We tested the hypothesis that growth and survival of aerial roots impose a limit on the height at which a primary hemiepiphyte can become successfully established within tree crowns and evaluate the implications for the vertical distribution of hemiepiphytes in the forest canopy. Density and spatial distribution, and growth and survival of aerial roots were studied in two common species of hemiepiphytic Araceae, Philodendron radiatum and Anthurium clavigerum, in a lowland tropical moist forest in Panama between March and October 2001. Additionally, root growth and survival were studied both in normal, unmanipulated, and experimentally cut roots to investigate the effect of damage on root resprouting and survival. Survival analysis revealed much greater survival of aerial roots of A. clavigerum than P. radiatum. In contrast, growth rates in the latter were on average about three times higher when compared to A. clavigerum. In both species, experimental cutting of the root invariably led initially to the development of dieback symptoms; however, plants responded to root damage by producing resprouts. A risk model for the two species suggests that only the fastest-growing roots of P. radiatum are likely to survive long enough to reach the soil from a host branch 6.8 m high, which equals the mean height of occurrence observed for the epiphytic stage of this species at our study site. In contrast, slow-growing appressed aerial roots of A. clavigerum may never be able to establish a connection with the soil from similar heights in the canopy before roots die. Consistent with our hypothesis, A. clavigerum is found much lower in the forest (it rarely exceeds ca 5 m).
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Vol. 36 • No. 4