Google (NSDQ: GOOG) has managed to outperform Wall Street’s expectations lately in part by dramatically reducing capital expenditures. But in a report this morning Bernstein Research’s Jeffrey Lindsay warns that the cutbacks are “unsustainable.” His argument:

—Despite the downturn, demands on Google’s datacenters are up, with the number of Google searches globally up 50 percent in April (over the previous April) and rapid growth in services like Gmail, Google Apps and YouTube.

—At the same time, the company’s capital expenditures are near recent historical lows (see chart after the jump), in part because it delayed the construction of two datacenters last year. Google spent $263 million on capital expenditures in the first quarter of the year, down from $842 million a year ago—an amount that was below depreciation. “Although Moore’s Law will help in the form of faster processor speeds and improvements in storage efficiency, we do not expect these effects to offset the rapid growth in raw storage capacity driven by demand for the company’s product and services,” he says.

Lindsay expects capital expenditures to jump to $2 billion in 2010 from $1.4 billion this year, and has lowered his earnings per share forecast slightly for next year. He believes Google will continue to benefit from the growth of search, which will help blunt the impact of the rise in cap ex.

• Earnings: Google Beats Expectations; Net Revenues Up 10 Percent