The long-term nutrient retention properties of forest ecosystems : a simulation investigation

A computer simulation model of the interplay between successional dynamics and long-term nutrient retention was constructed for the purpose of comparing the retention characteristics of two southern Appalachian forest stands. Descriptions of mechanisms of competition for light, water, and nutrients included in the model were tested by comparing the 12 year average diameter increment of trees in four stands from different slope positions in the Walker Branch Watershed (WBW) with model predictions. With only competition for light controlling growth in the model, predictions of diameter increment for lower-slope tulip poplar, mid-slope chestnut oak, upper-slope oak-hickory, and ridge-top pine stands were 0.36, 0.35, 0.33, and 0.39 cm•yr^-1, respectively. Including water limitation, predicted rates decreased to 0.32, 0.27, 0.27, and 0.26 cm•yr^-1 Addition of nutrient limitations caused rates to decrease to 0.29, 0.20, 0.22, and 0.20, respectively, which compare favorably to field measurements of 0.27, 0.19, 0.23, and 0.18 cm•yr^-1. In an assessment of long-term nutrient retention characteristics in the dry mid-slope forest of the WBW, in the Ridge and Valley Province of eastern Tennessee, losses of calcium from the soil pool during 500 years of simulation were large. In contrast, simulation of the dry mid-slope forest of the Hannah Branch Watershed (HBW), in the Great Smoky Mountains National Park, showed large losses of nitrogen and an increase of soil calcium. The differences in species dominance or community structure which developed during the succession of each forest were insufficient to produce the differences between the two forests. The greater cycling rate of the more productive HBW stand did not account for the differences. The differences were the result of the accumulated effects of short term imbalances between uptake and mineralization rates rather than the result of a chronic characteristic of the nutrient cycles. The balance of mineralization and uptake is dependent on site-specific characters varying through time. When tree uptake is similar to mineralization release, nutrients will be tightly held in the system. Losses are the result of releases from the litter layer in excess of quantities taken up by trees. Losses increase when tree uptake is limited by environmental conditions. The large calcium losses on the simulated WBW could have been caused by low nitrogen supplies limiting growth and uptake, while the HBW nitrogen losses could have been caused by calcium limitation. Many mechanisms, such as translocation alteration, tissue concentration alteration, and nitrogen fixation relationships, were shown to be capable of enhancing the adjustment of tree uptake to efficiently capture available nutrients and reduce nutrient loss. This study demonstrated the importance of spatial variability from plot to plot as well as the importance of temporal dynamics in assessing the causes of nutrient losses from an entire landscape. Although a general condition of calcium limitation might exist in the HBW, in any given year nitrogen might be the element limiting growth. Such transient conditions were likely due to the differential response of microbial mineralization processes and the tree uptake processes to climate and weather.