If seed is sown straight away after collection, it will stratify naturally in the ground. There may, however, be problems with birds, mice and shrews eating the seed over winter. As a result seed losses may be high. A layer of fine mesh can be placed over he seeds to protect them.
When stratifying seed naturally, extensive areas of ground may be occupyied by seed which may fail to germinate. In addition, this approach may also produce a crop of uneven aged young trees. On a small scale this may not matter too much, but when operating on a larger scale it adds to the labour, which is not desirable. Maceration and extraction prior to sowing may help some species to germinate.
For those species which need a really cold winter to break dormancy or which take two or three years to develop, in general, germination will be slower and more variable without adequate stratification.