Among the most oligotrophic of the bromeliads judging by substrates, plant structure and growth rates are the pulse-supplied forms, those species without phytotelmata, ant nests or soil roots to provide more continuous streams of essential ions (Fig. 1.3A,C). Aerosols, precipitation and canopy washes suffice instead. In fact, nutrient requirements for Type Five Tillandsia may rank among the most modest of all for vascular flora. Predictably, these plants grow slowly and produce durable foliage that sometimes contains exceptionally low concentrations of N, P and K compared with phytotelm and soil-rooted species (Tables 5.1, 5.5). Because drought and habitually scarce supplies of key elements promote similar leaf morphology and oblige low vigor, Brocchinia species native to the moist, base-poor savanna habitats of the Guayanan highlands of northern South America probably demonstrate bromeliad oligotrophy in its purest form.
Bromeliaceae illustrate several additional nutritional modes. Among the family's membership are several carnivores, at least 50 ant-fed species of two types, and another, much larger and more important group labeled the animal-assisted saprophytes (Table 5.4). Before moving on to describe these relatively novel arrangements, we need some information about the fertility of several kinds of habitats. Supplies of nutrients available to Bromeliaceae more or less fall into two categories distinguished by whether or not mutualists are involved. Few records beyond those for cultivated pineapple describe the media that support terrestrial Bromeliaceae, but given the diverse kinds of sites these plants occupy, species dependent on absorptive roots probably experience soils of widely varying qualities. Enough is known about sources for the epiphytes to warrant a brief survey.
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