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Ministry contact:George Harper Research ScientistB.C. Ministry of Forests and RangeResearch BranchP.O. Box 9519, Stn. Prov. Govt.Victoria, BC V8W 9C2(250) 387-8904George.Harper@gov.bc.ca

Ken Polsson Stand Modelling AnalystB.C. Ministry of Forests and RangeResearch BranchP.O. Box 9519, Stn. Prov. Govt.Victoria, BC V8W 9C2(250) 387-6948Ken.Polsson@gov.bc.ca

Extension NoteModelling Boreal Mixedwoods with the Tree and Stand Simulator (tass)

Introduction

Boreal mixedwood forests are a com-plex mixture of conifer and broadleaf species (Figure 1). These forests vary in structural, spatial, and temporal composition (Green 4). The management of boreal mixedwoods continues to evolve as our under-standing of mixedwood ecology, regeneration, and stand dynamics grows. Gap analysis of present boreal mixedwood management has suggested that several key issues need address-ing, including: lack of fundamental ecological data, inadequate forest inventory, and limited development of site productivity and growth and yield modelling tools tailored for boreal mixedwood forests (unbc ).

Modelling the growth and yield of boreal mixedwood forests is one of the main concerns of northern for-est managers. The key to improving boreal mixedwood management is accurate modelling, which forecasts mixedwood responses to silviculture practices, natural disturbance, succes-sion, and stand dynamics (Chen and Popadiouk ; Greene et al. ; unbc ; Hawkins and Maundrell 3). We have only limited knowl-edge and understanding of the growth,

yield, and stand dynamics of boreal and sub-boreal mixedwoods and their interactions with site productivity, silvicultural practices, economics, and other factors. Recent studies have high-lighted specific gaps in our knowledge and management applications in Brit-ish Columbia (unbc ; Hawkins and Maundrell 3; Green 4). Individual-tree responses and stand

Ministry of Forests and Range Forest Science Program

figure 1 Aspen–spruce mixedwood.

dynamics of boreal mixedwood forests differ from those of single-species stands (Man and Lieffers 1999; Chen and Popadiouk ).

Traditional long-term research trials cannot provide the necessary informa-tion quickly enough to guide timely decisions. Enhancing our knowledge base and modelling capabilities will provide resource managers with more biologically defensible, science-based decision-support tools.

Boreal Mixedwood Forest Dynamics

The boreal forests of western Canada are dominated by aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss). In Brit-ish Columbia, aspen is also a common associate of lodgepole pine (Pinus contorta var. latifolia Dougl.) on many sub-boreal and boreal interior sites (Meidinger and Pojar 1991).

Aspen has rapid juvenile growth, and commonly overtops conifers, such as white spruce and lodgepole pine. An overtopping broadleaf canopy influ-ences understorey trees, modifying microclimate by affecting available light, nutrients, and air and soil tem-peratures (Comeau 1; Pritchard and Comeau 4; Voicu and Comeau 6). Survival and growth of conifers can be optimized through silvicultural practices designed to modify light and temperature (McKinnon and Kayahara 3). Many management practices are designed to alter the tree species mixture and structure by manipulat-ing microclimatic conditions. This is typically achieved by modifying the overstorey tree canopy (usually aspen) (Ruel et al. ; Chen and Popadiouk ; McKinnon and Kayahara 3).

Studies of shade tolerance in boreal conifers show that tree growth and survival depends on species’ ability to acclimate their growth rates to changes in available light. Responses differ among conifer genera, with shade-in-

tolerant Pinus allocating more biomass to stem height growth under low light, whereas shade-tolerant Abies priori-tizes foliar growth to optimize light capture (Claveau et al. ; Claveau et al. 5; Claveau et al. 6). Shade-intolerant conifers, such as lodgepole pine, may suffer mortality and reduced growth under aspen competition. More shade-tolerant conifers, such as white spruce, reduce their height growth, live crown ratios, and root-to-shoot ratios when growing beneath aspen (Messier et al. 1999) (Figure ).

Tree growth responses to light are crucial to our fundamental under-standing of forest stand dynamics and individual-tree performance (Carter and Klinka 199; Wright et al. 1998; Comeau 1). Light is one of the key determinants of understorey growth and survival (Kaufmann and Ryan 1986; Chen et al. 1996; Williams et al. 1999). Consequently, extensive

research has focused on understanding and predicting light levels in relation to boreal mixedwood regeneration dynamics and understorey growth (Comeau et al. 1998; Greene et al. 1999; Lieffers et al. 1999; Messier et al. 1999; Pinno et al. 1; Lieffers et al. ).

Predicted light availability is a fundamental parameter underlying the simulation of individual-tree growth and yield models. Several spatial light models have been developed that estimate light dynamics within stands (Canham 1995; Brunner 1998; Stadt et al. 1; Groot 4). Many studies have measured forest light intercep-tion and transmission (Gendron et al. 1998; Pinno et al. 1; Pritchard and Comeau 4).

The tass Model

The Tree and Stand Simulator (tass) is the most widely used source of managed stand yield information in British Columbia. Yield tables gener-ated by tass are made available to the public and users via the software tipsy (Table Interpolation Program for Stand Yields) (Mitchell et al. 4). Together, these two systems predict managed stand yields for most coniferous species and two broadleaf species (red alder and trembling aspen) throughout British Columbia.

