This basal anchor sling from Rope Logic is used to anchor your single line to the tree. This concept was designed by Todd Kramer of Chicago. It allows the SRS climber to be lowered to the ground by his coworkers in case of an emergency rescue. It is comprised of a tachyon sling stitched onto a CT Rigging plate. The sling is adjusted in length with a prusik loop which is run through the plate and attached to the lowering device.
This sling can be used with different lowering devices (Petzl Rig for example) and backup configurations. A small port-a-wrap can be a good device to use because most ground workers will already be familiar with it.