Allocation to tree bark in pine and oak species in fire affected mixed forests across the Northern Hemisphere

Pines (Pinus, with 120 species) and oaks (Quercus, with at least 400 species), the two principal forest forming genera of the Northern Hemisphere occur in fire affected landscapes over a large geographical area from 120 degrees W to 150.E long, and from 10 degrees to 60 degrees N lat. We compiled studies of 116 such sites of which 75 sites had data of bark thickness. Taking a meta-analysis approach, here we have compared the absolute and relative bark thickness (bark thickness per unit stem diameter) of oak species sharing mixed stands to generalize adaptational significance of bark in relation to fire. For the meta-analysis we considered only natural forests with adequate evidences of fire history. We gave importance to relative bark thickness, which is less sensitive to tree size than absolute thickness while comparing species. At 100 cm girth the average absolute bark thickness was significantly higher for Diploxylon pines (3.65 cm) than for oaks (2.2 cm) and Haploxylon pines (1.65 cm). On an average, the relative bark thickness was higher for the fire adapted Diploxylon pines (6.3%) than for Quercus species, (4.4%). The resource allocation efficiency was higher in pine species than for oak species, emphasizing the importance of bark characters in their persistence and range expansion. The difference between species of two principal forest genera in relative bark thickness and hence success in fire affected landscapes across a wide geographical range strongly testifies its adaptational significance to fire.