Assessing UK Forest Structural Diversity

We collected data from 223 sample points distributed in 24 stands located in eight forests across the UK to consider different species, overstory, and geographical conditions (see Table 1). For each stand, we laid out ten sample points with a random systematic approach. We drew random transects on a desktop map and placed on them evenly spaced points, later identified in the field using a GPS receiver. The distance between points varied with the size of the stand. As most of the stands were originated by artificial planting, transects were not laid out parallel to each other to avoid following the planting lines. When carrying out the field survey, if a sample point fell in an open gap with no overstory, we relocated it under canopy cover if possible; otherwise, it was discarded (thus some stands had <10 sample points).
We assigned to each compartment a categorical variable named OV according to the overstory main species, with the following levels: “broadleaves” for mixed stands composed mainly of European beech (Fagus sylvatica L.) and oaks (Quercus petraea [Matt.] Liebl. and Quercus robur L.); “douglas” for Douglas fir (Pseudotsuga menziesii [Mirb.] Franco), sometimes associated with broadleaves; “larch” for European and Japanese larch (Larix kaempferii [Lamb] Carr. and Larix decidua Mill.); “pine” for Corsican and Scots pine (Pinus nigra subsp. laricio Maire and Pinus sylvestris L.); and “spruce” for Sitka spruce (Picea sitchensis [Bong.] Carr.).