Database for Estimating Tree Responses of Walnut and Other Hardwoods to Ground Cover Management Practices

The ground cover in plantings of walnut and other hardwoods can substantially affect tree growth and seed production. The number of alternative ground covers that have been suggested for establishment in tree plantings far exceeds the number that have already been tested with walnut and other temperate hardwoods. Knowing how other hardwood species respond to ground covers and which have responses statistically similar to that for walnut would greatly expand our knowledge base for making predictions and recommendations for ground covers in hardwood plantings. Data from over 110 reports of which nearly half included walnut species were compiled into a spreadsheet. The spreadsheet database compares multi-year growth and nut or fruit production of trees growing in different ground covers to that of trees in plots either with little or no management of the weedy vegetation or vegetation-free plots maintained through cultivation, herbicides, or mulches. For most hardwoods with moderate growth rates, growth as a percentage of growth in vegetation-free treatments is similar to the reduction observed for walnut saplings and pole-sized trees managed with forage legumes and grasses. In general, forage grasses have a greater impact on tree growth than do forage legumes. Although frequently used as a method of competition control, mowing does not alleviate the negative impacts of most ground covers on tree growth. Additional studies will likely need to be done before predictions similar to those for growth can be made on the effect of ground covers on fruit or nut production. INTRODUCTION Maximum tree growth and nut production of black walnut (Juglans nigra L.), butternut (Juglans cinerea L.), and Persian (English) walnut (Juglans regia L.) are expected to occur on well-drained, fertile sites maintained free of vegetation (Van Sambeek, 1989; Ramos, 1998; Alley et al., 1999). A vegetation-free approach to orchard management has significant environmental consequences including substantial soil erosion, soil compaction, wider fluctuations in soil temperature, and significant losses of soil moisture through surface evaporation (Ingels et al., 1998; Willoughby and McDonald, 1999). Benefits of orchard management with a ground cover include shading of soil to reduce surface evaporation, a turf to support equipment for maintenance and harvesting, and fixation of atmospheric nitrogen if it includes legumes (Ramos, 1998). Several studies have documented that an established stand of tall fescue (Festuca arundinacea Schreb.) can severely impact growth and nut production of black walnut and other hardwoods (Holt and Voeller, 1975; Schlesinger and Van Sambeek, 1986; Van Sambeek and McBride 1991; Alley et al., 1999). In contrast, the management of herbaceous legumes between trees can result in improved growth and fruit yields when compared to check treatments comprised of a mixed population of seedbank grasses and forbs (Haines et al., 1978; Van Sambeek et al., 1986; Van Sambeek, 2003). The objectives of this work are to describe a spreadsheet database being created using results from various publications and reports that can be used to statistically Proc. VIth Intl. Walnut Symposium Ed.: D.L. McNeil Acta Hort. 861, ISHS 2010 246 evaluate tree responses to different ground covers. Emphasis initially has been placed on compiling information from publications that have included walnut species along with other species. A secondary objective is to compare growth responses to common control treatments to determine what other tree species have responses similar to walnut species to broaden the inference basis for making recommendations on impact of different management practices. MATERIALS AND METHODS A key word search of the horticulture, arboriculture, and forestry literature and the literature cited within candidate publications produced more than 50 publications reporting on impact of ground covers on black walnut for one or more measures of tree growth or fruit production (Van Sambeek and Garrett, 2004). Publications were systematically analyzed to determine if reported studies included at least one nut tree or hardwood species with moderate growth rates, one or more treatments that could serve as a vegetation-free control or a resident vegetation control (non-managed plots usually with a succession of mixed forbs and grasses from a pre-existing seedbank), and response measurements over two or more years to minimize including reports with negative growth increment or growth under unique weather events. For those reports that met these criteria, information was extracted from the text, figures, and tables for each publication and recorded on a worksheet to facilitate entering information into an Excel 3.0 spreadsheet (Microsoft Corporation). The spreadsheet is structured with the first row containing unique variable names followed by rows with information from each report that includes source, site information, and responses of each tree species to various combinations of control and experimental ground cover treatments. To the extent possible letter codes are standardized as described below to facilitate eventual sorting of variables and conducting statistical analyses. The current spreadsheet database is designed to contain the following information: One column is coded for the publication or report that is the original source of the information by using the first letter of the last name of up to four authors followed by year published, i.e., this paper will probably be coded as V2009. The source code is also used to link to a separate database that has a full ANSI citation to the original publication or report. Four columns are coded to the study location and soils information. Two columns are used for the first eight letters of the county or nearest town and the two-letter state or three-letter country abbreviations. Two columns are coded to the first eight letters of the soil series name and a four-letter code is used to describe the predominant soil type, e.g., SILM for silty loam and SALM for sandy loam. Three columns are used to identify what tree species are included in the publication or report. One column is used for the common name and another column is coded using the first three letters of genus and the species name, i.e., JUGNIG for black walnut and JUGREG for Persian walnut. The third column places tree species within one of the following broad categories: walnut species (JGLNS), pecan and hickory (CARYA), oak (QURCS), ash (FRXNS), actinorhizal or nodulated tree legumes (N-FIX), fruit (FRUIT), short-rotation (BIOMS), other hardwoods (HRDWD), pines (PINUS), or other conifers (CONFR). Short-rotation species include most fast-growing trees, e.g., hybrid poplar (Populus hybrids) and silver maple (Acer saccharinum L.), that are expected to rapidly develop a closed canopy and quickly shade out the competing herbaceous ground cover. Three columns are used to indicate the year that seed of each tree species was germinated in the nursery or direct seeded at the study site, the year the last fall measurements were made before the spring establishment of any ground cover treatments, and the year for last reported fall measurements that are reported in the publication. Two columns are used to provide information on growing space per tree and percentage of that area covered by the experimental treatment. For closely-spaced trees, growing space (m) is calculated as distance between tree rows times spacing within