Many key commodities are on track to notch declines this year, underlining how fears of slowing growth, global-trade tensions and a persistently strong dollar have hammered prices for raw materials.

The Bloomberg Commodity Index is down by nearly 7% this year, led by a more-than-15% fall in oil prices. Other raw materials are also headed lower: Copper is off 16% this year, while iron ore has lost around 6%. Coffee prices have declined nearly 23% and lumber is down by around 28%.

Those slumps have come as investors pulled just over $11 billion from commodity-focused funds over the last six months, according to fund tracker EPFR Global. Net bullish bets by hedge funds and other speculative investors on oil prices stand at their lowest level since August 2016, data from the Commodity Futures Trading Commission showed.

Driving the declines are fears that trade tensions between the U.S. and China will hit global growth at a time when expansion outside of the U.S. is already lackluster. Investors worry that slowing growth would likely dent demand for commodities, which are used extensively in manufacturing and construction.

Those concerns might not dissipate soon. China, a top consumer of many raw materials, may be limited in the range of economic stimulus measures it can deliver after months of fighting to rid its economy of excess leverage, analysts said. The eurozone’s economy notched its weakest quarterly growth since early 2013 in the three months through September, and some investors are concerned that U.S. growth — a standout among major economies — may have peaked.

“The strength that we saw in the last two years is not there anymore,” said Michael Widmer, a commodities strategist at Bank of America Merrill Lynch. “Large parts of the global economy are now in a challenging environment.”

Mr. Widmer believes commodity prices will keep drifting lower in the first half of 2019, before stabilizing in the second half of the year, when he expects China to deliver a limited stimulus package.

Analysts at BNP Paribas expect global growth to drop to 3.4% in 2019, from an estimated 3.7% this year. That change doesn’t bode well for metals like copper, a key component in everything from smartphones to refrigerators. Despite limited growth in global copper supply, the slowdown will make it unlikely for prices to recover next year, said Harry Tchilinguirian, the bank’s global head of commodity-markets strategy.

A stronger dollar has also weighed on prices for raw materials, which are denominated in the U.S. currency and become more expensive to foreign investors when the dollar appreciates. The WSJ Dollar Index is up more than 5% this year, boosted by an outlook for higher U.S. interest rates and expectations that the U.S. economy will suffer less than others from trade frictions. Forecasts of higher U.S. interest rates have also dented gold, which struggles to compete with yield-bearing investments when borrowing costs rise.

Further declines in commodity prices would be an unwelcome development for countries like iron-ore exporter Australia and Brazil, which produces oil and sugar. The Australian dollar is down nearly 8% against its U.S. counterpart this year, while the Brazilian real has lost more than 14%. South Africa’s rand is down more than 12%, while the Chilean peso is off nearly 8%.

Heartened by recent signs that a trade truce between the U.S. and China may be sticking, some investors believe an end to the drop in the price of commodities may be in sight. Other market watchers are also convinced that the Fed may signal a new wait-and-see approach to tightening monetary policy after a widely expected rise in rates in December, a development that could slow the pace of rate increases next year and limit further gains in the dollar.

Meanwhile, a decision by the Organization of the Petroleum Exporting Countries and its allies to cut output starting in January may help curb a burgeoning glut of oil.

Hakan Kaya, a portfolio manager at Neuberger Berman, believes that politically driven risk aversion has been the main factor behind commodity-price declines and is increasing futures positions that would benefit from a rebound in oil and metals prices.

“I don’t remember any year when global politics impacted prices this much,” he said.