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Some of the costs and benefits of reactive nitrogen in the environment can be seen clearly in nitrogen-deficient habitats like the Bornean dwarf forests known as “kerangas.” Kerangas grow in acidic, sandy and podzolized soils. Essential elements enter the soil from decaying leaf litter, but most of these, magnesium, carbon, calcium and nitrogen in particular, leach away very quickly, and are only available in the top few inches. Phosphorus seems to leach away more slowly. Continual deposition of leaf litter is critical to the system, and disease, fire and logging or clearing for agriculture will convert kerangas to a barren habitat dominated by grasses and sedges known as padang. The kerangas-adapted she-oak (Gymnostoma nobile) bears nitrogen-fixing root nodules. Orchids obtain carbon via mycorrhiza; to what degree nitrogen is transferred this way is the subject of some controversy, but orchids show greater species diversity in the kerangas than any other plant family. Pitcher plants (Nepenthes spp.) trap insects in modified water-bearing leaves. At least one Bornean species, N. rajah, secretes a nectar that attracts tree shrews whose droppings are captured in the pitcher to nourish the plant. In perennially wet padang habitat, Bladderworts (Utricularia spp.) and Sundews (Drosera spp.) also trap small arthropods. Epiphytic ant plants (Hydnophytum spp.), form symbiotic relationships with ants, providing them shelter while receiving protection from the colony and nitrogen and other nutrients from its wastes. |
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