A resource capture efficiency index to compare differences in early growth of four tree species in northern England

At a trial established in Cumbria, northern England, significant differences in growth rate between tree species were apparent, with cider gum (Eucalyptus gunnii) and alder (Alnus glutinosa) exhibiting most rapid volume and biomass accumulation. Estimations were made of leaf area, specific leaf area, leaf area ratio (based on stem mass not whole tree mass) and length of growing season. These measurements were undertaken to explain tree growth difference and developing a growth potential index based on growing season length and leaf area. The high leaf area of cider gum and alder explained some of their superior growth, while alder also had the longest period in leaf, compared with ash (Fraxinus excelsior) and sycamore (Acer pseudoplatanus). The slow growth of ash can be explained by the short period in leaf and also the relatively low leaf area ratio. Leaf area to stem weight also differed between species with that of ash being relatively low. Specific leaf area was also low for ash, a trait shared with cider gum, which suggests that these species invest highly in each unit of leaf area. Of the tree species assessed, the length of the growing season was longest for alder, enabling it to maintain growth for a longer period. By multiplying growing season by leaf area a resource capture index was calculated and this explained 56% of the variation in stem dry weight between trees. The potential and limitations for using this index are discussed.