|The purpose of this study is to investigate the capabilities and short comings of SORTIE-ND as a predictive model for growth and stand dynamics in sub-boreal forests of central BC, especially those impacted by the Mountain Pine Beetle (MPB) epidemic. SORTIE-ND is an individual tree, spatially explicit model of stand dynamics that originated from the small scale disturbance model SORTIE developed and tested in the mid 1990ís for transitional oak-northern hardwood forests in the northeastern US (Pacala et al. 1996). SORTIE was designed to extrapolate fine-scale/short-term field measurements to large-scale, long-term forest dynamics (Pacala et al. 1996). In recent years SORTIE was parameterized for mixed forests in northwestern British Columbia and modified to be better suited for dealing with management issues (SORTIE/BC; Coates et al. 2004). SORTIE/BC has recently been restructured and reprogrammed in C++. The result is SORTIE-ND (www.sortie-nd.org) where ND signifies the modelís focus on local neighbourhood dynamics. Forest dynamics is the change of forest composition and structure over time. The spatio-temporal development of forests may be described as changes of tree populations due to birth and colonization, growth and death of trees. SORTIE-ND uses a combination of empirical and mechanistic sub-models to predict forest dynamics based on field experiments that measure fine-scale and short-term interactions among individual trees. The model structure is that of an ecological model rather than a traditional empirical growth and yield model. This model structure makes SORTIE-ND well suited for growth prediction in complex stands and recent model development (snag dynamics sub-model developed with a recent FSP grant) have made the model especially suited for growth predictions in complex structured MPB attacked stands. In order to practice sustainable forest management, it is a necessity to have growth models that can project future growth of stand types, including complex structured stands. A given growth model can never be good at predicting growth under all the different stand conditions found throughout BC in natural and managed stands. Thus, it is important to have a host of models that each have there strengths and weaknesses but can be utilized under the conditions where they are most suitable. In this way, SORTIE-ND would provide a good addition to the models that currently are most frequently used for timber supply analysis in BC. For a model to be used in timber supply analysis it is important that the model is evaluated and that efforts are made to obtain unbiased model predictions. Evaluation is defined as a process in which a modelís conceptual structure and predictions are described and assessed with regard to a specific purpose, for example, volume development of MPB affected stands over time. Consequently, this definition encompasses what is often referred to as validation and verification in the modeling community. Currently, SORTIE-ND is parameterized, but not calibrated. Parameterization is the process in which the parameters in an equation are fitted to a dataset. Calibration is the processes in which the predictions from a model are compared with observations and afterwards one or more parameters in the model are changed to produce predictions that match the observations. Most traditional growth and yield models have some kind of calibration performed in order to make the predictions realistic. Equally, SORTIE-ND will likely conform better with, for example, permanent sample plot (PSP) data if the model was calibrated. Calibration is, however, against the basic ideas from which the model was developed. As a research tool used to understand basic ecological questions such as successional dynamics, calibration is not appropriate as is does not necessarily allocate the changes to the correct processes. For predicting growth for Timber Supply Review the calibration of SORTIE-ND is more appealing since it could reduce biases in critical predictions, for example, volume development over time. The intent of this project is to evaluate overall model performance and determine if calibration of SORTIE-ND is desirable from the perspective of supporting timber supply analysis. The topic of model evaluation and validation has a rich literature. For example, a recent paper by Pinjuv et al. (2006) discusses an approach to the quantitative validation and comparison of different forest growth models. We will carefully review this paper and other related literature for insights into how to best evaluate a complex hybrid forest dynamics model like SORTIE-ND. |
Coates, K.D. Canham, C.D., Beaudet, M., Sachs, D.L. & Messier, C. 2003. Use of a spatially explicit individual-tree model (SORTIE/BC) to explore the implications of patchiness in structurally complex forests. For. Ecol. Manag. 186(1-3): 297-310. Pacala, S.W., Canham, C.D., Saponara, J., Silander, J.A., Jr., Kobe, R.K, Ribbens, E. 1996. Forest models defined by field measurements: II. Estimation, error analysis and dynamics. Ecol. Monogr. 66:1-43. Pinjuv, G., Mason, E.G., Watt, M. 2006. Quantitative validation and comparison of a range of forest growth models. Ecol. Manag. 236:37-46.