Management and analysis of 30-year Continuous Forest Inventory data on the Six Rivers National Forest

The purposes of this research were to test hypotheses concerning potential changes in the structure of the undisturbed portions of the Six Rivers National Forest between the period 1961-1963 and 1993-1995, and to create a user-friendly PC database of the Six Rivers Continuous Forest Inventory data. Data from 166 plots located throughout the Six Rivers National Forest were put into Microsoft ACCESS, a relational database management system. The stand density index (SDI) values for individual plots, as well as for individual species, were calculated, as were the contributions of particular species or groups of species to overall SDI. Analysis of the data indicates that overall SDI has increased during the study period. SDI for tolerant species has increased, while no change could be detected for intolerant species. The proportion of the SDI attributed to tolerant species as a group has increased relative to other species, and the proportion of SDI attributed to intolerant species has decreased. No significant change was found in the SDI for any individual intolerant species. Among the intolerant species, significant decreases were found for the proportion of SDI attributed to sugar pine, western white pine, and California black oak relative to all other species. Among more tolerant species, significant increases were recorded in the SDI for Douglas-fir, white fir, tanoak and madrone. Douglas-fir was the only tolerant species to have significantly increased its proportion of SDI relative to all other species. The results of this study lend credibility to the hypothesis that changes in stand density are due to the interruption of a frequent low-intensity fire interval. These results do not rule out the possibility that the interval of stand-replacing fires has not been interrupted and that the increases in stand density may primarily be due to natural succession. This work demonstrates that SDI is a very powerful tool in detecting changes in forest community ecology. Keywords: stand density index, community ecology, forest health, fire exclusion, fire interval, succession, shade tolerance, Continuous Forest Inventory, Forest Inventory and Analysis, Six Rivers National Forest.