The Effect of Physical Interactions and Understory Light Conditions on Long-term Stand Dynamics in White Spruce and Aspen Boreal Mixedwoods

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1 The Effect of Physical Interactions and Understory Light Conditions on Long-term Stand Dynamics in White Spruce and Aspen Boreal Mixedwoods Dan MacIsaac 1, Phil Comeau 2 and Ellen Macdonald 2 1 Canadian Wood Fibre Centre,, 2 Dept. Renewable Resources, University of Alberta, Edmonton, canadien

2 Trends in Aspen-Spruce Mixedwood Management Current emphasis is on retaining intimate mixtures of white spruce (Picea glauca) and aspen (Populus tremuloides) to meet a variety of ecological and management objectives White spruce understory protection through variable retention such as patch retention, disturbance-based management What are the implications of both species growing together? canadien

3 Individual White Spruce Release From Aspen Alberta Lees, Yang, MacIsaac White Spruce Stem Volume Growth Released at Spruce Age Class on Good Site Total Volume (m 3 /tree) Control N=30 Treated N= Year of measurement canadien

4 Implications of Single Tree Release Periodic spruce stem volume increases 55-93% between the treated and control trees (at a range of 10 to 60 years). Mechanism of growth response is light and physical abrasion (moisture not limited on these sites). White spruce shows incredible phenotypic plasticity in long term growth, under a range of stand conditions modelling even simple aspen-spruce mixtures is difficult. As aspen and spruce grow together, the slower spruce needs to eventually grow up through the canopy of aspen. What is the impact of close proximity? canadien

5 Research Objectives The research objective is to answer the following question: What is the importance of physical interactions (i.e., physical abrasion or leader whipping damage) as well as competition for light to the growth of white spruce into an aspen canopy, and how do fine-scale spatial patterns influence these interactions? canadien

6 Study Locations - Boreal Plains Ecozone Fire origin, white spruce-aspen stands, on upland mesic sites with clay-loam soils. Established in 1951, as part of a longterm CFS stand manipulation study on area-based release of white spruce from aspen competition. Measured in 1951, 1956, 1961, 1985, Source: Environment Study Sites canadien

7 White Spruce in Dense Understory Bertwell, SK Aug 1951 White Spruce Leader Whip Damage Bertwell, SK July 1959 canadien

8 canadien Aspen Removal Treatment Riding Mt., MB Plot 22 Sept 1951

9 Riding Mt Canopy Structure canadien

10 Physical Abrasion of Spruce by Aspen canadien

11 Tree Mapping and Measurements Data collected from 6 sites - 27 plots (8-60x60m, 19-20x20m) trees canadien

12 Hemispherical Analysis To quantify spatial variation in understory light conditions and crown architecture information, hemispherical images were taken during the period of maximum leaf cover. Images taken at a height of 1 m, spaced over a 4 m grid (36 images per plot). Analysis performed with Light Interception and Transmission Estimator (LITE) model (Comeau) Hemispherical Images Reserve, SK Leaf-On Sept-07 canadien Leaf-Off Oct-17

13 Full Plot Detailed Plot Design 60 m 20 m 4-6 plots at each site. At each site aspen removed in half the plots, the other half are uncut controls. Core Plot Location of hemispherical photography 5m x 5m understory vegetation plot canadien

14 canadien Stem Map: Locations of white spruce and aspen Big River Plot 11 (2001) - Control Distance West-East (m) Distance South-North (m) PB PB PB PB PB PB SB BW

15 White Spruce Height Growth in Control vs Release Plots Riding Mountain, MB Montreal Lake, SK Control Release 16 Control Release Height (m) 10 8 Height (m) Year Year canadien

16 White Spruce Growth in Control vs Release Plots Radial Increment Total Volume Increment (Individual Tree) Radial Increment at 1.3 m Ht (cm) Volume Increment (m3) Control Release 0 Big River Montreal Lake Riding Mountain 0.0 Big River Montreal Lake Riding Mountain Plots Plots canadien

17 White Spruce Growth By Leader Damage Class Height Increment by 1985 Leader Damage Class Total Volume Increment (Individual Tree) By 2002 Leader Damage Severity Class Height Increment (m) Total Volume Inc (m3) Damaged Not Damaged None 2. Light 3. Moderate 4. Severe canadien

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27 Conclusions White spruce trees growing in close proximity to aspen are negatively influenced by physical abrasion: Trees between 2-4 m are most at risk Significant effect on spruce growth and mortality, three-fold reduction in volume increment for trees with moderately to severely damaged leaders, compared to undamaged trees, based on analysis Light has a significant though lesser effect on spruce growth: regression of light levels with spruce growth: Understory light levels had a slightly better correlation with spruce growth than the amount of canopy gaps in the overstory. Light at top of spruce below the aspen canopy had the best fit (r 2 up tp 0.58) Spruce height was the best growth response variable to variations in light levels. Correlations from not as strong as those from a grid of hemispherical images was better than stem maps for regressions canadien

28 Conclusions (con t) Difference in growth between white spruce trees in control vs released plots accelerated since 1985, with a five-fold difference in total volume of individual trees in 2002 Stand tending operations may be required to remove these close competitors, while retaining some deciduous trees on site for ecological benefits. canadien

29 Acknowledgements We wish to thank the following for their support: committee members Drs. Christian Messier and Gitte Grover assistants Laura Chittick, Mark Schweitzer and John Cunnian Funding was provided by the NCE Sustainable Forest Management Network, Killam Trust Fund and Natural Resources,. canadien

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