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Source: After Benzing and Seemann (1978).

Source: After Benzing and Seemann (1978).

exhibited superior cation exchange capacity, higher base saturation values, and a preponderance of N over P and K. Neutral to moderately acid pH prevailed, but may not be typical: suspended humus collected in pluvial forest in northwestern Ecuador produced readings down to 3.8 (Bermudes and Benzing 1989).

Lesica and Antibus (1991) discovered that the epiphytes, including many Bromeliaceae, in humid lowland Costa Rican forest at La Selva root in substrates at least as fertile as those available to co-occurring terrestrial flora. However, the much larger volumes of soil on the ground probably assure greater total supplies for plants. Uneven rates of mineralization (higher below; e.g., Vance and Nadkarni 1990) further distinguish rooting media in the same forests, as does soil reaction. Whether this chemical mosaic contributes significantly, as Lesica and Antibus suggested, to the high diversity of local epiphytes, especially relatively root-dependent Bromeliaceae, remains to be seen.

More extensive sampling than at either Rio Palenque or La Selva allowed Nadkarni (1984) to determine that humic soils (histosols) suspended within the canopy of a lower montane rainforest in Costa Rica contained large fractions of the total on-site pools of several essential ions. A similar pattern prevailed in upper montane cloud forest in Colombia (Hofstede et al. 1993) and in biomass largely attributable to Tillandsia recurvata in the crowns of dwarfed Quercus virginiana in a coastal strand community in southwest Florida (Benzing and Seemann 1978; Table 5.11). Nadkarni also examined key processes that influence the mineralization of suspended humus, specifically the transformation of complexed N into plant-usable forms.

Less nitrification occurred in suspended compared with forest-floor litter, although microbial biomass was about the same in both compartments at Nadkarni's Costa Rican site. Cellulose discs embedded in canopy debris lost less weight than those worked into litter on the ground at the same location (Nadkarni 1986; Vance and Nadkarni 1990). Terrestrial samples weighed 23-45% less after eight weeks, while those incubated within suspended humus over the same interval changed little. Epiphyte-derived soils also harbored fewer detritivores, had lower water but higher fiber content, had a higher carbon/nitrogen ratio, and seemed to be dissipating polyphenols more slowly than phytomass decomposing on the ground. Densities of mites, adult beetles, holometabolous insect larvae, Collembola, amphipods and isopods averaged 2.6 times higher in earth compared with canopy soils. Only ants occurred at about equal densities in both media. Nadkarni and Matelson (1991) further concluded that the his-tosols suspended there largely develop in place. Except for the modest amounts of material intercepted in the shoots of phytotelm bromeliads, shed plant parts mostly fall to the ground.

Contrary to Nadkarni's findings for mats of epiphytes and suspended humus in Costa Rica, Paoletti et al. (1991) documented conditions favorable for rapid litter breakdown in the canopy of cloud forest at two sites in northern Venezuela. Up to fivefold greater detritivore densities (number of animals per unit volume of impounded humus) occurred in the shoots of resident phytotelm Bromeliaceae as on the forest floor (Fig. 8.15). Moreover, dried Psidium foliage placed in nylon mesh bags and incubated in the leaf axils of these epiphytes on average weighed 21-27% less after three months - about the same rate of loss recorded for samples buried in earth soil under the host trees. Additional surveys could help determine whether the soil fauna observed in phytotelm bromeliads also attack debris elsewhere in the canopy.

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