Mycorrhizae in Forest Tree Nurseries Michelle M. Cram and R. Kasten Dumroese Mycorrhizae are symbiotic fungus­ The intensive commercial seedling root associations. The colonization of production for reforestation typically roots by mycorrhizal fungi can benefit suppresses or delays colonization of the host by improving nutrient and water seedlings by mycorrhizal fungi. Fumiga­ uptake. In exchange, the host plant provides tion used to control pests can limit and the mycorrhizal fungi carbohydrates (car­ sometimes remove mycorrhizal fungi bon) from photosynthesis. A substantial from the upper 15 to 30 cm (6 to 12 portion of this carbon is ultimately trans­ in) of soil for weeks to several months. ferred to the rhizosphere and is estimated Some fungicides have also been found to to account for up to 15 percent of the suppress mycorrhizae development, espe­ organic matter in forest soils. Under most cially systemic fungicides (for example, environmental conditions, trees and other triadimefon) and fungicides applied as plants are naturally colonized by mycor­ soil drenches (for example, azoxystrobin, rhizal fungi. These mycorrhizal associations iprodione). High fertilization rates have are highly complex and dynamic, a result also been known to suppress mycorrhizae of the great diversity of mycorrhizal fungi, development, particularly when the fertil­ hosts, and terrestrial systems that interact izer is high in phosphorus (greater than and evolve with changes in hosts and 150 ppm). Despite the negative effect Figure I.6—Pisolithus tinctorius forms a yellow- environmental conditions. these common nursery treatments have on brown (ocher) mantle on pine feeder roots. Photo by mycorrhizae, the benefits of fertilization Michelle M. Cram, USDA Forest Service. Two types of mycorrhizae are found and pest control often outweigh the delay on trees: ectomycorrhizae and arbuscular of mycorrhizae formation on seedlings. mycorrhizae (syn. endomycorrhizae). Ectomycorrhizal fungi enter the root be­ tween cortex cells and often form a thick mantle outside of the short feeder roots that is visible to the naked eye (fig. I.6). Forest tree species with ectomycorrhizae include pine, firs, spruce, hemlock, oak, hickory, alder, and beech. Arbuscular mycorrhizal fungi enter the root cells and cannot be seen without the aid of a microscope (fig. I.7). Arbuscular mycorrhizae are especially effective at transferring carbon to soil in the form of glomalin, a sticky glue­like substance that is estimated to provide 30 to 40 percent of the carbon found in soils. Forest tree species with arbuscular mycorrhizae include cedars, cypress, ju­ nipers, redwoods, maple, ash, dogwoods, sycamore, yellow­poplar, and sweetgum. Agricultural crops used by forest nurser­ ies as cover crops also form arbuscular Figure I.7—Arbuscular mycorrhizal fungi within western redcedar root cells. Photo by Michael P. Amaranthus, Mycorrhizal Applications, Inc. mycorrhizae. 20 Forest Nursery Pests Mycorrhizae in Forest Tree Nurseries Ectomycorrhizal fungi are able to requires careful testing of a particular in hardwood beds following fumigation, infest fumigated fields or containers via mycorrhizal inoculant to ensure a positive container operations, or as requested by windblown spores produced by mush­ benefit­to­cost ratio before operational use. their clients. rooms and puffballs growing within or Artificial inoculation with mycorrhi­ Mycorrhizal fungi may be purchased, near the nurseries. These ectomycorrhizal zal fungi in the nursery is used to increase usually in the form of spore inoculum. fungi are often well­adapted to intensive seedling performance in situations known These products typically are applied in nursery systems. Thelephora terrestris is by researchers and managers to have the nursery as a dry granular to soil or common in most North American nurser­ consistently positive results. Pisolithus media before sowing or as a drench after ies (fig. I.8). In the Pacific Northwest, tinctorius, an ectomycorrhizal fungus, germination. Some container media are Laccaria laccata and Inocybe lacera are has been known to increase survival and sold with mycorrhizal fungi or spores also common forest nursery colonizers. growth of pine and oak seedlings on strip incorporated. It may be difficult to find In contrast, arbuscular mycorrhizal fungi mine spoils in the Eastern United States commercial products that contain only produce soilborne spores that are unlikely (fig. I.9). Artificial inoculation with ar­ one species of mycorrhizal fungi. Many to colonize container growing media and buscular mycorrhizal fungi (for example, products often combine multiple species can be slower to recolonize fumigated Glomus intraradices) for species such as of mycorrhizal fungi, other