- Additional Plants: ornamentals, trees
- Crop Production: forestry
Incorporating leguminous and non-leguminous cover crops into Fraser fir Christmas tree plantations appeared to improve soil biological and chemical properties as well as tree growth performance relative to a conventionally-managed fir production system. Groundcovers increased microbial biomass and soil available N after a 2-year study. Despite differences in external N fertilizer inputs and groundcover management, fir survival and growth in the novel and conventional systems were similar. However, allowing the cover crops, to grow continuously over the entire plot led to poor seedling performance likely because of a strong competition between cover crops and trees.
The production of plantation-grown Christmas trees is a significant agricultural industry in the United States (Koelling et al., 1992). An estimated 15,000 growers yearly harvest between 32 and 36 million trees contributing over $462 million to the nation economy (Nzokou et al. 2006; Chastagner and Benson 2000). The bulk of the production is centered in the northern half of the country and Michigan ranks third among the leading producing states. Many conifer species have long been grown as Christmas tree but during the last two decades, Fraser fir is increasing in popularity because of its superior post-harvest needle and moisture retention characteristics, and higher value compared to other species (Nzokou et al. 2006, Leuty 2005). However, Fraser fir is a very demanding species and requires close attention with respect to soil fertility, especially nitrogen needed to improve the tree growth and maintain the dark green color. In addition, weed control is also essential and rank among the top priorities in Christmas tree farming (Pollack et al. 1995). Conventional Fraser fir Christmas tree production requires heavy use of inorganic N fertilizer (100 to 150 lbs per acre every year) and herbicides (two glyphosate applications at 16 - 20oz per acre every year) in plantations. Such practices provide short term results but create an environment more prone to soil erosion, leaching, and development of weed resistance to the herbicides. In recent years, increased concerns have been raised over the environmental and health risks of the use of agrichemicals. Many studies have demonstrated that intensive management practices with excessive use of chemicals create a variety of economic, environmental and ecological problems (Huat and Bremner1982; Edwards, and Abivardi, 1998. Cox, 2002). Some of the potential problems are the increased contamination of groundwater systems. Excess Nitrogen is reportedly converted into nitrates know to be very mobile in soil, moving rapidly with deep percolation through the vadose zone to underlying groundwater (Smith et al. 1996a). For example, an investigation of the effects of nitrogen management in Fraser fir plantations conducted by Rothstein (2005) found that mineral N (NH4+ + NO3-) concentrations in water leaching below the rooting zone increased markedly with increasing N application rates. Additionally, a number of economic assessment studies on Christmas tree plantations reported that the high costs of inorganic fertilizers and herbicides reduce the profits for Christmas tree growers (Pollack et al. 1995; Nzokou et al. 2006). Despite the considerable environmental and economic costs associated with current N fertilizer and weed management practices in Christmas tree production, there is no published research work suggesting alternative management options for low input Christmas tree production systems. The choice of a cover crop with potential for fixing nitrogen biologically offers an attractive alternative for inclusion into the production system. This study was carried out to assess whether soil biology and fertility as well as Fraser fir survival and growth could be improved by incorporating leguminous and non-leguminous cover crops into Fraser fir (Abies fraseri) production system.
Most tree crop production systems are dependent on nitrogen availability and over-fertilization in such systems is common because most growers rely on synthetic fertilizers (Ingels and Miller, 1993). Cover crops offer a potentially valuable source of nitrogen, especially if the cover crop plants are nitrogen fixers such as white clover and other legumes (Shiferaw 1985; BSTID/NRC, 1994, Smith et al. 1996b). Healthy stands of white clover have been reported to produce 80 to 130 lb. nitrogen per acre, when killed the year after and is reported to be a first choice for “living mulch” systems planted between rows of vegetables or trees. They also offer an important way to control weed, conserve excess soil nitrate during winter and to supply nitrogen when needed by the trees in the spring and summer (Bowman et al., 1998). Cover crops in general have been used successfully in tree crop production to improve soil fertility, weed control (Schroth et al. 2000). Because of these and other benefits, growers should consider using cover crops rather than relying solely on synthetic nitrogen fertilizers for managing nitrogen. Several studies have have observed significant increases in aboveground biomass or volume production in mixtures of Eucalyptus spp. and a N2-fixing species compared to Eucalyptus monocultures. And this response is often attributed to increases in nutrient availability through N2-fixation or accelerated rates of nutrient cycling (Binkley et al. 1992; Kaye et al. 2000; Forrester et al. 2006; Jorge and Pekka 2005). The use of legume cover crops as nitrogen sources and as a mean to control weed in agricultural settings is quite well documented, however, applied research is still needed to evaluate this technology for Christmas tree cropping systems. For instance, the potential impact of the cover crop on the soil and tissue N in a production system including Fraser fir is not known. In addition the influence of the cover crop on the long term control of weed population should be investigated. Information on the overall N uptake patterns as well as the amount of supplemental fertilizer and herbicides growth needed to design cropping systems that reduce losses of applied fertilizers/ herbicides and optimize nutrients use.
The overall goal of this research was to develop a low input production system for Fraser fir in Christmas tree production with potential to meet the nutritional requirements of the trees, perform a more competitive biological weed control and improve the soil physical and biological conditions. The specific objectives of the research were to:
• Evaluate the impact of groundcover management practices on soil fertility, soil microbial properties and tree growth;
• Measure nitrogen flows between the cover crop, the soil and the tree to determine the best management practice and appropriate level of supplemental inorganic nitrogen for optimal growth;
• Determine the impact of ground cover management systems on weed populations