The practice of partial cutting is increasing in the Central and North Coast areas of the BC and is forecast to increase in application, especially with the adoption of EBM. Partial cutting is typically applied to meet non-timber objectives on the coast but appears to result in significant reductions in growth in the regeneration cohorts. In order to optimize management, practices supporting non-timber forest values, such as medium or high-retention partial cutting (> 20 40% of the original stand retained), need to be balanced by limiting reductions in timber growth and value. The effect of medium to high retention resulting from partial harvest on the growth of understory regeneration is poorly documented for wetter areas of the coast where Douglas-fir is not a dominant species. Although we understand that, in general, increasing retention results in reduced growth, quantification suitable for prescription writing and stand growth modelling is lacking. Simulations of overstory influence on understory with TASS are unsatisfying because the model uses second-growth trees, which tend to have different crown structures than the old-growth trees typically present in operational partial cutting.
Understanding the effects of residual stands on regeneration growth is pivotal, if timber values are not to be unduly compromised by the application of partial cutting to meet non-timber values. This is a critical issue for prescribing and reviewing foresters and for decision makers. Furthermore, timber supply impacts cannot be adequately modelled for these areas if the residual-overstory understory relationship is not quantified.
The study will be conducted at an existing partial cutting installation in the Rennell Sound area, Queen Charlotte Islands (CWHvh2 BEC unit). Regeneration data was last collected from the site in 2007 (Y08 2258) but overstory quantification and the relationship to understory growth has not yet been established. Other trials have examined the effects of partial cutting on regeneration and residual stand growth elsewhere on the coast or have examined the major timber species present in this trial in partial cutting trials in the ICH BEC unit. However, this installation is the only replicated partial cutting experiment that has yet been established that is applicable to the North and Central Coasts of BC. The project is particularly timely, as the Council of the Haida Nation, as well as other first nations supporting the EBM initiative on the Central and North Coasts, have expressed the desire to see the wide application of partial cutting practices.
This study will explore the effects of residual stands on understory light and regeneration growth and answer the essential question: How do increasing amounts of (old-growth) overstory affect regeneration growth? A secondary question is: Are densities of sitka spruce natural regeneration associated with residual amounts of sitka spruce in the overstory? This second question was prompted by our observation from Y08 2258 data that substantial amounts of sitka spruce natural regeneration were present in treatment areas with residual sitka spruce in the canopy. This is an important question, as natural regeneration in this area tends to be dominated by the less valuable western hemlock, rather than sitka spruce of western redcedar. The influence of other residual canopy species i.e. western hemlock and western redcedar, on achievement of natural regeneration, by species, will also be examined. A final question is: How is height growth of regeneration affected by below-canopy light levels in residual stands? This question was prompted by the observation during Y08 2258 that sitka spruce regeneration seemed to be growing in areas with quite low levels of understory light.
Our approach will be to collect data describing residual stand basal area, and composition by species, light, as percent above canopy level (PACL) and canopy openness; and understory regeneratio
Negrave, Roderick W.. 2009. Residual Stand Effects on Light Transmission and Understory Conifer Growth in Partially Cut Stands in the CWHvh2, Queen Charlotte Islands. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), British, Columbia
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