Forest Investment Account (FIA) - Forest Science Program
FIA Project Y102104

    Microclimate and tree growth as affected by western hemlock variable retention groups.
Project lead: Andrew Black (University of British Columbia)
Author: Black, T. Andy
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
In BC and around the world more trees are being left behind in forest openings such as through variable retention (VR). Leaving behind increased numbers of trees results in more forest edge environment compared to a clearcut and hence, a more variable and complex microclimate. The use of VR is a relatively new development in forest management in BC (Beese et al. 2001) so that very little data has been collected. Similarly there are few microclimate models that attempt to make linkages to tree and stand growth. Those that do are also in the research phase. Our intent is to characterize the patterns of VR stand microclimate (primarily group retention) and its impact on survival and growth of the next crop in coastal BC forests. We will transfer our findings to a microclimate driven growth model that will synthesize and help generalize the results of our research.

VR harvesting results in higher spatial variability in microclimate that may limit regeneration survival and growth. Increasing forest edge affects air and soil temperatures, soil water content, and solar irradiance; in turn these factors affect soil biological and chemical processes critical to tree establishment and growth (Spittlehouse and Stathers, 1990, Prescott et al. 1997). Research into the effects of novel silvicultural systems on microclimate has most often focused on the differences between open, forest edge and forest interior microclimates created by forest gaps and clear-cut strips rather than retention patches. Spittlehouse et al. (2004) showed that most of the changes in microclimate take place within one tree height on either side of a forest/opening edge. In BC, recent work on TASS (Mitchell, 1977) and SORTIE-ND (Canham, 1998) has focused on collecting fisheye photography linked to young stand height and diameter growth to characterize light or competition effects on individual trees. FORECAST (developed from Kimmins et al. 1999) is a hybrid–physiological model based on generalized relationships from the literature. We believe that we can make original contributions to this important body of work by undertaking detailed microclimate measurements that help reveal fundamental relationships between tree growth and environmental conditions. Fisheye photography can be used to help collect light information more cheaply across a variety of sites but fisheyes are simulations of light that cannot substitute for actual detailed light measurements. WFP has also undertaken similar fisheye work funded elsewhere (

Very little work has been done on the VR group retention that forms the bulk of retention systems in coastal BC: few group retention sites are more than a few years old and fewer still have been controlled experimentally to permit hypotheses tests. Unique silvicultural systems have been created that cannot be understood by simply generalizing light relationships determined from clearcut regenerated forests. WFP has a set of 10 large scale, recently established experimental sites (Beese et al. 2005) that can form the foundation for rigorous microclimate research. In Y083141 we used one of these sites (MH4902, a 60 ha Douglas-fir mixed retention site;; Smith et al. in preparation for Can J. For. Res, 2007). We also have data from an FIA project (see the FORGE link below) at TM188, a 100-ha dispersed Douglas-fir retention experimental site. For this project we propose to establish transects at the 100-ha Port McNeill group-amount retention site (5599). Here there is a 20-ha clearcut, a 20-ha uncut area, and three 20-ha sites with three different-sized retention patches (all at 15% total retention): small (0.1 ha), medium (0.25-0.5 ha) and large (0.8-1.0 ha). The same seed-source-stock type of western-hemlock has been planted throughout. Within all experimental areas ‘sector plots’ have been established to examine the effects of forest edge on growth (Iles and Smith, 2006, Smith et al., 2008). The sector plots are designed to sample growth-response gradients across VR edges; planted, natural and retained trees are spatially located within the sector plots. The sector plots are randomly located and oriented but sample N, S, E and W facing edges. For group retention or group removal treatments, four 9-degree sectors tied to a central 'pivot-point' are established with random angles in a minimum of 3 patches per treatment. Our plan is to establish permanent microclimate transects monitoring light, soil moisture, soil temperature and wind speed. In the first year we will establish transects in the large patch as we hypothesize that microclimate will be most affected there. Height and stem-diameter measurements of planted seedlings will be partly supported from other funding sources as an in-kind contribution. Due to complications in setting up the very long cables needed for deployment of the fixed transects around the large patch in the first year, installation was delayed until July, 2008. To ensure adequate growing-season data coverage, we will leave the fixed transects installed at the large patch until late summer 2009, then move them to the small patch area. We will not undertake a year-long measurement in the medium patch as originally planned, as there are no suitable medium sized patches where unhindered 30-m long transects can be established.

We plan to embed the microclimate relationships into the FORGE model (see FORGE already has the equations of Chen et al. (1993) for light, and Black and Spittlehouse (1980) for water balance and equations from Campbell and Norman (1998) for leaf temperature coded into a spatially explicit growth modelling framework (Smith et al, 1991). FORGE is based on a default of 5 m x 5 m grids populated with trees that grow leaf area and interact. An elevation model has been incorporated. Light, moisture and temperature affect net photosynthesis which in turn drives height and diameter growth. We are determining the impacts on growth from net photosynthetic curves in the seedlings (as part of this project) and from plot remeasurement data at other older Fluxnet Canada sites where we also have estimates of net photosynthesis (as an in-kind contribution). We are using detailed microclimate data to test the simulation predictions of FORGE. This project therefore addresses the important management issue of the impacts of variable retention on the microclimate and survival and growth of the next crop in Coastal BC forests. The results will have implications for all retention systems and for silvicultural prescriptions (planting locations, impacts of natural regeneration), habitat provision (structural components of the stand) and timber supply analysis (through yield prediction simulations). Plans for 2009-10 are to calibrate the FORGE model for the Port McNeill site as explained above.
Related projects:  FSP_Y091104FSP_Y113104
Contact: Nick Smith, (250) 327-9931,


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Updated April 29, 2011 

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