My friend Andrei noticed a link under one of the National Geographic photos that I had overlooked. It described a similar phenomenon at a park in Texas in 2007. During a wet period and an outbreak of midges and other aquatic and semi-aquatic insects, a tree near Lake Tawakoni became completely enmeshed in spider silk.
An arachnologist collected about 100 spiders from the webbing and identified species from 11 different families. The dominant species was Tetragnatha guatemalensis, a fairly common spider distributed throughout the Americas. This family, common name being long-jawed orb weavers, is riparian, typically building horizontal orb webs over water. They exhibit colonial behavior, tolerating neighbors to the extent of sharing thick, foundational silk strands within which they build their daily orb web. None are truly social. Their typical web looks nothing like the sheet webs covering these trees.
Research has shown that spiders' aggressiveness diminishes when food is plentiful. In flush times, it's common for a web-building spider to catch more prey than it can consume. What may have happened in Texas and Pakistan is floodwaters drove spiders whose normal habitat is edges of streams and rivers to refuge in trees. When rains subsided and the spiders began to seek places to build webs, flying insects were in such abundance that simple drag lines served as adequate traps. Instead of going to the trouble of constructing orbs, they just wandered around, leaving silk lines as all spiders do, too sated to bother with risky, aggressive interactions with other, similarly situated spiders.
The end result was a huge, disordered mass of silk covering entire trees and shrubs.