Section 3. The forester shall from the money appropriated annually for the expenses of his office recompense the forest wardens for the time spent by them in making investigations under his direction, as required by section thirty-seven of this chapter and section twenty-five of chapter forty-eight; provided, that he shall not be liable to make any such payment except upon the presentation of a duly itemized account or to pay for such investigations at a rate greater than that approved by him or in excess of the appropriation available for such payment. He may also expend such sums as are annually appropriated in making necessary arrangements for conventions of forest wardens to be held at a place within the commonwealth and in paying wholly or in part the traveling expenses to and from their towns of such forest wardens who attend these conventions; provided, that no money shall be expended in paying the traveling expenses of any one warden to or from more than one convention in one year.