Tosefta על מעשרות 3:4

One who found sheaves in a private domain -- they are forbidden under the laws of theft and they are liable in tithes. In a public domain -- they are permitted under the laws of theft and exempt from tithes. And the alum plants -- whether in a private domain or in a public domain -- are forbidden under the laws of theft and liable in tithes.

[A worker who] found produce that had been stacked (see Peah 1:6), he is forbidden from [removing some of it and] making it into a pile [of his own] under the laws of theft. [But if he found] scattered [produce], that is permitted under the laws of theft. In either case, he may separate Terumah and tithes on behalf of produce belonging to the homeowner and need not be concerned. [If] he found a harvesting basket, it is forbidden [to take produce out of the basket] under the laws of theft, and it is liable in tithes. How does he do it? He converts it to its monetary value [and eats it]. (See Yer. Maasr. III.1.10.)