Radial patterns of specific gravity variation in North American conifers

An extensive summary of publications reporting radial specific gravity (SG) variation for North American conifers is presented. SG varies from pith to bark in three distinct patterns before reaching asymptomatic values: Type 1 (SG increases with cambial age), Type 2 (SG is initially high at the pith, then decreases with cambial age, before increasing), and Type 3 (SG decreases with cambial age). Pinaceae have either a Type 1 or 2 pattern (majority are Type 2). Cupressaceae have a Type 3 pattern, but radial SG data do not exist for some species. We reviewed publications in table 7-1 of Panshin and de Zeeuw (the 1970 edition) which reports SG variation by species and Type (the 1980 edition has an equivalent table but does not reference publications), examining sampling strategies and radial patterns. For a small number of species, Type was either incorrectly interpreted or sampling made a conclusion regarding pattern of radial variation impossible. Specific examples of mislabeled Types reported by Panshin and de Zeeuw for Douglas-fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta), red pine (Pinus resinosa) and shortleaf pine (Pinus echinata), and western redcedar (Thuja plicata) are described. We conclude for nearly all species the pattern of radial SG variation is consistent; however, different patterns have been reported for eastern white pine (Pinus strobus) and subspecies of Douglas-fir and ponderosa pine (Pinus ponderosa).