Overstory density and harvesting method affect competition from understory vegetation. Klaus J. Puettmann and Matthew D. Duvall 1

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1 Overstory density and harvesting method affect competition from understory vegetation Klaus J. Puettmann and Matthew D. Duvall ABSTRACT. The retention of live vertical structure during harvesting operations is increasingly common in Minnesota. The growth of advanced regeneration can be significantly impacted by competition with understory vegetation. Because options for vegetation management are somewhat restricted beneath an overstory, silvicultural and forest harvesting techniques should be explored as tools for managing competing understory vegetation. We compared two stands that were partially harvested using different harvesting methods. Harvesting resulted in a range of residual overstory densities and different amounts of understory vegetation within each stand. A mixed birch-maple stand was harvested with a diameter limit cut in the summer of 994. The sale was felled by hand, and cable-skidder traffic was restricted to a defined skid-trail network, i.e., the openings (with measurement plots) were not trafficked. A jack pine plantation was strip cut in the summer of 995. This sale was also felled by hand, but cable-skidder traffic covered the full width of larger openings. Shrub vegetation made up the majority of the understory biomass and increased with increasing canopy openness on the birch-maple site but decreased on the jack pine site. The different response on the two sites likely reflects differences in soil compaction and setback of the understory vegetation during the harvest operation. These relationships show that harvesting treatments and residual overstory cover can be used to reduce understory competition with advanced regeneration. Harvests with residual overstory structure are increasingly common in Minnesota (Puettmann and Ek 998). Growth of advance regeneration is impacted by competing understory vegetation (Smidt and Puettmann 998). Vegetation management is more restricted beneath an overstory. Silviculture and harvesting can be tools for managing understory vegetation. OBJECTIVES Determine effects of overstory density and canopy type on understory resource availability and environments, using light as an indicator. Compare effects of overstory density and harvest method on the growth of competing understory shrubs. METHODS Overstory density was measured on both sites across the range of overstory densities. Basal area was measured at the center of each plot using a prism. Canopy openness (DIFN) was measured above the University of Minnesota Department of Forest Resources, 5 Green Hall, St. Paul, MN understory vegetation layer using a Licor LAI-2 Plant Canopy Analyzer. Shrub biomass was estimated in summer 997 in a - m x -m subplot at the center of each plot using allometric biomass equations (Smith and Brand 983). Soil compaction was measured across the light gradient using an Eijkelkamp penetrometer. The relationship between basal area and canopy cover (-DIFN) was modeled as: ( [] C= A( e b * BA ) ) where C is canopy cover, BA is basal area, A is an asymptote, and b describes the rate of canopy cover increase. Differences between sites were tested using ANCOVA where the data from both sites were pooled and site indicator variables were added to A and b: [2] (( b bc I BA C= ( A+ ( AC* I))( e + ( * ))* ) ) where I is the site indicator variable ( or ) and AC and bc are the adjustments to A and b when I =. At the jack pine site, increasing openness reflects increased skidder traffic, so the relationship between openness and shrub biomass was characterized by the relationship described by Lihai (994):

2 Table. Site descriptions of the two stands in this study Site Maple Birch (Fig. ) Jack Pine (Fig. 2) Geology Moraine Outwash Soil 59% Sand, % Clay 64% Sand, 3% Clay Sandy Loam Sandy Clay - Loam Stand Age (years) Dominant Species Acer saccharum (6%) Pinus banksiana (9%) (% Basal Area) Betula papyrifera (26%) Basal Area Range (m 2 /ha) Average Height (m) Average DBH (cm) Number of Plots 24 2 Harvest Prescription diameter limit cut strip cut: 2 cut, 4 uncut Harvest Date summer 994 summer 995 Harvest System tree-length tree-length Harvest Method hand felled, cable skidder hand felled, cable skidder Skidding System restricted: defined skid-trails dispersed across cut strips Figure. Photograph of shrubs beneath open canopy conditions at the maple-birch site (not included here). Figure 2. Photograph of shrubs across the cut strips and into the uncut strips at the jack pine site (not included here).

