Abstract
Here we report on a pilot study of the Living Root Bridges (LRBs) in the Indian State Meghalaya, which are grown with aerial roots of Ficus elastica, a facultative hemiepiphyte developing abundant aerial roots. Locals use these aerial roots to build living bridges, which strengthen themselves over time due to adaptive secondary growth and their capacity to form a mechanically stable structure via inosculations. An extensive inventory of LRBs in Meghalaya including data of location, altitude, approximate age and bridge length was performed in field studies. Root morphology was characterised by measurements of cross-sectional area and shape-related parameters and analysed in relation to the orientation of the roots. LRBs are found to occur mainly in the mountainous limestone rainforests where F. elastica may be native or traditionally cultivated. They cover an altitude range of 57–1211 m a.m.s.l. and display a length of 2 to 52.7 m. Some bridges are several hundreds of years old. Horizontally and vertically trained roots differ significantly in shape and cross-sectional area when approximately even-aged roots are compared. The results are discussed from an interdisciplinary perspective, considering the adaptive traits in the natural life cycle of F. elastica and possible applications in living architecture (Baubotanik).
Highlights
The special features of growth and mechanical properties of the aerial roots of Ficus elastica have been well known and utilised for centuries by the indigenous Khasi and Jaintia people in the subtropical moist broadleaf forest ecoregion of Meghalaya
In the Khasi and Jaintia Hills of Meghalaya, inhabitants of isolated villages have devised a practical and sustainable way of constructing living bridges grown with aerial roots of F. elastica in order to cross monsoon-swelled rivers
Where bridges made from bamboo or wood would rot and be swept away, and structures made from steel or concrete would be expensive, requiring outside investment and maintenance while rusting quickly and being damaged, the villagers made use of the mechanical strength of the aerial roots of F. elastica and their natural tendency to anastomose and form a mechanically stable network via inosculations, able to withstand mechanical stresses
Summary
The bridge-building technique obviously takes advantage of the mechanical strength of living aerial roots of F. elastica and their natural tendency to anastomose and form a mechanically stable structure via inosculations (Fig. 1). After reaching an adult stage, aerial roots (referred to as ‘roots’, unless specified as ‘subterranean’) emerge from the branches and are wound onto and directed across a deadwood framework (mainly bamboo) This anthropogenic process takes roots that generally hang vertically down and uses them horizontally to cross the river. The addition of handrails, a second deck, underpinning struts, or other features can further influence the bridge’s structural system This combination of continuous growth and maintenance leads to a high level of complexity in each bridge, making simple mechanical analysis unfeasible. In older hemiepiphytic Ficus elastica individuals the aerial roots anastomose, form inosculations (natural grafts) and build a scaffold around the host tree’s stem. Based on a study of Australian strangler figs Richard and Halkin[12] postulate that the presence of a strangler fig can have beneficial effects on a host tree by shielding it from cyclones
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