Forest Investment Account (FIA) - Forest Science Program
FIA Project Y081150

    Modelling Light, Site Quality and Crowding Effects on Growth of Understory Subalpine Fir in Lodgepole Pine Forests
Project lead: Simard, Suzanne (University of British Columbia)
Author: Simard, Suzanne W.
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
Managed forest stands will become increasingly complex in structure and species composition in the future because of the positive trends toward sustainable forest management. Foresters and land managers will need to understand the implications and trade-offs between timber production and other ecosystem values under new silvicultural strategies that will include continuous retention of canopy trees and management of multiple tree species with different life history characteristics. To design silvicultural strategies that optimise timber production and sustain other values, foresters and managers will require an understanding of how juvenile trees (seedlings and saplings) grow and survive across the full range of potential canopy retention levels and site qualities. Tree species preferences, expected growth rates and survival probabilities will likely shift among tree species across gradients of canopy retention, site quality and local neighborhood crowding by other juvenile trees. Using data collected in the Montane Spruce zone in linked FSP project, Y072028 (Improving predictions of juvenile tree growth in complex mixtures for sustainable forest management), we will model growth responses of juvenile subalpine fir to variation in residual lodgepole pine canopy cover, site quality, and crowding under a range of silviculture systems scenarios using the SORTIE model. The SORTIE model was originally developed as a small scale disturbance model in the early 1990’s (Pacala et al 1993; Pacala et al 1996). Recently, the model was re-parameterized with data from northern interior mixed species stands and the latest version of the model is now referred to as SORTIE-ND (Kobe and Coates 1997; Wright et al. 1998, 2000; Canham et al. 1999; LePage et al. 2000). SORTIE-ND is a resource-mediated, spatially explicit, mixed-species forest model that makes population dynamic forecasts for juvenile and adult trees and can test to a wide range of silvicultural systems. Initial attempts at calibrating the SORTIE-ND model for Northern Interior mixtures suggest that an improved understanding of the factors affecting juvenile tree growth would greatly enhance the model’s ability to make accurate growth predictions. Earlier work in northwestern BC has concentrated on the light-growth relationship for juvenile trees (Wright et al. 1998, 2000), since light has been shown to be a good proxy for predicting juvenile tree growth (Klinka et al. 1992, Pacala et al. 1994, Wang et al. 1994, Kayahara et al. 1996, Chen 1997, 1998, Coates and Burton 1999, Claveau et al. 2002), and is the resource most easily manipulated by silvicultural interventions. The light-growth relationship, however, has been shown to vary with site quality, especially at higher light levels (Drever and Lertzman 2001). Growth response to increasing light availability may be reduced, for example, on sites where soil and nutrient availability are limiting. Conversely, low soil resource availability may be compensated to a certain degree by high light availability. Competition for soil resources has also been shown to increase in importance relative to light with increased crowding (Simard and Sachs 2004). This research assesses how the light-growth relationship varies across a range of site qualities. In this proposal, we will use a modeling approach that builds on SORTIE-ND, allowing quantification and prediction of juvenile tree responses to different management strategies in complex structured, mixed species stands. The new SORTIE-ND submodel will be capable of making growth predictions for seedlings/saplings growing across a light gradient over a wide range of tree densities and site qualities, where site quality is defined by soil moisture regime. We will develop the submodel using the C++ programming language. C++ is a versatile programming language which will allow us to easily integrate the submodel into SORTIE-ND and possibly other existing growth models. SORTIE-ND is a resource-mediated, spatially explicit, mixed-species forest model that makes population dynamic forecasts for juvenile and adult trees. It has a flexible user-interface that allows the user to incorporate a wide range of silvicultural strategies (e.g., clearcutting, understory protection, understory planting, diameter limit harvesting, shelterwood, single or group selection, and variable retention). The development of this submodel is vital to the continued development of the SORTIE-ND model because current versions are unable to accurately predict juvenile tree growth at low light levels, in dense neighbourhoods of seedlings and saplings, and on sites drier or wetter than mesic. This modeling exercise builds upon a recent FSP project (Y061028 – “Improving predictions of juvenile tree growth in complex mixtures for sustainable forest management”) whereby we were able to identify important relationships among juvenile tree growth, specifically site crowding and resource availability (soil moisture regime). These findings will have been incorporated into the existing SORTIE-ND juvenile tree growth behaviours by end of this fiscal year (2006/07). However, initial parameterization suggests that the existing behaviours in SORTIE-ND are too simplistic and unable to capture the entire scope of our ecological findings. We believe that through the development of a new behaviour we will be able to incorporate all of our findings and drastically improve the model’s ability to predict the growth responses of juvenile trees to variation in residual canopy cover, site quality and crowding. The first step in the submodels development involves determining what variables will be used. For this study we will be using available light, site quality and crowding to predict juvenile tree growth. The next step in the developmental stage involves drafting a design of the submodel which will determine how the model functions. Using this draft as a framework, we will begin to develop the submodel’s data structure. This lists what kinds of data the submodel will require and what data structures it will utilize. We will then design the submodel’s actions which describe what will be done during each timestep (i.e., what tasks the submodel performs each time it is called upon by the SORTIE-ND model). Finally we will write our tests and prepare the appropriate test data. Once we are confident the submodel is functioning correctly we will imbed it into SORTIE-ND stand dynamics model. The last step will involve testing how the new submodel performs within SORTIE_ND. Final validation and testing of the model will be done using pre-existing permanent sample plot data. Once the model has been tested we will conduct a variety of simulations which will provide scientific information for developing guidelines and decision support tools for the management of structurally complex, mixed species, multi-cohort stands.
Related projects:  FSP_Y092150
Contact: Simard, Suzanne, (604) 822-1955,


Executive Summary (24Kb)

To view PDF documents you need Adobe Acrobat Reader, available free from the Adobe Web Site.

Updated August 16, 2010 

Search for other  FIA reports or other Ministry of Forests and Range publications.

Please direct questions or comments regarding publications to