A Natural History of the Central Appalachians

Chapter 07: Lower Plants

CHAPTER 07 LOWER PLANTS THE TERM LOWER PLANT has no precise meaning, and in the context of this chapter it simply refers to all the more primitive plants and plantlike organisms found in the forests of the Central Appalachians. This is an exceedingly diverse assemblage that includes vascular plants (ferns and lycophytes), bryophytes, lichens, and algae. Several very different taxonomic groups are involved, and the only feature they all have in common is that they are “green”: they can photosynthesize. Although many of the organisms considered in this chapter are not especially conspicuous, the forests and other habitats in which they are found would be quite different without them. This is especially true for the bryophytes, which are the dominant plants in some wetlands. Both ferns and lycophytes are often abundant in moist, high-elevation forests, and lichens are consistently present in a number of ecological settings, including old fence posts, rock outcrops and stone monuments, and the bark surface of living trees. Algae are microscopic organisms, and most species are not easily detected directly in the field. Some species, however, are easily observed as slimy masses on wet rocks or as green, felt-like mats on the surface of standing water. Lichens are usually discussed in the context of fungi, but lichens are more than just fungi. What we recognize as a lichen is a “composite” organism in which a particular kind of fungus is intimately associated with a certain type of eukaryotic green or prokaryotic blue-green alga (or cyanobacterium). The vegetative body (or thallus) that results from the combination of these two different organisms is a truly remarkable structure that bears little resemblance to either of its two component parts. Indeed, most lichens are so different from fungi that it would be hard to imagine the two being confused. The true nature of lichens was not comprehended until about 150 years ago, when the German mycologist Heinrich Anton de Bary first suggested the possibility that what had been considered previously as an autonomous organism actually consisted of two different organisms. The partnership formed by the two organisms living together allows both to survive under conditions unsuitable for either partner alone. The algal part of the lichen (or photobiont), through the process of photosynthesis, produces organic molecules (often simple carbohydrates such as glucose) that are used by the fungal part (or mycobiont). In a simple sense the alga is providing the fungus with food. In return, the mycobiont creates a favorable microenvironment for the photobiont, which is protected during periods of desiccation, shielded from excessive solar radiation, and provided with mineral nutrients that are either extracted by the mycobiont from the substrate upon which it grows or deposited directly upon the upper surface of the thallus from the atmosphere. Their unusual partnership has allowed lichens to be widespread and enormously successful in some ecological situations. For example, they are often the first macroscopic organisms to colonize bare soil and rock surfaces, and the term nature’s pioneers is frequently used in this context. From an ecological standpoint, the ability of lichens to colonize such surfaces represents the first step in a successional sequence that ultimately can lead to a diverse assemblage of higher plants. The lichens that grow on rock surfaces enhance the weathering process that results from such things as the freeze-thaw cycle, and the lichen thallus intercepts particulate matter blowing or flowing (in water) across the rock surface. Over long periods (sometimes measured in centuries), enough soil and soil-like material builds up to allow the spores of mosses to germinate and grow. Once established, the mosses accelerate the rate of succession by allowing more soil particles and organic debris to accumulate. Eventually the spores of ferns that can tolerate high levels of light and the seeds of hardy grasses, sedges, and other plants reach favorable microsites on the rock surface and allow the first higher plants to become established. In time, conditions improve to the point that the seedlings of certain trees and shrubs are able to survive. Once this stage of succession has been achieved, the transition to a more diverse assemblage of higher plants is well under way. FIGURE 55 Fires that often followed logging in red spruce forests during the late nineteenth century often burned away the organic mat, leaving behind little more than bare rocks 115 07 LOWER PLANTS [18.191.5.239] Project MUSE (2024-04-19...