Does the shelterwood method to regenerate oak forests affect acorn production and predation?
Introduction
In Canada, red oak (Quercus rubra L.) occurs in the southeastern mixed coniferous–deciduous and deciduous forests, from east of Lake Superior to Nova Scotia. Although red oak can be a common main canopy species in these northern forests, it is hard to regenerate and sustain its current level of stocking in the overstory.
Many factors contribute to the difficulty of maintaining oak by natural regeneration in these forests. Typically, natural regeneration of red oak forests occurs after disturbances (e.g., fire) that open the forest canopy and understory, thus favoring oak seedling growth and recruitment into the overstory by providing more light in the forest understory (Crow, 1988, Dey, 2002). Fire suppression, however, has resulted in the displacement of oaks by more shade-tolerant species (Abrams, 1992), and historical trends suggest the replacement will continue (Nowacki et al., 1990). The natural regeneration of oaks is further complicated because acorn production by red oak is highly variable among individuals and fluctuates from year-to-year (2–7 years interval between large crops) for individual trees (Healy, 2002, Johnson et al., 2002). This natural variability in acorn production makes it hard to time silvicultural practices with a good seed crop to favor oak regeneration. Variability in seed production is due, in part, to the (1) inherent periodicity of good to bumper acorn crops, (2) differences in individual tree characteristics such as age, size of crown and crown dominance, (3) stochastic environmental factors that affect flowering, pollination and seed development, and (4) predation of oak flowers and acorns by insects and vertebrates (Christisen and Kearby, 1984, Sork et al., 1993, Dey, 1995). In addition, some red oaks are consistently good acorn producers, whereas others are not, although they may be dominant trees in the site (Healy et al., 1999, Healy, 2002, Johnson et al., 2002).
Crown size, health and vigor are major determinants of acorn production of oaks (Downs and McQuilken, 1944, Christisen, 1955, Sharp and Sprague, 1967, Goodrum et al., 1971, Sork et al., 1993). The potential to produce acorns is partially related to stand density and the degree crowns are exposed to direct sunlight (Johnson et al., 2002). Compared to forest-grown trees, open-grown oaks have (1) larger, wider crowns with greater surface area and volume, (2) more leaf area and foliage biomass, (3) higher densities of live, healthy branches, (4) greater live crown ratios, and (5) greater crown exposure to direct sunlight; all of which are correlated with acorn production.
Oak seedling establishment occurs mainly in years when production of sound acorns exceeds predation (Downs and McQuilken, 1944, Gysel, 1957, Christisen and Kearby, 1984). Predation by insects, mammals and birds may result in low acorn survival, especially in years of low to moderate acorn production (Marquis et al., 1976). It has been hypothesized that in years of mast production seed predators become satiated, leaving a greater number of seeds available for dispersal and germination (Murphy, 1968, Janzen, 1971). Insects can destroy the entire crop in low seed years, and a large proportion of the acorns in highly productive years (Galford et al., 1988). Many insects, such as weevils (e.g., Curculio spp., Conotrachelus spp.), moths (e.g., Melissopus spp.) and cynipid wasps, attack acorns causing damage that prevents germination (Kearby et al., 1986, Weckerly et al., 1989, Johnson et al., 2002). Mammals such as deer (Harlow et al., 1975), squirrels (Short, 1976), chipmunks (Pyare et al., 1993) and mice (Gómez et al., 2003) are known predators of acorns and can consume high proportions of the acorn crop in years of poor to moderate production (Sork, 1984). However, mammals and birds also play a beneficial role in oak seed dispersal, and their caching of acorns promotes germination (Darley-Hill and Johnson, 1981, Jensen and Nielsen, 1986).
Currently, the shelterwood system is one of the primary methods used to encourage the regeneration of oak forests (Hannah, 1987, Dey and Parker, 1996). To create the shelterwood for oak regeneration, trees are harvested from below (i.e., from the lower diameter classes first) until the desired overstory stocking is achieved. Non-oaks are preferentially removed, leaving co-dominate and dominate oaks in the overstory to produce acorns. Reductions in stand stocking may promote acorn production by providing more growing space for oak crown expansion and development, and by modifying environmental conditions that influence acorn production and predation. Few studies have scientifically evaluated the effects of thinning or shelterwood harvesting on acorn production of individual trees and stands, and their results were inconsistent. There is evidence that thinning oaks in young stands (e.g., 7–22 years old) may increase tree diameter growth and oak stocking in the upper crown classes (Johnson et al., 2002), thereby increasing the amount of acorn bearing crown area in the stand. Stand density, however, may be weakly related to acorn production because it has little influence on acorn crop size in years of poor production, and in years of high production, a few large, inherently good seed producers can generate the majority of a bumper acorn crop, irrespective of stand density (Greenberg and Parresol, 2002). In Massachusetts, Healy (1997) thinned young (i.e., 40 years old) stands to relate stand stocking to differences in acorn production where red oak was growing with white pine and red maple. It was found that reducing stand stocking from 100% to 40% (according to Gingrich, 1967) significantly increased acorn production per tree, and that differences between the thinned and uncut stands were the greatest in years of poor acorn production. At the stand-level, Healy (1997) reported that acorn production that was averaged over a 3-year period was not significantly different in thinned or uncut mature stands (62–82 years old) in New England, although on an annual basis, he observed that acorn yields were consistently greater in thinned stands than uncut stands; the lack of significance was attributed to large year-to-year variation in acorn production during his short sampling period. Because little is known about the influence of shelterwood harvesting on acorn production of individual trees and stands, we initiated a study to experimentally test for differences in acorn production and predation by insects and mammals between stands being regenerated by the shelterwood method and uncut stands in central Ontario.
Section snippets
Study area and site description
The study area was located in central Ontario, Canada, in the Great Lakes-St. Lawrence forest region, an area characterised by mixed deciduous–coniferous forests where common tree species include red maple (Acer rubrum L.), sugar maple (Acer saccharum Marsh.), American beech (Fagus grandifolia Ehrh.), red oak, yellow birch (Betula alleghaniensis Britton), white ash (Fraxinus americana L.), white pine (Pinus strobus L.), red pine (P. resinosa Ait.), jack pine (P. banksiana Lamb.) and eastern
Treatment effect on acorn production
The oak density (t = 0.41, P > 0.695), DBH (t = 0.28, P > 0.793), tree height (t = 0.10, P > 0.922), and total oak crown area (t = 0.85, P > 0.424) were similar between harvested and control plots (Table 1).
Visual inspection of oak canopies showed no acorn production in 1994 and 1995, thus, no acorn traps were deployed. First production of acorns after the shelterwood harvest treatment occurred in the fall of 1996 (3 years after treatment). In 1997, the crop was moderate in size, and it was significantly lower
Acorn production
Acorn production varied considerably (i.e., from no seed crop to mast production) over the 4-year period that acorn development was monitored. Acorn production by red oaks is known to be cyclical and to have pronounced year-to-year fluctuations (Downs and McQuilken, 1944, Sork et al., 1993, Healy, 2002). An average production of 3232 acorns per tree is considered a maximum yield for forest-grown red oak trees during mast production years (Downs and McQuilken, 1944, Sork et al., 1993). Our oaks
Acknowledgements
We thank Blake Laporte, J. Van Loon, K. Kloosterman, C. Patterson, and A. Sauvé for assisting in the field. This work was funded in part by the Ontario Forest Research Institute, Ontario Ministry of Natural Resources.
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