What causes my cats matted fur? I think its sap from the bushes she crawls into. Others say long haired cats get knots when they shed.
All cats shed, and even short haired cats can get mats when they shed; long haired cats are even more prone to it. The best way to prevent that is by regular grooming. (Some cats will self-groom efficiently enough, but may develop hairball problems). Try brushing once or twice a week to see if that helps. If sap is causing the problem, I would expect it to be sticky. If the mats seem to be dry and not brittle, then shedding is most likely the problem.