Aerial canopy walkways are a relatively simple, flexible, and inexpensive method of studying a broad swath of the forest canopy. For short-term studies (e.g., a few days or weeks), structures held together and supported by ropes and high-strength industrial tapes have been used. For longer-term use (months and years), aerial walkways consist of platforms and bridges linked together to form pathways through the trees. The platforms are supported by stainless-steel or galvanized aircraft cable (tensile strengths of 12, 000 and 14,000 kg, respectively). The bridges are suspended from above or supported from below with these cables.
The platforms and bridges have netting and handrails made of rope or steel cable. Overhead cables and other strong attachment points are provided with safety lanyards, which allow users of the walkways to remain tethered at all times. Cables supporting bridges and platforms are bolted with galvanized steel bolts through the trees and are secured from the other side by washers and nuts. Suspending the structures on cables prevents structural members from rubbing against the tree in the wind. Trees in the walkways are also guyed with rigid seven-strand cable to stabilize and counterbalance the weight of these platforms and bridges. Encircling the tree trunks or limbs with cables or ropes should not be done to avoid risk of injuring the cambium.
Factors in selection of a site for an aerial walkway include:
a) appropriateness and accessibility of the site for the specific research,
education, or tourism project involved;
b) nearness of the site to ridgetops, clearings, or other physical
features that may increase the incidence of lightning or wind damage;
c) placement of trees of sufficient size, soundness, and proximity
to facilitate efficient and economical construction of a walkway;
d) trees that would enable future expansion of the walkway;
e) access to a range of canopy levels and trees to maximize use of
the structure.
Several factors affect the cost and materials that are required to construct a walkway. In tropical wet forests, expensive stainless steel is required, at least for the main bridge support cables. Stainless steel cable, when used for all support cables of both bridges and platforms, raises the materials cost of an average project by 15-20%. Stainless steel generally lasts for 40-50 years; galvanized cable will generally last for less than 10 years.
Pressure-treated wood, rot-resistant wood, or aluminum are good choices for platform decking, support joists, and bridge treads. The greater expense of aluminum plus possible metallic noise problems must be balanced against the greater weight of wood and its tendency to deteriorate more rapidly than aluminum.
Access methods to the platform include wood or aluminum ladders bolted
to the tree trunks, a block-and-tackle arrangement of polyester rope and
pulleys, and counterweight systems. A counterweight system requires extreme
caution and extensive training in safety procedures.
