UNITED NATIONS — When the environmentally oriented United Nations launches its ambitious building-refurbishing project next month, dozens of stately cherry trees, among others, will have to be cut down, the man charged with executing the renovation plan, Michael Adlerstein, said yesterday in an admission reminiscent of young George Washington’s “I cannot tell a lie.” The $1.876 billion Capital Master Plan renovation project is scheduled to be launched May 5 with a ground-breaking ceremony for a temporary building that will house conference rooms for diplomats during the six-year construction period. The temporary conference building will be erected in the well-manicured gardens of the northern portion of the U.N. campus, an area known as the North Lawn, where rows of trees were planted 30 years ago among rare artistic artifacts and perennial and annual flowering plants.

Secretary-General Ban has raised environmental issues to the top of the U.N. agenda and made them a central theme of his administration. Yesterday, Mr. Ban said “global warming” was one of the issues he planned to discuss with Pope Benedict XVI, who is scheduled to visit Friday. Mr. Ban is Korean, and many in the region he hails from cherish the short-lived cherry blossom season and travel to areas where the trees are abundant to adore the blooms.

The cherry trees in full bloom yesterday under the U.N. flag are doomed. As the temporary building is being erected on the North Lawn, “25 out of 300 trees will have to come down, including cherry trees,” Mr. Adlerstein acknowledged yesterday.

Mr. Adlerstein added, however, that after consulting with old colleagues at the New York Botanical Garden — where he was vice president and architect before joining the United Nations last July — solutions were found for dealing with many of the trees: some will be replanted elsewhere, he said, and shoots of cherry and other trees will be sent to a specialized nursery. The shoots will be preserved until they can be replanted at the end of the renovation period, in a process akin to “tree cloning,” Mr. Adlerstein said.

Once the renovation project is completed, in mid-2013, according to the plan, “We are going to green the compound and the surrounding neighborhood” around First Avenue and 48th Street, Mr. Adlerstein added. “We are going to plant 150 new trees in this area as we landscape the gardens,” he said.

Of the renovation plan’s budget, $27.9 million was set aside for “sustainability measures,” which include, in addition to the tree planting, up to a 30% reduction in energy costs, according to the U.N. Web site.

Next month’s groundbreaking ceremony will mark the end of major disagreements among member states over finances and other provisions of the plan, which delayed the renovation by years and significantly increased its budget in the process. To bring the project back to within budget, Mr. Adlerstein plans to shorten the construction period to six years. The project, to be carried out by the Swedish-owned Skanska USA Building Inc., is financed by member states according to their dues assessments. America’s share is just under a quarter of the budget.

As construction begins on the north lawn, United Nations employees currently working at the iconic Secretariat building will start exiting it in “a couple of months,” according to the Capital Master Plan’s spokesman, Werner Schmidt. The relocation will be completed by the summer of 2009, leaving the building empty to be gutted and stripped.

Last month, the U.N. leased a 15-floor office space in a Madison Avenue building owned by real estate investor Sheldon Solow, whose construction projects in the neighborhood include the old Con Ed building, situated just south of the U.N.compound. About 1,800 employees will move to that glass tower, while others will be transferred to several other Midtown buildings already leased by the organization, as well as to the Dag Hammarskjold Library on the U.N. campus.