3 [3] Mc= Mu* e ( k* C) where Mu is the mass of shrubs in undisturbed areas, Mc is the biomass at canopy openness C, and k describes the rate of decrease for shrub mass. At the maple-birch site, the plots were not trafficked, so the relationship between openness and shrubs in this study is similar to that in Huffman et al. (994): ( [4] M = A( e bc * ) ) where M is shrub biomass, C is canopy openness, A is an asymptote, and b is the rate of shrub biomass increase. RESULTS AND DISCUSSION The relationship between overstory basal area and canopy cover is strong for both sites (fig. 3), but the slope and asymptote of these relationships differed between the sites (table 2). Canopy cover increases more rapidly with increasing basal area and is higher at similar basal areas beneath the maple-birch canopy than the jack pine canopy. Thus, compared to jack pine overstories, the same basal areas in a maplebirch overstory might lead to lower light availability for advanced regeneration and other understory vegetation. The soil compaction data confirm that open areas at the jack pine site were heavily trafficked while those at the maple-birch site were not (fig. 4). Table 2. ANCOVA (equation 2) concerning the effect of the two different sites on the relationship between overstory basal area and canopy cover. * indicates that a parameter is significant at a =.5. Parameter Estimate Standard error A.68*.33 AC.32*.6 b.8*.7 bc.9*. The relationship between canopy openness and shrub biomass is highly variable but follows the trends described by equations 3 and 4 (fig. 5). At least in the first years after harvest, the benefit of opening up the overstory and freeing up resources for understory vegetation might be superseded by negative impacts from the harvesting operation. These may include soil compaction and/or physical damage to shrubs during the harvesting operation. CONCLUSIONS Residual stand structure and harvesting method can be used to manage understory vegetation and thus influence competitive conditions for advanced regeneration as: overstory density influences understory light (and likely other resource) conditions and growth of understory vegetation Jack pine Maple - Birch Basal Area (m2/ha) Figure 3. The relationship between overstory basal area and canopy cover (-DIFN) in two sites.

4 6 5 p= Canopy Openness Jack Pine Maple-Birch Maple-Birch Skid Trail Figure 4. The relationship between soil compaction and overstory density in two sites. traffic of harvest equipment can reduce the growth of competing understory vegetation. When applying these methods it is important to consider that the relationship between overstory structure and understory growing conditions varies between canopy types. ACKNOWLEDGMENTS The authors acknowledge the support of the Iron Range Resources and Rehabilitation Board, Minnesota Department of Natural Resources, Cloquet Forestry Center, and the land departments of Itasca and St. Louis Counties. REFERENCES Puettmann, K.J.; Ek, A Status and trend of silvicultural practices in Minnesota. Northern Journal of Applied Forestry. (in prep.). Smidt, M.F.; Puettmann, K.J Overstory and understory competition affect underplanted eastern white pine. Forest Ecology Management. 5: Smith, W.B.; Brand, G.J Allometric biomass equations for 98 species of herbs, shrubs, and small trees. Res. Note NC-299. U.S. Department of Agriculture, Forest Service, North Central Research Station. 48 p. Huffman, D.W.; Zasada, J.C.; Tappeiner, J.C Growth and morphology of rhizome cuttings and seedlings of salal (Gaultheria shallon): effects of four light intensities. Canadian Journal of Botany. 72: Lihai, W Effects of skidding traffic on soil properties and growth reduction of seedlings. In: Proceedings, International seminar on forestry operations under mountainous conditions. China.

5 4 2 Jack Pine Maple-Birch Canopy Opennes Predicted Shrub Biomass Figure 5. The relationship between overstory canopy openness (DIFN) and understory shrub biomass in two sites. The upper portion of the figure displays regression fits and the lower portion shows residuals from these regression fits.