(Bloomberg) -- Investors should watch for 10-year Treasuries to move above 2.6%, a threshold that would mark an end to the three-decade bond bull market and be a more important barometer than the Dow Jones Industrial Average passing 20,000, according to billionaire bond manager Bill Gross.

"It is the key to interest rate levels and perhaps stock price levels in 2017," Gross, manager of the $1.8 billion Janus Global Unconstrained Bond Fund, wrote in a monthly investment outlook released Tuesday. "Investment happiness and/or despair may lie ahead over the next 12 months depending on it."

Stocks have rallied on hopes that the surprise victory of President-elect Donald Trump will lift U.S. gross domestic product growth to 3% from the past decade's 2% average, according to Gross. But the slower pace, popularized as "the New Normal" by Pimco after the 2008 financial crisis, while Gross was the firm's chief investment officer, will be challenging to exceed because of an aging population, technological advancement and the retreat of globalization, he wrote today.

"President-elect Trump tweets and markets listen for now," Gross wrote. "Trump's policies may grant a temporary acceleration over the next few years, but a 2% longer term standard is likely in place that will stunt corporate profit growth and slow down risk asset appreciation."

The Dow Jones Industrials ended Monday at 19,887 after gaining 8.5% since the Nov. 8 election. Yields on 10-year Treasuries, which surged to a high of 2.5967% on Dec. 15, retreated to 2.3647% as of Monday.

The Janus unconstrained fund returned 5.3% last year, outperforming 56% of its Bloomberg peers. It has returned about 4.5% since Gross took over after leaving Pimco in September 2014.

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