Adaptations of strangler figs to life in the rainforest canopy
Susanne Schmidt A C and Dieter P. Tracey BA School of Integrative Biology, The University of Queensland, Brisbane, Qld 4071, Australia.
B Department of Environment, 168 St Georges Terrace, Perth, WA 6000, Australia.
C Corresponding author. Email: Susanne.Schmidt@uq.edu.au
D This paper originates from a presentation at ECOFIZZ 2005, North Stradbroke Island, Queensland, Australia, November 2005.
Functional Plant Biology 33(5) 465-475 https://doi.org/10.1071/FP06014
Submitted: 17 January 2006 Accepted: 24 March 2006 Published: 2 May 2006
Abstract
Figs are rainforest keystone species. Non-strangler figs establish on the forest floor; strangler figs establish epiphytically, followed by a dramatic transition from epiphyte to free-standing tree that kills its hosts. Free-standing figs display vigorous growth and resource demand suggesting that epiphytic strangler figs require special adaptations to deal with resource limitations imposed by the epiphytic environment. We studied epiphytic and free-standing strangler figs, and non-strangler figs in tropical rainforest and in cultivation, as well as strangler figs in controlled conditions. We investigated whether the transition from epiphyte to free-standing tree is characterised by morphological and physiological plasticity. Epiphyte substrate had higher levels of plant-available ammonium and phosphate, and similar levels of nitrate compared with rainforest soil, suggesting that N and P are initially not limiting resources. A relationship was found between taxonomic groups and plant N physiology; strangler figs, all members of subgenus Urostigma, had mostly low foliar nitrate assimilation rates whereas non-strangler figs, in subgenera Pharmacocycea, Sycidium, Sycomorus or Synoecia, had moderate to high rates. Nitrate is an energetically expensive N source, and low nitrate use may be an adaptation of strangler figs for conserving energy during epiphytic growth. Interestingly, significant amounts of nitrate were stored in fleshy taproot tubers of epiphytic stranglers. Supporting the concept of plasticity, leaves of epiphytic Ficus benjamina L. had lower N and C content per unit leaf area, lower stomatal density and 80% greater specific leaf area than leaves of conspecific free-standing trees. Similarly, glasshouse-grown stranglers strongly increased biomass allocation to roots under water limitation. Epiphytic and free-standing F. benjamina had similar average foliar δ13C, but epiphytes had more extreme values; this indicates that both groups of plants use the C3 pathway of CO2 fixation but that water availability is highly variable for epiphytes. We hypothesise that epiphytic figs use fleshy stem tubers to avoid water stress, and that nitrate acts as an osmotic compound in tubers. We conclude that strangler figs are a unique experimental system for studying the transition from rainforest epiphyte to tree, and the genetic and environmental triggers involved.
Keywords: biomass allocation, carbon-13, epiphyte, Ficus, hemiepiphyte, nitrate, nitrogen-15, rainforest, resource use, stomatal density, tuber, water stress.
Acknowledgments
We thank the Rainforest CRC for access to the canopy crane and Dale Dixon for help with fig taxonomy. We are indebted to Dieter’s father, the late Geoff Tracey, for help finding and identifying the figs.
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