Other Nutrient-Conserving Mechanisms
Direct transfer of nutrients from litter to roots and root concentration near the soil surface are only two of several mechanisms that have evolved in tropical forests to conserve nutrients (Montagnini and Jordan 2002). Others include:
• Aerial roots. Mats of living and dead bryophytes, lichens, club mosses, bro-meliads, ferns, orchids, and other epiphytes often occur on the branches and stems of rain forest trees. Adventitious tree roots penetrate the mats. Morphological evidence for the role of such roots in nutrient transfer includes abundant root hairs, unsuberized root tips, and the presence of en-domycorrhizal hyphae (Nadkarni 1981).
• Rapid soil drainage. The high concentration of soil organic matter near the surface of tropical rain forests results in a relatively stable aggregation of soil particles. Water can drain rapidly through the pores that exist between these stable aggregates, and there is limited contact between soil particles and drainage water, thus limiting leaching potential (Nortcliff and Thornes 1978). When forests are cleared, the soil organic matter breaks down, the aggregates lose their stability, and the soil becomes less permeable. As a result, there is more runoff and erosion.
• Scleromorphic leaves. Evergreen scleromorphic leaf types are commonly associated with nutrient-poor tropical forests (Medina 1995). The young leaves of tropical evergreen species may be susceptible to insect attack, but as they mature they become thick and tough. Their resistance to herbiv-ory, fungal infection, and attack by other predators and parasites increases (Coley 1982, 1983). Such leaf characteristics may be advantageous to plants in nutrient-poor areas where leaf replacement is energetically expensive (Chapin 1980). In many species, mobile nutrients such as phosphorus, nitrogen, and potassium are translocated from leaf to twig before leaf abscission (Charley and Richards 1983).
• High nutrient use efficiency in leaves. Leaves in tropical forests tend to be low in nutrients, and thus their "nutrient use efficiency" is high. In other words, the trees are able to synthesize leaves with limited nutrient availability (Vitousek 1982, 1984). In most lowland forests leaves tend to use phosphorus efficiently, whereas in Spodosols and in montane forests, nitrogen use efficiency tends to be highest.
• Epiphylls. Leaves in the humid tropics are often covered with epiphylls such as mosses, lichens, and algae. Some of the lichens and algae fix nitrogen (Forman 1975). Almost all epiphylls appear to be effective in scavenging nutrients from rainwater (Jordan et al. 1979).
• Secondary compounds in plants. Secondary plant compounds are those compounds in plants that do not play a role in the metabolism of the plant. These secondary compounds may serve as chemical defenses against pathogens and herbivores. The concentration of secondary compounds in plants of nutrient-poor regions is often high, because replacement of nutrients to synthesize new leaves is energetically expensive (Janzen 1974).
Some nutrient conserving mechanisms are not unique to the tropics. Monk (1966) suggested that the sclerophyllous needles of pine trees at high as well as at low latitudes conserve nutrients. They are long-lived, and resistant to leaching, insect attack, and decay. As mentioned before, surficial root mats of decaying litter interspersed with roots, important in low-nutrient forests of the tropics, also occur in mature beech stands on Ultisols in the Piedmont of southeastern USA (Jordan, pers. observ.).
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