Herbicide-free plantations of oaks and ashes along a gradient of open to forested mesic environments

Abstract

This study seeks to assess whether valuable hardwood species, red oak (Quercus rubra L.), bur oak (Quercus macrocarpa Michx.), white ash (Fraxinus americana L.) and red ash (Fraxinus pennsylvanica Marsh.), could be successfully underplanted in certain early successional vegetation types (alders, poplars, birch) in shelterwood systems, without the use of herbicides to control competing vegetation, as opposed to traditional plantations in abandoned fields. In the Eastern Townships region of southern Québec, six different plantation sites, occurring along a gradient from open to forested mesic environments, were selected for this study: (1) old-field sprayed with herbicide; (2) old-field without herbicide; (3) alder (Alnus rugosa [Du Roi] Spreng.) stand; (4) aspen (Populus tremuloides Michx.) stand; (5) red pine (Pinus resinosa Ait.) plantation; (6) sugar maple (Acer saccharum Marsh.) forest. All of the forested sites were herbicide-free. After eight years of growth, results show that hardwood species can be successfully grown herbicide-free using shelterwood systems in mesic forested environments in southern Québec. Planted trees have shown equal and even greater growth gains in forested sites in comparison with the open environments during the last year of growth. Species showed clear differences in growth response between forested environments. In forested sites, bur oak and both ash species had superior growth gains in the alder stand, while red oak had the highest growth gain in all other forested environments. Maximum height growth gain in forested environments was obtained in the aspen stand by red oak. Deer (Odocoileus virginianus) browsing has affected oak and ash species differently. Both ash species had unexpected reduced growth in the open environment with herbicide application, where conditions of resource availability are optimal. Although under heavy browsing pressure early on, red oak has had continuous growth gains in open environments. Both ash species and bur oak achieved better growth in the youngest stands (i.e. old-field and alder stand), whereas red oak achieved better growth in a relatively older stand (i.e. aspen stand) in our spectrum of the successional gradient. Past human activity has resulted in the disappearance of much of the hardwood forests in southern Québec. However, the resulting successional stands provide an exceptional management opportunity for the herbicide-free reintroduction of valuable hardwood tree species, using shelterwood cuts and an integrated plantation system.

Introduction

Almost all successful valuable hardwood reforestation programs in recently abandoned fields have relied on repeated herbicide applications for weed control during the early years of plantation establishment (Davies, 1985, Cabanettes et al., 1995, Thompson and Schultz, 1995, Cogliastro et al., 1997). Without herbicide treatments, most of the planted trees stagnate and poor growth is recorded (Von Althen, 1985, Miller et al., 1987, Cogliastro et al., 1993). Truax and Gagnon (1993) showed that mulching has potential to improve survival and growth of hardwood seedlings in recently abandoned fields in southern Québec, as a possible alternative to herbicide applications. However, Lambert et al. (1994), studying the same plantations with enzyme assays, concluded that a combination of mulching plus herbicide application always resulted in superior growth. The same result was obtained in the United Kingdom with hardwoods planted in fields (Davies, 1988).

A goal of Québec’s recent Forest Protection Strategy (Gouvernement du Québec, 1995) is to reduce and even eliminate herbicide use in forestry. This environmental concern is also of current interest in the US and in Europe (Johnson, 1992, Teclaw and Isebrands, 1993, Ferm et al., 1994, Gemmel et al., 1996). Because of proven effectiveness of herbicide applications in hardwood reforestation (most of it done in abandoned fields, with low deer density or with fencing), the question was raised about the continued relevance of planting valuable hardwoods in recently abandoned fields in southern Québec. Could it be possible to find an alternative to herbicide use, perhaps by using environments other than recently abandoned fields? In these other environments, the planted tree species would grow in a more integrated plantation system for several decades, more suited to their basic autecology, with the objective of producing an acceptable growth gain without herbicide use.