Our newest version of tass (tass iii; Figure 3) is a multi-canopy ap-plication that includes the light model trayci (Brunner 1998), which simu-lates spatially explicit understorey light levels (Gersonde et al. 4). Presently, tass iii development aims to improve the model capacity, precision, and accuracy of predicting mixedwood and complex stands. Simulating boreal mixedwood stand dynamics requires additional func-tional parameters to model tree-level crown and height growth responses to light, as well as to other micro-environmental variables. Stand-level information is also required to cali-

figure Complex canopy spatial and temporal dynamics: foliage den-sity, clumping, and canopy gaps.

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needed to apply quantitative predic-tions to the landbase (Figure 4).

Data from over 1 mixed conifer–broadleaf psps, predominantly from British Columbia, Alberta, and Manitoba, have been compiled and summarized. We have also obtained data from mixedwood research trials from both internal and external sources. These data are particularly valuable, since they document re-sponses to thinning, partial cutting, and other silvicultural practices.

Summary

Modelling the growth and yield of bo-real mixedwoods is a logical progres-sion in our model development efforts, building on past successes with single-species simulations of conifers and broadleaves. The complexity of model-ling mixed-species stands presents challenges to our current knowledge base and biological understanding. Key issues need to be addressed through further research and data collection.

Research is needed to discern the response of boreal species to changes in soil moisture, soil temperature, and air temperature following harvesting and silvicultural practices. Commit-ment to long-term field data collection and research will ensure that we have the necessary information to support growth and yield model predictions.

brate and validate stand-level model output. Adding quantitative relation-ships derived from field research and observations will improve model accuracy and reliability. Our focus on the biology of understorey tree growth, light relationships, and de-velopmental patterns will ensure that the model primarily reflects biologi-cal principles, rather than empirical functions.

Approach

The objective of this modelling proj-ect is to provide biologically realistic growth and yield estimates for boreal mixedwood stands across British Columbia.

To support tass modelling and improve our knowledge and under-standing of boreal mixedwood stand dynamics, we reviewed information from published and unpublished sources. Tree- and stand-level data were obtained from research groups and institutions across Canada. Tree-level information will be used

to formulate light-dependent growth functions to incorporate individual-tree growth in response to simulated light resources (trayci predictions) in the tass iii model. Stand-level information will be used to adjust the model to accurately predict field expectations of size-class distribution, density, and volume.

Tree-level data—spruce and pineOur information review identified 48 and 3 studies for spruce and pine, re-spectively, with both light and growth information on height, diameter, and crown expansion. Most of these stud-ies measured diameter and/or height increment (Astrup 5a, 5b). Un-der aspen-dominated canopies, spruce (mainly white spruce) height incre-ment tended to follow an asymptotic pattern. There was a rapid increase in height increment in stands with up to 5% of available above-canopy light, and height gains diminished to less than % where light levels were above 5%. For pine, there was insufficient information to define this relationship in aspen-dominated stands.

As Astrup (5a, 5b) and oth-ers (MacPherson et al. 1; Machado et al. 3; Claveau et al. 5; Voicu and Comeau 6) have indicated, tree growth can be affected by many factors in addition to light availability. This drives the wide variation in ob-served growth at any given light level (Harper et al. 6). Other studies have also compiled boreal mixedwood research, synthesizing light and tree growth information (Lieffers et al. 1999; Man 3; McKinnon and Kaya-hara 3; Man and Greenway 4).

Stand-level data—permanent sample plotsHistorically, permanent sample plot (psp) data have been used to support stand-level calibration and final model adjustments. This process refines mod-el output to emulate field observations, and provides the statistical confidence

figure 3 TASS III simulation, aspen–spruce mixedwood.

figure 4 TASS III simulation, aspen–spruce mixedwood.

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Developing and refining spatial, tree-level models such as tass will provide the needed flexibility to quan-tify the uncertainties associated with succession and site-specific conditions encountered in boreal mixedwood stands (Green 4). By maximizing both detail and flexibility, spatial tree-level models facilitate the evaluation of size-class output, stand treatment options, utilization criteria, and har-vesting options (Zedaker et al. 1987).

This project was designed to ad-dress the need for a provincial boreal mixedwood growth and yield model. Simulations of mixedwood stand dynamics will help define information needs and support sustainable timber supply projections. The development of tass iii will enhance our ability to explore innovative silviculture and adaptive management options, and enable sensitivity analyses to support science-based management decisions.

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Acknowledgements

Financial support was provided by the Forest Investment Account (fia) For-est Science Program (fsp).

The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the Government of British Columbia of any product or service to the exclusion of others that may also be suitable. This Extension Note should be regarded as technical background only.

CitationHarper, G. and K. Polsson. 7. Modelling boreal mixedwoods with the Tree and Stand Simulator (tass). B.C. Min. For. Range, Res. Br., Victoria, B.C. Exten. Note 8. <http://www.for.gov.bc.ca/hfd/pubs/Docs/En/En8.htm>