FRANKFURT — Havens can pose their own risks.


That is the reality that holders of Germany’s government bonds now must face. The value of German bonds — considered a shelter from Europe’s debt storm during the last two years — has started to fall, now that investors sense a calming in other European waters.


On Wednesday the yield — or effective interest rate demanded by markets — on the benchmark 10-year German bond rose as high as 2.07 percent, an 18 percent increase from last week. Later in the day, it fell back somewhat, to 1.98 percent.


Because bond yields rise as their prices fall, that means investors holding them now feel proportionately poorer than they did last week. A lot of those holders are big European banks, which have enough problems already without having to worry about their bunds, as German bonds are known.


The spiking yields partly reflect a shift by hedge funds and asset managers into debt issued by countries like Italy and Spain. Those offer a much higher interest rate — but are evidently not considered quite as risky as they were just a few months ago, as Europe shows signs of having muddled through the worst of the crisis.


As long as that crisis was raging, Germany’s relatively robust economy and aura of financial discipline made its bunds seem such a safe bet that the yield fell well below the rate of inflation as their prices rose.


Investors were effectively paying the German government to keep their money safe — as holders of United States Treasury bonds have been doing in recent years. (Holders of Treasuries are also now experiencing a similar decline in the market value of their haven assets.)


The change in European sentiment is good news for Italy and Spain because it means at least some investors no longer regard their bonds as impaired.


Italy’s 10-year bond was yielding 4.9 percent Wednesday — still a higher interest rate than Germany’s, but well off the 7 percent interest rates and more that investors were demanding that Italy pay last November.


But there is a dark side for European banks, which have loaded up on haven bunds the last two years. Europe’s 90 biggest banks had holdings of German government debt that regulators valued last June at 489 billion euros ($645 billion).


Some analysts even speak of a bursting bund bubble.


“The bund is probably the most widely held asset in European banks,” said Carl Weinberg, chief economist at High Frequency Economics in Valhalla, N.Y. “Any further decay in a widely held asset like bunds would be unwelcome, and would contribute to a contraction in credit that I think is the next big problem for Europe.”


One problem is that many banks booked gains from German bonds when they underwent official stress tests last summer. The profits helped offset other losses and reduced pressure on the banks to raise more capital. It could be a problem for some banks if those bund profits proved to be ephemeral.


Some economists and European policy makers have watched the decline in German bonds with concern.


Otmar Issing, the former chief economist of the European Central Bank, who remains influential with central bankers, said in a conference in Frankfurt on Tuesday that the price of bunds and United States Treasuries had become “distorted” because of their haven status.


Peter Praet, a member of the executive board of the European Central Bank, said at the same conference that banks had been benefiting until recently from the safety status of their German bonds, which made it easier for them to satisfy regulators’ demands that the banks reduce risk.


As a sign of the bunds’ rising value on the open market last year, the yield on the 10-year bunds went from about 3.5 percent in April to as low as 1.67 percent in September. Now some of those gains are turning into losses.


“What is the potential impact of that distortion?” Mr. Praet asked.


The answer is probably that no one knows. Detailed information on banks’ current bond holdings is lacking.