Natural regeneration studies of oaks and ashes have revealed that they are intermediate in shade tolerance and early- to mid-successional species (Sander, 1990, Johnson, 1990, Schlesinger, 1990, Kennedy, 1990). This is an indication that these species could potentially be successfully planted in environments differing from a recently abandoned agricultural field, especially in the case of red oak (Quercus rubra L.). Much research has been done on the natural and artificial regeneration problems of red oak in North America (Bundy et al., 1991, Pope, 1993, Lorimer et al., 1994, Kruger and Reich, 1997, Buckley et al., 1998). Abrams (1996) concludes that this species is experiencing regeneration failure throughout its range. Changes in disturbance patterns related to anthropogenic causes, such as fire frequency and intensity, cutting regime and increased deer browsing, may be responsible.

As reported by several authors (Horsley and Marquis, 1983, Tilghman, 1989, Kittredge and Ashton, 1995, Stromayer and Warren, 1997), deer (Odocoileus virginianus) browsing is an important factor that can negatively affect the establishment of natural regeneration in hardwood forests. Moreover, in southern Québec, the deer population has doubled in the last decade (MEF, 1998). This increase in deer populations has had a profound impact on artificial regeneration as well. Most of the recent research uses fencing and repulsing methods, even in shelterwood and clear-cut systems (Teclaw and Isebrands, 1993, Gemmel et al., 1996, Kruger and Reich, 1997, Zaczek et al., 1997).

Shelterwood methods have been used extensively as a natural and artificial regeneration system for oaks and other hardwoods in the United States (Johnson, 1984, Hannah, 1991). However, the success of these methods depends greatly on herbicides to control non-oak species that also benefit from the canopy opening, especially on mesic sites, but also on dry sites (Johnson et al., 1986, Johnson et al., 1989, Sims and Loftis, 1989, Loftis, 1990, Crow, 1992). Consequently, even shelterwood systems which prescribe herbicide applications on mesic sites would not be acceptable as a reforestation solution in southern Québec, in view of the current goal of the Forest Protection Strategy. Our previous studies have shown that it was possible to use plantation environments other than abandoned fields for regenerating red oak without herbicide in southern Québec (Truax et al., 1994a). However, only one mature hardwood forest was assessed as a shelterwood system for underplanting, and a broader spectrum of vegetation communities, in terms of age, structure and composition, need to be assessed for their potential to receive valuable hardwood plantings.

It has been shown that vegetation changes occurring during succession from open to forested environments produce changes in soil chemistry, soil nitrogen mineralization, soil organisms (including mycorrhiza), available light (quantity and quality) and vegetation dynamics (Bergeron et al., 1988, Myster, 1993, Zhou and Sharik, 1997, Boerner et al., 1998, Doyon et al., 1998). Palik and Pregitzer (1991) have shown that red oak, white ash (Fraxinus americana L.) and red maple (Acer rubrum L.) establish themselves in pioneer successional stands (Populus dominated), but each at different times. We can hypothesize that the site characteristics of these stands meet the basic growth requirements of oaks and ashes at their seedling and sapling stages.

Human activities in forest and agricultural lands in southern Québec have contributed to the maintenance of a complex vegetation mosaic, ranging from recently abandoned fields, to pioneer successional stands, and to late-successional forests on mesic sites. Moreover, pioneer successional stands, composed mostly of mixtures of Betula populifolia Marsh., Populus tremuloides Michx., Populus grandidentata Michx., Acer rubrum L., Alnus rugosa (Du Roi) Spreng. and Salix spp., are abundant in the region. These successional stands may have a high potential as a reforestation alternative to herbicide use, by underplanting with valuable hardwoods and by manipulating their canopy as in shelterwood systems. Furthermore, valuable hardwood regeneration is insufficient in southern Québec, especially for oaks, of which only few mature stands are found. However, valuable hardwood species historically had a much greater abundance before widespread deforestation by settlers (MacKay, 1985, Simard and Bouchard, 1996, Kesterman et al., 1998).

This study seeks to address the issue of herbicide-free valuable hardwood reforestation by asking three questions: (1) Can the use of different types of vegetation, other than abandoned fields, be appropriate for herbicide-free shelterwood establishment of valuable hardwoods (red oak, bur oak [Quercus macrocarpa Michx.], red ash [Fraxinus pennsylvanica Marsh.] and white ash) in southern Québec? (2) Can deer browsing negatively affect species differently depending on planting environment? (3) Is it possible to propose an integrated and herbicide-free plantation system for valuable hardwoods in southern Québec?