Several areas of the Botanic Garden are roped off. A sign explains that is to protect visitors from the risk of falling branches as a result of Summer Branch Drop. That can occur when trees are drought stressed, and when they take up water after a period of drought. It is unpredictable, and not related to other indicators of tree health.
The Drought FAQ page of the Botanic Garden website mentions that branch drop can affect any tree but is particularly known to affect cedars, pines, oaks, beeches, chestnuts and poplars, as well as old trees.
Rain taken up by branches after drought can weaken them to the point that branch drop can occur – usually six to eight hours after heavy rain.
While some trees are sacrificing branches, others such as this Yoshino cherry, are curling up their leaves to limit transpiration.
Meanwhile, no real rain for almost a month.