Forest Investment Account (FIA) - Forest Science Program
FIA Project Y072141

    Effects of VR harvesting on microclimate, survival and growth of trees in British Columbia
Project lead: Black, Andrew
Contributing Authors: Smith, N.; Cascadia Forest Products Ltd.; Black, T. Andy
Imprint: [Vancouver, BC] : University of British Columbia, 2007
Subject: Forest Investment Account (FIA), Variable retention harvesting, Trees, British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
The proposed research aims at determining the impacts of variable retention silvicultural systems on stand microclimate and the consequent response of planted seedlings, and incorporating the findings into an operational microclimate-tree growth model. This research fits into the FSP Timber Growth and Value Program: Theme 1 (Basic research on tree growth and stand growth development) 1.1. Complex Stands. Variable retention systems cover substantial parts of the forest land base in BC and elsewhere. Many forest companies and organizations are examining the advantages of leaving more trees unharvested as individuals or in groups. Following clearcut harvesting, radiant energy exchange shifts to near the ground surface so that planted seedlings are subjected to microclimate (temperature, atmospheric humidity and soil water content) extremes, which affect their survival and growth rates. On the other hand, variable retention harvesting results in higher spatial variability in microclimate. Low light levels near the unharvested trees may limit the growth of planted seedlings. Air and soil temperature and water content, and solar irradiance effects of variable retention also influence soil biological and chemical processes (e.g. mineralization of organic matter), which are critical to the establishment and growth of planted seedlings. Microclimatic information on variable retention systems is needed to identify the best type and extent of tree retention for optimal seedling survival and growth. Chen et al. (1993a) found that the highest microclimatic variability occurred at the edge rather than in the interior of an old-growth forest patch or the adjacent clearcut. We will modify the forest photosynthetically active radiation (PAR) model of Chen et al. (1993b) to calculate the spatial distribution of soil temperature, soil water content, and direct and diffuse PAR. This model will be used to test and improve the user-friendly stand-level cellular automata driven forest growth model (Forest Growth Engine, FORGE) recently developed by N. Smith and colleagues at Weyerhaeuser BC Coastal Ltd. This model links tree and stand growth to modifications in microclimate. The specific objectives of this study are to: (1) Quantify seasonal and annual changes in the microclimate in different variable retention systems in west coast forests. (2) Assess the impact of changed microclimate on the survival and growth rates of planted seedlings. (3) Model the impact of variable retention systems on seedling microclimate. (4) Test and improve the forest growth model (FORGE). While previous research has examined the transition in microclimates that occurs at the forest edge between the conditions of the forest interior and the open, the ongoing BCFSP project (Y072141, hereafter referred to as the 'parent project') to which this proposal is linked directly fills a critical need for a microclimatic study that focuses specifically on VR systems. The goals of the parent project are to validate the light model of the forest growth model FORGE and to modify the model (1) to more accurately describe the photosynthetic response of Douglas-fir seedlings growing in VR systems and (2) to include the effects of energy (solar and longwave radiation) and water (precipitation and evaporation) fluxes on seedling microclimate (temperature and moisture). Microclimate data (PAR, soil temperature and soil moisture content) collected along fixed sensor transects installed in each of the VR systems (dispersed, group and mixed group/dispersed) is being used to validate the model. A climate station used to measure half-hourly mean values of precipitation, solar irradiance, total and diffuse PAR, wind speed, air temperature and relative humidity, provides clearcut (reference) data to drive the model and produce output, which is then compared to the fixed transect data. Photosynthetic response experiments are performed on seedlings growing alongside the fixed transects periodically throughout the growing season to determine the effects of changing soil moisture and temperature, and phenological stage. In response to research team concerns that the study site reflect the predominant form of VR harvesting used by the collaborating partner (Cascadia Forest Products), the study site was moved to a mixed group/dispersed retention site located off the Malahat (MH) Highway on Vancouver Island from a dispersed VR site near Powell River at the beginning of May, 2005. While this new site presents logistical advantages of having group and dispersed VR treatments in very close proximity, in order to preserve the continuity of the data record and improve the robustness of our forest growth model validation, we propose to extend the deployment of the fixed transects from six months (May-October) to a full year in each VR type and to include additional photosynthetic response measurements in mid-winter and prior to leaf flush. Our aim is to capture seasonal changes in seedling light, temperature and moisture environment associated with fall, winter and spring solar elevation angles that would otherwise be missed, and to determine seedling microclimate associated with leaf flush at the beginning of spring and potentially during episodic, mid-winter periods of anomalous warmth and/or sunshine. Further, our validation of FORGE has been complicated by the fact that the slope of the terrain at the new site varies significantly over the area covered by the fixed transects. In order to properly validate the model, we will need to (1) modify FORGE to accept a user-input site map of elevation contours and (2) modify the fundamental algorithms in FORGE used to calculate view factors and direct beam attenuation.
Related projects:  FSP_Y061141FSP_Y083141


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Updated August 16, 2010 

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