparameterized model to simulate ecosystem attributesduring primary succession at five elevations, andconducted single-factor experiments with the model toidentify the specific influences of variations intemperature, nutrient inputs, and rainfall on modeledecosystem characteristics.Simulated aboveground productivity (ANPP), net N andP mineralization, and biomass element pools allincreased through time at each elevation, and alldeclined with increasing elevation at each point intime. After 3500 y of succession none of theseattributes had reached a stable asymptote, butasymptotes were approached more quickly, andsuccession was therefore faster, at lower than athigher elevations. Simulated soil organic matter(SOM) pools increased with elevation, despite thatplant productivity declined. These results, andsimilar comparisons among rainfall regimes, suggestthat SOM pools were more sensitive to factorscontrolling decay than production rates.Within elevations and temperature regimes, nutrientavailability was the most important factor controllingsimulated rates of plant productivity, biomass, anddetritus accumulation during ecosystem development. Through time, SOM accumulations alleviated nutrientlimitations to plants, but simulated productivityremained highly dependent upon externally suppliednutrients even after 20,000 y. Rainfall had two maineffects on nutrient availability within the model: (1)it increased rates of leaching, and thus depletednutrient supplies; and (2) it exacerbated soil floodingand thereby decreased nutrient turnover rates. Highrainfall on windward Mauna Loa maintains oligotrophicconditions through time despite continuous N and Pinputs.