Direct seeding of ash (Fraxinus excelsior L.) and sycamore (Acer pseudoplatanus L.): The effects of sowing date, pre-emergent herbicides, cultivation, and protection on seedling emergence and survival

摘要

Direct seeding is an alternative method to conventional planting for woodland establishment that has several potential advantages. This study reports the effects of sowing date, pre-emergent herbicides, and cultivation and protection on the emergence and survival of direct-sown ash (Fraxinus excelsior L.) and sycamore (Acer pseudoplatanus L.) on three sites in southern England situated on former pasture and arable land, and on a windblown site in beech (Fagus sylvatica L.) woodland. Few seedlings emerged at the woodland site probably due to excessive seed predation by small mammals. At the other sites, the effects of sowing date on emergence reflected seasonal variation in soil moisture content and temperature. Periods of soil waterlogging in seed beds during winter resulted in greatly reduced seedling emergence of early sown seeds, and seedling emergence of both species ceased once the daily maximum soil temperature exceeded 25 super(o)C, which was in late April in this study. Analysis of the thermal time requirements for complete emergence of both species under the recorded soil-temperature regime suggested that the latest sowing date that would allow emergence before soil temperatures were too high was the beginning of March. Ash seedlings were particularly vulnerable to freezing injury after spring frosts that occurred during the seedling emergence period in one experiment. This study also confirmed earlier work identifying the potential of the herbicides napropamide, pendimethalin, and isoxaben for use on direct seeded ash and sycamore, and the results show the importance of adequate vegetation management for maintaining seedling survival and growth. For winter sowing, protecting seed beds with fleece was found to enhance seedling establishment, but reduced subsequent seedling survival after the protection was removed. Compared with cultivated soil, direct-drilling tree seed between the stubble of preceding cereal crop had no effect on seedling emergence, and increased seedling survival.