Not only was site preparation important but also the choice of tree species. Before ploughing and fertiliser were available much effort was put into the selection of tree species. Some trees demand fertile soils to grow well. Such trees are called exacting species and they include beech, ash, elm, oak and silver fir. At the other extreme are a few trees that will accommodate themselves to poor soils. They are called accommodating species and include pines, spruces, birch and willow. This distinction has huge consequences for the trees that can be used on the land available for forestry. It can be observed that, in general, broadleaf trees are more exacting species than conifers and it is for this reason that conifers became to dominate the poor upland plantations.

The conifers used in forestry shifted over time. Initially Scots pine was the most planted tree because it was believed that this hardy species grows well on both dry and wet soils. By 1933, the experiments at Inverliever had shown that only a few conifers were suitable for planting on wet peat soils. Scots pine thrives in areas where Sitka spruce, lodgepole pine and other spruces are likely to fail because it is too dry. Scots pine does best on well-drained sands, gravels and other well drained sites. Because of this Scots pine was dismissed as a useful tree on wet peat and banned to the drier sandy soils in the east and on drier heathlands. The consequence was that during the 1930s Sitka and Norway spruce overtook pines as the most planted trees. Norway spruce was planted on moist waterlogged sites of medium to high fertility, including the less acid peats, but because Norway spruce is less accommodating the proportion of planting fell rapidly against Sitka Spruce and Lodgepole pine from the 1940s onwards. Lodgepole pine resembles Scots pine and, like the latter, it is tolerant of poor soils. However, logdepole pine can tolerate wet conditions much better than Scots pine and is therefore widely planted at high elevations on the poorest western soils and it will grow well with only low inputs of fertiliser. Its hollow roots bring air into the ground and provide it with the capacity for drying out peat. Lodgepole pine is widely planted as a nurse to provide shelter, usually in mixtures of Sitka spruce. However, this practice almost ceased by the late 1960s in favour of Sitka spruce.

The use of Sitka spruce had a profound impact on the shape of British forestry. From the late 1920s it was realised that it was a tough tree that was easy to establish and hard to kill, and it was regarded as the most suitable tree for growing on the wet upland parts of Scotland. Sitka spruce is highly productive, tolerates high levels of exposure, and grows well on a wide variety of sites. It grows well on drained peats, is highly productive on gley soils and grows best on well-drained deep soils in suitable high rainfall areas. It was in conjunction with research into new planting techniques for marginal grounds that Sitka spruce began to assume its dominant position on difficult sites. Because of its suitability for planting on the upland peats of Scotland, more research has been devoted to Sitka spruce than to any other single tree species.