Our recent studies with ash and sycamore seeds have shown the importance of soil conditions on survival of direct sown tree seed (Jinks et al. 2005). Seed of both species are particularly sensitive to soil waterlogging, so seedling emergence on wet areas of our sites is much lower than at drier ones.
This local variation diversifies seedling density and composition across a site, which enhances the ‘natural’ appearance of new woodland established by direct sowing species mixtures compared with traditional transplanting methods.
Such variation is a potential advantage for creating native woodland, but might be a disadvantage when considering direct seeding as a way of establishing more commercial crops where it is more desirable to minimise such variation.
We’ve also found that the process of germination and seedling emergence is particularly sensitive to high soil temperatures in spring and early summer. Results so far suggest that emergence stops once soil temperatures exceed 25oC; in some species like ash, ungerminated seeds return to dormant state and may be capable of germination the following spring.
In southern England, soil temperatures can exceed this threshold in April, so seed needs to be sown well in advance if germination is to be completed before then.
It’s not just heat and desiccation that can reduce seedling emergence, but spring frosts can also wipe out entire cohorts of newly emerged seedlings of sensitive species like ash. Basically, exposed fields are a more extreme environment for tree seedling emergence than woodland.