The economy will be vibrant and the real estate investment environment steady through 2000, predicted Hugh F. Kelly, chief economist at Landauer Realty Group, the research arm of Grubb & Ellis Co., New York. At a news briefing, Mr. Kelly noted the substantial population growth among 55- to 64-year-olds will create opportunities for retailing, resorts and recreational development, which in turn will require entertainment and recreational ventures with commercial facilities to support them. The dynamics of the property markets dont suggest any immediate threat of sudden volatility in the real estate industry, according to the group. The key will be basing projections for space needs on levels of demand, not on expectations of continued growth. He also forecast that revitalized downtowns will be a major trend between 2000 and 2010, spurring increased institutional interest in the office sector. But Mr. Kelly sounded a cautionary note, warning that construction on a national basis is likely to outstrip net absorption in 2000. If its sustained beyond one year, it could compromise investor interest.