Two broken limbs can cause the campus buses to reroute their services. That is, if they are tree limbs.

Within the past month, two tree branches have fallen near the East Campus bridge, disrupting traffic on Campus Drive and Main Street, although no one was injured.

Some experts said recent drought conditions throughout the state, including in Durham County, may have weakened the structure of tree branches, causing the collapses.

"Many of the trees on campus have shown signs of stress brought on by the heat and dry conditions of the summer," Joe Jackson, assistant director of grounds, sanitation and recycling services, wrote in an e-mail. "Trees could have suffered from other little problems within their structure that did not become as pronounced until certain conditions [brought them out], such as a drought."

Although the Facilities Management Department did not perform extra analysis in response to the recent incidences, Jackson said the department routinely inspects trees.

"We have had a certified arborist inspect the trees to determine as best as possible the extent of decay and any other structural defects. We performed additional safety pruning as a result of this inspection," Jackson said. "We don't have a situation on campus where trees that are eminently dangerous are still there."

He noted that the department keeps an inventory of trees on campus, including information such as species, age and environmental conditions and maintenance activities to help foster healthy forest community.

Aging trees are especially vulnerable, since they struggle with adversities specific to the urban environments, Jackson added.

Duke Forest Resource Manager Judd Edeburn wrote in an e-mail that lightning strikes, wounds, soil conditions and age are also factors that might have caused the tree crashes.

"Various wood rotting fungi such as [those] in the genera Armillaria, Ganoderma and others are likely [to be] the source of decay in trees in the area," he added.

It is difficult to foresee whether more tree branches will collapse on campus, experts said.

"I will say that predicting the timing of breakage from internal tree damage is very difficult, though one might expect such events to be most common in storms with wind, ice or snow, when stresses on weakened areas are greatest," Edeburn said.

Students said they were not worried about the recent crashes.

"If there was a possibility of [the trees] falling on me, I would be concerned," freshman Melanie Plageman said.