Developing relative stand density index for structurally complex mixed species cypress and pine forests

Stand Density Index (SDI) is a numerical value that captures intensity of competition within forest stands. It is a tool for managing density of trees, controlling crown development and maintaining degree of forest health through decisions made on initial planting density and thinning schedules. However, classical Reineke's SDI (Reineke, 1933) has been found to be unsuitable for mixed species and structurally complex forest stands. Alternative measures of SDI are being explored. Natural forests in Taiwan cover an area of approximately 1.5 million ha with 75% classified as mixed species forests. Current SDI research in Taiwan focuses on single-species natural and plantation forests such as Chamaecyparis formosensis and Cryptomeria japonica forests. No study has investigated and developed SDI for mixed species natural forests in Taiwan. Because management of these forests is crucial for conservation and protection against soil erosion, it is necessary to develop SDI for these forests to establish management guidelines. Therefore, relative SDI was developed for cypress- and pine-dominated forests using data from the 4th Taiwan National Forest Inventory following the approach by Ducey and Knapp (2010) which uses intersections between the mixed-species equation and previously-published density equations to establish a relative SDI model. Plots with at least 30% of basal area per hectare of target species were used for model fitting. During model fitting, it was discovered that the lack of species-specific specific gravity estimates played an important role in model significance. Lastly, three different relative SDI models for each forest type were produced that provide preliminary estimates of minimum, mean and maximum relative SDI for forest stands. The major result from this study was that relative SDI could be tentatively calculated and graphically presented for the mixed-species and structurally complex cypress and pine forests that allows a manager to design silvicultural strategies to meet forest management objectives.