It is not a physical necessity to remove the trees in order to install the guardrail. It is a convenience for the contractor.
It also appears that no environmental assessment was ever performed on this project. According to the City Environmental Quality Review (CEQR) procedure, the lead agency of any city project should perform such an analysis when it would lead to "the removal or destruction of large quantities of vegetation or fauna" or "the impairment of the character or quality of important historical, archeological, architectural or aesthetic resources (including the demolition or alteration of a structure which is eligible for inclusion in an official inventory of such resources)."
According to the Parks Department, the parkway consists of three traffic lanes going in each direction, two large center plots, one small plot, two marginal streets, a bridle path, and an aqueduct shaft.
"An opulent thoroughfare linking two of Parks’ brightest jewels, the Bronx and Pelham Parkway, is a graceful testament to the beauty Mulally and the New Parks Act brought to the Bronx, " the agecny says on its website.