biological soils. This delay in colonization can limit incense cedar, redwood, giant sequoia, organisms (for example, bacteria and production of species highly dependent and western redcedar significantly Trichoderma spp.), and nutrients, making on arbuscular mycorrhizae. increase seedling density and growth in each ingredient’s benefit difficult to as­ Artificial inoculation with mycor­ the nursery, and survival and early growth sess. When testing mycorrhizal products rhizal fungi can be beneficial for a variety after outplanting on some sites. Many that also contain nutrients, it is imperative of tree seedlings and sites; however, other examples exist of nurseries that use to include a nutrient control to determine there are many documented cases in artificial inoculation of mycorrhizal fungi if seedling response is caused by the fungi which artificial inoculation resulted in no measurable benefit or significant losses in seedling survival and growth. The symbiotic association between plants and mycorrhizal fungi requires a balanced exchange of carbon to expanded nutrient and water uptake to be mutually benefi­ cial. If the environmental conditions that plants are growing in are fully adequate for growth, then mycorrhizal fungi may add little benefit to offset their carbon use. Rapid colonization of seedlings by mycorrhizal fungi naturally present in the nursery or at a typical outplanting site may also render artificial inoculation un­ necessary. Results of artificial inoculation will vary greatly depending on the host, species or strain of mycorrhizal fungi, and environmental conditions of the nursery and outplanting sites. The complexity of interactions between mycorrhizal as­ Figure I.8—Thelephora terrestris fruiting bodies on Figure I.9—Pisolithus tinctorius (Pt) fruiting bodies sociations and environmental conditions slash pine. Photo from USDA Forest Service Archive. form under a Virginia pine originally inoculated with Pt and planted on a strip-mine spoil. Photo by Michelle M. Cram, USDA Forest Service. Forest Nursery Pests 21 Mycorrhizae in Forest Tree Nurseries or the nutrients. Managers interested in soil under desired tree species and mixed increase arbuscular mycorrhizae levels in using or testing a particular mycorrhizal with container media before sowing host the subsequent summer crops. In forest species may have to special order a single seeds, such as alfalfa and grasses. After tree nurseries, more information is needed species product. the host plants have matured and are well on the interactions of mycorrhizae, cover infected (assistance from university or crop, tree species, and application timing Some nurseries produce their own State extension services may be required to deploy a rotational cropping system mycorrhizal inoculum. Ectomycorrhizal to determining the presence and species), for managing seedlings dependent on inoculum is made by grinding up the the soil and host roots can be cut up and arbuscular mycorrhizae. fruiting bodies (puffballs, truffles, etc.) added to container media (10 percent by and adding the spores to soil, dusting The potential of mycorrhizae to volume) or applied to beds before sowing. seeds, or mixing the inoculum with water positively affect seedlings survival and and drenching containers or beds after Common cover crops used in bareroot growth will continue to draw efforts at germination (fig. I.10). Nurseries that forest nurseries for 1­ to 2­year rotations using artificial and cultural techniques to use pesticides known to suppress mycor­ can help boost arbuscular mycorrhizae produce a superior mycorrhizal seedling. rhizae should delay artificial inoculation populations and increase organic matter A better understanding of the mycorrhizal until after the pesticide applications content. This increase in mycorrhizal system for each nursery, tree species, and are finished. The primary limitation to fungi, however, will most likely be lost outplanting site is needed to determine producing ectomycorrhizal inoculum is if fumigation occurs between cover the best cultural or artificial inoculation finding sufficient quantities of fruiting cropping and seedling production. Many practices. Ultimately, any practices used bodies. In contrast, arbuscular mycor­ nurseries that fumigate in the fall will in forest nurseries must increase seedling rhizal inoculum can be grown in pots use a winter cover crop, such as rye performance and have an acceptable with fast growing host plants. Original grass and oats, as living mulch. Winter benefit­to­cost ratio. inoculum is collected in the field from cover crops in other agriculture systems Selected References Allen, M.F.; Swenson, W.; Querejeta, J.I.; Egerton­Warburton, L.M.; Treseder, K.K. 2003. Ecology of mycorrhizae: a conceptual