Analysis of factors regulating ecosystem development on Mauna Loa using the Century model

We used the Century model to evaluate environmental controls over ecosystem development during the first 3500 y of primary succession on pahoehoe (i.e ., relatively smooth, solid) lava flows of wet, windward Mauna Loa, Hawaii. The Century model is a generalized ecosystem model that simulates carbon, nitrogen and phosphorus dynamics for plant-soil systems. Preliminary result s indicated the need to modify the model to include the effects of soil C a ccumulation on soil water storage and drainage. The modified model was para meterized to simulate observed values of aboveground productivity, biomass and soil element pools on a 3400-y-old site at 700 m elevation. Testing the model parameters at 1660 m elevation indicated that N inputs were lower an d soil water drainage rates were slower at the higher elevation. We applied the modified and fully parameterized model to simulate ecosystem attribute s during primary succession at five elevations, and conducted single-factor experiments with the model to identify the specific influences of variatio ns in temperature, nutrient inputs, and rainfall on modeled ecosystem chara cteristics. Simulated aboveground productivity (ANPP), net N and P mineralization, and biomass element pools all increased through time at each elevation, and all declined with increasing elevation at each point in time. After 3500 y of succession none of these attributes had reached a stable asymptote, but asy mptotes were approached more quickly, and succession was therefore faster, at lower than at higher elevations. Simulated soil organic matter (SOM) poo ls increased with elevation, despite that plant productivity declined. Thes e results, and similar comparisons among rainfall regimes, suggest that SOM pools were more sensitive to factors controlling decay than production rat es. Within elevations and temperature regimes, nutrient availability was the mo st important factor controlling simulated rates of plant productivity, biom ass, and detritus accumulation during ecosystem development. Through time, SOM accumulations alleviated nutrient limitations to plants, but simulated productivity remained highly dependent upon externally supplied nutrients e ven after 20,000 y. Rainfall had two main effects on nutrient availability within the model: (1) it increased rates of leaching, and thus depleted nut rient supplies; and (2) it exacerbated soil flooding and thereby decreased nutrient turnover rates. High rainfall on windward Mauna Loa maintains olig otrophic conditions through time despite continuous N and P inputs.