The temperature has been in the thirties in London this week, which according to CBC's climate dashboard, that's approximately 10 C above average. It follows a very dry May.  Hodgins said it's important to recognize that if the leaves coming off your tree are falling in clumps, it's likely the work of thirsty squirrels looking for moisture in the juicy leaves or flushing out their nests. "Clusters of five or six leaves are squirrels. Individuals leaves falling are a symptom of a tree conserving water," Hodgins said.  She said vulnerable species of trees right now are silver maples, willows and London plane trees, or Sycamores.