Symbolism, Imagery, Allegory
The neighborhood where the Lisbons and the narrators live is attacked by blight; specifically, all the elms are dying. One by one the Parks Department is condemning the neighborhood trees. The neighbors hate to see the trees go (Dutch Elm Disease killed most of the elms in North America by the 1980s). Most families came out to watch the cutting, but the sisters watch from an upstairs window "their faces cold-cream white," as the first branches came down (4.100).
When the men return to take the main trunk, the girls run across the lawn and, holding hands, surround the tree. They argue with the men to leave the tree to survive or die on its own; they claim that the tree won't necessarily die (they're right, some don't). The girls refuse to budge and the men eventually leave without finishing the job. All the neighbors felt like applauding, and the girls' stand made the newspapers. The trunk of the tree remains on the Lisbon lawn, at least for now, sick and wounded but alive.
It's hard not to see the similarities between the dying tree and the Lisbon sisters, who are gradually dying emotionally and wasting away physically themselves, shut up in their crumbling, neglected house. Are the girls fighting for their own lives? Saving the tree because they don't believe they can save themselves? Trying to salvage something from their lives?
There's another important aspect to the symbolism of the elm: contagion. Dutch Elm Disease is a really contagious fungal disease. It can pass from a sick tree to the trees nearby, which is one reason why the Parks Department wants to take down the Lisbons' tree—to protect the others. This mirrors how the neighbors view Cecilia's suicide: contagious in some weird way. They avoid the Lisbons for that reason; they don't want their kids to start thinking morbid thoughts. After the rest of the girls die, the neighbors believe that Cecilia's death caused the other girls' suicides.
Her suicide, from this perspective, was seen as a kid of disease infecting those close at hand. In the bathtub, cooking in the broth of her own blood, Cecilia had released an airborne virus which the other girls, even in coming to save her, had contracted. (4.54)
Un-fun fact: The study mentioned by Eugenides in the novel was made up, but unfortunately, there's scientific evidence for the existence of "suicide contagion." It's not a virus, of course, but it's been shown that in the wake of suicides, particularly celebrity suicides, people who may have been thinking about killing themselves can get pushed into action.