What took place behind the belt of the river trees?

Dave died behind the belt of river trees; he was shot by his master. 

"Morning found him too weak to travel. At harness-up time he tried to crawl to his driver. By convulsive efforts he got on his feet, staggered, and fell. Then he wormed his way forward slowly toward where the harnesses were being put on his mates. He would
advance his fore legs and drag up his body with a sort of hitching movement, when he would advance his fore legs and hitch ahead again for a few more inches. His strength left him, and the last his mates saw of him he lay gasping in the snow and yearning toward them. But they could hear him mournfully howling till they passed out of sight behind a belt of river timber.

Here the train was halted. The Scotch half-breed slowly retraced his steps to the camp they had left. The men ceased talking. A revolver-shot rang out. The man came back hurriedly. The whips snapped, the bells tinkled merrily, the sleds churned along the trail; but Buck knew, and every dog knew, what had taken place behind the belt of river trees."