Hemiepiphytic Trees: Ficus as a Model System for Understanding Hemiepiphytism

Abstract

Woody hemiepiphytes that have an epiphytic juvenile growth stage differ crucially in physiology and ecology from common trees. A relatively high degree of ontogenetic plasticity confers these plants stress tolerance during the epiphytic stage and sufficient competitiveness later as independent trees. The genus Ficus consists of about 500 hemiepiphytic and about 300 non-hemiepiphytic woody species. Ecophysiological comparative studies between hemiepiphytic (Hs) and non-hemiepiphytic (NHs) Ficus tree species reveal that the existence of an epiphytic growth habit even only for a part of their life cycle involves profound changes that persist to a large degree in their terrestrial growth stage. When growing under similar conditions, both as saplings and mature trees, the Hs have physiological traits resulting in conservative water use and drought tolerance contrasting with more prodigal water use and drought sensitivity in NHs. Divergence in water related functional traits between the two groups are centrally associated with a trade-off between xylem water flux capacity and drought tolerance. Two distinct groups of life history traits for Hs and NHs have evolved—epiphytic regeneration with a slow starting growth rate but enhanced ability to tolerate water deficits in the upper canopy environment and regeneration in the forest understory with an initial burst of growth to rapidly gain a relatively large seedling size that can better survive risks related to terrestrial regeneration. Evidence shows that the underlying physiology distinguishing these two growth forms mostly involves divergences in adapting to contrasting water regimes but not light conditions, contrary to the conventional hypothesis that hemiepiphytism evolved for gaining access to higher irradiance in the canopy than on the forest floor.