SANTA CRUZ DE LA SIERRA, Bolivia (Reuters) - In Bolivia's farm region of Santa Cruz, a worsening fuel shortage is starting to hit farmers' ability to harvest their crops, a concern for the embattled South American country where agriculture has become a key economic driver.

The fuel scarcity, which has led to long lines at the pump, stems from a slide in foreign currency reserves over the last decade and dwindling local gas production that is reaching crisis levels. The situation is rattling the government of President Luis Arce, which has sought to cap prices with subsidies.

"If there is no fuel, producers will go deeper into debt," said Joel Eizaguirre, a soybean producer in the lowland region of Santa Cruz, the main farm belt of the country.

"We're going to be left with producers who will start to make other choices, and it's going to affect everyone."

Jaime Fernando Hernandez, manager of oilseed and wheat group ANAPO, said that if there was not enough diesel for the farm machinery and tractors, then a large amount of food - including soy, corn and sorghum - could be lost. This would send ripples through the food chain and affect production of livestock, chicken, milk and eggs.

"The impact in terms of productivity and food production could be truly catastrophic," he said.

Bolivia's government, under growing pressure due to the dollar and fuel crisis, has moved to try to ease imports, letting state energy firm YPFB use cryptocurrency to pay for fuel cargoes and to pay companies.

Farmer Eizaguirre said he would prefer to pay more for fuel than to not have enough, referring to the parallel exchange rate that has hit more than 11 bolivianos per dollar versus 6.86 at the controlled official rate amid the hard-currency shortage.

"Personally, I'd rather have fuel cost 11 bolivianos than not have enough fuel to harvest our grain, or not be able to plant during this approaching winter," he said.