|SORTIE-ND is a spatially-explicit individual tree model that originated from the small scale disturbance model SORTIE (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-ND was parameterized and developed as a growth model for mixed forests in northwestern British Columbia (e.g. Kobe and Coates 1996, LePage et al. 2000, Canham et al. 2004, Astrup 2006) and modified to be better suited for dealing with management issues (Coates et al. 2004). Growth models for silvicultural regimes such as underplanting, partial cutting, and utilization of secondary structure in mountain pine beetle (MPB) attacked stands should be: (1) capable of simulating mixed-species stands, (2) process-oriented due to limited relevant long-term data, (3) spatially-explicit due to the high degree of spatial heterogeneity, and (4) good at predicting understory tree growth. The stand level growth model SORTIE-ND possesses these features. Consequently, there is increasing interest in applying SORTIE-ND for simulation of complex stand growth in both boreal and sub-boreal British Columbia. In a previous FSP project, we performed an evaluation of SORTIE-ND as a growth model for mixed aspen-spruce stands (Astrup 2006). SORTIE-ND was evaluated in terms of its conceptual structure, a sensitivity analysis was performed, and the model predictions were compared to independent permanent sample plot data. The evaluation suggested that SORTIE-ND is a suitable model for growth prediction in complex mixed-species stands. However, the evaluation also illustrated topics where further model development can improve the models robustness and predictive ability. The most critical topic for model development is related to crown allometry (crown diameter and crown length) (Astrup 2006). In SORTIE-ND, crown allometry affects the prediction of understory light, which is a critical resource for prediction of tree growth and mortality in complex stands. Thus, improvement of the crown allometry functions will improve the models ability to predict understory tree growth and mortality. Crowns of similar sized trees do generally not overlap. Thus, crown radius of an individual tree is strongly dependent on local neighbourhood composition and structure. Crown shyness is the empty space that can be observed between individual crowns of similar-sized trees. Crown shyness is believed to be caused by breakage of branches caused by crown collisions during wind events (Rudnicki et al. 2002). Additionally, crown shyness is believed to be more evident in boreal and sub-boreal forests where branches are brittle during cold winter conditions (Lieffers et al. 2001). Crown depth is generally determined by light availability which in turn is dependent upon the surrounding tree neighbourhood structure. In SORTIE-ND, crown length of an individual tree is a linear function of tree height while crown radius is a nonlinear function of diameter at breast height (DBH). These relationships are completely independent of the surrounding trees (Coates et al. 2004). Simultaneously, SORTIE-ND allows individual crowns to overlap. Thus, both crown radius and crown length are often overestimated in dense stands and underestimated in open stands. The result is a potential bias where understory light availability is underestimated in dense stands and overestimated in open stands. In SORTIE-ND, growth of individual understory trees is predicted based on light availability while mortality of individual understory trees is a function of recent growth (Coates et al. 2004). A bias in understory light predictions can consequently lead to a bias in understory tree growth and survival. To resolve this potential bias and make SORTIE-ND a more robust growth model for complex stands, we propose to create a new model of crown allometry. Our primary objective is to develop and parameterize a distance-dependent model of crown allometry for SORTIE-ND. We intend to do this with a two year project. In year 1, we will collect the required data, develop, and test alternate statistical models. In the second year, we will incorporate the findings into SORTIE-ND, undertake model testing, and complete reporting and extension of our results. Our secondary objective is to measure and describe crown shyness in complex stands. Some studies related to crown shyness have been undertaken in western Canada (Rudnicki et al. 2002), but never for mixed-species stands. Thus, an investigation of crown shyness is of academic interest. Additionally, the study of crown shyness can potentially be useful for implementing rules about crown shyness into stand-scale simulation models used in BC (e.g. SORTIE-ND, TASS, PrognosisBC, and FORECAST). The results will be applied to forest management through the use of SORTIE-ND. Thus, our results can aid in predictions of growth of complex stands such as stands with mountain pine beetle attack, partial cutting, or variable retention. |
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