By Clare Baldwin

NEW YORK (Reuters) - Two companies' shares fell in their debuts on the New York Stock Exchange on Friday, capping a week of poor performance for initial public offerings as investors grow choosier at year-end.

Chinese radiotherapy and medical imaging company Concord Medical Services Holdings Ltd <CCM.N>, opened 9.1 percent below its IPO price, and used car and truck auctioneer KAR Auction Services Inc <KAR.N> opened 5 percent below its IPO price.

Of the eight IPOs scheduled this week, one was withdrawn after the company was acquired, three were pulled due to bad market conditions and most of the rest traded lower on their first day.

Even the best performing IPO for the week -- Pebblebrook Hotel Trust <PEB.N>, a real estate investment trust that plans to buy hotels -- rose just 2.2 percent on its first day of trading. Stocks often rise 10 to 12 percent on their first day of trading.

Analysts attributed the lackluster performance to a host of factors, noting that investors are less willing to take risks at the end of the year and that few of the IPOs looked particularly strong.

"There are no compelling, must-do IPOs this week," said IPOfinancial.com President David Menlow. "I think they just don't see anything there that makes a table-pounding argument to buy the stocks."

Investors' reluctance to take risk is more a matter of the time of the year than a broader fear of initial public offerings, Menlow said.

Concord Medical's shares fell as much as 12.6 percent from their initial offering price in morning trading on Friday.

The China-based company, which raised about $132 million in its IPO, sold its shares for $11. It had originally planned to sell shares for between $9.50 and $11.50 apiece.

On Thursday, another China-based medical company struggled in its Nasdaq debut. China Nuokang Bio Pharmaceutical Inc <NKBP.O> closed down 3.7 percent at $8.67, after pricing for $9 late on Wednesday, below its expected pricing range.

Concord Medical operates a network of radiotherapy and diagnostic imaging centers in China. As of September 30, the company operated 83 centers in 36 cities there. The company, which sold 12 million American Depositary Shares, said it would use the proceeds to expand and develop research centers, and for general corporate purposes.

Underwriters were led by Morgan Stanley, JPMorgan and China International Capital Corp Hong Kong Securities Ltd.

Concord Medical posted net revenues of 205.7 million yuan ($30.1 million) for the nine months ended September 30, up 101.6 percent from a year before. It posted net income of 89 million yuan, up 122.3 percent from a year earlier.

A STRUGGLING INDUSTRY