Ten Year Results of The Bear Mountain Mixedwood Trial (EP 1077): The Effect of Overtopping Aspen Canopies on White Spruce Seedling Growth and Seedling Microclimate

In a previous research note (Coopersmith and Hall 1999) we described the objectives and initial research findings of the Siphon Creek (Fort St. John) Installation of Experimental Project (EP) 1077. A second EP1077 installation, Bear Mountain, will be the focus of the present report. The previous research note (PG-17) concentrated on the effects of overtopping aspen canopies on understorey spruce growth and yield, and proposed that a simple height to diameter ratio (HDR) could be a useful tool to evaluate the degree of overtopping by both broadleaf and herbaceous vegetation. HDR has been shown to be highly dynamic, reacting quickly to changes in the growth conditions experienced by white spruce (Picea glauca) seedlings, especially changes to their light environment. HDRs as low as 40 have been observed for well-established, open-grown 10year-old spruce at Siphon Creek. These ratios rise quickly as competition for light increases. HDRs appear to have stabilised at or near 70 for white spruce saplings beneath trembling aspen (Populus tremuloides) cover at 5000 and 10,000 stems per hectare (sph) treatments at Siphon Creek (Coopersmith and Hall 1999). In this report we again compare height and diameter growth of white spruce seedlings beneath various densities of aspen cover to see if our 1998 research results at Siphon Creek are repeatable. In addition, we will examine in detail the effects of overtopping broadleaf canopies on the seedling microclimate of the underplanted spruce. Overtopping broadleaf cover can affect understorey conifer crops directly, by limiting the amount of light for photosynthesis and shifting the ratio of red to far red light (Man and Lieffers 1997), and indirectly, by altering seedling microclimate (air temperature and humidity, soil temperature and soil moisture [Man and Lieffers 1997, 1999, Childs and Flint 1987]). These indirect effects can have both positive and negative affects on understorey conifers. Sheltering overstories of nurse crops can substantially reduce growing season frosts, especially so-called “radiative” frosts which occur on clear-sky nights (Stathers and Spittlehouse 1990). Such frosts occur frequently in areas of the Boreal White and Black Spruce (BWBS) biogeoclimatic zone during the growing season, and can cause substantial damage to young conifer plantations (Dang et al. 1992). Because they produce large amounts of leaves in less time than do conifers, broadleaf nurse crops can transpire substanTen Year Results of The Bear Mountain Mixedwood Trial (EP 1077): The Effect of Overtopping Aspen Canopies on White Spruce Seedling Growth and Seedling Microclimate By Dave Coopersmith1, Bob Sagar2 and Doug Thompson3