Arbuscular mycorrhizal inoculation following biocide treatment improves Calocedrus decurrens survival and growth in nursery and outplanting sites

Commercial production of tree seedlings often includes various biocidal soil treatments for disease control. Such treatments can be effective in eliminating or reducing disease organisms in the soil, but may also eliminate non-targeted beneficial soil organisms, such as mycorrhizal fungi, that improve seedling performance, both in the nursery as well as the outplanted environment. The arbuscular mycorrhizal fungal (AMF) relationship has been verified for some important western coniferous species such as Calocedrus decurrens (Torr.) Florin (incense cedar), Sequoia sempervirens, (D.Don) Endl (coastal redwood) and Thuja plicata J. Donne ex D. Don (western red cedar). This study was designed to determine the response of Calocedrus decurrens after soil fumigation with and without the addition of phosphorous fertilizer and a commercial mycorrhizal inoculant containing Glomus intraradices. Calocedrus seedling performance was monitored in both the nursery and outplanted environments. At the nursery, non mycorrhizal seedlings had significantly less foliar phosphorous levels and uneven growth even when phosphorous fertilizers were applied. Mycorrhizal inoculation at the nursery significantly improved height growth and improved seedling uniformity on treated plots. Seedlings from the nursery beds were then outplanted on two reforestation sites. Mycorrhizal inoculation at the nursery improved survival and growth of seedlings at the outplanted site. Introduction Under natural conditions, most plants live in close beneficial association with soil microorganisms called mycorrhizal fungi. These fungi colonize plant roots and extend the root system into the surrounding soil to form an essential link between plant and soil environment. Mycorrhizal mycelia are extensions of the plant root system and are more effective in nutrient and water absorption than plant roots by themselves. The relationship is mutually beneficial because the fungus receives essential sugars and other compounds from the plant to fuel its activities and in return it increases plant nutrient and water uptake, increases plant resistance to disease and extends protection against a wide variety of environmental extremes (Harley and Smith, 1983; Allen 1991). All conifer species are known to form and be dependent upon the mycorrhizal relationship in their native habitats. Commercial production of tree seedlings often includes various biocidal soil treatments for disease control. Such treatments can be effective in eliminating or reducing disease organisms, but may also eliminate non-target beneficial soil organisms such as mycorrhizal fungi (Menge 1982; Trappe et al. 1984; Kough et al.1985). Research has shown mycorrhizal fungi as critical to the uptake of water and nutrients and seedling survival across a wide range of host and field conditions (Amaranthus and Steinfeld 2003; Steinfeld et al. 2003; Miller et al. 1998; Jackson et al. 1998). However, nursery conditions in which water and nutrients are amply provided, can decrease the need and observed benefits of the mycorrhizal relationship. This is especially true when phosphorous is readily available (Browning and Whitney 1992; Harley 1978). Numerous practitioners, however, have observed stunting and uneven growth of conifers following biocidal treatments even after soil analysis reveals adequate levels of soil fertility. Many of these cases of uneven growth and nutrient deficiencies following biocidal treatment have documented improved growth and nutrition when inoculated with the appropriate mycorrhizal fungus (Bartschi et al. 1981, Parke 1982; Parke et al. 1983). In these cases, poor growth of many conifer species despite adequate soil fertilization, may be due to the coarse root systems lacking root hairs which mycorrhizal fungi augment by providing increased surface area and enzymes activity to release immobile soil nutrients such as phosphorous, zinc, copper and others (St. John 1979). Mycorrhizal fungi can profoundly effect seedling performance in the field by mediating nutrient and water uptake and protecting against environmental extremes in the narrow window for seedling establishment (Harley and Smith 1983; Amaranthus et al. 2004; Steinfeld et al. 2003). A typical forest site generally contains many mycorrhiza-forming fungal species (Amaranthus et al. 1996; but populations can be dramatically reduced or eliminated following site disturbance (Amaranthus and Trappe 1993, Dumroese et al. 1998, Perry et al. 1987.) Seedlings inoculated at the nursery with the appropriate mycorrhizal fungi before outplanting have the ability to more quickly assimilate site resources during the critical period for seedling establishment. The arbuscular mycorrhizal fungal (AMF) relationship has been verified for some important western coniferous species such as Calocedrus decurrens (Torr.) Florin (incense cedar), Sequoia sempervirens, (coastal redwood) and Thuja plicata (western red cedar). This study was designed to determine the response of Calocedrus decurrens after soil fumigation with and without the addition of phosphorous fertilizer and a commercial mycorrhizal inoculant. Calocedrus performance was monitored in both the nursery and outplanted environments.