Development of a Simulation System for Testing Hardwood Plantation Silviculture

Hardwood plantations are becoming increasingly important in the United States. To date, many foresters have relied on a conifer plantation model as the basis of establishing and managing hardwood plantations. The monospecific approach suggested by the conifer plantation model does not appear to provide for the development of quality hardwood logs similar to those found in natural hardwood stands. Thus, there is interest in creating mixed species plantations to simulate natural hardwood stand development.

A simulation system, CherrybarkSQ, was developed to provide a platform for investigating the impacts of mixed species management of hardwood plantations on the stem quality of cherrybark oak (Quercus pagoda). Stem wood quality was defined by the proportion of clear wood (without knots) grown over a knotty core expressed by a Clear Wood Index (CWI) value. The construction of CherrybarkSQ consisted of developing models for predicting first-order branch characteristics and the occlusion of first-order branches for a distance-dependent individual tree model. CherrybarkSQ tracks the production of the knotty-core and clear wood over time through relationships developed between crown length and branch diameter and branch diameter and the overwood needed to occlude the branches.

CherrybarkSQ was used to simulate the development of four hardwood plantation designs, three cherrybark oak monoculture designs (Pure, PureThin and Pure25) and a cherrybark oak and sweetgum mixed design (Mix) over a 50-year period. Among designs with similar initial stand density (Pure, PureThin and Mix), the Mix design produced the lowest CWI, an indication of greater clear wood production. The Mix plantation design consistently out-performed the Pure and PureThin designs. The low-density Pure25 design had the lowest CWI and the largest average diameter.

The results of the CherrybarkSQ simulations indicate that when initial stand density is similar, a mixed species approach to cherrybark oak plantation management produces greater amounts of clear wood. A low-density monospecific approach produces a similar proportion of overwood, but the distribution of that overwood is on a shorter branch-free bole compared to the longer branch-free bole of trees at the greater density. The impact of hardwood plantation management decisions on clear wood or stem quality production, and therefore value, offers a great opportunity for evaluating hardwood plantations. Models and evaluations like those provided by CherrybarkSQ can assist with the interpretation, management and future development and value of mixed species plantations.