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74 Chapter 7 Life on the Forest Floor The Soil The forest floor consists chiefly of soil, which makes it a good place to start. By tradition, northern forest soil is described as “poor,” which it obviously isn’t for conifers. It simply means poor for agriculture. One of the most distinctive soils is podsol, the typical soil in conifer forests . (For brevity, let’s ignore the fact that its modern name is “spodosol.”) This soil is easily recognizable if you expose it by digging a hole, or even more easily by seeing it exposed in an eroding stream bank. It has a thin, dark top layer of packed needle-leaf litter gradually decaying into humus (newly decomposed material with organic ingredients only). Immediately below that is a layer, often thick, of almost pure white sand. The white sand is sterile, but comparatively nourishing red or brown sandy soil lies below it. Sometimes, however, this “good” layer is useless to trees because the mineral-bearing water draining slowly down into it cements the soil into a hardpan impenetrable to tree roots. The soil is cold, acidic, and sometimes wet. It is too acidic for many flowering plants. Among the few that are at home growing in it are members of the heath family (Ericaceae). Most of them are shrubs, sub-shrubs, and woody ground-creepers, for example, kinnikinnick (Arctostaphylos uva-ursi), Labrador tea (Rhododendron (was Ledum) groenlandicum), blueberries and huckleberries (Vaccinium spp.), cranberries (Oxycoccus spp.), salal and creeping snowberry (Gaultheria spp.), swamp laurel or life on the forest floor 75 lambkill (Kalmia spp.), and leatherleaf (Chamaedaphne calyculata). Of these, some prefer open forests on dry, often sandy ground; others grow in patches of wetland in the forest. All are adapted to acid soil of low fertility, whether wet or dry. Forest Flowers A group of flowering plants that grow only in conifer forest are members of the family Monotropaceae. They are non-green, lack chlorophyll, and so cannot photosynthesize. Therefore, they must obtain their sugars readymade . Examples of them, shown in figure 7.1, are candystick (Allotropa virgata), pinesap (Monotropa hypopithys), Indian-pipe (Monotropa uniflora ), and gnome-plant (Hemitomes congestum). These plants used to be known as saprophytes, meaning they obtained their sugars from dead organic matter, but it is now known that, in fact, Figure 7.1. (a) Candystick. (b) Pinesap. (c) Indian-pipe. (d) Gnome-plant. 76 the world of northern evergreens no plant can do this. They are parasites on fungi that in turn are parasites on conifers, which makes them epiparasites on conifers.1 The fungi concerned are the mycorrhizal fungi described in chapter 4, where they were not called parasites because the attachment between fungus and conifer is mutualistic. One could say, though, that each was parasitic on the other. (The terminology that has developed is confusing.) Hence, we are dealing with a three-part linkage (figure 7.2) in which a conifer, a fungus, and a parasitic plant (in this case, Indian-pipe) are all connected. It’s possible that the pinesap contributes a growth stimulant to the fungus; even if it does, its contribution to the threesome is probably negligible. The epiparasitic Figure 7.2. The linkages among a conifer and its mycorrhizal fungus, plus a mycoheterotrophic plant, in this case Indian-pipe. life on the forest floor 77 plant can also be described as myco-heterotrophic. A heterotrophic plant is one that cannot feed itself (in contrast to an autotrophic plant that photosynthesizes its own sugars). The prefix myco is from the Greek mukes, a mushroom. The plants already listed are not the only myco-heterotrophs. Some orchids , too, such as the various coralroots (Corallorhiza spp.), behave in the same way. Another group of flowering plants that grow among conifers belong to the wintergreen family, Pyrolaceae. These green plants are believed, on strong evidence, to be ancestral to the non-green plants just considered. Examples are pipsissewa (Chimaphila umbellata), single delight (Moneses uniflora), and one-sided wintergreen (Orthilia secunda), shown in figure 7.3. To summarize, the most common flowers on the coniferous forest floor belong to three families: one, of plants that prefer acid soil; a second, of epiparasitic plants; and a third, of ancestors of the epiparasites. A plant in one of these categories truly belongs in the forest. Others are “intruders” that have chanced to find hospitable spots. Figure 7.3. (a) Pipsissewa. (b) Single delight. (c) One-sided wintergreen. 78 the world of northern evergreens The Floor of...


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