What's the matter with the Panhandle's elm trees?

Many of the Panhandle's elm trees have very little foliage, despite having reached the middle of the summer season. For park users, this is a loss, both for the lack of shade beneath the trees, and also because we will miss out on the seasonal leaf change of the elms, our park's main source of autumn colours. 

Elm trees on Saturday, August 1, with few leaves

Some of the elm trees in the park have a good amount of leaves, such as the one pictured below located near the intersection of Fell @ Lyon, but the greater number of them are quite bare. 

What accounts for this unusual phenomenon? One might suspect the prolonged drought, but the Panhandle is irrigated. Also, a local tree expert explained a more compelling theory: the warm local weather of the last year, in particular, the few nights this winter with cold temperatures, caused the tree's reaction. Their physiology is confused and they don't know whether or when to foliate. The elm trees in the Panhandle were brought from colder climates more typical of the eastern United States. 

image from The California Weather Blog

It's unclear why a few elm trees have normal leaf growth while the majority are bare, but there could be micro-climates at work, or perhaps it's related to aspects of the tree such as age or gender (and it might be a different species altogether - I am not really certain). A more ominous question is how many seasons like this one can a tree tolerate before it just gives up? More research into phenology and climate change is needed in order to understand what's happening in our parks and wildlands.

Fall color - a thing of the past? Picture from 12/2/2011