Why do oak trees throw twigs away? Often people assume it's a sign of drought stress or ageing. Some years ago, I studied this phenomenon in a research project.
Most twigs are shed in autumn, when photosynthesis has ceased and drought is not an issue any more.
Most shed twigs are one year old and leafless. Their shedding does not reduce transpiring leaf area.
Twig abscission (cladoptosis) is no immediate reaction to drought stress.
Most twigs are shed by vigourously growing trees, so it is not a sign of ageing either.
RUST, Steffen; ROLOFF, Andreas. Reduced photosynthesis in old oak (Quercus robur): the impact of crown and hydraulic architecture. Tree Physiology, 2002, 22. Jg., Nr. 8, S. 597-601. pdf
RUST, Steffen; SOLGER, Alexander; ROLOFF, Andreas. Bottlenecks to water transport in Quercus robur L.: the abscission zone and its physiological consequences. Basic and Applied Ecology, 2004, 5. Jg., Nr. 3, S. 293-299. html
RUST, Steffen; ROLOFF, Andreas. Acclimation of crown structure to drought in Quercus robur L.—intra-and inter-annual variation of abscission and traits of shed twigs. Basic and Applied Ecology, 2004, 5. Jg., Nr. 3, S. 283-291. html